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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. 相似文献
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Simulated recharge to a shallow confined aquifer indicates that locally concentrated recharge through microtopographically determined recharge areas (having elevation differences of less than 2 cm) can have a substantial effect on the spatial variability of nitrate-N concentrations [N-NO3] in the upper meter of an aquifer. For an aquifer that is confined and near pressure equilibrium with the watertable, hydraulic mounds formed under recharge areas can cause temporary large local increases in hydraulic gradient and subsequent flux from the aquitard to the upper aquifer. These hydraulic surges carry nitrates from the aquitard to the upper aquifer, where they move as local plumes and disperse according to the properties of the prevailing aquifer flow system, until discharged, reduced to ammonia, or denitrified. Under conditions of maximum local recharge the ratio of aquifer nitrate concentration to till nitrate concentration ([N-NO3]a/[N-NO3]t) may approach an upper limit of 1. Understanding these processes is important for interpretation of nitrate and other solute measurements near the aquifer-aquitard boundary. 相似文献
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