5 WATER QUALITY 5.1 INTRODUCTION 5.2 METHODOLOGY

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1 5 WATER QUALITY 5.1 INTRODUCTION The proposed capital dredging, offshore disposal of dredged material and beach nourishment would redistribute sediments across Poole Harbour and Poole Bay with the potential to change water quality conditions. This section identifies the water quality characteristics of Poole Harbour and Poole Bay, and assesses the magnitude and significance of changes associated with the proposed project with reference to a number of EC Directives concerning water quality. 5.2 METHODOLOGY Data collection The existing water quality conditions have been characterised using information yielded from a literature review, including a detailed report by the Marine Biological Association (MBA) (2003), water quality survey data available from the Environment Agency and suspended solids data described in Section 3. Assessment of significance The principle approach to assessing water quality effects has been to predict impacts against the assessment criteria established as water quality standards under the EC Directives (and relevant UK legislation) covering dangerous substances, bathing waters and shellfish waters. Where possible, impact magnitude has been assessed quantitatively, allowing a comparison between a predicted environmental change in water quality and water quality assessment criteria. If a quantitative assessment cannot be made, then impacts have been predicted qualitatively. Assessment criteria for dangerous substances The EC Dangerous Substances Directive was adopted in 1976 to control pollution caused by certain dangerous substances on the aquatic environment. The Directive established List I substances, which are regarded as particularly dangerous because of their toxicity, persistence and bioaccumulation; pollution by these substances must be eliminated, and List II substances, which are regarded as less dangerous but which have a deleterious effect on the aquatic environment; pollution by these substances must be reduced The Dangerous Substances Directive stipulates uniform emission standards (UESs, also known as limit values) and environmental quality standards (EQSs) as approaches for the control of List I substances. All member states are required to establish EQSs for List II substances on a national level. EQSs for List II substances have been implemented in the UK by the Surface Waters (Dangerous Substances) (Classification) Regulations 1997 and The EQSs for List I and List II substances form the assessment criteria for water quality concerning dangerous substances. and Beneficial Use Schemes EIA 135 Poole Harbour Commissioners

2 5.2.5 The EQSs for selected List I substances are shown in Table 5.1. The table is based on the information presented in Guidelines for managing water quality impacts within UK European marine sites (Cole et al., 1999). Table 5.1 Selected List I dangerous substances* Substance** EQS Type Marine EQS (annual average, µg.l -1 ) Estuarine EQS*** (annual average, µg.l -1 ) Mercury (dissolved) Annual average Cadmium (dissolved) Annual average HCH (Lindane) **** Annual average Total DDT Annual average ppddt Annual average Pentachorophenol Annual average 2 2 Aldrin Annual average Dieldrin Annual average Endrin Annual average Isodrin Annual average Total 'Drins' Annual average Hexachlorobenzene Annual average Hexachlorobutadiene Annual average Carbon tetrachloride Annual average Chloroform Annual average ,2-dichloroethane Annual average Trichloroethyleme Annual average Perchloroethylene Annual average Trichlorobenzene Annual average * EQS List I taken from and WRc (October 1999) **total concentration (i.e. without filtration) unless specified *** in the UK, standards for estuaries are the same as for marine EQSs, as established under the Surface Waters (Dangerous Substances) (Classification) Regulations 1989 **** all HCH isomers, including Lindane All member states are required to establish EQSs for List II substances on a national level. EQSs for List II substances have been implemented in the UK by the Surface Waters (Dangerous Substances) (Classification) Regulations 1997 and The EQSs for selected List II substances are shown in Table 5.2. Table 5.2 Selected List II dangerous substances* Substance EQS Type Marine and Estuarine EQS (annual average, µg.l -1 ) Arsenic (dissolved) Annual average 25 Chromium (dissolved) Annual average 15 Copper (dissolved) Annual average 5 Lead (dissolved) Annual average 25 Nickel (dissolved) Annual average 30 Tributyl tin Maximum concentration Zinc (total) Annual average 40 * the full EQS List II is available on and Beneficial Use Schemes EIA 136 Poole Harbour Commissioners

3 5.2.7 The risk of an adverse impact on water quality relates to the chemical contaminants and organic material associated with the sediment, and how these substances are affected by disturbance during dredging. An equilibrium partitioning approach is used to derive significance criteria to establish whether the sediment in the dredging areas contains sufficient concentrations of contaminants to pose a threat to water quality. Sediment significance criteria derived from equilibrium partitioning provide a set of values that may be tentatively considered as safe sediment concentrations (Webster and Ridgway, 1984). That is, sediment with concentrations less than these values are not expected to adversely affected water quality Sediment significance criteria for water quality (C sed ) are calculated using published partition coefficients based on organic carbon content (K oc ), water EQSs (C w/cr ), and the sediment s total organic carbon content (TOC), where C sed = K oc C w/cr TOC. The mean TOC 0.59% for the dredging areas is 0.59%. The derived criteria are shown in Table Criteria are derived for a selected number of contaminants. Criteria for metals, two PAHs and total PCBs are derived where partition coefficients are available. A criterion for TBT can be derived, but it is so low that it is way below detection limits and cannot be compared to the sediment survey data for the dredging areas. Criteria for organochlorine pesticides are derived only for ppddt and g-hch but, as shown in these examples, do not provide a useful comparison because all sediment survey data for the dredging areas are recorded at concentrations below detection limits. Table 5.3 Sediment significance criteria derived from equilibrium partitioning Substance C w/cr Marine EQS (µg/l) K oc * TOC (%) C sed (mg/kg) Arsenic (dissolved) 25 13, Cadmium (dissolved) , Copper (dissolved) 5 1,700, Lead (dissolved) , Mercury (dissolved) 0.3 8, Zinc (total) , Fluoranthrene 8 45, Benzo(a)pyrene 8 300, PCB ,000*** Total DDT , g-hch , * partition coefficients taken from Webster and Ridgway (1984) unless stated ** partition coefficients taken from Waldock et al (1989) *** average partition coefficient for PCB congeners **** partition coefficient for ppddt Assessment criteria for bathing waters The quality of bathing waters in England and Wales is monitored against standards laid down in the Bathing Water Regulations 1991, which give effect to the EC Bathing Waters Directive (76/160/EEC). Compliance with the Directive is monitored against microbiological standards. Water samples are taken in the bathing waters during the and Beneficial Use Schemes EIA 137 Poole Harbour Commissioners

4 bathing season (20 samples from May to September) and analysed for total coliform bacteria and for faecal coliform bacteria (as Escherichia coli). The mandatory standards, which should not be exceeded, are 10,000 total coliforms per 100ml of water and 2,000 faecal coliforms per 100ml of water In order for bathing water to comply with the Directive, 95% of the samples (19 out of 20) must meet the mandatory standards, plus other criteria. There are more stringent guideline criteria which stipulate that 80% of samples must not contain more than 500 total coliforms or 100 faecal coliforms per 100ml and 90% of samples must not contain more than 100 faecal streptococci per 100ml. The guideline criteria are preferable, but not a statutory basis for monitoring bathing water quality The mandatory bathing water quality criteria form the assessment guidelines for water quality concerning bathing waters In December 2000 the European Commission put forward its new thinking on the broad principles for revising the EC Bathing Waters Directive. Although the revision was then expected to be published in June 2002, it has yet to occur. The Commission s intentions include extending the scope of the Directive. For example, according to DEFRA s interpretation, the term bathing could be broadened to cover activities and recreation involving direct bodily contact with water, such as surfing and kayaking, but with a likely emphasis on activities on inland waters rather than coastal waters. In addition, the bathing season could be extended to 12 months and require more extensive monitoring. There also could be a requirement for active beach management, enhanced public information and suggested new standards for water quality based on concentrations of pathogens suggested by the World Health Organisations. Assessment criteria for shellfish waters The Shellfish Waters Directive (79/923/EEC) is designed to protect the aquatic habitat of bivalve and gastropod molluscan species of shellfish in order to contribute to high quality edible shellfish products. The species covered by the Directive include oysters, mussels, cockles, scallops and clams, but not shellfish crustaceans such as crabs, crayfish and lobsters. The Shellfish Waters Directive (79/923/EEC) is implemented in the UK under the Shellfish Waters (Shellfish) (Classifications) Regulations Shellfish waters are monitored for various parameters based on water quality standards established by the Shellfish Waters Directive. These parameters include suspended solids, salinity, dissolved oxygen (DO), organo-halogenated substances (e.g. PCBs, organo-chlorine pesticides), metals and coliforms. For each substance, the Directive specifies the minimum number of samples to be taken, the water quality standards to be met and the percentage of samples that must meet these standards. The standards are either a numeric limit or a descriptive standard (see Table 5.4). According to the Environment Agency (see the water quality standards have been met if the following percentage of the samples analysed do not exceed the limit values: 100% for metals and organo-halogen compounds; 95% for salinity and dissolved oxygen (DO); 75% for other substances; and, No evidence of harm to the shellfish from organo-halogenated compounds. and Beneficial Use Schemes EIA 138 Poole Harbour Commissioners

