In Focus: Reducing water pollution from agriculture

Water pollution from agriculture is a serious threat to river health, but farmers can employ a wide range of strategies to reduce their business’s impacts. This article outlines effective strategies to reduce agricultural water pollution from nutrients, sediment, agrochemicals, microplastics, and indirect water pollutants like ammonia. It also highlights the regulations on water quality that farmers must comply with.

Some of the strategies outlined are relatively inexpensive and straightforward to implement, delivering financial savings due to improved resource-use efficiency, fewer inputs and greater resilience. Others require initial capital expenditure or involve income foregone, but government support is frequently available for these. Check capital grants and Environmental Land Management schemes payments in England here, and small grants as well as the forthcoming Sustainable Farming Incentive in Wales here.

Interventions can be combined in bespoke ways. Examining farming systems holistically helps to identify where strategies can dovetail or where a more structural change would outperform a series of different actions. Finally, it is important to remember that reducing agricultural water pollution is an iterative process. Farmers do not need to do everything all at once, shortlisting some actions for long-term investment, provided that they comply with the regulations.

Water pollution occurs when substances enter water at levels that harm the functioning of the ecosystem. This includes the addition of substances not naturally present.

Water pollution can be divided into point-source pollution, discharged from a discrete point (e.g., a leaking slurry store), and diffuse pollution, which is caused by an action spread across a large area (e.g., nitrate leaching).

Agricultural water pollution occurs through numerous pathways:

1. Surface runoff: The flow of water over the land carries soluble substances and anything which the flow of water can transport (e.g., soil, material attached to soil particles, manures etc.) into watercourses.
2. Leaching: When water moves downward through layers of soil and rock, soluble substances can dissolve into the water and be transported away from the crop – either into groundwater or into sub-surface flows. When contaminated groundwater upwells through springs or other flows, it pollutes surface waterbodies.
3. Drain-flow and ditch-flow. Field drains collect soil water containing leached nutrients. Ditches subsequently offer a rapid route for leached nutrients and surface runoff to enter watercourses.
4. Direct discharge into watercourses. This category covers point-source pollution, where leaking liquid from slurry, silage, and manure stores, and washings from yards, machinery, and processing, flow directly into watercourses. Livestock which urinate and defecate directly into watercourses and cause sediment pollution via bank erosion fall within this category, as do sprays of fertilisers and agrochemicals that land in watercourses.
5. Roof drainage from intensive livestock sheds. Some ammonia from intensive pig and poultry settles on shed roofs. When it rains, this ammonia dissolves and can be carried into watercourses.
6. Indirect ammonia pollution. Ammonia in livestock manures and manufactured fertilisers easily volatises, but sooner or later it settles on soil or is deposited by rainfall. From here, nitrogen can be leached or transported in surface runoff.

In England and Wales, diffuse agricultural water pollution is regulated according to the actions that farmers take, rather than the amount of water pollution they cause (with the exception of point-source pollution incidents reported to the Environment Agency or Natural Resources Wales).

In England, farmers must legally follow the ‘Farming Rules for Water’ – a set of eight actions, with statutory guidance – which are defined in ‘The Reduction and Prevention of Agricultural Diffuse Pollution (England) Regulations 2018’. If farmers do not comply with these, the Environment Agency can impose civil or criminal sanctions, although they will generally prioritise advice before enforcement. Farmers must also comply with The Water Resources (Control of Pollution) (Silage, Slurry and Agricultural Fuel Oil) (England) Regulations 2010, and guidance on this can be found here.

The Farming Rules for Water regulate how and where manures and manufactured fertilisers must be stored and applied. There are minimum distances from watercourses that fertilisers must be stored and livestock feeders positioned. During certain soil conditions (waterlogged, flooded, snow-covered, or recently frozen land) fertilisers must not be spread. Farmers must take “reasonable precautions to prevent significant soil erosion and runoff”, including measures to prevent poaching by livestock and them accessing watercourses. Farmers must plan nutrient applications in advance according to soil and crop needs, as determined by periodic soil testing.

