Appendix B: Tyrsting Valley

Transcription

1 Appendix B: Tyrsting Valley Appendix B: Tyrsting Valley A plan for protecting the groundwater at a proposed well field located in Tyrsting buried valley, Denmark This Groundwater Protection Plan (GPP) is made for an area designated for groundwater protection pursuant to the Danish Environmental Objectives Act, the Water Supply Act and the Spatial Planning Act. The GPP is based on a detailed survey of hydrogeological conditions and land use together with an assessment of all known potential sources of contamination in the area. 1 Introduction Tyrsting is located in the northern part of Vejle County, Denmark. Here several buried valleys crosscut the Quaternary and Tertiary deposits that house and protect most of the groundwater resources in the area. The quality of the groundwater in the buried valleys is generally excellent, but the valleys sometimes contain pathways allowing contaminants to penetrate to great depths from where they may spread to other areas or even to other water districts. A detailed understanding of the geological and hydrological conditions pertaining in the buried valleys is therefore essential if effective plans are to be drawn up to ensure sustainable groundwater protection. Present situation and problem The Tyrsting project area is located on intensively farmed land south of the town of Brædstrup (Fig. B1). In the 1980s the water supply wells in the area became so contaminated by nitrate that a new centralized water supply had to be established based on wells located in the buried valley. Recent analyses show that the nitrate has now penetrated down to as much as 100 m.b.s. in the valley, thus indicating the existence of "windows" allowing contaminants to penetrate to great depths. The GPP thus aims to localize these windows, elucidate their hydraulic properties and, in collaboration with local stakeholders (farmers, waterworks, etc.), to ensure sustainable use of the area for farming while concomitantly protecting the groundwater. The project area has been extensively investigated as part of scientific studies of the Tertiary deposits in Jutland. Moreover, the Royal Veterinary and Agricultural University has performed an intensive study of the physical and social structure of farming in the area. Both studies provide substantial input to the present project to develop a GPP. Vejle County has performed detailed geological studies in the area including drilling, TEM, seismic surveys, chemical analyses, aquifer tests, etc. as part of ongoing Danish efforts to safeguard drinking water resources. The proposed pilot project is nevertheless necessary to obtain a full understanding of this area, which is very complex due to the crosscutting buried valley. Furthermore, the pilot project will provide a broader insight into the hydraulic influence of buried valleys, knowledge that will be useful for groundwater protection planning in other areas with buried valleys. Aim The aim of the project is to demonstrate how areas with buried valleys should be investigated and how the results can be utilized in land use planning and groundwater administration. Geophysical and geological techniques and mapping methodologies will be improved so that they can be used as a tool for investigating groundwater resources in buried valleys. The focus is on establishing a GPP for protecting and improving the groundwater in collaboration with local and regional stakeholders. The problem In the near future the only existing waterworks in Tyrsting valley, Bjerrebo Waterworks, must considerably increase its level of groundwater 289

2 KLAUS PETERSEN & SANNE ØSTERGAARD NIELSEN Fig. B1: Project area Tyrsting valley. abstraction to meet increasing demand for pure drinking water from nearby towns. Before discovery of the buried valley the possibilities for groundwater abstraction in the area were negligible as all the minor waterworks located in the vicinity had been closed down due to nitrate and pesticide contamination. These waterworks abstracted water from aquifers located at depths down to around 60 m.b.s. With the discovery of Tyrsting valley and its deeply buried aquifers the necessary groundwater resource to meet future demand seems to have been localized. The natural next step is to safeguard the aquifers. In the present project preliminary groundwater abstraction scenarios have thus been modelled to identify the groundwater recharge area and the capture for the proposed well field. 290 Groundwater modelling is becoming a valuable indirect planning tool for spatial and land use planning. It provides local authorities, planners and policy makers with improved knowledge of where and how groundwater protection measures should be implemented, as is well illustrated by the present Tyrsting valley pilot project. By simulating an annual abstraction rate of 1 3 million m from the deepest part of the aquifer it has been possible to obtain an idea of the groundwater recharge area feeding the aquifer and the groundwater capture for the proposed well field. The groundwater capture for the proposed 2 well field covers 8.3 km of the valley and spreads across the municipality of Brædstrup and Nr.