5 Table 5.4 Selected imperative standards for shellfish waters Parameter Units Standard Suspended solids mg.l -1 A discharge affecting shellfish waters must not cause the suspended soil content of the waters to exceed by more than 30% the content of waters not so affected Salinity Parts per thousand (i.e. g.l -1 ) <=40 parts per thousand A discharge affecting shellfish waters must not cause their salinity to exceed by more than 10% the salinity of the waters no so affected Dissolved oxygen % saturation Average of individual values >70% and an individual measurement may not indicate a value lower than 60% unless there are no harmful consequences for the development of shellfish colonies Organohalogenated substances - The concentration of each substance in the shellfish waters or in the shellfish flesh must not reach or exceed a level which has harmful effects on the shellfish and their larvae Metals (Ag, As, Cd, Cr, Cu, Hg, Ni, Pb, Zn) mg.l -1 The concentration of each substance in the shellfish waters or in the shellfish flesh must not reach or exceed a level which has harmful effects on the shellfish and their larvae The synergic effects of these metals must be taken into consideration 5.3 EXISTING ENVIRONMENT Introduction The existing environment for water quality is established using published information and raw data acquired from the Environment Agency. A review of general water quality is provided in Sections and 5.3.3, and is described in more detail with regard to the following EC Directives on dangerous substances, bathing waters and shellfish waters, and their assessment criteria: A detailed review of water quality in Poole Harbour is presented by a recent study by the MBA (2003) made on behalf of the Environment Agency and English Nature. The objectives of this study were to characterise water quality and to identify areas where water quality conditions might result in effects on habitats and species for which the Poole Harbour SPA is designated. The study also considered whether permissions, activities and sources are likely to have a significant effect on the site The study reviewed published literature and unpublished reports and interrogated raw data sets, notably of the Environment Agency. Some of the key findings were that current chemical compliance monitoring is restricted to the main Harbour, where water quality generally confirms to statutory limits. Nevertheless, zinc concentrations in water (and seaweed) from the Lake Pier area were found to be high and may reflect additional sources from antifouling substances and sacrificial anodes. The MBA (2003) reported that much of the marine site, notably the north Harbour and secondary embayments, is subject to eutrophication. Various bioassays have confirmed that water quality is relatively good in the Harbour as a whole, with some deterioration in biological condition inside Holes Bay Water quality monitoring data for the years 2000 to 2003 were provided by the Environment Agency. The data are recorded from seven sites within the study area and Beneficial Use Schemes EIA 139 Poole Harbour Commissioners

6 including six sites in Poole Harbour and one site in Poole Bay. The Poole Harbour sites are Wareham Channel (buoy 82), Poole Bridge, Poole Harbour (buoy 36), Harbour Entrance, South Deep, Hutchins (buoy 71) and Poole Bay (see Figure 5.1). The Environment Agency s data are summarised as mean values and are presented in Table 5.5. The data are used in the subsequent sections of this report to provide data about the existing environmental conditions. Table 5.5 Summarised water quality data for Poole Harbour and Poole Bay (Environment Agency ) Determinand Wareham Hutchins Poole Poole Harbour South Poole Channel Buoy Bridge Harbour Entrance Deep Bay Temperature ( o C) Salinity (g/kg) ph Susp. solids (mg/l) Turbidity (FTUs) Chlorophyll a (µg/l) No data DO (% saturation) DO (mg/l) BOD (mg/l) No data TON (µg/l) No data Nitrate (µg/l) No data Nitrite (µg/l) No data Ammonia (µg/l) No data Orthophos. (µg/l) No data Cadmium (µg/l) <0.25 No data < Mercury (µg/l) 0.01 No data Arsenic (µg/l) 1.1 No data No data Chromium (µg/l) 0.5 No data No data Copper (µg/l) 1 No data No data Lead (µg/l) <2.5 No data <2.5 <2.5 No data <2.5 <2.5 Nickel (µg/l) 3 No data No data 3 3 and Beneficial Use Schemes EIA 140 Poole Harbour Commissioners

7 Table 5.5 (continued) Determinand Wareham Hutchins Poole Poole Harbour South Poole Channel Buoy Bridge Harbour Entrance Deep Bay Silver (µg/l) <1 No data No data <1 No data <1 <1 Zinc (µg/l) 4.9 No data No data OC compounds (µg/l) * No data * * * * * Coliforms (no./100ml) 529 No data No data F. strep. (no./100ml) No data No data 47 No data No data No data No data * Virtually all data recorded as below the detection limit Dangerous Substances Water quality monitoring data for the years 2000 to 2003 were provided by the Environment Agency and are summarised in Table 5.6. For establishing the baseline conditions for Dangerous Substances, one site (of seven) has been taken to represent Poole Harbour, namely the site at buoy 36 in the North Channel, adjacent to the Middle Ship Channel, near Salterns Marina. Another site has been taken to represent Poole Bay (see Table 5.7) Poole Harbour The results for the determinands covered by the Environment Agency s monitoring data indicate that no parameters breach the EQSs established under the EC Dangerous Substances Directive. Poole Bay The results for the determinands covered by the Environment Agency s monitoring data indicate that no parameters breach the EQSs at the Poole Bay site, except for one result for cadmium and one result for chromium. Both these results were significantly higher than any other results recorded by the Environment Agency, were recorded on different days, and possibly represent anomalous situations compared to normal (i.e. the median concentrations). and Beneficial Use Schemes EIA 141 Poole Harbour Commissioners

8 Wareham Wareham Channel Channel (Buoy (Buoy 82) 82) South South Deep Deep Hutchins Hutchins (Buoy (Buoy 71) 71) Poole Poole Bridge Bridge Harbour Harbour Entrance Entrance Poole Poole Harbour Harbour (Buoy (Buoy 36) 36) Poole Poole Bay Bay Figure 5.1 Date: Oct 2004 Client: BoP and PHC Scale: Not to scale Project: Poole Harbour Approach Channel Deepening and Beneficial Use Schemes Title: Environment Agency water quality monitoring sites Source: ARCS Charts under license from the UK Hydrographic Office Key:

9 Table 5.6 Summarised dangerous substances data for Poole Harbour ( ) Determinand Minimum Maximum Mean (µg/l) EQS* (µg/l) No. of Data No. of Data (µg/l) (µg/l) Exceeding EQS Cadmium <0.25 <0.25 < Mercury < Arsenic < Chromium < Copper < Lead <2.5 <2.5 < Nickel < Zinc < HCH (Lindane) <0.002 <0.002 < ppddt <0.001 <0.001 < Aldrin <0.001 <0.001 < Dieldrin <0.001 <0.001 < Endrin <0.001 <0.001 < Isodrin <0.001 <0.001 < Total 'Drins' Hexachlorobenzene <0.001 <0.001 < Hexachlorobutadiene < < < * For the purposes of this comparison, the lower value of the marine and estuarine EQSs is used (see Tables 5.2 and 5.3) and Beneficial Use Schemes EIA 143 Poole Harbour Commissioners

