Water quality: the role of sediment

Excessive sediment can significantly impact the condition of freshwater habitats, resulting in a deterioration of water quality. It can have a greater impact than more standard pollutants such as phosphorus and nitrogen. Researcher Daire Ó hUallacháin explains further

Freshwater systems are under pressure from multiple-stressors. To-date, much research has focused on the impact of nutrients (e.g. phosphorus and nitrogen) or pesticides (e.g. MCPA) on water quality. However, recent reports by the Environmental Protection Agency (EPA) and the Agricultural Sustainability Support and Advisory Programme (ASSAP) suggest that sediment is a much greater threat to water quality than previously thought.

Sediment reduces the quality of freshwater habitats

Sediment is a naturally occurring material, derived from the weathering and erosion of underlying bedrock and steam banks, which is then subsequently transported downstream. However, excessive sediment can significantly impact the condition of freshwater habitats, resulting in a deterioration of water quality. Sediment can have a greater impact on freshwater insects (key indicators of water quality) than more standard pollutants such as phosphorus and nitrogen. In some extensive catchments, particularly those that contain sensitive species (e.g. the Freshwater Pearl Mussel), excessive sediment is likely to be the dominant threat to water quality.

Sources of sediment

Sources of sediment are varied and widespread. Recent studies by Teagasc utilising ‘sediment fingerprinting’ (i.e. matching instream sediment to potential sources) highlight that channel banks, roads, and agricultural sources were the dominant sources of sediment in agricultural catchments. Forestry, including established, unmanaged, forest plantations, can also contribute significant amounts of sediment to freshwater systems.

From an agricultural point of view, sediment sources are influenced by soil drainage characteristics and land use type. Catchments with poor drainage are more likely to deliver larger amounts of sediment than those with good drainage. Where arable land (and associated periods of low ground cover) occur on poorly drained soils, the highest sediment loads are likely to occur. Agricultural activity can also influence channel bank sources of sediment. Artificial drainage (surface ditches and sub-surface drains) can result in high flow and associated channel bank erosion. Cattle access to watercourses (image 1) can also result in channel bank erosion and sediment delivery to watercourses.

Reducing the impact of sediment

Teagasc research indicates that, although still a threat, sediment lost from Irish catchments, is low compared to values for the UK and mainland Europe. This is largely attributed to greater density of landscape features such as hedgerows and drainage ditches on Irish farms. These natural mitigation features can intercept flow and sediment, preventing it from reaching watercourses. Such landscape features are valuable for water quality, but also for biodiversity and carbon storage, therefore landowners should be encouraged and advised on how best to manage them for multiple environmental functions.

Targeted management practices can help to further reduce sediment inputs to freshwater ecosystems. Arable soils could benefit from temporary sediment control measures, (e.g. sediment fences) when rainfall intensity is high and groundcover is low. Poorly drained grassland catchments could benefit from appropriately managed buffer zone (image 2) or riparian wetlands. Ongoing Teagasc research (e.g. Smarter_BufferZ) is exploring the effectiveness of some of these approaches.

Key messages to reduce and manage sediment

Management practices are needed to reduce sediment delivery to freshwater. Such practices will prevent the loss of nutrient-rich topsoil from the land, resulting in environmental and agronomic benefits. These strategies typically aim to reduce the source of sediment, or break the pathway between source and river.

• Management practices to reduce sources of sediment include soil conservation practices. Arable land on poorly-drained soils is a high soil loss risk. In-field mitigation measures such as rapid establishment of crop cover after drilling are practical approaches to reduce the space and time soils are bare.
• Maintenance of existing landscape features is a priority to prevent soil losses from agricultural catchments to rivers and streams.
• Establishment of stabilising structures for riverbanks, or establishing riparian wetlands and buffer zones, will have benefits for water quality, but also for biodiversity and carbon storage

Sediment: Flux, provenance and ecological impact - 6124 (PDF)

Cattle access to watercourses: Environmental and socio-economic implications - 6758 (PDF)

Image 1: Cattle access to water can result in bank erosion and sediment delivery to watercourses.

Image 2: Appropriately targeted and managed riparian buffers can prevent sediment from entering watercourses