Mitigation Options for Phosphorus and Sediment (MOPS): reducing pollution in run-off from arable fields.

Abstract

Recent figures show that seven out of 10 river stretches in the UK now achieve either good or very good status under the general quality assessment criteria (Environment Agency, 2009). However, this also means that three rivers in every 10 fail to achieve this level, and many rivers and lakes fail to meet the more stringent criteria required under the EU Water Framework Directive (European Parliament, 2000). Although the water quality in UK rivers and lakes has improved over the past two decades, the improvement is principally due to the control of point source pollutants, such as outfalls from sewage treatment works, which are easily identified and can be tackled through regulation. Further improvements in water quality are unlikely to be achieved so easily, as other pollution sources are diffuse - scattered across the landscape - and difficult to identify and control. Agriculture is generally considered to be responsible for the majority of diffuse pollution inputs to surface waters: latest available data place the contribution of agriculture to diffuse pollution at between 25 per cent for phosphorus and 70 per cent for sediment (Defra, 2009).
Two of the principal diffuse pollutants of concern for widespread water quality are sediment and phosphorus. Sediment is a pollutant in its own right, with high sediment loads making rivers turbid and reducing light levels which affect ecosystem function. The deposition of sediment causes blockage of fish spawning sites in gravel beds and the silting up of rivers and lakes (Owens et al, 2005). Sediment inputs to streams have other impacts, being indicative of soil erosion which reduces agricultural productivity, and reducing flood storage capacity. Sediment is also associated with the transport of other pollutants, including toxic metals, pathogens and nutrients (Edwards and Withers, 2008). Sediment and nutrients have a strong influence on the trophic status (primary productivity) of surface waters, and high levels of sediment and nutrients can lead to eutrophication, excess growth of aquatic plants which may lead to toxic algal blooms, deoxygenation of water, and fish kills (Harper, 1992). Levels of phosphorus are of particular concern in inland waters, as phosphorus is the limiting nutrient for aquatic growth (Mainstone and Parr, 2002). Phosphorus in surface run-off is largely transported in particulate form, bound to sediment particles, but can also be lost as phosphate in solution (Haygarth et al, 2000). Losses of phosphorus from agriculture are often high, as agricultural systems traditionally have high inputs of phosphorus applied in fertilisers and manures to enhance productivity. In the UK, the average agricultural phosphorus surplus has been estimated at around 16 kg ha^-1 per year (Withers et al, 2001). As diffuse pollution sources are scattered across the landscape, current options available focus on two main catchment management approaches: the prevention of soil erosion and pollutant loss from agricultural fields, and the trapping of pollution before it reaches the river network. A number of UK publications document and describe potential catchment management approaches, for example, the Department for Environment, Food and Rural Affairs (Defra) Diffuse Water Pollution from Agriculture User Manual (Cuttle et al, 2007) focuses on agricultural diffuse pollution mitigation measures. However, the evidence available illustrating that these measures actually work is limited. Trialling and testing catchment management measures and generating data to determine their effectiveness is difficult for two reasons. First, because of the size of the research area needed for trials - many of these measures take place at field, farm and catchment scale, which is difficult to replicate to give statistically robust results and may be difficult to relate to water quality measurement locations, and second, because of the natural variability in landscapes - no two sites are the same, and transferring trial data between locations relies on a number of assumptions being made which results in uncertainty in results. The Mitigation Options for Phosphorus and Sediment (MOPS) projects are two collaborative research projects, funded by Defra to determine the effectiveness of a number of management options which have potential to mitigate diffuse pollution from agricultural land. The projects focus on a variety of in-field and edge-of-field options, suitable for different farm and soil types. MOPS1 (2005-2008) trialled different tillage options for in-field surface run-off losses from winter-sown cereals. MOPS2 (2008- 2013) is considering two further possible areas for diffuse pollution control, in-field surface run-off losses from spring-sown crops, and losses at the edge-of-field in run-off through ditches and drains. This paper summarises the results of MOPS1, reports progress on MOPS2 and presents some initial data.