TEMPORAL TRENDS IN GROUNDWATER QUALITY OF THE BIG SPRING BASIN, 1982-1992

R.D. Rowden, G.R. Hallberg, R.D.Libra


Iowa Department of Natural Resources, Geological Survey Bureau,
1993 Midwest Groundwater Conference
Champaign-Urbana, IL

ABSTRACT


Monitoring in the 270 sq. km. Big Spring groundwater basin in northeastern Iowa has documented agricultural impacts on water-quality. Historic data have shown regional increases in nitrate-N in groundwater paralleling increases in fertilizer-N rates and corn acreage during the 1960s and 70s. Through demonstration projects significant improvements in N input efficiency have been made; from 1980 to 1989 basin farmers reduced fertilizer-N rates for corn by 20%, with no yield loss. Basin monitoring shows that, while annual flow-weighted (fw) nitrate concentration changes parallel changes in discharge, annual fw atrazine concentrations do not. Analysis of water-quality changes is also complicated by climatic variations, subsequent storage effects, and system time-lags. Therefore, before declines in nitrate-N and atrazine loads can be attributed to improved management and source reduction, overall system variations must be considered.

Under the Payment-In-Kind set-aside program in 1983 the basin area in corn production was reduced by about 33% (relative to 1982). This reduction in corn acreage and N-loading was reflected by a significant reduction in nitrate-N in the groundwater in Water-Year 1985 (WY-85). Annual fw mean nitrate-N concentrations dropped from 10.2 mg-nitrate-N/L in WY-83, and 9.6 in WY-84, to 6.9 mg/L in WY-85. Declining discharge during this period also contributed to declining nitrate-N concentrations. In contrast, annual fw mean atrazine concentrations increased from 0.28 g-atrazine/L in WY-83 and 0.45 in WY-84 to 0.70 g/L by WY-85. From WY-88 to WY-89 annual discharge declined from 26,000 acre-feet (ac-ft) to 12,700 ac-ft, annual fw mean nitrate-N concentrations decreased from 9.5 mg/L to 5.7 mg/L, but annual fw mean atrazine concentrations increased from 0.13 g/L to 0.61 g/L. Annual discharge, loading and fw means for nitrate and atrazine vary by factors ranging from two to ten during the ten year period and underscore the importance of long-term monitoring for nonpoint-source contamination.

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