What’s the dirt on sediment pollution from agriculture?

The FAO have just released a review on global water pollution from agriculture. At over 200 pages long (I should know, I’ve just finished reading the entire thing!), I’ve decided against squeezing it all into a single blog post. I’ve therefore decided to concentrate on sediment pollution, as nitrates and phosphates already receive a relatively high level of attention. Furthermore, sediment pollution is the main pollutant I’m studying as part of my PhD!

Overview of water pollution

In the EU alone, 38% of water bodies are under significant pressure from agricultural water pollution.

The annual costs of water pollution from agriculture is £billions, most of which is borne by society due to being non-market externalities. An externality in this context is a negative outcome of farming which isn’t experienced on the farm itself but by the environment/local community/other stakeholders; farmers don’t/won’t (possibly shouldn’t?) then pay to mitigate these problems. These costs are only going to increase, with emerging issues such as dam siltation from sediment pollution becoming an increasing (and very costly) issue.

Nitrogen pollution alone is estimated to cost the EU $35-230 billion/year due to the associated damage to ecosystems, decreased water quality, and impacts on human health. Although I’m not covering nitrogen pollution much in this blog post, this estimation gives a good indication of just how costly water pollution is, particularly as there area multitude of other pollutants which are also expensive to ameliorate.

Source IWMI
Water pollution sources/pathways. Picture source: IWMI, 2017

Global drivers of water pollution from agriculture

As I’m sure most of you are aware, the global population is set to reach 9.8 billion by 2050. That’s a lot of mouths to feed, particularly when the majority of this growth is expected to occur in developing countries.

Just to make things worse, humans are eating too much; average calorific intake per day is now around 2800 calories, with many countries (US, Italy, Egypt, Turkey) exceeding 3500 calories per day! Dietary preferences are also changing, with consumers expecting to eat more meat, dairy, eggs, fish, and oil, all of which are problematic in terms of sustainability (and often welfare!).

The total number of livestock has increased from 7.3 billion units in 1970 to over 24.2 billion units in 2011; it’s 2018 now, so that number will be much higher! The current global trend is to increase meat consumption: we consumed on average 23g/person/day in 1961, and  >43g/person/day in 2013. This is a huge issue when we consider the huge demand for resources meat production leads to; 70% of all agricultural land is now dedicated to livestock production (that’s 30% of global land surface!).

According to the FAO (2018), livestock is the largest global source of water pollution if the land used for feed crops is taken into account

There are many heated discussions about future meat consumption occurring at the moment, with numerous studies finding that current intake is unsustainable. My current opinion is that if we eat meat, it shouldn’t be often, and it should be high quality, higher priced (to ensure the farmers are paid fairly), high welfare, and sustainably reared (e.g. pasture fed). Picture source: Pixabay.

Global fish production has also skyrocketed, with almost 1/2 of fish now produced through aquaculture, which can also have profound effects on water quality.

Waste is another huge issue, with over 1/4 of food production is lost along the food supply chain. Additionally, many consumers take our varied, accessible supply of food for granted so don’t think about how to reduce their own food waste.

In order to feed the world, the global area of arable land is expected to increase by  around 70 million hectares by 2050.

Furthermore, some of the innovations developed to increase productivity have nasty side effects; although irrigation increases food security, it is also associated with decreasing water quality due to runoff and leaching of contaminants, many of which are harmful to human health and expensive to remove.

Impacts of water pollution from agriculture

Health: Poor water quality can lead to an increased burden of a variety of diseases

Environment: Poor water quality can lead to decreased biodiversity, eutrophication (see image below), dead zones, visual impacts, bad odours (e.g. from slurry, which is basically liquid poo!), less opportunities for recreation (would you want to swim/canoe in murky water?), and increased greenhouse gas emissions

Productive activities: Poor water quality can also lead to decreases in agricultural productivity, decreased market value of harvested crops, decreased tourism, and less fish/shellfish catches.

Eutrophication – this is when too many nutrients in a river (from water pollution) lead to the rapid growth of algae; this then reduces oxygen/light availability, affecting the survival of the entire ecosystem.


Sediment consists of solid particles of soil, which contains minerals and organic matter. It is not only a physical pollutant but it is also able to carry chemical contaminants and pathogens, making it an important environmental concern. Sediment is the most important pathway for pollutants which have low solubility (i.e. don’t readily dissolve in water) such as phosphates, some metals, and pesticides. Sediment pollution happens when soil is transported off of the land, eventually ending up in water courses along with any of the pollutants it’s carrying.

Soil erosion is estimated to lead to annual fluxes of 23-42 million tonnes of nitrates each year, and 14.6-26.4 million tonnes of phosphate from farmed land each year. Current soil erosion rates are 100-1000x higher than natural erosion rates (as some erosion has always happened anyway), but because it’s so horrifically high since industrialisation of farming, it has dramatically overtaken rates of soil formation; this means we are losing soils to an extent where we may not have any left in 30-50 years.

This is a satellite image of the UK during a storm event; you can see the sediment leaving the country and entering the sea. Soil loss is a huge issue and we could end up unable to farm in 30-50 years if current rates of losses continue! Picture source: WWF (2018)

Losing sediment is not only costly to health and the environment, but also to the farmer; when they lose soil, they also lose nutrients vital for plant growth. Farmers then compensate for this loss by using expensive fertilisers.

Global soil erosion costs $77-140 billion each year for phosphate losses alone!

