Surface Water Quality and
Subsurface Drainage
How subsurface drainage works...
Conventional drainage methods involve installing underground (subsurface) drainage pipes that outlet into a ditch or stream. The water table is lowered to the depth of the drainage pipe by the force of gravity.
A water quality benefit of subsurface drainage is that more water infiltrates the soil and there is less runoff. Runoff carries sediment and attached nutrients to surface waters. Sediment loss is reduced by up to 65% and phosphorus loss by up to 45% on cropland with subsurface drainage. An adverse effect of subsurface drainage is that water soluble chemicals and plant nutrients (such as nitrate-nitrogen) in shallow ground water can move from the soil to surface waters by means of the drainage system.Nitrate-nitrogen concentrations greater than 10 mg/L in drinking water supplies (U.S. EPA maximum contaminant level for drinking water) may impact human health. Nitrate can also contribute to the condition of hypoxia, or low dissolved oxygen levels in freshwater and marine systems. Agriculture is not the only source of nitrate. Sources of nitrogen include crop and lawn fertilizers, crop residues, animal manures, organic soils, septic tank effluent, municipal sewage wastes, industrial wastes, and industrial atmospheric by-products.