3 Appendix B: Tyrsting Valley Snede (Fig. B2). The area around Tyrsting valley is intensively farmed, the most important crops being cereals and Christmas trees. The livestock density is high and mainly accounted for by pigs and dairy cattle. Much of the area has previously been designated as a nitrate-sensitive groundwater protection area. The new vulnerability mapping confirms the high vulnerability to nitrate due to the limited thickness of the clay cover above the aquifer. Even though the main aquifer is located m.b.s., nitrate poses a risk to the upper aquifers and hence may possibly cause problems for the deep aquifer if the upper and deep aquifers are hydraulically connected or if water flows down the sidewalls to the deep aquifer. The main problems in the area are the high vulnerability of the upper aquifers to nitrate and the risk of groundwater contamination in connection with the use of herbicides in the Christmas tree plantations. The town of Brædstrup, population 3,000, situated northeast of the valley is supplied with water by Brædstrup waterworks located north of the town and which abstracts groundwater from aquifers north of Tyrsting valley. Much of this area has been designated as a Particularly valuable groundwater abstraction area pursuant to Danish legislation and hence is subject to land use and other restrictions. The Tyrsting valley water resource is also of interest to Brædstrup Waterworks, which, like Bjerrebo Waterworks, is looking for a new well field to meet future needs. Horsens and even Aarhus could also be interested in the valley groundwater resource to supply their populations if the resource exceeds 1 million m 3 per year. No waterworks presently abstract water from the buried valley itself. Bjerrebo waterworks, owned by Brædstrup Municipality, is located on the side of the valley and abstracts groundwater from m b.g.s in the upper, poorly protected aquifers. The waterworks is permitted to abstract 10,000 m 3 per year. 2 Protection measures and risk assessment Table B1 shows measures to protect the groundwater in Tyrsting valley. Integration of the GPP in spatial planning and land use planning In the future the knowledge of the area gained through the pilot project will be incorporated in the planning work carried out by the regional and local authorities. 3 Hydrogeology Aquifers The buried valley contains several aquifers located at different depths but hydraulically connected to some extent. The GPP focuses on the deepest aquifer, where the groundwater resource is qualitatively and quantitatively best. The deep aquifer is located m b.g.s and extends along almost the whole of the valley floor. The sediments in the deep valley are comprised of meltwater sand and gravel. The base of the aquifer consists of Tertiary clay, and the upper boundary consists of a mixture of sand and till. Groundwater capture and groundwater recharge areas The groundwater capture covers 8.3 km 2 and stretches from Mattrup forest in the east to Gripstrup in the west (Fig. B2). The groundwater recharge area (infiltration ) covers 4 km 2 of the groundwater capture and is mainly located in the western part of the outside the valley itself. The vulnerability of the capture (and groundwater recharge area) is very high with the upper layers (0 30 m) containing less than 5 m of clay in 70% of the. 291

4 KLAUS PETERSEN & SANNE ØSTERGAARD NIELSEN Table B1: Measures for groundwater protection with concrete actions and accountable authorities. Measure Background/Aim Authority Time frame Mapping of disused wells and boreholes in the groundwater capture Boreholes and wells are open holes connecting the surface with the upper aquifers. They may allow contaminants to penetrate directly to the aquifers. Moreover, old boreholes and wells are often in poor technical condition Brædstrup/Nr. Snede Municipality 2006 Environmental inspection of livestock holdings and large cereal farms in the groundwater capture In connection with the annual environmental inspection the municipality will assess the risk to the groundwater and make recommendations regarding appropriate use of pesticides, fertilizer, etc. Brædstrup/Nr. Snede Municipality 2006 Local ban on the application of sewage sludge in the groundwater capture Banning the use of sewage sludge as fertilizer in the groundwater capture will reduce the risk of groundwater contamination Brædstrup/Nr. Snede Municipality 2006 Information about appropriate behaviour in the groundwater capture To further reduce the risk of groundwater contamination by informing citizens, farmers and others of appropriate behaviour in the groundwater capture through distribution of a folder and through personal contact Brædstrup/Nr. Snede Municipality and the waterworks in the groundwater capture 2006 Information meeting for farmers about appropriate farming practice in the nitrate-sensitive areas To inform farmers of the location of the nitratesensitive areas and of appropriate farming practice in these areas Vejle County 2006 Mapping of Christmas tree plantations and pesticide use in the area Mapping of land use in the nitrate-sensitive areas Additional vulnerability mapping in the western part of the groundwater capture To assess the risk to the groundwater associated with Christmas tree production in the area To facilitate calculation of nitrate losses using the Daisy model. It may be necessary to regulate land use if nitrate loss from the soil is too high Vejle County 2006 Waterworks To complete vulnerability mapping of the area Vejle County 2006 Table B2: Hydrological data for estimating infiltration to the buried valley. Groundwater recharge area Precipitation Evaporation Estimated runoff Infiltration to the buried valley 4 km mm/year 530 mm/year 275 mm/year 145 mm/year 292

