AGRICULTURAL DRAINAGE AND WATER-QUALITY IN MISSISSIPPI DELTA

Drainage is an essential land-management practice for growing crops in the Lower Mississippi Valley and Gulf coastal areas. Current crop production and water-management methods pose significant potential for surface acid ground-water contamination. Management practices need to be identified, evaluated, and refined so that threats to water quality can be eliminated without significant loss in productivity. Sediment and nutrient (N, P, K) losses were measured from research plots with surface drainage only and from plots with both surface and subsurface drainage for all or part of the decade 1982-1991. During the 1987 growing season, losses of pesticides (atrazine and metolachlor) were also measured. Subsurface drainage was effective in reducing surface runoff by an average of 35%, and thus reduced soil loss by 31%, nitrogen loss by 17%, phosphorus loss by 31%, and potassium loss by 27%. For the 1987 season, losses of preemergent applied herbicides from the plots with subsurface drainage were less than one-half that from plots with surface drainage only: 1.4% versus 3.2% of applied for atrazine, and 1.1% versus 2.4% of applied for metolachlor.