Water quality 37. Background

Transcription

1 6 Water quality Background A good quality chalk river has very low loads of fine particles in suspension or in the gravel bed. This, along with relatively low concentrations of nutrients (phosphate and nitrate), limits algal growth and results in water that is naturally very clear. The calcium-rich, high alkalinity waters support a characteristic flora and fauna, and high productivity of plants, invertebrates and fish. Clean river gravels are essential as spawning beds for salmonid fish and habitat for many species of aquatic invertebrates. Occasional silty areas are also important for such organisms as lamprey larvae and the burrowing mayfly Ephemera danica. The main threats to the water quality of chalk rivers are sediment-laden runoff and nutrient inputs from sewage works and farmland. Toxic substances in industrial effluent, sewage and urban runoff, and pesticides from agriculture and watercress farms, may also have significant impact, although the extent of this is unclear. Very good water quality is critical for all chalk rivers Dennis Bright The state of river water quality General conditions The biological and chemical condition of rivers in England and Wales is determined by the General Quality Assessment (GQA), which is based primarily on organic pollution. This type of pollution comes mainly from sewage and to a lesser extent from some industries, agriculture and urban areas. The biological grading is based on the presence or absence of indicator invertebrate groups and provides a broad assessment of water quality. High nutrient discharges from sewage treatment works reduce river water quality Dennis Bright River Itchen near Alresford 36 Water quality 37

2 In the 2000 GQA survey, 89 per cent of chalk rivers were of good or very good biological quality (Figure 11) up from 72 per cent in Only two stretches had poor water quality: the Lee downstream of Luton and the Wandle below Croydon. These are both affected by urban runoff, and probably by misconnected drains. Chemically, 83 per cent of chalk rivers were graded as good or very good quality, compared with 64 per cent in In England as a whole in 2000, 66 per cent and 64 per cent respectively of rivers were biologically and chemically of good or very good quality. Some 63 per cent of the river length does not achieve both grade a biological quality and grade A chemical quality, though more analysis is required to show where this shortfall is significant. > Figure 12 Phosphate concentrations in chalk rivers, > Figure 11 Biological water quality of chalk rivers, 2000 Nutrients In temporate regions, plant growth in fresh waters is limited mainly by the availability of the nutrient phosphorus. Too much phosphorus may cause eutrophication. This is when excessive plant growth disrupts plant and animal communities. Under these conditions, algae may smother flowering plants such as Ranunculus and coat the river bed. This increases siltation and makes conditions unsuitable for salmonid spawning and many invertebrate species. For csac chalk rivers (flow above 2.5m 3 /s), a guideline phosphate standard of 60µg P/l has been proposed (Pitt et al., 2002). This limit is used indicatively here to make a preliminary assessment of phosphate levels in all chalk rivers. Elevated phosphate levels in bed materials can reduce root stability in Ranunculus. 38 Water quality 39

3 > Figure 13 Water company investment in projects improving sewage discharges to chalk rivers, In 2000, only 23 per cent of chalk river GQA stretches had average phosphate concentrations below the 60µg P/l guideline standard for csac chalk rivers (Figure 12), compared with 27 per cent in Some 61 per cent of chalk river stretches exceeded 100µg P/l of phosphate. Ten per cent exceeded 1000µg P/l. Preliminary comparison with the guideline phosphate standard in chalk rivers designated as csacs indicate that some stretches of the Itchen and Lambourn are compliant. Most of the Wensum and Hampshire Avon, though, are noncompliant (Pitt et al., 2002). Small headwater streams are not included in the GQA survey. Available data suggest that the guideline phosphate standard (60µg P/l) for csac chalk streams (flow less than 2.5m3/s) may only be met in those with minimal agricultural influence and few point source discharges, such as the upper Bure and Nar (Demars and Harper, 2002). Sewage works and agriculture are the biggest sources of phosphorus entering rivers. Agriculture is the main source of nitrate. This is exemplified in the Hampshire Avon (Case study 4), though the situation varies from river to river. The sewage derived phosphorus contribution occurs throughout the year and usually dominates the supply of available phosphorus in rivers during the summer growing season. Water companies are introducing phosphate removal at major sewage works, which is reducing the loads. This is being carried out primarily where receiving waters have been identified as eutrophic or at risk of becoming so under the Urban Waste Water Treatment Directive, and for other designated rivers, notably under the Habitats Directive and for SSSIs. In the current phase of investment between 2000 and 2005, an extra 40 sewage works on chalk rivers will have treatment to remove phosphate (Figure 13). Phosphate removal at sewage works can reduce this source by about 75 per cent (Case studies 5 and 6). The resulting reduction in river phosphorus concentrations varies according to the relative contributions from sewage works and other sources. There are numerous scattered sources of phosphorus inputs to water. These include livestock, crops, fish farms, watercress farms and small sewage works. So far, phosphate stripping at major sewage works has led to some big reductions in river concentrations, but the cumulative effects of other inputs have kept levels relatively high. The ecological responses to lower inputs of phosphorus from sewage have so far been limited. Even large reductions in phosphorus loads may leave river concentrations above the thresholds at which biological changes are expected. It may also be too soon in some cases to assess ecological change because river sediments may continue to be a source of phosphorus for some time. There may also be a biological time-lag in the response of individual species to altered conditions. Further investment in major sewage treatment works is continuing to cut the largest sources of phosphorus to rivers, but achieving further reductions in chalk river ecosystems will require action on other fronts. This includes better farm nutrient management and may require additional controls at some smaller sewage works and better practices in fish and watercress farms. Over the past 10 years, phosphorus has been used increasingly on some watercress farms to force multiple crops. Much of this fertiliser flushes straight into rivers. Erosion and surface runoff from ploughed land and livestock areas can be significant sources of phosphorus to rivers (Dampney et al., 2002). 40 Water quality 41

