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GROUP 4 Diffuse Water Pollution

Working group members and contributor

Diffuse Water Pollution

Outline of the issue

5.1 Diffuse pollution to soils is generally associated with atmospheric deposition and certain land management activities. Atmospheric deposition impacts on soil functioning are dealt with in the section of the soil strategy and dissolved organic carbon issues will be addressed in the separate section on organic soils. Here we consider the role of land management in terms of the impact of diffuse pollution on soil functioning and soils acting as a sink and buffer to these pollutants.

5.2 A significant proportion of our soils are managed to maximise their function as a medium for food and timber production. To meet these demands, soils have been managed in an increasingly intensive manner. In turn this increased intensity of use has compromised some soils to fulfil their role in functioning as a filter and buffer to retain and mediate pollutant inputs. In turn this loss of function has resulted in pollutants reaching surface and groundwaters. Soil and water quality are inextricably linked and to a significant extent soil functions determine water quality. Soils also regulate the flow of water in the environment and hence affect the transport pathways of potential pollutants to water. Soil management is therefore integral to the maintenance and improvement of water quality and to achieving the environmental objectives of the Water Framework Directive ( WFD).

Strategic Objective

To identify the role of soils and their management in regulating diffuse pollution inputs to waters, to assess these impacts in Scotland and associated mitigation measures to establish a linkage between the Scottish Soil Strategy and the Water Framework Directive.

5.3 The WFD⁴ is a wide-ranging and ambitious piece of European environmental legislation which became law in Scotland at the end of 2003 through the Water Environment and Water Services (Scotland) Act 2003 ⁵ ( WEWS act). It establishes a new legal framework for the protection, improvement and sustainable use of surface waters, transitional waters, coastal waters and groundwater across Europe.

5.4 Diffuse pollution is estimated to be one of the most significant pressures putting water bodies at risk of not meeting the environmental objectives of the WFD. Diffuse pollution arises from a wide range of activities, including both urban and rural land management and atmospheric deposition. Inputs may be minor so as not to cause contamination of the soil. However, collectively over time and under certain environmental conditions the cumulative effect of their loss to waters can be significant and impact on water quality and ecology. For the purposes of this report only the most significant diffuse pollution inputs on soils that result in pressures on water quality will be considered ie those from rural and urban land use activities. More specifically the soil functions described above, their role in protecting and improving water quality and the impact of the pressures outlined below on those functions and water quality will be considered.

5.5 A common definition of diffuse pollution is (D'Arcy et al., 2000);

"Pollution arising from land-use activities (urban and rural) that are dispersed across a catchment or subcatchment, and do not arise as a process industrial effluent, municipal sewage effluent, deep mine or farm effluent discharge."

5.6 Some further characteristics which define diffuse water pollution are given at Box 5.1.

Impact of soils and their management on the water environment

5.7 In considering the impact of diffuse pollution to waters derived from soil management it is necessary to consider their type, source and transport. Soils under intensive land-use can be left without a vegetated surface, they can also be subject to high inputs of organic and inorganic fertilizers and host pathogens derived from livestock slurries and manure. In turn where these situations exist and are not adequately managed they will give rise to diffuse pollution. The impact of these pollutants on the water environment depends on the type of pollution. For example, soils, when transported to the water environment, can be a pollutant in their own right by smothering gravels in rivers and lochs and reducing light penetration in estuaries and coastal waters. They are also important in the transport of other pollutants such as pesticides, nutrients and faecal pathogens that are attached to soil particles. In addition, a range of physical and chemical soil properties determine the loss and transport of potential pollutants. This buffering capacity of soil is essential to the protection of the water environment. Some loss of soil and associated pollutants to the water environment is of course natural and essential to the maintenance of ecological status. However, the impact of anthropogenically accelerated rates of soil loss and their associated pollutant loadings on the ecology of the water environment can be significant (Williams, 2007).

Sources of diffuse pollution

5.8 The Scottish Government consultation on Diffuse Pollution from Rural Land Use (Scottish Government, 2007) identified the main activities from rural land use posing a risk to the water environment. A modified version of these activities is listed in Table 5.1 together with an indication of their relative contribution to the pollutants of concern. Note these activities apply to all rural land use including forestry, agriculture and inputs from septic tanks. In all of these soil (properties and functions) plays a key role in determining the magnitude of the pollutant and its potential impact on the water environment.

