What is it?

Diffuse Pollution (DP) is defined as pollution that arises from geographically dispersed sources. Individual sources may be minor (i.e. there is no single point source) but collectively can have a large impact on a water course. For example, sheet run off from fields inputs a large amount of sediment into the river and surface water drains in urban areas collectively contribute significant DP.

Types of DP include silt/soil, farm chemicals (e.g. pesticides), organic wastes (slurry, silage), fertilisers, oil/fuel from road surface, contaminated road dust, road salt and de-icers, sewer leakage and domestic products from drains.

DP has a serious impact on salmon and trout populations, as well as other species that depend on a clean aquatic environment, such as freshwater pearl mussels, lampreys and water voles. Impacts on salmon and trout include:

1. Fine sediments increase mortality of eggs and newly-hatched fry, by preventing oxygen reaching them within the river substrate
2. Fine sediments reduce the amount of habitat in the watercourse available for juvenile salmon, so lower numbers of salmon are supported
3. Fine sediments clog up spawning gravels, reducing the area in which salmon and trout can deposit eggs
4. Fine sediments increase water turbidity and can reduce the feeding success (and therefore growth rate) of juvenile salmon and trout
5. Fine sediments can reduce sunlight penetration, thereby lowering primary productivity of the stream and potentially reducing food supply for juveniles.
6. Chemicals in run-off, if not lethal, can have sub-lethal effects such as damage to olfactory (sense of smell) system, reducing their capacity for imprinting and homing, detecting predators and communicating with the opposite sex at spawning grounds (link to factsheet).
7. Organic wastes decompose and decrease oxygen content of the stream, increasing fish mortality in warm conditions.
8. Organic wastes decompose and release nutrients into the water. Excessive nutrients can cause algal blooms, and may use up so much oxygen in the water that fish cannot survive.
9. Input of fertilisers (nitrogen/phosphorous) accelerates plant growth, affecting habitat suitability and oxygen levels

Surveying of the Dee catchment was carried out by the Project Biologist in 2007. As an output of the survey, the major sources of DP were quantified in a RDT Report (Prioritisation of Habitat Restoration Based Upon the 1999 River Dee Habitat Survey & Subsequent Updates, 2007). The main causes of DP identified within the Dee catchment are: 

1. Livestock accessing watercourses, causing damage to river banks and adding sediment and organic wastes to the water.
2. Heavily drained cultivated land and road drainage schemes which input high levels of sediment and nutrients into the river.
3. Sediment input from forestry operations and run off from forestry areas.
4. Others include salt/grit run off from roads, run off from construction/development works.

The Dee Fisheries Management Plan (2009) classed the reduction of diffuse pollution as the top priority for the RDT and Dee DSFB.

Run-off from agricultural (Cattie burn) and forestry (Culsten burn) land runs directly into the watercourse, depositing DP. No ripaian buffer strips interupt the flow.

What work is being done?

Previous work undertaken by Dee DSFB to reduce DP entering the Dee included the creation of 80 km of buffer strips, which intercept sediment and nutrients in surface run off and prevent bank erosion from cattle. In addition, 67 silt traps were constructed on small 'feeder' streams and ditches to collect fine sediments; these are regularly maintained. Some of thise work was undertaken as part of the LIFE project.

Electrofishing surveys undertaken to determine the success of these habitat improvement works (see RDT 2009 Survey report) show that the addition of buffer strips significantly increase the number of salmon parr in the water course. For example, in the Cattie burn, parr numbers (catch per unit effort) increased from 0.033 to 0.33 parr between 2008 and 2009.

The improvements to the watercourse and salmon habitat brought about by riparian fencing are likely to take 5-10 years to develop, as the river banks takes time to stabilise and begin to heal, and fine sediments in the watercourse have to be washed out via natural processes. Once these effects are fully realised, we hope to see more substantial improvements to juvenile salmon numbers.

In Spring 2010, the RDT launch a scheme to support projects undertaken by land managers on Deeside that include riparian and other farm improvements that will mitigate and reduce DP (see River Dee Enhancement Project). This is by supporting applications to The Scottish Rural Development Programme (SRDP), a scheme funded primarily by Scottish Government. The RDT and Dee DSFB’s scheme aims to increase the uptake of applications to SRDP for funding projects that will deliver a reduction in DP. The aim is to deliver 12 km of buffer strip per year.

 New fencing installed on the Beltie burn (above right) which created a wider buffer strip than before (above left)

 Examples of silt traps on 'feeder' burns of the Coy and Beltie burns