SHALLOW GROUNDWATER AND SURFACE WATER MONITORING of the SILVER CREEK SUB-BASIN within the BIG SPRING BASIN 1986-1995: A Summary Review
R.D. Rowden, R.D. Libra, H. Liu
Iowa Department of Natural Resources, Geological Survey Bureau
Technical Information Series 38, 1999, 94 p.
The 4.39 mi2 Silver Creek sub-basin is located in the north-central portion of the 103 mi2 Big Spring groundwater basin in Clayton County, Iowa. Precipitation, surface water and groundwater discharge, and the concentrations and loads of various chemicals have been monitored within and around the Big Spring basin since 1981. This report summarizes the results of monitoring at sub-basin tile-line site L22T (85 acres) and surface-water site L23S (4.39 mi2, or 2,810 acres) during water years (WYs) 1986-1995. During this period, precipitation has varied from 22.94 inches in WY 1988 to 47.27 inches in WY 1991. The driest consecutive two-year period in the state's history, WYs 1988 and 1989, preceded the wettest consecutive two-year period since monitoring began in WY 1981. Annual basin precipitation increased from 22.94 inches in WY 1988 and 24.32 inches in WY 1989 to 37.87 inches in WY 1990 and 47.27 inches in WY 1991. The precipitation total for WY 1992 was 35.74 inches. Water Year 1993 had the second-greatest annual precipitation total during WYs 1981-1995 at 46.47 inches, or 141% of the long-term average. The annual precipitation for WY 1994 was 30.42 inches, or 92% of normal, and precipitation for WY 1995 was 29.28 inches, or 89% of normal. The WY 1988 and 1989 totals were 70% and 74%, and the WY 1990 and 1991 totals were 115% and 143% of the long-term average precipitation of 32.97 inches. The increased precipitation during WYs 1990-1993 generated both runoff and infiltration recharge. During WY 1993, annual groundwater discharge totaled 188.7 acre-feet (ac-ft) at L22T, 58,186 ac-ft at Big Spring, and 5,720 ac-ft at L23S. These were the highest annual discharges for L22T and L23S during WYs 1986-1995, and for Big Spring during WYs 1982-1995. The lowest annual discharge from L22T, 13.8 ac-ft, occurred in WY 1990. The lowest annual discharges for Big Spring and L23S, 12,672 ac-ft and 552 ac-ft, occurred in WY 1989.
The lowest annual flow-weighted (fw) mean nitrate-N concentrations and nitrate-N loads for L22T and L23S during WYs 1986-1995, and for Big Spring and the Turkey River during WYs 1982-1995, occurred in WY 1989. Annual fw means and loads were 15.0 mg/L and 622 pounds for L22T; 2.0 mg/L and 2,998 pounds for L23S; 5.7 mg/L and 195,000 pounds for Big Spring; and 2.6 mg/L and 1.6 million pounds for the Turkey River. The highest annual fw mean nitrate-N concentrations occurred in WY 1991, and greatest nitrate-N loads occurred in WY 1993. The fw mean for L22T was 31.3 mg/L for both WYs 1990 and 1991. For WY 1991, the annual fw mean for L23S was 12.0 mg/L, 12.5 mg/L for Big Spring and 9.9 mg/L for the Turkey River. The annual nitrate-N loads for WY 1993 were 9,103 pounds for L22T, 138,951 pounds for L23S, 1,796,013 pounds for Big Spring, and 32.4 million pounds for the Turkey River.
Atrazine is the most consistently detected herbicide in the Big Spring basin. During WYs 1982-1995, atrazine was detected in 83% of the samples from L22T, 68% of the samples from L23S, and 95% of the samples from Big Spring. The highest annual fw mean atrazine concentration for L22T, 0.40 µg/L, and L23S, 6.75 µg/L, occurred in WY 1989. The highest annual fw mean for Big Spring, 1.17 µg/L, occurred in WY 1991. The greatest annual atrazine loads, 0.063 pounds for L22T, 22.5 pounds for L23S, and 135 pounds for Big Spring, all occurred in WY 1991. For L22T, the lowest annual fw mean, 0.10 µg/L, occurred in WYs 1993 and 1994, and the smallest annual atrazine load, 0.013 pounds, occurred in 1987. For L23S, the lowest fw mean atrazine concentration, 0.09 µg/L, occurred in WY 1995, and the smallest atrazine load, 0.40 pounds, occurred in WY 1987. At Big Spring, the lowest annual fw mean atrazine concentration, 0.12 µg/L, was recorded in WY 1995, and the smallest annual atrazine load, 9.2 pounds, was recorded in WY 1988. During WYs 1986-1995 annual fw mean atrazine concentrations for the Turkey River varied from 1.90 µg/L in WY 1990 to 0.25 µg/L in WY 1992, and atrazine loads varied from 407 pounds in WY 1988 to 3,386 pounds in WY 1993. Alachlor, cyanazine, and metolachlor were also detected at many monitoring sites within the Big Spring basin and in the Turkey River during WYs 1982-1995.
Basin monitoring during WYs 1982-1995 indicates that while fw mean nitrate concentrations and loads generally parallel changes in annual groundwater and surface-water discharge, fw mean atrazine concentrations and loads do not. Relatively high concentrations and loads of atrazine have occurred during some years with low groundwater and surface-water discharge and low concentrations and loads have occurred during some years with high groundwater and surface-water discharge. During WYs 1988-1989 annual groundwater and surface-water discharge and annual fw mean nitrate concentrations and loads decreased to the lowest levels during the period of monitoring, while annual fw mean atrazine concentrations and loads increased significantly. From WY 1989 to WY 1991, annual groundwater and surface-water discharge increased, and fw nitrate concentrations and atrazine loads increased to record levels, while fw atrazine concentrations declined at L22T and L23S, and increased at Big Spring. From WY 1992 to WY 1993, annual discharges and nitrate-N loads increased to record levels, and annual atrazine loads increased, but remained relatively low, while fw nitrate concentrations decreased slightly at all three sites. Flow-weighted atrazine concentrations decreased slightly at L22T, and increased slightly at L23S and Big Spring. The interpretation of these water-quality changes is complicated by climatic variations, subsequent storage effects, and system time lags. Water Year 1993 was the first year of monitoring at Big Spring that annual fw mean nitrate concentration decreased while annual discharge increased. This decline in fw mean nitrate concentration may be due to increased leaching of nitrate-N during WYs 1990-1992, or it is possible that the gradual reductions in nitrogen fertilizer applied within the basin are beginning to affect changes in the water quality of Big Spring. Five years of data collection and analysis were necessary to establish the water-quality significance of input changes from the Payment-In-Kind set-aside program in 1983, when the basin area used for corn production was reduced by about 33% relative to 1982. It will take additional years of monitoring and analysis to fully ascertain the changes in water quality caused by smaller magnitude land use changes and gradual improvements in nitrogen management within the basin. Climatic variations, storage effects and time-lags complicate the interpretation of changes in water quality related to land use changes, and illustrate the need for detailed, long-term monitoring of nonpoint-source contamination. Therefore, before declines in fw mean nitrate-N and atrazine concentrations and loads can be attributed to improved management and source reduction, overall system variations must be considered.