Table 5.1 Activities posing risk to the water environment

X = major contribution,
x = minor contribution
*soil as transport mechanism and other properties which affect pollutant transport to the water environment
FIO- faecal indicator organisms,
SS- suspended solid,
Pest - pesticides

Transport of diffuse pollutants from land to water

5.9 There are a number of factors which influence the transport of pollutants from the land surface to freshwaters such as the type of soil, slope of land, drainage conditions and position in the landscape. An understanding of these will help identify measures for mitigation of diffuse pollution. Overland flow is the primary pathway for transporting pollutants attached to soil particles (such as phosphorus and faecal pathogens). In contrast sub-surface flow is the primary route for transporting soluble pollutants (such as nitrate). In the rural environment soil properties which influence mobilisation and run-off include, for example, soil structure, aggregate stability and organic matter content. In the urban environment soils can act as valuable sinks for pollutants and can regulate the flow of water flowing from impervious surfaces.

What are the main pressures on soil properties affecting diffuse water pollution?

5.10 Soils play an essential role in buffering potential pollutants. Their ability to buffer will depend on the type of pressure (Table 1), the amount of exposure to the pressure and inherent properties of the soil to ameliorate, assimilate and transport the potential pollutant to a water body.

5.11 The diagram in figure 5.1 gives an example of the soil properties determining the impacts on water quality of a typical pressure. As presented in paragraph 5.19 there is a requirement to further develop these pressure-state- impact-response type relationships in a quantitative manner.

Figure 5.1 Soil / water relationship

The impact in Scotland

5.12 In 2005 SEPA carried a comprehensive risk assessment of water quality in Scotland ( SEPA, 2005) This has now been updated by the Significant Water Management Issues Report ( SEPA, 2007). Both of these reports have identified diffuse water pollution from agriculture as being the most significant pressure placing water bodies at risk of not meeting the environmental objectives of the WFD. As part of this work the Diffuse Pollution Screening Tool (Anthony et al., 2005) estimated the annual sediment loss to surface waters has been estimated at 878.400 t with diffuse source accounting for ~85% of the load. Speirs & Frost (1987) attribute increases in erosion to the increased coverage of winter cereals and the creation of fine seed beds. The evidence for soil erosion is summarised in the Soil Erosion and Compaction sub-group submission. In addition to soil erosion, the evidence for soil's ability to fulfil its function of protecting the water environment by filtering and buffering pollutants and regulating the flow of water must also be considered. The importance of diffuse pollution relative to point sources for the major pollutants is summarised in the table 5.2.

Table 5.2 Importance of diffuse pollution relative to point source for the major pollutants

(Source Anthony et al., 2005)

5.13 Diffuse pollution of the water environment could be viewed as an indicator of soils' ability to fulfil that function. The evidence suggests that in some parts of the country this function has been exceeded (about 25% of Scotland's river water bodies for example). However, in the rest of the country soils are adequately protecting water quality.

5.14 The impact of suspended solids in terms of quantification of the load to the water environment and subsequent impacts on ecology is not well understood. This is partly because monitoring of suspended solids occurs generally at low flows and therefore the majority of the load (up to 80%) is missed (Greig et al., 2005). In addition, no standard for suspended solids in rivers is included in the new WFD classification scheme.

5.15 Diffuse pollution from the leaching of nitrates from soils under intensive agricultural management has been recognised as a major issue for many waters in Scotland. In these areas identified as Nitrate Vulnerable Zones ( NVZs) farmers are required to manage the land through an action programme aimed at minimising losses of nitrate from soils.

Economic impact

5.16 There can be substantial costs associated both with the sedimentation of watercourses and with diffuse pollution impacts. As an illustration, a UK survey in 1986 estimated the cost of sediment-related problems in urban drainage systems to be ~ £50-60 million per year (D'Arcy et al., 2001 and references therein). SEPA's SWMI report ( SEPA, 2007) also assess the socio-economic impacts of diffuse pollution to be significant.