Sedimentation is caused by a range of farming activities, including (but not limited to):

  1. Land clearance
  2. Excessive tillage (ploughing!)
  3. Overgrazing
  4. Uncovered soils
Excessive tillage on land vulnerable to soil erosion can lead to sedimentation

An increase in sediment within rivers leads to a number of problems, primarily caused by increased turbidity (cloudiness). High turbidity prevents sunlight from reaching the water body, leading to decreased photosynthesis, and thus, less food. Furthermore, predators that rely on sight will no longer be able to find prey, and equally, prey may not be able to hide from predators; this sends the entire ecosystem out of balance, and species which don’t rely on sight will thrive, whilst others will decline. Reduced sunlight can also lower the temperature of the water body, affecting breeding cues.

Fine sediment can also clog up the gills of fish and destroy spawning habitats by forming a blanket over the fish’s spawning patch. High rates of sedimentation can disrupt hydraulics and transport capacity of rivers and in extreme cases, lead to flooding as the depth of the river is reduced by large amounts of sediment.

Dunlop et al 2005
Impacts of turbidity on aquatic ecosystems – from Dunlop et al (2005)

The type of soil in the area affects the likelihood of erosion; whilst clay rich soils are generally quite resilient, high silt soils are often very vulnerable to runoff as the particles are easily detached, tend to crust, and therefore runoff is likely.


FAO 2018
Key mechanisms of soil erosion leading to sedimentation (FAO, 2018)

Sediment can also reduce the storage capacity of reservoirs, which we rely on for drinking water; this can also reduce the effectiveness of hydroelectric power stations and irrigation systems.

It has been estimated that by 2050, 64% of the world’s current reservoirs will be filled with sediment

What can be done to reduce sediment pollution?

1. Policy responses

Due to its complexity and variability, managing water pollution requires a number of responses. According to the FAO (2018), policy makers need to act on:

  1. The key drivers of agricultural expansion and intensification (e.g. unsustainable dietary shifts to increased meat consumption, unnecessary food waste)
  2. Limiting the export of pollutants from farms (to stop sediment reaching the watercourse in the first place!)
  3. Protecting water bodies from loads (e.g. with buffer strips)
  4. Restoring affected ecosystems

Unsurprisingly, remediation is more expensive than preventing pollution in the first place; furthermore, in many cases, being reactive often doesn’t work if the damage is irreversible.

2. Regulatory instruments

These are essentially rules that farmers are expected to follow; however, most of them are very difficult, expensive and complicated to enforce, leading to questionable results. This is because there are so many different pollutants which can travel into rivers through various pathways at different rates. Furthermore, even if a farmer mitigates for water pollution, there is usually a time lag before results are seen in the waterbody.

3. Economic instruments

This could be in the form of taxing farmers which are causing pollution, or compensating/rewarding those that undertake measures to prevent pollution. Studies have found that incentivising farmers is successful in the short term but the funding needs to be sustained or the farmers may revert back to old practices.

4. Education/awareness

Schemes such as Catchment Sensitive Farming provide free advice to farmers on how to reduce water pollution from agriculture.

On-farm responses to reduce sedimentation

This is quite complex as the best measures to use for reducing pollution depends on the soil type, farm type and a number of other factors; land degradation can be avoided best by recognising the capability of the land. However, here is a brief overview of the recommendations made in the FAO report:

Crop farmers

  1. Optimise timings and amounts of any inputs (e.g. fertilisers/pesticides etc.)
  2. Apply fertilisers to vegetables little and often with some precision
  3. Split fertiliser applications across the plant’s most responsive growth phase to reduce leaching
  4. Ideally use slow release fertilisers (although these are expensive!)
  5. Use green manure (by leaving uprooted/sown crop parts to wither on the field; this is a nifty trick often used by organic farmers!)
  6. Minimum tillage – according to one study, using less heavy machinery can reduce sediment export by 68%, leading to an 81% reduction in phosphate loss!

It is important to note that using agrochemicals inefficiently is not only costly to the environment but also to the farmers due to lost production and loss of the fertilisers they’ve paid for!

Livestock farmers

  1. Match stocking densities to the land capability, and use long-term weather forecasting to estimate how much feed is available (this will prevent overgrazing; bare land leads to increased soil erosion)
  2. Rotational grazing – this allows the fields to recover whilst increasing the quality of the pasture
  3. Use the right type of animals for different types of pasture at different times of the year – for example, whilst sheep graze close to the base of plants, exerting high pressure on plants during dry periods, cattle graze higher up so may be more suitable for grazing during this time
  4. Locate water troughs strategically – keep them away from water bodies as livestock trample land and can cause ‘poaching’, leading to soil erosion
  5. Don’t use fire to control woody weeds unless carefully controlled
  6. Integrate trees within the pastures – this also provides shade, shelter, and soil stability
  7. Plan drainage systems carefully so that any runoff spreads out rather than becoming concentrated
  8. Minimise how much livestock pasture grazes on steep slopes
  9. Protect stream banks (e.g. through riparian buffer strips)
  10. Restore degraded pasture using a mixture of species
  11. Manage feed carefully, ideally using food with high nutrient digestability (although in my opinion, cattle should be pasture fed as much as possible, on land which is unsuitable for growing human available food – visit Pasture for Life for more information!)
An example of a riparian buffer strip

Thanks for reading; as always, please feel free to make comments! The full FAO report is available here.


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