In Wales, legal requirements to minimise agricultural water pollution are set out in The Water Resources (Control of Agricultural Pollution) (Wales) Regulations 2021. They are significantly more technical and stringent than their English counterparts. Welsh readers should refer to Welsh Government’s Guidance for Farmers and Land Managers for details.

An effective programme to reduce water pollution from agriculture will tackle both the sources of pollution and the pathways through which they enter watercourses. The rest of the article outlines available strategies. Actions required by the above legislation are not listed.

Healthy soils simultaneously tackle the source of pollution (fewer inputs are needed for crops) and pathways for pollution (by reducing surface runoff and leaching). Regenerative agriculture proposes the following recipe for well-functioning soil that holds and absorbs more water: minimise soil disturbance and keep it covered, feed its microbes with living roots, increase the diversity of crops, and build soil organic matter through cover crops, leys, and/or livestock grazing.

Cover crops: Covering the soil over winter reduces surface runoff and soil erosion by increasing surface roughness. Living roots feed the soil food web, which improves soil texture, infiltration rate, organic matter, and crop nutrient availability. Simultaneously, living plants absorb nutrients like nitrogen, which would have otherwise been lost to overwinter leaching.

Cultivating compacted soils and loosening compacted soil layers in grasslands (through subsoiling, slitting or spiking): These strategies reduce surface runoff by increasing infiltration rate and water retention, and improves the soil’s biological diversity with benefits as above.

Reduced frequency of ploughing (i.e., minimum-till or zero-till): On uncompacted soil, minimising soil disturbance improves its structure and organic matter content, reducing runoff and improving the soil’s fertility and resilience to pests and diseases. Less pollution from sediment and sediment-bound nutrients can occur if there is less unconsolidated sediment on the surface.

Diverse soil microorganisms: The richer the microbes around crop roots (rhizobiome), the more nutrients the crop can access and the better protected it is against pests and diseases. High dosage of fertilisers reduce the crop’s investment in its rhizobiome and increase the sugariness of its biomass, which attracts crop pests and diseases. Applying pesticides further weakens the rhizobiome – a vicious cycle which leads to ever-increasing volumes of chemical and fertiliser inputs. By healing degraded soils, farmers can cut their chemical use.

Reducing the total inputs to the farming system by improving efficiency is more economical for farmers and better for the environment.

Thinking holistically: An integrated nutrient plan examines all sources of fertility and aims to maximise existing or free sources of fertility – mineralisation of soil organic matter by microorganisms, manure from livestock, nitrogen fixation from legumes – whether through mixed farming, cover cropping, or arable leys. At a landscape scale, livestock farms can trade manures with arable farmers to achieve similar effects.

Nutrient planning: A Nutrient Management Plan, based on soil sampling, helps to increase the likelihood of optimal nutrient applications and well-balanced soils. Using a fertilisation recommendation system and laboratory analysis of manures can assist.

Applying fertilisers in a series of smaller doses, synchronised to plant growth: By reducing total application volume, farmers reduce the amount of nutrients available to leaching and surface runoff. Precision farming techniques, spatially variable application rates, and chlorophyll monitors (to avoid overapplication) can help.

Reducing ammonia volatilisation: Switching away from urea, adding a urease inhibitor to urea, and injecting slurry and liquid fertilisers into the soil can reduce nitrogen loss from fertilisers, reducing indirect ammonia pollution and increasing nitrogen use efficiency (NUE).

Choosing crops with fewer fertility requirements. Many niche break crops – such as quinoa, hemp, buckwheat, linseed, rye, lupins, beans and peas (as well as root crops) – have lower input requirements compared to common cereals, and can improve NUE across a rotation. NUE and disease susceptibility should be considered alongside yield when selecting cultivars or varieties.

Relocating gateways and feeding troughs from high-risk areas: Changing where livestock congregate and machinery manoeuvres reduces pollution from livestock poaching and wheel rutting. Tracks and concrete yards can attract and funnel water, increasing the velocity of surface runoff, so drains and culverts to redirect flows can help.

Fencing livestock away from waterbodies: This prevents livestock trampling from causing bank erosion, and urine and faeces from directly entering waterbodies.