The impact on fish and invertebrates

5.17 The impact of sedimentation and suspended solids on fish, particularly salmonids, is very well documented (Greig et al., 2005). River and lake sediment loads, both in suspension and deposited in the bed, can impact upon fish by affecting spawning site selection, intergravel survival, swim-up fry emergence and juvenile and adult survival. There is also considerable evidence that sedimentation and turbidity are significant contributors to declines in populations of aquatic organisms: Tsui & McCart (1981), Wagener & La Perriere (1985). Ryan (1991) concluded that a 12 to 17 percent increase in interstitial fine sediment may be associated with a 16 to 40 percent reduction in the total abundance of invertebrates.

Loch of Strathbeg case study

Loch of Strathbeg is a 915 ha wetland reserve in north east Scotland. It has been designated for a range of biological and geological features at the international, national and local levels as a SSSI, Ramsar site, and a Special Protection Area ( SPA). A number of conservation features at the loch are currently in unfavourable condition. Many of these features depend on water quality. Heavy siltation, and increasing levels of nutrients from the surrounding agricultural land, are thought to be the main pressures on the loch. Restoring the site and its catchment area are likely to cost in the range of £350,000 to £1 million.

Climate Change

5.18 Predicted changes in climate are likely to exacerbate the pressure on the water environment from diffuse pollution. For example it is predicted that rainfall over the eastern part of Scotland will decrease both in the winter and summer and that the rainfall distribution will change with an increase of 20% in the amount of precipitation in the autumn ( SNIFFER, 2006). This is likely to increase the potential for diffuse pollution especially when soils are bare and unprotected. Higher intensity rainfall can lead to more erosion and suspended solid generation and washing off of microbial contamination from fields. Conversely lower flows during the summer could reduce the ability of water bodies to dilute higher concentrations of soluble inputs from diffuse pollutants. It has been observed that both winters and springs are warmer than previously and the frost free season is longer, which has resulted in a change in the growing season (Barnett et al., 2006). There is also potential for impacts through more indirect effects of climate change such as changes in land use, including the growth in biofuel production, which will have an, as yet undetermined, impact of diffuse water pollution. In addition, some mitigation options such as the use of natural or constructed wetlands built to buffer and filter diffuse pollutant runoff may increase the emissions of the greenhouse gases methane and nitrous oxide. The trade off between these processes needs further evaluation.

Data/evidence gaps

5.19 Although we have considerable knowledge about soil management and its influence on accelerated erosion and loss of other pollutants, there are a number of significant gaps in our knowledge of the extent and the impacts of soil loss and associated pollutants on water quality in Scotland. The understanding of the impacts of diffuse pollution on the ecology, particularly of our rivers, needs attention. Gaps in the evidence base and the subsequent research needs are listed below. It is suggested that these ideas are discussed in more detail by a soil and water 'task and finish group' of relevant experts and land management practitioners.

• Quantification of land-use and soil management in relation to diffuse pollution and the impact on water quality and ecology in Scotland to provide the evidence base for policy development.
• Investigation into the impacts of predicted land use, soils and climate change on diffuse pollution in Scotland. This would include likely changes under CAP reform, increased forest coverage, soil sealing and other development pressures.
• Monitoring of suspended and deposited solids in representative catchments to understand sources and transport pathways, ecological impacts, remediation measures and recovery times.
• The determination of the spatial and temporal status of soil P and N concentrations and their relationship to the loss and leaching across water bodies in Scotland
• The generation of the relative sources and the magnitude in terms of source apportionment of diffuse pollutants on a catchment by catchment basis.
• An assessment of the impact of agricultural land drainage schemes on diffuse pollution transport in Scotland.
• An assessment of how much reduction of diffuse inputs to soil is required to achieve tolerable levels that are technically and economically viable.
• Develop a standard for the ecological impacts of suspended solids in surface waters.
• Identify key factors and use a critical loads type approach to assess the buffering capacity of soils for the key diffuse pollutants.

Relevant Policies

5.20 In this section we have considered only those policies (including strategies and regulations) relevant to soil related impacts on diffuse water pollution.