10 Table 5.7 Summarised dangerous substances data for Poole Bay ( ) Determinand Minimum (µg/l) Maximum (µg/l) Mean (µg/l) EQS (µg/l) No. of Data No. of Data >EQS Cadmium < Mercury < Arsenic < Chromium < Copper < Lead <2.5 <2.5 < Nickel < Zinc < HCH (Lindane) <0.002 <0.002 < ppddt <0.001 <0.001 < Aldrin <0.001 <0.001 < Dieldrin <0.001 <0.001 < Endrin <0.001 <0.001 < Isodrin <0.001 <0.001 < Total 'Drins' < Hexachlorobenzene <0.001 <0.001 < Hexachlorobutadiene < < < Bathing Waters There are 21 bathing waters within the study area: 5 in Poole Borough, 9 in Bournemouth Borough, and 7 in Purbeck District. The locations of these bathing waters (using the positions of their water quality monitoring points) are shown on Figure 5.2. Bathing water quality is established under the EC Bathing Water Directive in accordance with the assessment criteria described in Section Tables 5.8, 5.9 and 5.10 identify the status of water quality achieved at bathing waters in Poole Borough, Bournemouth Borough and Purbeck District respectively. Water quality status is classified as excellent, good or poor, relative to the requirements of the Bathing Waters Directive. Bathing waters classified as excellent or good comply with the mandatory water quality criteria of the Directive and, in the case of an excellent classification, the more stringent guideline criteria. Bathing waters classified as poor fail to meet the Directive s mandatory criteria. and Beneficial Use Schemes EIA 144 Poole Harbour Commissioners

11 Poole Poole Harbour Harbour Lake Lake Swanage Central Central Swanage Studland Studland Knoll Knoll House House Shell Shell Bay Bay North North Poole Poole Harbour Harbour Sandbanks Sandbanks Poole Poole Shore Shore Road Road Sandbanks Sandbanks Bournemouth Bournemouth Hengistbury Hengistbury East East Bournemouth Bournemouth Southbourne Southbourne Christchurch Christchurch Mudeford Mudeford Sandbank Sandbank East East Bournemouth Bournemouth Fishermans Fishermans Walk Walk Bournemouth Bournemouth Durley Durley Chine Chine Branksome Branksome Chine Chine Bournemouth Bournemouth Pier Pier Poole Poole Sandbanks Sandbanks Car Car Park Park Poole Poole Harbour Harbour Rockley Rockley Sands Sands Bournemouth Bournemouth Alum Alum Chine Chine Bournemouth Bournemouth Boscombe Boscombe Pier Pier Figure 5.2 Date: Oct 2004 Client: BoP and PHC Approx scale: 1cm = 1.4km Project: Poole Harbour Approach Channel Deepening and Beneficial Use Schemes Title: Location of designated bathing waters Source: ARCS Charts under license from the UK Hydrographic Office Key:

12 All the bathing waters within Poole, Bournemouth and Purbeck areas have exhibited excellent and good water quality since the 1990s. For a more detailed breakdown of the Environment Agency s data for each bathing water for each year, the reader is referred to Your Environment What s in your Backyard section of the Environment Agency s website Table 5.8 Bathing water quality at Bathing Waters within the Borough of Poole Bathing Water Poole Shore Road G E G E E G E E E E E E E E E Sandbanks Poole Harbour P G G G G G E E E E G E E E E Sandbanks Poole Sandbanks E E E E E E E Car Park Poole Harbour G G G G G G G G G E G E E E E Lake Poole Harbour Rockley Sands G G G G G G G G G G G G G G E Bathing Waters Classifications: E = Excellent, G = Good, P = Poor Table 5.9 Bathing water quality at Bathing Waters within Bournemouth Borough Bathing Water Christchurch G G G G G G G E E E E E E E E Mudeford Sandbank East Bournemouth G G G E E E G E E E E E E E E Hengistbury East Bournemouth E E E E Southbourne Bournemouth E E E E E E E E E E G Fishermans Walk Bournemouth P G G G G G G G G G G Boscombe Pier Bournemouth Pier G G G E G G G E G G G G G G G Bournemouth E E E E G E E E E E E Durley Chine Bournemouth E E E E Alum Chine Branksome Chine E E E E Bathing Waters Classifications: E = Excellent, G = Good, P = Poor and Beneficial Use Schemes EIA 146 Poole Harbour Commissioners

13 Table 5.10 Bathing Water Quality at Bathing Waters within Purbeck District Bathing Water Shell Bay North G E E E E E E E E E E E E E E Studland Knoll G E G G E E E E E E E E E E E House Swanage Central G G G G P G E E G E G E E E E Kimmeridge Bay G E G G P P G E E E G E E E E Lulworth Cove G G G G G E G G G G G E E G E Durdle Door East G E G E E E E E E E E E E E E Durdle Door West G E E E E E E E E E E E E E E Bathing Waters Classifications: E = Excellent, G = Good, P = Poor Shellfish Waters Poole Harbour Poole Harbour is split into three designated shellfish waters under the EC Shellfish Waters Directive: Poole Harbour North, South and West (see Figures 5.3 and 5.4). Poole Harbour North includes all of the area north of a line between the Harbour entrance and Patchins Point, excluding Holes Bay and Parkstone Bay to the north, and Wareham Channel and Lychett Bay to the west. Poole Harbour South includes all of the area south of a line between the Harbour entrance and Patchins Point. Poole Harbour West includes most of Wareham Channel (but not its western extent), but excludes Lychett Bay Water quality within the Harbour is regularly monitored by the Environment Agency at monitoring points representing the three shellfish waters. The monitoring points are at Salterns Main Channel (NGR SZ ) (now marked on Admiralty Charts as North Channel), South Deep (NGR SZ ) and Wareham Channel (NGR SY ) for Poole Harbour North, South and West shellfish waters respectively (see Figures 5.3 and 5.4) Water quality in the Harbour has, in the past, failed the Shellfish Waters Directive standards. For example, the Salterns Main Channel site failed the Directive in 1996 and 1997 due to elevated levels of nickel and zinc, with copper levels also exceeding Directive standards in The Environment Agency s monitoring data ( ) is summarised as minimum, maximum, mean and median values in Tables 5.11, 5.12 and and Beneficial Use Schemes EIA 147 Poole Harbour Commissioners

14 Key: Monitoring station Poole Harbour Approach Channel Deepening and Beneficial Use Schemes Poole Harbour North and Poole Harbour South Shellfish Waters Figure 5.3 BoP and PHC Source: CEFAS Date: Oct 2004 Scale: NTS

15 Key: Monitoring station Poole Harbour Approach Channel Deepening and Beneficial Use Schemes Poole Harbour West and Poole Bay Shellfish Waters Figure 5.4 BoP and PHC Source: CEFAS Date: Oct 2004 Scale: NTS

16 Table 5.11 Summarised water quality data for Poole Harbour North Shellfish Water Parameter Minimum Maximum Mean Median Suspended solids (mg.l -1 ) Salinity (g.l -1 ) Dissolved oxygen (% saturation) Organo-halogenated substances (µg.l -1 ) Virtually all data recorded as < detection limit Metals (dissolved) (µg.l -1 ) As Cd Cr Cu Hg Ni Pb Zn <1 <0.025 <0.5 <0.5 <0.01 <3 <2.5 <4 1.5 < < < < <0.025 < <0.01 <0.3 <2.5 <4 Faecal coliforms pre. (no. per 100ml) Faecal coliforms conf. (no. per 100ml) Note: < means less than Table 5.12 Summarised water quality data for Poole Harbour South Shellfish Water Parameter Minimum Maximum Mean Median Suspended solids (mg.l -1 ) Salinity (g.l -1 ) Dissolved oxygen (% saturation) Organo-halogenated substances (µg.l -1 ) Virtually all data recorded as < detection limit Metals (dissolved) (µg.l -1 ) As Cd Cr Cu Hg Ni Pb Zn <1 <0.25 <0.5 <0.5 <0.01 <3 <2.5 < < < <0.25 < <0.01 <3 <2.5 <4 Faecal coliforms pre. (no. per 100ml) Faecal coliforms conf. (no. per 100ml) Note: < means less than and Beneficial Use Schemes EIA 150 Poole Harbour Commissioners

