Reducing the Environmental Impact of Land Drainage

By Ivan Kelly, ASSAP Adviser, Teagasc Galway/Clare

Appropriately designed and constructed land drainage will lead to improved nutrient uptake by the plant. This can lower losses of nutrient and sediment to watercourses, however care must be taken to ensure that drainage channels do not become pathways for losses of nutrients.

Water quality in Ireland has been falling over the last number of years. After agriculture, hydro morphology which includes land drainage and channel straightening is the second biggest pressure on water quality in Ireland. The recently published Teagasc Manual on Land Drainage (2^nd edition) details some of the risks and mitigation measures when carrying out on-farm drainage work.

What are the Risks associated with land drainage?

Soil Nutrients:

The channelling of water in drainage networks increases the speed of water delivery to rivers and by passes the soil’s ability to remove nutrients such as nitrate, Phosphorus, and also pathogens, sediment and pesticides from the drainage water. Drainage also by-passes potential methods to mitigate diffuse pollution such as riparian buffers beside rivers. In rivers, nitrogen and phosphorus loss can result in excessive plant and algal growth. This reduces the amount of oxygen in the river and suffocates sensitive fauna. Excessive fine sediment in a river can smother the stream bed habitat and clog the gills of many sensitive mayfly species.

Flooding:

Drainage will reduce flooding on the drained lands. But drainage will also lead to more rapid movement of water from the land surface to the receiving river or lake. This greatly alters the hydrology of the catchment with the potential for down stream flooding of agricultural land or urban settlements in floodplains.

Carbon Footprint:

The drainage of mineral soils affects a reduction in greenhouse gas emissions both directly through lower nitrous oxide emissions, which tend to be higher in poorly drained soils, and indirectly through the benefits of extended grazing. However, the drainage of high organic content and peat dominated soils will result in substantial CO2 emissions to the atmosphere, which would dwarf any non CO2 benefit. Therefore current climate change mitigation policy dictates that further drainage of high organic content and peat soils cannot be justified.

Biodiversity:

Wetlands (such as fens, marshes, ponds) and species-rich wet grassland habitats make a very important contribution to farmland wildlife, and contribute to the environmental sustainability of farmland. A variety of species including dragonflies, butterflies, frogs and wading birds are dependent on wet grassland and wetland habitats, therefore drainage would have serious impacts on ecological conservation.

What are the options for reducing the impacts of draining?

1. Alternative measures:

In some circumstances, as outlined previously, the installation of new land drains can have an excessive environmental impact. Consider alternatives measures to land drainage and consult with an agricultural advisor regarding an appropriate land management plan.

2. Drain Cleaning:

If cleaning an open drain/ditch, vegetation should be removed from the drain bed and one bank only. The other bank should be left undisturbed throughout that season. Any maintenance to surface water drains should only be carried out during the months July to September. Fish and their spawning grounds are protected under the Fisheries Acts (1959 – 2010). In-stream works should not be carried out without prior consultation and approval of Inland Fisheries Ireland (www.fisheriesireland.ie)

3. Weather:

Aim to carry out drainage operations in good weather conditions where soil erosion and sediment loss during and following excavation works is minimised. Drainage installation under the wrong conditions can also lead to compaction and decreased effectiveness e.g. - in the case of mole drains.

4. Sediment Traps:

When positioned strategically in existing surface drains, these minimise the risk of sediment and nutrients reaching larger watercourses during drain installation.

5. Backfilling Closed Drains with Soil:

Ensure that a soil blinding layer of >15 cm is used above the gravel in closed drains to prevent the direct entry of fertiliser nutrients or pesticides to drainage waters.

6. Water Table Management:

The amount of water leaving the system from drainage needs to be balanced with the crop and habitat needs. Programmes such as the Pearl Mussel Project are working with farmers on water table management with measures such as drain blocking on those organic soils that were previously drained.

7. Riparian Margin:

The riparian margin adjacent to the drain can also play an important role in the protection of biodiversity, water quality and flood control. Vegetation in this margin should be allowed to regenerate naturally and the use of herbicides, pesticides and fertiliser should be excluded from this area. Periodic grazing or cutting of this riparian vegetation will prevent scrub dominance and will provide benefits for a variety of species associated with agricultural ecosystems.

If you intend to undertake land drainage works that (a) exceed 15 hectares, or (b) the works are to be carried out within (or may effect) an NHA, a proposed NHA, an SAC, an SPA or a nature reserve or (c) the proposed works may have a significant effect on the environment, screening by DAFM is required. Teagasc recommends that any drainage scheme, even where screening by DAFM is not obligatory, should be viewed by a professional to assess any possible environmental impact.