5 Appendix B: Tyrsting Valley The available vulnerability charts do not cover the whole capture, though, especially not the western part. Additional mapping of the western part should therefore be accorded high priority in the GPP. Well field, water balance and resource exploitation The waterworks scenario includes one proposed 3 production well abstracting 1 million m of groundwater per year. The well screen is located m.b.s. The well field could possibly be extended with two additional wells in the valley located within a distance of 2 3 km NNW of the first well. The amount of groundwater that could be abstracted from the proposed well without over3 exploitation is around 600,000 m per year. The total amount of groundwater that could be abstracted from the whole valley is estimated at 3 around 2 3 million m per year depending on recharge in areas further north and northeast in the valley that have not yet been assessed. The hydrological data for estimating infiltration to the buried valley are listed in Table B2. The annual infiltration in the entire project area (and hence in the groundwater recharge area) is 145 mm per year. Fig. B2: The map shows the limits of the buried valley and the limits of the groundwater capture and groundwater forming area for two scenarios: a former situation where groundwater is abstracted from the upper valley aquifer and without knowledge of an underground valley structure; the actual situation where the buried valley is known and groundwater is abstracted from the deep aquifer in the valley. Note that the groundwater capture and groundwater forming area largely are located outside the valley limits. 293

6 KLAUS PETERSEN & SANNE ØSTERGAARD NIELSEN Groundwater chemistry and quality The quality of the groundwater is generally very good at all depths in the buried valley, although nitrate has been detected in concentrations of 19 mg/l at a depth of m.b.s. The concentration is below the limit value of 50 mg/l for drinking water. The nitrate is located 130 m above the deep primary aquifer in the buried valley. No evidence has been found for the presence of hazardous substances such as pesticides, organic solvents, heavy metals, etc. in the medium-to-deep parts of the valley. The detection of nitrate at a depth of m indicates the existence of hydraulic connectivity between the surface layers and the medium-todeep aquifers. This is not unexpected since the clay cover is generally less than 5 m thick in 70% of the project area. Nitrate has not been detected in the deeper parts of the valley. The groundwater in the deepest part of the aquifer is classified as D, i.e. strongly reduced and characterised by the presence of methane and the absence of oxygen and nitrate, and only low concentrations of sulphate (<20 mg/l) (see Chap part I of this book for explanation of water types). The groundwater in the upper-tomedium levels is B C2, i.e. characterised by the presence of nitrate, high concentrations of calcium and magnesium and medium high concentrations of iron and sulphate. The age of the groundwater has been identified as relatively young in the upper-to-medium levels (around 30 years or younger), whereas that in the deeper parts is relatively old (at least more than 50 years). Besides classification of water type, the groundwater has been assayed for naturally occurring problematic substances such as arsenic, which is highly toxic. Apart from arsenic, no other substances posed a problem as regards groundwater quality. The arsenic problem can be solved by the traditional method of oxidizing the water, which removes most of the arsenic. Aquifer vulnerability One of the main aims of the geological and geophysical investigations was to identify groundwater recharge areas so that these can be protected in order to reduce contamination of the groundwater. The vulnerability of the buried valley to infiltrating nitrate and hazardous substances has been evaluated. Vulnerability analysis The vulnerability zoning is based on an assessment of accumulated clay thickness (0 30 m.b.s.), groundwater type, groundwater recharge areas (infiltration) and hydraulic head. The clay thickness is the dominant parameter, with vulnerability to nitrate being defined as clay cover thickness <5 m and vulnerability to hazardous substances as clay cover thickness <15 m. Clay thickness has not yet been mapped in the western part of the groundwater capture. The project area is generally highly vulnerable to both nitrate and hazardous substances. Nitrate Vulnerability to nitrate is divided into 3 classes: High (red), medium (yellow) and low (light blue). High vulnerability s are designated as groundwater protection areas pursuant to Danish legislation. The nitrate vulnerability classes are characterised in Table B3. Table B3: Simplified classification of nitrate vulnerability. Vulnerability /Parameter High Medium Low Clay thickness <5 m 5 15 m >15 m Water type A, B C2 C1 and D Aquifer (hydraulic Unconfined/ Unconfined head) Confined Confined 294