4 > Case study 4 Long-term nutrient increases in the Hampshire Avon > Figure 14 Phosphate and nitrate trends in the Hampshire Avon, > Case study 5 Land use change and nutrient controls in the River Kennet, Wiltshire Nutrient concentrations have increased in many rivers over the past century as a result of population growth and agricultural intensification. In the Hampshire Avon, levels of phosphate have trebled and levels of nitrate, another key plant nutrient, have doubled since the 1950s (Figure 14). The species composition of rooted plants in the river and the diatom flora in the headwaters are indicative of nutrient enrichment. There is growth of blanketing algae downstream of Salisbury sewage works (see Environment Agency, 2002b). Source: Environment Agency, 2002a The promotion of good agricultural practice is essential River Kennet near Hungerford The growing pressures from people and farming over the 20th century are illustrated by changes in the River Kennet. Between 1931 and 1991, the human population in the catchment grew from about 55,000 to 175,000. Numbers of sheep and pigs increased substantially, while the area of cereal crops nearly trebled to 58,000ha, largely in place of permanent grassland. As a result of these changes, annual phosphorus inputs to the river increased from 94 to 247 tonnes by More than half of this now comes from sewage, and a fifth each from livestock (mainly cattle) and arable land (Figure 15). At the same time nitrogen inputs have risen from 1,720 tonnes to 4,050 tonnes per year, two-thirds from agriculture and a tenth from sewage. Phosphate removal treatment was introduced at Marlborough STW on the Kennet in The soluble reactive phosphorus load was reduced by about 75 per cent. This lowered average summer concentrations downstream from 550µg P/l to around 70µg P/l after treatment. Phosphorus release from sediments was not considered significant. The sewage effluent contribution is now comparable with that from diffuse sources, which dominate after rainfall. Algal blooms still occur, and improvement will depend on changes in arable and livestock practices, better buffering from these agricultural inputs, and further phosphate removal at other STWs that discharge to the Kennet, these are planned under the AMP3 programme. > Table 3 Annual inputs of phosphorus and nitrogen to the upper Hampshire Avon > Figure 15 Phosphorus inputs to the River Kennet, Source Tonnes of Percentage of Tonnes of Percentage phosphorus/year phosphorus nitrogen/year of nitrogen inputs inputs Atmospheric/natural Inorganic fertiliser , Livestock Sewage treatment works Industry (fish farms) Total , Source: Parr et al., 1998 Source: Jarvie et al., 2002; Whitehead et al., Water quality 43

5 > Case study 6 Phosphorus loads and controls in the River Wensum, Norfolk The River Wensum is affected by dense growths of blanket weed and other filamentous algae and low dissolved oxygen during the summer. Concentrations of phosphorus are high in stretches impacted by discharges (100 to 4,000µg P/l) when compared with those in the wooded catchment of the River Bure (4µg P/l). Point discharges contributed nearly 90 per cent of the Wensum phosphorus load between 1990 and Two-thirds was from sewage, mainly from two large treatment works (Table 4). Phosphate stripping at East Dereham and Fakenham sewage works reduced their phosphorus loadings by 64 per cent and 76 per cent respectively in 2000, though loads from all point source loads remained substantial. Though concentrations of phosphorus in the river were greatly reduced they remained relatively high at over 60µg P/l. The numerous small discharges and diffuse sources will make achieving further reductions in phosphorus inputs very challenging. Rooted plants in the Wensum do not appear to be limited by existing concentrations of phosphorus in water or sediments, though understanding responses to this nutrient is complicated by other factors such as water velocity, nitrate and carbon availability. > Table 4 Phosphorus loads before and after controls at two sewage treatment works on the Wensum Source Source: Demars and Harper, 2002 Phosphorus loads (total phosphorus kg/day) 1 before STW controls after STW controls East Dereham STW Fakenham All STW Industrial effluents Diffuse sources Total Sediment can wash off fields and into the river. 1 Recorded at Costessy Mill in the lower Wensum catchment. 2 Runoff from land and small unconsented discharges; the increase after controls is related to higher rainfall. Phosphate fertiliser use has fallen slightly since the mid-1990s, but this followed a long-term doubling of phosphorus derived from farmland after the 1930s (Whitehead et al., 2002). Many lowland soils still have phosphorus levels in excess of crop requirements. In some cases, elevated phosphorus concentrations are appearing in groundwaters. These may eventually increase concentrations in rivers. Increasing nitrate concentrations in groundwaters and chalk rivers are a serious long-term concern. Many chalk aquifers and rivers provide drinking water, which may eventually need to be treated to remove nitrate. Silt Cleaning spawning gravels of silt using high pressure water jets. Excessive silt smothers river bed gravels, blocking spaces for invertebrates and rooting plants. It reduces the intra-gravel flow that supplies oxygen and removes waste products from salmonid eggs and larvae. It also increases the sediment concentrations of bio-available phosphorus, because arable soils are a big source of the eroded particles. Chalk rivers are especially vulnerable to sedimentation because of their lack of high scouring flows. This is exacerbated by river flows reduced by abstraction and by over-sized channels. There are long, silty stretches upstream of the frequent mills and weirs on rivers like the Wensum (Demars and Harper, 2002). Artificial gravel cleaning was undertaken historically. This can only partly alleviate the problem, and is a very short-term, costly local remedy. A freeze-core from a heavily silted chalk river bed 44 Water quality 45