17 Table 5.13 Summarised water quality data for Poole Harbour West Shellfish Water Parameter Minimum Maximum Mean Median Suspended solids (mg.l -1 ) Salinity (g.l -1 ) Dissolved oxygen (% saturation) Organo-halogenated substances (µg.l -1 ) Virtually all data recorded as < detection limit Metals (dissolved) (µg.l -1 ) As Cd Cr Cu Hg Ni Pb Zn <1 <0.25 <0.5 <0.5 <0.01 <3 <2.5 <4 1.6 < < < < <1 <0.25 < <0.01 <3 <2.5 <4 Faecal coliforms pre. (no. per 100ml) Faecal coliforms conf. (no. per 100ml) ,000 10, Note: < means less than Poole Bay In addition, Poole Bay is also designated as a shellfish water (see Figure 5.4). The monitoring point for Poole Bay is south of Bournemouth Pier (NGR SZ095890). The Environment Agency s monitoring data ( ) is summarised in Table Table 5.14 Summarised water quality data for Poole Bay Shellfish Water Parameter Minimum Maximum Mean Median Suspended solids (mg.l -1 ) < <3 Salinity (g.l -1 ) Dissolved oxygen (% saturation) Organo-halogenated substances (µg.l -1 ) Virtually all data recorded as < detection limit Metals (dissolved) (µg.l -1 ) As Cd Cr Cu Hg Ni Pb Zn <1 <0.25 <0.5 <0.05 <0.01 <3 <2.5 < < < <0.25 < <0.01 <3 <2.5 <4 Faecal coliforms pre. (no. per 100ml) Faecal coliforms conf. (no. per 100ml) <10 < <10 <10 Note: < means less than It is worth noting that there is additional information in Section 3.3 about suspended solids concentrations in Poole Harbour and Poole Bay. This information is not directly related to the EC Shellfish Waters Directive. and Beneficial Use Schemes EIA 151 Poole Harbour Commissioners

18 5.4 POTENTIAL IMPACTS ASSOCIATED WITH THE APPROACH CHANNEL DEEPENING Construction phase Potential effect on water quality due to release of dangerous substances during capital dredging The capital dredging has the potential to disturb sediment and release sediment-bound chemical contaminants into the overlying water column. A release of chemical contaminants could increase concentrations in Poole Harbour and Poole Bay, and affect the water quality with regard to the EQSs required by the Dangerous Substances Directive This assessment addresses whether baseline concentrations in the interstitial water of the in situ sediment (i.e. when the solids and liquid phases are in equilibrium) are present in concentrations that exceed EQSs established for the EC Dangerous Substances Directive. For this assessment, mean contaminant concentrations recorded by a sediment survey of the dredging areas (see Sections to ) are compared to significance criteria derived from equilibrium partitioning (see Table 5.15) The comparison shows that most mean concentrations of contaminants in the sediment of the dredging areas are below the significance criteria. This means that the interstitial water of the sediment mostly contains metals, PAHs, PCBs and organochlorine pesticides at concentrations below EQSs for dangerous substances However, mean arsenic and mercury concentrations in all the dredging areas and the mean fluoranthrene (a PAH) concentration in the Little Channel exceed the significance criteria. This means that the interstitial water of the sediment contains these substances at concentrations above EQSs for dangerous substances. The magnitude of the exceeding concentrations over the EQSs significance criteria is less than one order of magnitude (i.e. a factor of 10) Dredging would release sediment into the overlying water column. The sediment and the contaminants in the interstitial water would undergo significant dilution in the water column as they are dispersed throughout Poole Harbour and Poole Bay. In the case of arsenic, mercury and fluoranthrene, a dilution of 10 would be sufficient to reduce interstitial concentrations to below EQSs. Hence, one litre of interstitial water needs only 10 litres of water in the overlying water column to dilute its concentrations of arsenic, mercury and fluoranthrene to concentrations below EQSs. and Beneficial Use Schemes EIA 152 Poole Harbour Commissioners

19 Table 5.15 Comparison of sediment criteria with mean sediment concentrations in the proposed dredge areas Substance C sed Criteria (mg/kg) C sed Swash Channel (mg/kg) C sed Middle Ship Channel (mg/kg) C sed Turning Basin (mg/kg) C sed Little Channel (mg/kg) Arsenic (dissolved) Cadmium (dissolved) Copper (dissolved) Lead (dissolved) Mercury (dissolved) Zinc (total) Fluoranthrene Benzo(a)pyrene < Total PCB 0.17 < < < < ppddt <0.01 <0.01 <0.01 <0.01 g-hch <0.01 <0.01 <0.01 < Clearly, the sediment released during dredging would undergo sufficient dilution and dispersion throughout Poole Harbour and Poole Bay, as shown by the numerical modelling of suspended solids (see Section 3.6), to not change concentrations of arsenic, mercury and fluoranthrene to the extent that EQSs would be significantly affected. On this basis, it is concluded that dredging would result in a short term, localised (i.e. in the vicinity of the dredger) impact of minor adverse significance on water quality since it would increase the concentrations of some dangerous substances in the water column. After initial dilution, the dangerous substances released by dredging would be at concentrations below EQSs and, therefore, no impact is predicted beyond the immediate vicinity of the dredging. Mitigation and residual impact The release of contaminants into the water column as a result of the proposed dredging is not possible to mitigate. The residual impact would be of minor adverse significance in the vicinity of the dredger, with no impact beyond this area. Potential effect on bathing water quality during capital dredging The proposed capital dredging has the potential to disturb sediment and release sediment-bound bacteria into the overlying water column. Subsequently, bacteria may be transported to designated bathing waters around Poole Harbour and Poole Bay, and affect the water quality with regard to the standards required by the Bathing Waters Directive The capital dredging would not occur within designated bathing waters, although numerical modelling (see Section 3.6) of the dispersion of suspended solids arising from capital dredging indicates that the hydrodynamic forces within Poole Harbour and Poole Bay may transport sediment to bathing waters (see Figure 3.8). Accordingly, there and Beneficial Use Schemes EIA 153 Poole Harbour Commissioners

20 appears to be an impact pathway for bacteria associated with sediment released by dredging to reach bathing waters However, there are a number of reasons why bacteria released from sediment in the Middle Ship Channel, Turning Basin and Little Channel (the Swash Channel is considered uncontaminated) would not significantly affect bathing water quality. These are as follows: Source concentration of bacteria. The sediments of the Middle Ship Channel, Little Channel and Turning Basin contain various concentrations of faecal coliforms (2-8,000/ml) and streptococci (0-35/ml) (see Tables 4.14 and 4.15). The mean concentration for faecal coliforms in the sediment is 2,867/ml. Source distribution of bacteria. Bacteria are only present in significant concentrations in the upper few centimetres of the sediment. Based on 5cm (allowing some leeway over the 4cm quoted by the Environment Agency, 2000a) depth of contamination, then the volume of bacteria-contaminated material approximates to 4.9% of the total volume of the Middle Ship Channel, Turning Basin and Little Channel. Therefore, dredging of the surface sediments (i.e. 4.9% by volume) poses the highest risk of an impact on water quality and dredging of the deeper sediment (95.1% by volume) poses no or very little risk. Sediment dispersion in the water column due to dredging. The material would be dredged from the seabed and retained in the dredger s hopper. There would be some overflowing of fine-grained material to optimise the hopper loading. The total loss of fine-grained material would be 15% of the total volume, of which only 4.9% of the lost material may be considered contaminated by bacteria. Bacteria die-off. The time taken between dredging and the bacteria reaching bathing waters is important because bacteria die over short periods of time to the extent that they may be inactivated either before or not long after reaching bathing waters. Bacteria can become inactivated over a matter of hours, even in winter when the temperature is lower and sunlight hours are less. For example, Sinton et al (1999; in Environment Agency, 2000a) identify T 90 die-off rates for faecal coliforms in sunlight hours to be 7.7 hours and in dark hours to be 63 hours during winter. Some degree of die-off would occur during dredging. Bacterial die-off is not quantified specifically for this assessment, but using an average of 0.8 days for T 90 of E. coli in sea water (Anon, 1999; in Environment Agency, 2000a), it is clear that there is the potential for 90% of bacteria to die-off within less than 24 hours of dredging, which will substantially reduce the concentration of bacteria affecting the bathing waters. Dredging duration. Based on a dredger with a hopper capacity of about 1,500m 3 working on a 3 hour dredging and disposal cycle, the duration of the works for dredging of sediment that is potentially bacteria-contaminated is about 3.5 days (assuming continuous dredging and disposal). This time period is a very small proportion of the total dredge period (estimated to be around 7 months), and indicates that any impact on water quality will be a short-term effect. In practice, the 3.5 days is likely to be spread over a much larger proportion of the whole period of dredging, which would have the effect of increasing the duration, but reducing the intensity, of the impact. and Beneficial Use Schemes EIA 154 Poole Harbour Commissioners

