Diffuse Water Pollution

Pesticides, fertilisers, microplastics, hydrocarbons and sediments from construction sites, roads and agricultural land are just some of the types of diffuse pollution that contaminate our water, causing harm to human health and the environment.

Point and diffuse are the two sources of water pollution, the main difference being the site of exposure. Point source pollution has a specific site of origin and can therefore be more easily controlled through regulations. On the other hand, diffuse pollution has many rural and urban sources from where potential pollutants are transported through rainfall and similar pathways to accumulate in waterbodies. Due to the dispersed nature of the pollutants and the complex routes, diffuse pollution is generally harder to control and monitor.

Diffuse water pollution therefore continues to reduce the quality of water and affects many areas reliant on the resource, such as human health through contaminated drinking water, and causes catastrophic environmental incidents such as fish kills and ecosystem destruction. In June 2020 the Government released the 25 Year Environment Plan Progress Report stating that diffuse pollution from rural areas was responsible for 40 % of rivers failing to meet ‘good ecological status’ standard. Only 16 % of water bodies met this standard according to the Environment Agency (EA).

The sources of the pollutants entering waterways varies between rural and urban areas. However, because of the dispersed nature of the pollutants, the area they can affect and accumulate in, may be far from the initial source. The main pollutants of concern are excess nutrients such as nitrates and phosphates, microplastics, oils and chemicals, pesticides, sheep dip, organic pollutants from manure, bacteriological contamination, sediment, and sulfuric and nitric acid.

Agricultural practices are one of the main sources of diffuse pollution. Processes such as the use of fertilisers, manure, pesticides, and sheep dip, as well as physical processes to the land releasing sediment, all contribute to diffuse pollution.

Fertilisers containing nitrates and phosphates are used widely in farming to add the necessary nutrients for crop growth. However, excess nutrients can percolate through the soil into the groundwater or be washed off the land through heavy rainfall leaching into surface waters. These excess nutrients can lead to increased algae growth, forming algal blooms in the water. Not only can variations of these algae be toxic, making it unsafe for recreational activities, but they also lead to a reduction of dissolved oxygen. This causes a general decline in the quality of the aquatic environment and the rapid depletion of oxygen levels can lead to occurrences such as fish kills.

Pesticides that are toxic to animals and insects are used to control pests damaging crops. Their mechanisms into surface and groundwaters are similar to fertilisers via surface run-off and leaching respectively. In the water systems they cause loss of aquatic life due to toxicity and persist in the ecosystem. DDT is an example of a pesticide that has been banned in the UK for almost 40 years due to its impacts on the environment.

Sediment also reduces oxygen supplies in water. It is most commonly associated with the disturbance of land through agriculture and construction.

Diffuse pollution in urban areas is dominated by road and other surface run-off, whereby rain transports pollutants such as oil and chemicals. Leaking sewers into surface water drains is also a problem, with a historic paper from the EA (2007) reporting 1 in 5 households having foul drains connected to surface water drains. This produces bacteriological contamination in the water sources and is an illegal practice.

Large levels of sulphur and nitrogen are produced from transport and industrial emissions and their impacts are widespread due to transfer to water bodies from the atmosphere as acid rain. This degrades the quality of the water by lowering the pH. Aquatic ecosystems are very sensitive to changes in the environment and acid rain can decrease the pH of a lake to a threshold that the fish and plants can no longer survive.

An article published by the International Union for Conservation of Nature (2017) identified microplastics as an increasing source of diffuse pollution with car tyres accounting for 28 % of microplastics in the ocean. Microplastics ingested by marine wildlife bioaccumulate, causing damage to their growth and reproduction. The concentration of microplastics also increases higher up the food chain to now include the human population.

The increasing impact of diffuse water pollution originally led to the development of the Water Framework Directive (WFD) in 2000 by the European Union. The WFD set a target for all inland and coastal water bodies to reach good quality status by 2015, and although this target was not met, various regulations and initiatives to reduce diffuse pollution were introduced. These include river basin management plans (2009/2010), Reduction and Prevention of Agricultural Diffuse Pollution (England) Regulations 2018, and the Catchment Sensitive Farming initiative advising farmers on improving practices to reduce pollution (2015).

Impacts of some pollutants once entrained in the water can be managed, such as reducing levels of nitrates in the water for drinking by water treatment or reversing the algal blooms by addition of oxygen and adding routes of degradation for the excess nutrients that accelerate the effects of eutrophication. Whilst it is possible to address some pollutants this way, it does not however, reduce the negative impacts of pollutants that occur prior to treatment.

Supporting the agricultural industry in reducing their contribution to diffuse pollution through chemical use and physical processes, is important to maximise the effectiveness of the regulations. Through initiatives and incentives, with regular monitoring and testing of soil and groundwater, a dynamic system can be implemented that gives scope for changing conditions and agricultural needs whilst also minimising diffuse pollution.

Both the sources and the pathways of pollution need to be understood and controlled to properly manage the pollutants and associated impacts of diffuse pollution. Slowing down the pathway between source and receptor is one mechanism through which pollutants can be broken down, thus reducing the amounts of pollutants ultimately entering the water.

Sustainable drainage systems (SuDS) allow pollutants to settle out and therefore have longer to degrade. For example, ponds or wetlands can be positioned between the source and receptor to give a slower rate of transport, thereby encouraging the pollutants to settle out whilst also reducing flood risk. In urban areas particularly, green infrastructures would be beneficial to surface water runoff for example, increasing the proportion of porous surfaces through planted verges by roads which could act as a filter for water before entering the water bodies.

I see one of the biggest challenges for reducing diffuse pollution being the continual growth of population and the increasing pressure it places on resources and infrastructure. The demand for housing will persist, and with developments being such a large contributor to diffuse water pollution through sediment and wrongly connected sewers, surface water management on these sites should be a priority. It also follows that the wastewater companies will have higher volumes of effluent to treat, so must have the infrastructure in place to deal with the increasing requirement for treatment to avoid environmental incidents.

Alongside the support and incentives provided to those contributing to diffuse pollution, there should be stringent enforcement of regulations where necessary.

Integrating new technologies that emerge into current practices could help further mitigate or even eliminate some sources of diffuse pollution. Green fertilisers for agriculture are already being created to tackle nutrient pollution, and filters are being added to washing machines to capture microplastics released from clothes. In the future we could see the use of Goodyear’s biodegradable and self-regenerating tyre to reduce microplastics, and CSIRO’s genetically modified coloured cotton reducing toxic chemicals entering water bodies through the dyeing process. Consideration should however be given to ensure any new technologies applied to tackle diffuse pollution are not solving one problem by creating another.

Hannah Corran, Environmental Scientist at John F Hunt Water Technology