Improved drainage management: Managing water table levels requires a delicate balance to avoid increasing water pollution, but significant reductions are available. If waterlogging is an issue, clearing ditches and drains can improve infiltration, whereas in well-drained soils letting drains and ditches deteriorate can reduce leaching losses.

Cultivating across slope: Where safe, consider cultivating and driving along the slope contour. Similarly, mounting tines behind tractor wheels can disrupt their tramlines. Both strategies reduce the number of downslope channels for runoff, which would increase its velocity and potential for soil erosion.

Calibrating sprayers and spreading machinery: This avoids patchy fertilisation, which is a cost to farm businesses and leads to losses in overapplied areas. Using wide aperture nozzles to achieve the coarsest appropriate particle size, checking the wind forecast, and keeping the spray boom as low as possible can reduce sprayer drift. Machinery operators should be aware of product buffer zone widths, and could be trained to better calibrate machinery.

Cultivating land for crop establishment in spring rather than autumn: This strategy reduces the amount of unconsolidated material exposed to winter rainfall and surface runoff. It also provides time for cover crop to establish, and aims to reduce soil compaction.

Planting earlier varieties and harvesting them earlier in the autumn: When harvesting machinery traverses wet soils, it can compact them and increase surface runoff risk. Early varieties reduce the risk that soils will be wet at harvest, and also allow autumn-sown cover crops longer to establish.

Slurry and manure are significant sources of water pollution, both when stores leak (point source) and after being spread on fields (diffuse pollution).

Sufficiently large, covered slurry stores: Larger stores put farmers in control of when and where they spread slurry and manure, so they are not forced to spread during high-risk times. As quicker fixes, avoid diluting slurry by using less water when hosing down (e.g., scraping down material first and using a pressure washer), and fit stores with an impermeable cover to keep out rainwater.

Separating out clean and dirty water: It is advisable to separate clean rainwater from slurry- and manure-contaminated waters, to avoid adding unnecessary liquid to slurry stores and pollutants to surface runoff. Maintaining guttering, unblocking drains, fitting a roof over a farmyard manure store, and installing cross drains could help.

Converting slurry into a more manageable, more transportable product: Making it easier for farmers to handle, transport and store slurry and manure enables them to trade this fertility resource and have more control over spreading. Ways to achieve this include slurry separators, anaerobic digestion, composting manures, belt drying and incineration (for poultry manures). Anaerobic digestion and incineration provide energy and heating.

Adding alum (aluminum sulphate) to poultry litter: Highly effective, this reduces ammonia emissions by converting ammonia to ammonium, which is less susceptible to loss via volatilisation and leaching.

Installing ammonia scrubbers in intensive (pig and poultry) livestock sheds: Removing ammonia from ventilation gases is a best-practice-solution strongly advised through the Environmental Permitting Regulations for intensive pigs and poultry. It reduces water pollution via reduced nitrogen deposition to surrounding land.

Cooling and/or acidifying slurry and manure: Using heat exchange to cool slurry and/or acidifying it can reduce ammonia emissions and make the resulting product a more efficient fertiliser.

Reducing N and P content of livestock rations: Although many livestock diets are already finely tuned, some savings may be possible by reducing the N and P content of rations to avoid cautionary oversupply, which decreases nutrient concentrations in excreta. On large feeding operations, supplying different diets according to animal age and stage can improve the efficiency of nutrient use.

Injecting slurry and manure into soils: Band injection, trailing shoes, and dribble bars reduce ammonia volatilisation and the risk of nutrient loss via surface runoff.

The best way to reduce the risk of water pollution from agrochemicals is to use less of them.

Integrated Pest Management (IPM): IPM involves reducing pesticide applications by monitoring and identifying pests, taking preventative measures, such as good plant hygiene and soil health, harnessing naturally occurring pest control and judiciously using targeted pesticides where other methods have not worked. Linking Environment and Farming (LEAF) has produced a good introduction to IPM.

Using narrow-action pesticides specific to the pest or disease afflicting the crop. Broad-spectrum mixes can compromise natural enemies and the soil food web, which leads to a long-term decline in the ecosystem’s in-built defences against pests and diseases. Consider targeting applications only to known pest hotspots.