21 Bacteria dilution in the water column. If the mean concentration for faecal coliforms in the dredged material is taken to be 2,867/ml and the significance criteria is taken to be the mandatory bathing water standard, which is 2,000/100ml, then a water dilution of 150 will be sufficient to reduce faecal coliform concentrations in the sediment to a level below the bathing water standard, before die-off is taken into account For this assessment the volume of potentially bacteria-contaminated sediment that would be released into the water column during dredging is taken to be the 4.9% of the material lost to the water column during dredging (132,150m 3 ). This gives a total release volume of 6,475m 3. If 6,475m 3 of material containing faecal coliforms at a mean concentration of 2,867/ml is released into the sea during dredging, a dilution of 150 will require M litres or 971,250 m 3 of sea water. This is a relatively small volume of Poole Harbour. For example, the water volume in the Middle Ship Channel, Turning Basin and Little Channel is currently 6,420,000m 3 (assuming a mean declared depth of 6m below CD and an area of 107 hectares) at low water and before a further 881,000m 3 of sediment will be removed. Accordingly, this dilution is easily achievable in the water over which the bacteria would be dispersed, particularly as dispersion extends beyond the navigation channels and into the wider areas of Poole Harbour, including the bathing waters Based on the above assumptions, the dilution necessary to reduce faecal coliform concentrations to below mandatory standards for bathing waters is easily achievable in considering the water volume within Poole Harbour. This conclusion is based on dilution and does not take into account bacteria die-off (e.g. 90% in 0.8 days) that will be occurring as bacteria are dispersed and diluted across Poole Harbour. Accordingly, there would be a short-term impact of minor adverse significance on water quality at and around the dredging areas, but this would not manifest itself as an impact at the bathing waters, even if there is a pathway and, therefore, no impact is predicted at bathing waters. Mitigation and residual impact No mitigation measures are possible and there would be a localised, short term residual impact of minor adverse significance. No residual impact is predicted at bathing waters. Potential effect on shellfish water quality during capital dredging The capital dredging has the potential to disturb sediment and release sediment-bound contaminants into the overlying water column. Subsequently, physical, chemical and bacteriological contaminants may be transported to designated shellfish waters around Poole Harbour and Poole Bay, and affect the water quality with regard to the standards required by the Shellfish Waters Directive This impact is considered with regard to three water quality parameters included under the Shellfish Waters Directive: and Beneficial Use Schemes EIA 155 Poole Harbour Commissioners

22 Suspended solids; Dissolved oxygen; and, Metals and organo-halogenated compounds. Part 1: Suspended solids There is no quantified water quality standard for suspended solids under the Shellfish Waters Directive, but the Directive requires that a discharge affecting shellfish waters must not cause the suspended solids content of the waters to be exceeded by more than 30% in 75% or more of samples compared with waters not so affected. Therefore, the standards of the Directive have been met if 75% of samples do not exceed the limit value As identified in Table 5.5, the mean suspended solids concentrations at Poole Harbour North, South, West, and Poole Bay shellfish waters (2000 to 2003) are 4.9mg/l, 6.6mg/l, and 3.85mg/l respectively. However, there is some degree of uncertainty about whether these data could be considered background concentrations for the purposes of assessing the impact of dredging on shellfish waters. For example, it is believed that the Environment Agency take samples for suspended solids from near the water surface, where concentrations are typically much lower than other parts of the water column. Also, sampling does not appear to represent a particular state of the tide, so avoids higher concentrations that may occur on the largest spring tides and/or during stormy periods when wave action could increase sediment loads in the water The Environment Agency identify general background concentrations of 10mg/l or less in Poole Harbour, and 50mg/l in the intertidal areas and creeks. In contrast, HR Wallingford identify depth-averaged mean concentrations on spring tides of 20mg/l or more (30mg/l near bed) in Poole Harbour with peak concentrations of 50mg/l regularly experienced in calm conditions and mg/l in wavey conditions, and peak concentrations in the order of mg/l. The impact of dredging on shellfish waters is assessed against this range of suspended solids concentrations. In view of the points made in paragraph , it is likely that the concentrations identified by HR Wallingford are more representative of the range of suspended sediment concentrations occurring in Poole Harbour The worst-case scenario is modelled for sediment plumes generated during the dredging of fine-grained sediment in the Middle Ship Channel (see Section 3.6). The modelling predicts the following magnitudes of suspended solids concentrations over background conditions: Maximum suspended solids concentrations above 500mg/l in the streamline of dredging in the Middle Ship Channel, reducing to concentrations below 50mg/l within 500m of either side of the dredger; and, Peak suspended solids concentrations of more than 100mg/l throughout Poole Harbour and into Poole Bay (in and around the Swash Channel), reducing to mean concentrations of between 10-50mg/l HR Wallingford (see Appendix 3) conclude that the predicted suspended solids concentrations generated during dredging will lead to peak increases in concentrations in excess of those normally experienced. However comparison of the mean measured concentrations suggests that except in the immediate vicinity of the dredging the and Beneficial Use Schemes EIA 156 Poole Harbour Commissioners

23 average concentration increases induced by dredging will be within the range of variation of natural background conditions. It must be noted, however, that the dredging will greatly increase the frequency of these higher concentration events during operations. On tidal flats and in South Deep the predicted increases are smaller and the natural range of concentration experienced larger and in these locations the effect of dredging will be much less significant Therefore, the numerical model predictions suggest that the dredging particularly where dredging overflows fine-grained material - may intermittently cause the suspended solids content of the waters to be exceeded by more than 30% the content of waters not so affected. Figure 5.5 shows the predicted peak concentrations of suspended sediments arising from capital dredging in the Middle Ship Channel in relation to monitoring stations for shellfish waters in Poole Harbour. It can be seen from Figure 5.5 that there is the potential for this to result in a breach of the water quality criteria defined in the EC Shellfish Waters Directive. However, whether or not a breach of the Directive would occur depends on whether 75% or more of water samples would be found to have a suspended solids content of 30% or more than unaffected waters. Figure 5.5 Predicted peak concentration of suspended sediment arising from dredging of silty sediment in the Middle Ship Channel in relation to the location of monitoring stations for shellfish waters (Poole Harbour North, West and South) A key consideration in determining whether or not the EC Shellfish Waters Directive would be breached is, therefore, the predicted duration of elevated suspended solids concentrations due to dredging at monitoring points and the frequency of monitoring. It is predicted that the greatest elevation in suspended solids would occur within the immediate vicinity of the operating dredger, with concentrations being significantly lower away from the streamline of the dredging. and Beneficial Use Schemes EIA 157 Poole Harbour Commissioners