6 > Figure 16 Estimated annual soil sediment delivery to chalk rivers Though silting up of chalk rivers is a common concern, there have been few systematic studies of the impacts. In a survey of trout spawning beds that included 11 chalk streams, in each case there was more than 25 per cent of fine sediment (less than one millimetre diameter). This silt content is well above thresholds known to allow 50 per cent survival of salmonid larvae, although the medium and coarse sands at these sites may help to maintain sediment permeability and fish survival (Milan et al., 2000). In the River Itchen, all gravels investigated at sites near Winchester had fines contents at harmful levels. Salmon egg and larval survival is very low (Riley et al., 1999). The use of high pressure water jets was successful in cleaning gravels and increasing egg survival. Artificial channel narrowing to speed up flows helped to keep gravels clean, but in unmodified channels fine particles returned to their previous high levels within months. The main sources of sediment inputs to rivers are: intensive soil cultivation throughout catchments, particularly where soils are compacted and left exposed to heavy winter rain; heavy trampling and bank erosion by livestock, particularly near watercourses, and soil surface damage by free range pigs; effluent from sewage works, fish and watercress farms; run-off from urban surfaces, especially construction sites; the breakdown of aquatic plant and animal material High loads of fine soil particles to rivers are caused by land use that compacts and exposes vulnerable soils to erosion, and creates rapid routes for them to be washed into watercourses. The risk of erosion varies across chalk catchments, with some areas more vulnerable due to their soil type, slope and land use (Figure 16). The origin of fine sediments in chalk rivers seems to be predominantly arable land, including cultivated riparian land. Some 89 to 97 per cent of the fine sediment in salmonid spawning gravels in the Test, Itchen and Kennet comes from the surrounding land surface. Bank erosion is localised and often related to damage by livestock (Walling et al., 2001; GeoData Institute, 2002). Grade A water quality should be the objective for all chalk rivers Dennis Bright Outlook Better quality discharges to chalk rivers should arise through the current round of sewage treatment improvement schemes to be implemented by This will be followed by further schemes to be agreed for Under the Habitats Directive, rivers designated as csacs or SPAs should be protected from any adverse effects of consented discharges as the Environment Agency will be reviewing all relevant environmental licences and permissions between 2004 and Remedial action is likely to require improvements to some sewage effluent and other industrial discharges. Many chalk river stretches will not meet phosphate targets without action being taken to control diffuse sources, especially from farmland. The Code of Good Agricultural Practice for the Protection of Water (Ministry of Agriculture, Fisheries and Food, 1998), agri-environment programmes and voluntary initiatives have great potential to reduce sediment and phosphorus loss if more widely implemented. Defra s current review of agricultural policy in relation to diffuse pollution is looking at these issues. 46 Water quality 47

7 The coordination of controls on diffuse and point source pollution should be achieved through integrated river basin management under the Water Framework Directive, which also requires surface water and groundwater management to be integrated. Implementation of the Directive is an extension of the Environment Agency s catchment management approach, and will be particularly important for chalk rivers. We therefore expect some further improvements to effluent discharges and chalk river water quality over the next few years. To have real impact on diffuse pollution, though, concerted action will be needed. A note of caution should also be added regarding potential risks to water quality that have not been fully evaluated. We do not have a clear picture of the extent of pesticide impacts on chalk rivers. Nationally there is concern about the ecological effects of endocrine-disrupting substances from sewage treatment works and other sources. We need to keep a watching brief to ensure that such issues do not compromise the improvements we hope to achieve through the priority actions highlighted here. Conclusions Nearly 90 per cent of the total chalk river length is of good to very good biological quality. But only 37 per cent achieves both very good biological and very good chemical quality. Only 23 per cent of the total chalk river length would meet a guideline standard for phosphate that has been considered appropriate for csac chalk rivers. Silting-up of river bed gravels is widespread in chalk rivers. 48