PATTERNS AND TRENDS OF SALINITY AND NITRATE-N IN AN INTENSELY DEVELOPED BURIED VALLEY AQUIFER, SOUTHEAST NEBRASKA

Spatial and temporal patterns of water quality were examined in a buried valley aquifer located between the towns of Chester and Fairbury in southeast Nebraska. The unconfined aquifer consists of western-source fluvial sands and gravels deposited in a bedrock valley and buried by Pleistocene loess. It is laterally bounded and underlain by low permeable Cretaceous shale and limestone. Dakota sandstone containing brackish water underlies the eastern end of the aquifer. Predominant land-use is row-crop agriculture irrigated from about 350 wells. Local resource managers are concerned about the sustainability of the ground-water resource. In order to understand the impacts of development on water quality, conductivity measurements were taken from water samples of 124 irrigation wells during the summer of 2004. Also, 17 wells in 4 transects across the aquifer were sampled for water chemistry, 7 of which were analyzed for stable isotopes and 5 for tritium. Water from the westernmost wells is a calcium bicarbonate type with low concentrations of sodium, nitrate-N, and salinity and no detectable tritium, suggesting the meteoric water recharged prior to 1952. Water from the easternmost wells is a calcium sodium bicarbonate type with detectable concentrations of tritium and significantly higher sodium, nitrate-N, and salinity suggesting a mixed water source from modern meteoric recharge and some intrusion of older meteoric brackish water from the Dakota sandstone. The salinity hazard of water from some eastern wells is high, but it is suitable for irrigation on the well to moderately well drained soils of the area. Also, the sodium hazard is low for all wells sampled. Conductivity values were generally higher for wells located near the aquifer margins than those along the central axis of the aquifer indicating some laterally induced recharge of more brackish water from surrounding bedrock formations. Results indicate the aquifer is significantly impacted by agricultural practices. Nitrate-N exceeded 10 ppm in 7 wells, while the mean for all 17 wells was 9.3 ppm. Increasing trends of nitrate-N was observed in 5 irrigation wells with long-term records. Public water supply wells for the towns of Chester, Reynolds, and Fairbury show a long term increasing trend of nitrate-N with some stabilization in recent years.