24 It is predicted that significant elevations in suspended sediment concentrations would occur for a period of about 2 to 3 months during the dredging of silty material, although there would not be continuous dredging of silty material over this period. The remainder of the capital dredging comprises dredging of sandy material which is not predicted to significantly increase suspended sediment concentrations for prolonged periods Water quality monitoring for the Shellfish Waters Directive is undertaken by the Environment Agency. Based on recent monitoring data it can be seen that monitoring is undertaken fairly regularly throughout the year, approximately on a monthly basis although occasionally more than one sample is taken per month On the basis of the duration of dredging of silty sediment, it is estimated that the capital dredging could, assuming a precautionary approach, result in exceedence of the suspended solids criteria as defined in the Shellfish Waters Directive for more than 75% of samples at a particular monitoring location. This is considered a worst case because suspended sediments arising from dredging would not continuously affect a particular monitoring station, with dispersion of sediment within shellfish waters being dependant on location of dredging in the Middle Ship Channel and current direction and velocity in relation to the timing of dredging. However, if it is assumed that suspended sediment concentrations could exceed those of unaffected water by more than 30% for up to 3 months (i.e. the maximum period over which silty material would be dredged), there is the potential for an estimated 25 to 30% of water sampled to exceed the criteria of the Directive, thereby resulting in a breach of the Directive Given the above, it is considered that there is the potential for an impact of moderate adverse significance to arise on water quality in that the suspended sediments criteria in the Shellfish Waters Directive could, under a worst case, be breached. However, this impact significance does not necessarily apply to the shellfish resource that the Directive is ultimately designed to protect. Potential impacts on shellfish are discussed in Section 9. Part 2: Dissolved oxygen The Shellfish Waters Directive requires that the average of individual water quality monitoring data should achieve greater than 70% saturation and an individual measurement may not indicate a value lower than 60% saturation unless there are no harmful consequences for the development of shellfish colonies. As identified in Table 5.5, the mean dissolved oxygen levels at Poole Harbour North, South, West, and Poole Bay shellfish waters (2000 to 2003) are 111.2%, 109.2%, 106.1%, and 108.1% saturation respectively. The mean values are well above the >70% saturation standard. Also, the minimum DO levels at Poole Harbour North, South, West, and Poole Bay shellfish waters (2000 to 2003) are 79.4%, 74.2%, 83.2%, and 76.2% saturation respectively. The individual DO measurements are well above the 60% saturation standard required for individual measurements A quantitative prediction of the impact on dissolved oxygen of the capital dredging works has not been undertaken. However, the sediment conditions and research for other dredging projects suggest that the potential magnitude of a reduced DO concentration would be very small. and Beneficial Use Schemes EIA 158 Poole Harbour Commissioners

25 The majority of the sediment to be dredged from the Swash Channel, Middle Ship Channel, Turning Basin and Little Channel does not exhibit the conditions that would promote significant oxygen depletion. The baseline data indicate that the majority of the sediment will yield a low oxygen demand as indicated by typical redox values of below - 200mV, typical TOC values of <1% and typical ammonia concentrations <4mg/kg. Also, the water column into which dredged material would be released has good ambient DO concentrations and relatively good tidal flushing in the open harbour. Although Poole Harbour may be vulnerable to DO depletion, this vulnerability is greatest in semienclosed water areas (i.e. Holes Bay, Wareham Channel, and the tidal River Piddle) and at night during summer months when high nutrient levels encourage diurnal and seasonal fluctuations in DO due to algal activity (MBA, 2003). The dredging will occur over winter months There may remain some concern that some parts of the Little Channel and the Middle Ship Channel may contain higher amounts of oxygen-demanding material, and that the capital dredging, particularly of the Little Channel, could cause oxygen-demanding material to affect Holes Bay. For example, two TOC values were recorded at >4%, although these are not particularly high values, and ammonia values in the Little Channel are higher (up to 20mg/kg) Dredging research indicates that the risk that dredging may reduce DO in Poole Harbour is small. As an example, water quality investigations were undertaken for this issue concerning dredging across the tidal River Tyne as part of an EIA for an immersed tube tunnel project: the New Tyne Crossing (Posford Haskoning on behalf of Arup, 2002). For this project, dredging would disturb alluvial silt with high organic matter contents (up to 13%); the organic loading reflecting the proximity of a nearby sewage treatment works. The River Tyne itself was particularly sensitive to DO reduction since it is an important salmon river and experiences regular fish kills due to low DO at locations tens of kilometres upstream. Sediment oxygen demand tests and subsequent numerical modelling identified that, even at the most sensitive times of the year, the maximum reduction in DO concentrations in the water column would be 0.092mg/l near bed and 0.045mg/l at the surface. In terms of % saturation, 0.092mg/l represented 1% of the saturated DO level, which is a very small magnitude of change over background With mean background DO concentrations greater than 100% saturation and dredging research indicating that DO reduction to be small, it is unlikely that dredging would reduce DO substantially such that DO will fall below the 60% saturation standard required for individual measurements under the Shellfish Waters Directive. Consequently, no impact is predicted. Part 3: Metals and organo-halogenated compounds A comparison of sediment survey data with Canadian sediment quality guidelines for the protection of aquatic life indicates that the sediments within the dredging areas generally contain low levels of contaminants at concentrations below the Canadian TEL (see Sections to ). This comparison included the metals and organohalogenated compounds included under the EC Shellfish Waters Directive. According to the CCME (1999), contaminant concentrations below the TEL are in the minimal effect range within which adverse effects rarely occur on biological resources. and Beneficial Use Schemes EIA 159 Poole Harbour Commissioners

26 In addition, an assessment of water quality changes relating to the EC Substances Directive covers the metals and organohalogenated compounds included under the EC Shellfish Waters Directive. Comparison to significance criteria based on equilibrium partitioning shows that the interstitial water of the sediment generally contains concentrations of metals and organohalogenated compounds (i.e. PCBs and organochlorine pesticides) at concentrations less than the EQSs established under the EC Dangerous Substances Directive, except in the cases of arsenic and mercury. However, the interstitial water released during dredging will undergo significant dilution and dispersion throughout Poole Harbour and Poole Bay and will quickly dilute arsenic and mercury and the other dangerous substances to concentrations below EQSs Based on this evidence for dangerous substances, it is believed that the magnitude of change to concentrations of metals and organohalogenated compounds would not compromise water quality criteria under the EC Shellfish Waters Directive. Hence, there would be no impact to shellfish water quality in terms of the concentrations of substances in the shellfish waters will not reach or exceed a level which has harmful effects on the shellfish and their larvae due to dredging. Part 4: Conclusion Overall, it is predicted that, under a worst case scenario, there is the potential for an impact of moderate adverse significance on shellfish waters due to suspended solids concentrations being increased by dredging, but no impact due to changes in DO, metals or organohalogenated compounds. In terms of the impact on suspended solids, it should be recognised that the shellfish waters cover extensive areas within Poole Harbour and Poole Bay. For example, the Poole Harbour North shellfish water covers most of the area north of a line extending from the Harbour s entrance to Patchins Point, excluding Holes Bay and Parkstone Bay. This means that the entire length of the Middle Ship Channel and the Turning Basin are situated within the Poole Harbour North shellfish water and that it would be almost impossible not to significantly increase suspended solids concentrations at some points within the shellfish water, particularly in the near vicinity of the dredger In summary, dredging would: Intermittently increase suspended solids by more than 30% the content of waters not so affected, thereby potentially breaching the requirements of the EC Shellfish Waters Directive within the designated shellfish waters within Poole Harbour and Poole Bay. However, it should be noted that 25% of water samples need to exceed this criteria in order for a breach to occur; May reduce DO but not so substantially that DO will fall below the 60% saturation standard required for individual measurements under the Shellfish Waters Directive; and, May increase the concentrations of substances in the shellfish waters but not so substantially that the concentrations will reach or exceed a level which has harmful effects on the shellfish and their larvae. and Beneficial Use Schemes EIA 160 Poole Harbour Commissioners

