R.D. Rowden, G.R. Hallberg, R.D.Libra
Iowa Department of Natural Resources, Geological Survey Bureau,
1993 Midwest Groundwater Conference
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 µg-atrazine/L in WY-83 and 0.45 in WY-84 to 0.70 µg/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 µg/L to 0.61 µg/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.
Contact (217)333-1724 or (217)244-2788 for a list of abstracts from the 1993 Midwest Groundwater Conference.