Hydraulic Effects of Crop Management Systems on Nitrate Variability in a Confined Aquifer

Abstract

Nitrate movement to saturated glacial till (at about 3.5 m) and a shallow confined aquifer (6 m) was evaluated for five years (1992 to 1996) under three dryland cropping practices. Cropping systems were Biological (using no manufactured chemicals or fertilizers), Conventional (cropping practices used in east-central North Dakota), and Integrated (using integrated pest management practices). Following a wet year in 1993 the average nitrate-N concentration in the vadose zone did not differ (p ≤ 0.05) between treatments. Largest nitrate increase in the saturated till occurred under the Conventional treatment, and the least occurred under the Biological Treatment (p ≤ 0.05). Nitrate increased until 1995, and then began to decrease. Most had not reached their 1992 levels by 1996. Nitrate increased most, and for the longest period, in the Carrington aquifer under the Biological treatment, and least under the Conventional treatment (p ≤ 0.05). Larger nitrate-N in the aquifer under the Biological treatment was not caused by additional leaching of nitrate-N from the soil zone, but by indirect hydrologic effects of crop systems on flux from the overlying till to the aquifer. Reduced dry matter production and surface cover under cool wet conditions increased runoff to microtopographic low areas, and enhanced local root-zone drainage. Local increases in drainage caused local increases in hydraulic gradient from the till to the aquifer, which caused local and temporary influxes of nitrate-laden water from the aquitard to the aquifer. Under all treatments temporal and spatial variability of nitrate-N concentrations were large, and increased mean nitrate-N values were caused by expanded spread of the data. Under all treatments there were sample positions and times with small or negligible nitrate-N concentrations.