27 Mitigation and residual impact The effect of the dredging on suspended sediment concentration, in terms of its effect in relation to the Shellfish Waters Directive, is not possible to mitigate because technically the breach of the Directive occurs in the vicinity of the dredger where elevated concentrations are unavoidable (the dredging occurs within the designated boundary of designated waters). However, whether or not a breach would occur depends on the frequency with which the suspended solids criteria of the Directive is exceeded. Under a worst case scenario, the residual impact would be of moderate adverse significance. No mitigation measures are required with respect to other water quality effects. No residual impacts are predicted on dissolved oxygen and metals and organohalogenated compounds with respect to the Shellfish Waters Directive. Operational phase Potential effect on water quality due to release of dangerous substances during maintenance dredging Maintenance dredging would be required during the operational phase with the potential for the release sediment-bound chemical contaminants into the overlying water column. A release of chemical contaminants could increase concentrations in Poole Harbour and Poole Bay, and affect the water quality with regard to the EQSs required by the Dangerous Substances Directive This assessment assumes that the sediments subject to maintenance dredging have the same characteristics as the sediments to be removed by capital dredging. Therefore, the same assessment method and significance criteria apply to this impact (see Sections to 5.4.6) As for capital dredging, the sediment released into the overlying water column would contain low concentrations of most contaminants and slightly elevated concentrations of arsenic, mercury and fluoranthrene. The same arguments with respect to the potential for impact apply here as described above for the capital dredging. During maintenance dredging, the sediments and the contaminants would undergo significant dilution and dispersion in Poole Harbour and Poole Bay. On this basis, it is concluded that maintenance dredging will have a short-term impact of minor adverse significance on water quality since it will increase the concentrations of some dangerous substances in the water column, but after initial dilution, the dangerous substances released by dredging will be at concentrations below EQSs. and Beneficial Use Schemes EIA 161 Poole Harbour Commissioners

28 Mitigation and residual impact The release of contaminants into the water column as a result of the proposed dredging is not possible to mitigate. The residual impact would be of minor adverse significance. Potential effect on bathing water quality during maintenance dredging Maintenance dredging would not occur in bathing waters although numerical modelling (see Section 3.6) of the dispersion of suspended solids indicates that the hydrodynamic forces within Poole Harbour and Poole Bay could potentially transport sediment (derived from maintenance dredging) to bathing waters situated in these areas. Accordingly, there appears to be an impact pathway for bacteria to reach bathing waters This assessment assumes that the sediments subject to maintenance dredging have the same characteristics as the sediments to be removed by capital dredging. Therefore, the same assessment method and significance criteria apply to this impact as for capital dredging Given the above and the fact that the volume of material to be dredged during maintenance campaigns is significantly less than that dredged during capital dredging, the dilution necessary to reduce faecal coliform concentrations to below mandatory standards for bathing waters is easily achievable in considering the water volume within Poole Harbour. Accordingly, it is predicted that there would be a short-term impact of minor adverse significance on water quality at and around the maintenance dredging areas, but this would not manifest itself as an impact at the bathing waters and, therefore, no impact is predicted at bathing waters. Mitigation and residual impact No mitigation measures are possible and there would be a localised, short term residual impact of minor adverse significance. No residual impact is predicted at bathing waters. Potential effect on shellfish water quality during maintenance dredging As for capital dredging, the potential for impact is considered with regard to three water quality parameters included under the Shellfish Waters Directive: Suspended solids; Dissolved oxygen; and, Metals and organo-halogenated compounds This assessment assumes that the sediments subject to maintenance dredging have the same characteristics as the sediments to be removed by capital dredging. Therefore, the same assessment method and significance criteria apply to this impact as for capital dredging. and Beneficial Use Schemes EIA 162 Poole Harbour Commissioners

29 As for capital dredging, there is the potential for maintenance dredging to result in a breach of the Shellfish Waters Directive given that dredging would result in suspended sediment concentrations by 30% or more compared with unaffected waters. However, the period of time of elevated suspended sediment concentrations associated with maintenance dredging would be very limited (about 2 weeks at most) and, therefore, it is considered that a breach of the Shellfish Waters Directive would not occur given that the standards of the Directive have been met if 75% of samples do not exceed the limit value. As a result, no impact is predicted on shellfish waters. Similarly, no impact is predicted due to changes in DO, metals or organohalogenated compounds. Mitigation and residual impact No mitigation measures are required. No residual impacts are predicted on suspended sediment concentrations, dissolved oxygen and metals and organohalogenated compounds with respect to the Shellfish Waters Directive. 5.5 POTENTIAL IMPACTS ASSOCIATED WITH THE OFFSHORE DISPOSAL OF DREDGED MATERIAL Construction phase Potential effect on water quality due to release of dangerous substances during the disposal of dredged material Like capital dredging, the offshore disposal of dredged material would release sedimentbound chemical contaminants into the water column. A release of chemical contaminants could increase concentrations in the water column, particularly Poole Bay, and affect the water quality with regard to the EQSs required by the Dangerous Substances Directive An assessment of this impact can be made by comparing mean contaminant concentrations recorded by a sediment survey of the dredging areas (and hence the material to be disposed of) (see Sections to ) to significance criteria derived from equilibrium partitioning (see Table 5.3). The comparison (see Table 5.15) identifies that the interstitial water of the sediment to be disposed of contains metals, PAHs, PCBs and organochlorine pesticides at concentrations below EQSs for dangerous substances except arsenic, mercury and fluoranthrene, which exceed EQSs by less than one order of magnitude As is the case for dredging, the sediment released during offshore disposal would undergo significant dilution and dispersion throughout Poole Bay, as shown by the numerical modelling of suspended solids. On this basis, it is concluded that offshore disposal would have a short-term impact of minor adverse significance on water quality since it would increase the concentrations of some dangerous substances in the water column, but after initial dilution, the dangerous substances released by dredging would be at concentrations below EQSs. and Beneficial Use Schemes EIA 163 Poole Harbour Commissioners

30 Mitigation and residual impact The release of contaminants into the water column as a result of the proposed dredging is not possible to mitigate. The residual impact would be of minor adverse significance. Potential effect on bathing water quality during the disposal of dredged material Like capital dredging, the offshore disposal of dredged material would release sedimentbound bacteria into the water column. A release of faecal coliforms could increase concentrations in the water column, particularly Poole Bay, and affect the water quality with regard to the Bathing Waters Directive Disposal would not occur within bathing waters, but numerical modelling (see Section 3) of suspended solids indicates that the hydrodynamic forces within Poole Harbour and Poole Bay would transport sediment to bathing waters situated in these areas. Accordingly, there appears to be an impact pathway for bacteria in the dredged material to reach bathing waters The significance criterion for this impact is the mandatory concentration for faecal coliforms (e.g. E. coli) at bathing waters (2,000/100ml) There are many factors that affect the potential for bacteria to impact on bathing waters, and these are described below. Source concentration of bacteria. The sediments of the Middle Ship Channel, Little Channel and Turning Basin contain various concentrations of faecal coliforms (2-8,000/ml) and streptococci (0-35/ml) (see Tables 4.14 and 4.15). The mean concentration for faecal coliforms in the sediment is 2,867/ml. The sediments of the Swash Channel are located away from major sewage point sources (e.g. Poole STW), comprise medium and coarse-grained sediment (sand and gravel) which limits bacterial survival rates, and are considered to be uncontaminated. In addition, the sediments of the Swash Channel are more likely to be used for beach nourishment rather than be disposed of, and are not considered any further in this impact assessment. Source distribution of bacteria. Bacteria are only present in significant concentrations in the upper few centimetres of the sediment. Assuming disposal of the total volume of the dredged material from the Middle Ship Channel, Turning Basin and Little Channel (881,000m 3 ), which is not realistic because some of this material will be used for beach nourishment, then the volume of bacteria-contaminated material approximates to 42,800m 3 (i.e. 4.9%). Therefore, disposal of the surface sediments (i.e. 4.9% by volume) poses the highest risk of an impact on water quality and disposal of the deeper sediment (95.1% by volume) poses no or very little risk. Bacteria die-off. The time taken between disposal and the bacteria reaching bathing waters is important because bacteria die over short periods of time to the extent that they may be inactivated either before or not long after reaching and Beneficial Use Schemes EIA 164 Poole Harbour Commissioners

