Controls on nitrate?￠????N concentrations in groundwater in a Missourian claypan watershed

摘要

Nitrogen (N) fertilizer applications have resulted in widespread groundwater nitrate?￠????N (NO 3 ?￠????N) contamination in the U.S. Corn Belt. Goodwater Creek Experimental Watershed (GCEW) is an agricultural watershed in the claypan soil region of northeastern Missouri with a network of 96 wells at depths of 2.7?￠????15.7?￠????m. The objectives of this study were to (1) inspect the spatial and temporal variations of NO 3 ?￠????N concentrations in GCEW's groundwater, particularly with well depth at scales ranging from individual well, well nest, and field to the entire watershed during the period 1991 to 2004; (2) understand the processes controlling the variability of NO 3 ?￠????N concentrations in groundwater at various scales within GCEW; and (3) compare groundwater NO 3 ?￠????N concentrations in GCEW to other agricultural watersheds in the U.S. Nitrate?￠????N concentrations were determined in more than 2000 samples collected from 1991 to 2004. Despite the low hydraulic conductivity of the claypan soils, considerable NO 3 ?￠????N contamination of the glacial till aquifer occurred, with 38% of the wells exceeding 10?￠????mg?￠????L ?￠????1 . Groundwater recharge by preferential pathways through the claypan appeared to be the primary mechanism for NO 3 ?￠????N movement to the aquifer. Changes in concentration with depth steadily increased to 8.5?￠????10?￠????m and then decreased with further depth. This pattern was consistent with decreased hydraulic conductivity in the Paleosol layer at 8.5?￠????10?￠????m, denitrification below this layer, and mixing of recent contaminated water with older uncontaminated water in the lowest strata. Only 19?￠????23% of sampled wells exceeded 10?￠????mg?￠????L ?￠????1 in nonclaypan agricultural watersheds over the continental U.S., suggesting that groundwater in GCEW was more susceptible to NO 3 ?￠????N contamination than nonclaypan watersheds. These results demonstrated that preferential flow through the soil and hydraulic conductivity of the subsurface strata controlled NO 3 ?￠????N transport in this claypan watershed.