Controlled versus Conventional Drainage Effects on Water Quality

Fertilized cropland is a potential nonpoint source of nitrogen and phosphorus, which can contribute to the nutrient enrichment of surface water ecosystems. Research has shown that agricultural drainage water may contain fertilizer nutrients and pesticides. Nitrogen and phosphorus are present in drainage outflow primarily because of the addition of fertilizer, which results from the change in land use following drainage improvements instead of from the mere installation of drainage. Yet, public concern for the environment has led to a general criticism of agricultural drainage for degrading water quality. Controlled drainage has been recognized in some states as a best management practice to reduce the transport and delivery of nitrogen and phosphorus to sensitive surface waters. This article summarizes and compares water quality impact associated with conventional and controlled drainage practices. Nitrogen and phosphorus reductions of 30% to 50%, resulting from controlled drainage are reported in several studies. Several researchers believe further reductions are possible by carefully managing fertilization rates and timing to match crop uptake. Further minimization of the off-site impact of agricultural production will require a better understanding of the mechanisms of fate and transport of nutrients and pesticides and their interaction with drainage and other water-management alternatives such as controlled drainage.