Using correct application rates and spray-drift reduction methods, and ensure contractors are properly trained and briefed: These strategies are discussed in the general farm management section above.

Best practice in handling agrochemicals: Significant pollution from agrochemicals occurs through drips, spills, and washing out sprayers and containers. Agrochemicals should be transferred between vessels over an impermeable surface – whether a plastic tray or a dedicated concrete bund. Intermediate bulk containers (IBCs) can store washings. Washings should not be discharged without treatment through a biobed or biofilter (impermeable bunds and containers respectively containing organic material which degrades agrochemicals over time).

Plastics can degrade into small fragments known as microplastics, which impair crop growth and plant health at high concentrations.

Avoid using thin plastics to mulch vegetables: Using plant residues like compost, woodchip or green manures to mulch avoids the plastic issue entirely and has many soil-health benefits. Another option is thicker mulch films, which are easier to retrieve and less liable to break down in-situ.

Try to find alternatives to plastic-coated fertilisers, pesticides and seeds: Plastics are added to agrochemicals to slow or control their release and to bind treatments to seeds, which theoretically reduce water pollution. However, these microplastics end up in the soil when the coatings degrade. Try to source alternatives, or consider how a combination of strategies from the sections above can negate the need for plastic-coated products.

As a line of defence, farmers can trap sediment and nutrients carried in surface runoff before these reach watercourses by slowing the velocity of surface runoff, either via vegetation or sudden changes in water depth. Plants in these areas absorb nutrients and reduce the rate at which nutrients leach.

Swales, bunds, and sediment traps or ponds: These relatively cheap barriers are designed to fill with water and initiate the process described above. When full of sediment, it can be excavated and spread as a fertiliser.

Widening ditches and creating ditch wetlands: Depending on the flood risk of the land, consider reprofiling existing ditches and inserting in-ditch barriers to create linear wetlands, which allow nutrients and sediment to settle. Wetland plants take up nutrients as well as encouraging sediment deposition.

Riparian buffer strips: The most well-known option, an area of field immediately adjacent to a watercourse which is vegetated with grasses and/or trees. Alongside the above mechanisms, denitrifying bacteria thrive in riparian soils and remove nitrogen from the soil by converting it into gaseous nitrous oxide. Mowing the strip and removing cut material can prolong the effectiveness of buffer strips by reducing the rate at which buffer strips saturate with phosphorous. In-field grass buffer strips are also valuable, though less effective.

Constructed wetlands: The most involved type of barrier, these begin with a sediment trap or pond and end with a heavily vegetated wetland area (which can be zero discharge if planted with rapidly growing and transpiring species like willow).

Some of the most effective ways to reduce total water pollution from agricultural land involve changing land use, either at a farm scale or in targeted high-risk areas.

Arable reversion, either to livestock grazing, conservation grassland, or woodland: According to research by ADAS, arable reversion in high-risk areas can reduce nitrate losses by 80-90%, and particulate P and sediment pollution by 50% (unless livestock poach the ground). This is mainly the result of ceasing to apply substantial quantities of fertilisers.

Farmland to biomass cropping involving willow, poplar or miscanthus: Alongside the benefits mentioned above, biomass cropping removes nutrients from the land, which counteracts saturation of P in soil.

Other forms of environmental delivery: Most forms of environmental land use are likely to reduce surface runoff and leaching, and can be funded through environmental markets, payments from Defra’s Environmental Land Management scheme or Welsh Government’s forthcoming Sustainable Farming Scheme, and/or ecotourism.

Finally, changing the design of the farming system so it is not oriented around absolute yield and a treadmill of inputs can deliver major improvements to water quality. This might involve transitioning towards more regenerative or organic agriculture.

Research shows that it is often more profitable for farms to reduce their yields if this avoids bought-in inputs.

One study found that lowland livestock farms could increase their profits by 45.3% on average, upland livestock by 39.1%, lowland dairy by 32.7%, and lowland arable by 9.5% by farming at the landscape’s ‘maximum sustainable output’. In arable systems, it could be worth investigating whether the income from obtaining an extra tonne of yield outweighs the costs of the fertiliser and pesticides used to achieve this.