7 Appendix B: Tyrsting Valley Vulnerability to nitrate is high in the northwestern third of the groundwater capture, medium-to-low in the central third and high in the southern third (Fig. B3). Other contaminants No hazardous substances have been detected in the deepest aquifer in the buried valley, although herbicides have been detected in the upper/nearsurface aquifers that can be traced back to the Christmas tree plantations in the area. The hazardous substances mainly encompass pesticides, pesticide degradation products, organic solvents and oil products. At present, no clear understanding exists of hazardous substances and their behaviour (composition and transport) in different environments. As a consequence, no means is yet available to unequivocally classify vulnerability to hazardous substances. The present assessment therefore takes a conservative approach based on whether or not the clay cover is 15 m thick (Table B4). Based on this definition 90% of the whole project area is vulnerable to hazardous substances. Table B4: Simplified classification of vulnerability to hazardous substances. Vulnerability High Low Clay thickness <15 m >15 m Fig. B3: The map illustrates the vulnerability on the subject of clay cover for the actual situation with an abstraction from the deep valley aquifer. The clay-thickness is divided in three levels (see the legend for details). Note that the overall clay-cover is very sparse and thereby the natural protection against pollution is very limited. The land-use in this area will have a high impact on the groundwater quality e.g. with respect of nitrate and pesticides. 295

8 KLAUS PETERSEN & SANNE ØSTERGAARD NIELSEN In summary, most of the groundwater recharge area and groundwater capture for the proposed well field is classified as vulnerable, as is most of the project area as a whole. The groundwater resource in the deepest aquifer in the buried valley is presently uncontaminated, but the situation might change if effective groundwater protection is not implemented. The risk thus exists that a future well field in the valley could become contaminated. 4 Land use and sources of contamination 5 References Water4all (2005): Sustainable Groundwater Management - a handbook of best practice to reduce agricultural impacts on groundwater quality. 240 pp. ndbuch_e_1_72dpi.pdf [Oct.27, 2006]. Zonering. Detailkortlægning af arealer til beskyttelse af grundvandsressourcen, Danish Environmental Protection Agency Guideline No. 3, Agriculture The groundwater capture is mainly farmland with livestock holdings and Christmas tree plantations. The livestock holdings are mainly pig holdings and dairy farms, with nitrate leaching being greatest from the cattle holdings and dairy farms. Livestock density in the area is very high, almost approaching the maximum permitted density. It is difficult to re-allocate manure application to other fields in the same area as there are few fields that do not already receive manure. The mapping shows that about 60% of the area is highly vulnerable to nitrate and hazardous substances including pesticide. Woodland is only present in the southern part of the groundwater capture in the form of Mattrup forest, about 70% of which is conifer and the remainder is broadleaf. Woodland does not usually pose a contamination problem because nitrate leaching is negligible. However, large numbers of Christmas trees are produced in Mattrup forest, and the area is very sandy. In the early spring, when the producers apply herbicides to the Christmas tree plantations, the soil is bare and there is a risk that herbicides will leach to the groundwater during major storm events. Another problem posed by conifer forest is that pine needles falling on the ground and acid precipitation lower the soil ph, possibly resulting in the leaching of heavy metals in the long term. 296