31 bathing waters. Bacteria can become inactivated over a matter of hours, even in winter when the temperature is lower and sunlight hours are less. For example, Sinton et al (1999; in Environment Agency, 2000a) identify T 90 die-off rates for faecal coliforms in sunlight hours to be 7.7 hours and in dark hours to be 63 hours during winter. Some degree of die-off will occur during disposal. Bacterial die-off is not quantified specifically for this assessment, but using an average of 0.8 days for T 90 of E. coli in sea water (Anon, 1999; in Environment Agency, 2000), it is clear that there is the potential for 90% of bacteria to die-off within less than 24 hours of dredging, which will substantially reduce the concentration of bacteria affecting the bathing waters. Disposal duration. Based on a dredger with a hopper capacity of about 1,500m 3 working on a 3 hour dredging and disposal cycle, the duration of the works for 42,800m 3 of bacteria-contaminated sediment is about 3.5 days (assuming continuous dredging and disposal). This time period is a relatively small proportion of the total dredge period (estimated to be around 7 months), and indicates that any impact on water quality will be a short-term effect. In practice, the 3.5 days is likely to be spread over a much larger proportion of the whole period of dredging, which would have the effect of increasing the duration, but reducing the intensity, of the impact. Sediment dispersion in the water column due to disposal. Disposal of dredged material would release sediment from the dredger s hopper to fall through the water column onto the seabed. Only a small proportion (3%) of the released sediment remains in the water column after 5 minutes (see Section 3.6). The rest (97%) descends to the seabed where the remaining silt and sand would be winnowed out by tidal flows, leaving a small quantity of coarser gravel behind. Hence, only 3% of the 42,800m 3 bacteria-contaminated is immediately released into the sea of Poole Bay; although a proportion of the remaining 97% would become resuspended at a later date. The proportion of bacteria-contaminated material that can be re-suspended would be restricted by the amount of material buried during the initial disposal, the amount of material buried by the subsequent disposals that will occur every 3 hours afterwards, and amount of material re-suspended prior to the bacterial die-off. Bacteria dilution in the water column. If the mean concentration for faecal coliforms in the dredged material is taken to be 2,867/ml and the significance criteria is taken to be the mandatory bathing water standard, which is 2,000/100ml, then a water dilution of 150 will be sufficient to reduce faecal coliform concentrations in the sediment to a level below the bathing water standard, before die-off is taken into account For this assessment the volume of bacteria-contaminated sediment released into the water column during disposal is taken to be the 3% of 42,800m 3 that is immediately released into the water column, plus another 10% of 42,800m 3 that is re-suspended from the seabed after disposal (the rest is buried and/or subject to bacteria die-off). This gives a total release volume of 5,564m 3. If 5,564m 3 of material containing faecal coliforms at a mean concentration of 2,867/ml is released into the sea during disposal, a dilution of 150 will require 834.6M litres or 834,600 m 3 of sea water. This is a relatively small volume of Poole Bay. For example, assuming an average water depth of 15m in and around the disposal ground, the bacteria-contaminated sediment will need to be and Beneficial Use Schemes EIA 165 Poole Harbour Commissioners

32 diluted evenly over an area of hectares. Accordingly, this dilution is easily achievable in the water over which the bacteria will be dispersed, particularly if dispersion extends as far as bathing waters Based on the above assumptions, the dilution necessary to reduce faecal coliform concentrations to below mandatory standards for bathing waters is easily achievable in considering the water volume within Poole Bay, particularly if bacteria are dispersed as far as bathing waters. This conclusion is based on dilution and does not take into account bacteria die-off (e.g. 90% in 0.8 days) that will be occurring as bacteria are dispersed and diluted across Poole Bay. Accordingly, there will be a short-term impact of minor adverse impact on water quality at and around the disposal ground, but this should not manifest itself as an impact at the bathing waters and, therefore, no impact is predicted. Mitigation and residual impact No mitigation measures are possible and there would be a localised, short term residual impact of minor adverse significance. No residual impact is predicted at bathing waters. Potential effect on shellfish water quality during the disposal of dredged material This assessment assumes that the concentrations of contaminants in the sediments subject to disposal are at the same concentrations in the sediments to be removed by dredging. Therefore, the same assessment method and significance criteria apply to this impact This potential impact is considered with regard to three water quality parameters included under the Shellfish Waters Directive: Suspended solids; Dissolved oxygen; and, Metals and organo-halogenated compounds. Part 1: Suspended solids There is no quantified water quality standard for suspended solids under the Shellfish Waters Directive, but the Directive requires that a discharge affecting shellfish waters must not cause the suspended solids content of the waters to exceed by more than 30% of 75% or more of samples compared with waters not so affected. Therefore, the standards of the Directive have been met if 75% of samples do not exceed the limit value As identified in Table 5.5, the mean suspended solids concentration at Poole Bay shellfish waters (2000 to 2003) is 3.85mg/l. However, there is some degree of uncertainty about whether these data could be considered background concentrations for the purposes of assessing the impact of dredging on shellfish waters (see Section ). and Beneficial Use Schemes EIA 166 Poole Harbour Commissioners

33 The worst-case scenario is modelled for sediment plumes generated during disposal of fine sediment at the offshore disposal ground in Poole Bay (see Section 3.6). The modelling predicts the following magnitudes of suspended solids concentrations over background conditions: Peak suspended solids concentrations above 500mg/l in the streamline of disposal. Peak suspended solids concentrations of the order of 5-200mg/l for the most part of Poole Bay. Depth averaged peak suspended solids concentrations of the order of mg/l at the maerl beds and over 250mg/l at the Sabellaria beds HR Wallingford (see Appendix 3) notes that the predicted suspended solids concentrations generated during disposal should be put into the context of the proportion of time over which these peak values are attained. The mean concentration increases experienced at these locations (during the first spring-neap cycles) are of the order of 25mg/l or less except at the point of disposal Therefore, the numerical model predictions suggest that the disposal operations may intermittently cause the suspended solids content of the waters to be exceeded by more than 30% the content of waters not so affected. Figure 5.6 shows the predicted peak concentrations of suspended sediments arising from offshore disposal in the south-east quadrant of the disposal ground in relation to monitoring stations for shellfish waters in Poole Bay. It can be seen from Figure 5.6 that there is the potential for this to result in a breach of the water quality criteria defined in the EC Shellfish Waters Directive. However, whether or not a breach of the Directive would occur depends on whether 75% or more of water samples would be found to have a suspended solids content of 30% or more than unaffected waters The same considerations with respect to whether or not a breach of the Shellfish Waters Directive is likely to occur apply here as described in paragraphs to for capital dredging. It is, therefore, considered that there is the potential for an impact of moderate adverse significance to arise on water quality in that the suspended sediments criteria in the Shellfish Waters Directive could, under a worst case, be breached. However, this impact significance does not necessarily apply to the shellfish resource that the Directive is ultimately designed to protect. Potential impacts on shellfish are discussed in Section 9. Part 2: Dissolved oxygen The Shellfish Waters Directive requires that the average of individual water quality monitoring data should achieve greater than 70% saturation and an individual measurement may not indicate a value lower than 60% saturation unless there are no harmful consequences for the development of shellfish colonies. As identified in Table 5.5, the mean dissolved oxygen level at the Poole Bay shellfish water (2000 to 2003) is 108.1% saturation. and Beneficial Use Schemes EIA 167 Poole Harbour Commissioners

34 Figure 5.6 Predicted peak concentration of suspended sediment arising from offshore disposal of silty sediment in relation to the location of monitoring stations for shellfish waters (Poole Bay) A quantitative prediction of the impact on dissolved oxygen of the disposal has not been undertaken. However, the sediment conditions and research for other dredging projects suggest that the potential magnitude of a reduced DO concentration would be very small (see Sections ) With mean background DO concentrations greater than 100% saturation and dredging research indicating that DO reduction to be small, it is unlikely that dredging will reduce DO substantially such that DO will fall below the 60% saturation standard required for individual measurements under the Shellfish Waters Directive. Consequently, no impact is predicted. Part 3: Metals and organo-halogenated compounds A comparison of sediment survey data with Canadian sediment quality guidelines for the protection of aquatic life indicates that the sediments to be disposed of generally contain low levels of contaminants at concentrations below the Canadian TEL (see Sections to ). This comparison included the metals and organo-halogenated compounds included under the EC Shellfish Waters Directive. According to the CCME (1999), contaminant concentrations below the TEL are in the minimal effect range within which adverse effects rarely occur on biological resources. and Beneficial Use Schemes EIA 168 Poole Harbour Commissioners