5.21 With the exception of the Nitrates Directive (91/676/ EEC), there are currently no mandatory measures to reduce diffuse pollution to waters. However, there are a number of policy instruments and codes of good practice that relate to the protection of soil and which could have a beneficial effect on diffuse water pollution. The most relevant are:

• Prevention of Environmental Pollution From Agricultural Activities ( PEPFAA, (Scottish Government, 2005))
• Good Agricultural and Environmental Conditions ( GAEC, (Scottish Government, 2006))
• General Binding Rules for reducing diffuse pollution ( GBR, (Scottish Government,2007))
• Farm Soils Plan (Scottish Government, 2005b)
• Forests and Water Guidelines (Forestry Commission, 2004)

Figure 5.2 Measures impinging on Soil

5.22 These fall into measures which are regulatory, economic or voluntary in nature (see Figure 5.2) Together these instruments provide a comprehensive set of measures by which the impacts of diffuse pollution may be reduced. Additionally these measures will also have significant add-on benefits for biodiversity, landscape and potentially farm businesses. However, what is less well developed is the mechanism through which the measures can be implemented effectively and at the appropriate scale.

5.23 Many of the measures and mechanisms are now in place to mitigate diffuse water pollution. However, the implementation of these measures and mechanisms is a major gap and resources will be required to fund an effective implementation strategy e.g. via awareness raising and knowledge exchange.

References

Anthony, S., Betson, M., Lord, E., Tagg, A., Panzeri, M., Abbott, C., Struve, J., Lilly, A., Dunn, S., DeGroote, J., Towers, W. and Lewis, D. 2005 Provision of a screening tool to identify and characterise diffuse pollution pressures: Phase II. Final Report to SNIFFERWFD19

D'Arcy BJ, Ellis JB, Ferrier RC, Jenkins A and Dils R (eds.) 2000 Diffuse Pollution Impacts, the environmental and economic impacts of diffuse pollution in the UK. Chartered Institution of Water and Environmental Management ( CIWEM).

Forestry Commission 2004 Forests and Water Guidelines
http://www.forestresearch.gov.uk/PDF/fcgl002.pdf/$FILE/fcgl002.pdf

Greig, S. et al., 2005 The most relevant study linking this impact to the cause is: The Impact of Land Use on Salmonids: A Study of the River Torridge Catchment, National Rivers Authority, R & D Report: 30.

Ryan P. 1991 Environmental effects of sediment in New Zealand streams: a review. New Zealand Journal of Marine Freshwater Research, 25: pp. 207 - 221.

Scottish Government 2005a, Scottish Government:
www.scotland.gov.uk/Publications/2005/03/20613/51368

Scottish Government 2005b, Scottish Government:
www.scotland.gov.uk/Publications/2005/12/01130314/03142

Scottish Government 2006, Scottish Government:
www.scotland.gov.uk/Publications/2005/12/0990918/09199

Scottish Government 2007 Scottish Government:
www.scotland.gov.uk/Publications/2007/09/05093837/0

SEPA, 2005
http://www.sepa.org.uk/pdf/publications/wfd/Article_5_Scotland_River_Basin.pdf.

SEPA, 2007
http://www.sepa.org.uk/wfd/news/index.htm.

SNIFFER, 2006 Climate Change Trends
www.sniffer.org.uk/climatehandbook/

Speirs, R. B. and Frost, C. A., 1987 Soil water erosion on arable land in the UK. Outlook on Agriculture, 4, 1-11.

Implementing the Water Environment and Water Services (Scotland) Act 2003: Diffuse Water Pollution from Rural Land Use: General Binding Rules and related provisions Consultation
http://www.scotland.gov.uk/Publications/2007/09/05093837/0

Tsui, P. and McCart, P. 1981 Effects of stream-crossing by a pipeline on the benthic macro-invertebrate communities of a small mountain stream. Hydrobiologia, 79: pp. 271 - 276.

Wagener, S. and La Perriere J. 1985 Effects of placer gold mining on primary production in subartic streams of Alaska. Water Research Bulletin, 22: pp. 91 - 99

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