AND SURFACE-WATER MONITORING in the BUGENHAGEN SUB-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 41, 2000, 150 p.
The 1,105 acre Bugenhagen 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. In 1986, as part of the Big Spring Demonstration Project, the Bugenhagen sub-basin was selected to be a “model” showcase for implementation of improved farm management and soil conservation using Best Management Practices. Landuse changes were tracked using farm surveys and aerial photographs that were digitized into a Geographic Information System. This report summarizes the results of monitoring at sub-basin tile-line sites BTLUE (250 acres), BTLUW (116 acres), BTLD (18 acres) and surface-water site BOOGD (736 acres) during water years (WYs) 1986-1995. During this period, precipitation has varied from 22.94 inches in WY 1988 to 47.28 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 47.28 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 372 acre feet (ac-ft) at BTLUE, 182 ac-ft at BTLUW, 38.5 ac-ft at BTLD, and 58,186 ac-ft at Big Spring. Monitoring at BOOGD ended in WY 1992 so discharge data for WY 1993 was not available. These were the greatest annual discharges for BTLUE and BTLUW during WYs 1987-1995, for BTLD during WYs 1987-1994, and for Big Spring during WYs 1982-1995. The greatest annual surface-water discharge for BOOGD during WYs 1986-1992, 1,020 ac-ft, occurred in WY 1991. The smallest annual discharges for BTLUE (10.5 ac-ft), BTLD (0 ac-ft), BOOGD (57 ac-ft), and Big Spring (12,672 ac-ft), occurred in WY 1989. The smallest annual discharge for BTLUW, 9.6 ac-ft, occurred in WY 1988.
Land-use changes significantly altered the sub-basin’s hydrology. Prior to WY 1985 there were no tile intake terraces within the 418 acre upper sub-basin. Cumulatively, 34 acres, or 8.1% of the upper sub-basin were diverted into terraces in WY 1985; 113 acres, or 27.0% in WY 1986; 161 acres, or 38.5% in WY 1987; and 228 acres, or 54.6% of the upper sub-basin area drained into intake terraces and discharged through BTLUE and BTLUW in WY 1988.
Comparison of land-use changes within the sub-basin with annual flow-weighted mean NO3-N and atrazine concentrations and loads from monitoring sites showed time lags. Corn acreage and total N applied within the upper and total sub-basins were greatest during WY 1990, at 339 and 680 acres, and 58,748 and 115,365 pounds (lbs), and smallest in WY 1987, at 137 and 389 acres, and 23,128 and 67,231 lbs. Within the lower sub-basin, the greatest amount of fertilizer, 60,760 lbs, was applied during WY 1989, the greatest corn acreage, 341 acres, was planted during WY 1990, and the smallest corn acreage and amount of nitrogen applied, 207 acres and 30,066 lbs occurred in WY 1993. Annual fw mean NO3-N concentrations for BOOGD, BTLUE and BTLUW were greatest during WY 1991, at 16.0, 26.8 and 25.7 mg/L, and smallest during WY 1989, at 2.0, 3.9 and 3.5 mg/L. The greatest annual fw mean NO3-N concentration for BTLD, 16.0 mg/L, also occurred in WY 1991, but the smallest, 9.8 mg/L, occurred in WY 1988. The greatest NO3-N loads for BOOGD, BTLUE and BTLD, 44,336, 12,606 and 1,565 lbs, occurred during WY 1991, and the greatest load for BTLUW, 7,957 lbs, occurred in WY 1993. The smallest NO3-N loads for BOOGD, BTLUE, BTLUW, and BTLD (317, 111, 103, and 0 lbs, respectively) occurred during WY 1989. At Big Spring, the greatest fw mean NO3-N concentration, 12.5 mg/L, occurred in WY 1991, the greatest NO3-N load, 1,796,013 lbs, occurred in WY 1993, and the smallest annual fw mean and load, 5.7 mg/L and 194,928 lbs, occurred in WY 1989.
The number of corn acres treated with atrazine and total pounds of atrazine applied within the upper sub-basin ranged from 160 acres in WY 1995 and 300 lbs in WY 1989 to 0 acres and 0 lbs in WYs 1991 and 1994. Within the lower sub-basin, the number of corn acres treated with atrazine and lbs of atrazine applied ranged from 220 acres and 433 lbs in WY 1987 to 32 acres and 32 lbs in WY 1988. For the total sub-basin, the number of corn acres treated with atrazine and lbs of atrazine applied ranged from 363 acres in WY 1995 and 697 lbs in WY 1987 to 55 acres and 61 lbs in WY 1988. At BOOGD, annual fw mean atrazine concentrations and loads ranged from 3.32 µg/L and 9.2 lbs in WY 1991 to 0.17 µg/L and 0.1 lbs in WY 1987. At BTLUE, fw mean atrazine concentrations ranged from 5.32 µg/L in WY 1989 to 0.17 µg/L in WYs 1994 and 1995, while atrazine loads ranged from 2.1 lbs in WY 1991 to 0.02 lbs in WY 1987. At BTLUW annual fw means ranged from 9.82 µg/L in WY 1990 to 0.12 µg/L in 1995, while loads ranged from 1.6 lbs in WY 1991 to 0.007 lbs in WYs 1988 and 1994. For BTLD, annual fw means ranged from 0.13 µg/L in WY 1994 to 0 µg/L in WY 1990, and annual loads ranged from 0.01 lbs in WY 1993 to 0 lbs in WYs 1989 and 1990. For Big Spring, the greatest annual fw mean atrazine concentration and load, 1.17 µg/L and 135 lbs, occurred in WY 1991, the smallest fw mean, 0.12 µg/L, occurred in WY 1995, and the smallest atrazine load, 9.2 lbs, occurred in WY 1988. Atrazine has been detected in 95% of the samples analyzed for pesticides from Big Spring during WYs 1982-1995. During WYs 1986-1995, atrazine was detected in 93% of the samples from BOOGD and BTLUE, 91% of the samples from BTLUW, and 25% of the samples from BTLD.
The annual data from the Bugenhagen sub-basin during WYs 1986-1995 supports observations from other sites within the Big Spring basin, showing that annual fw mean NO3-N concentrations and loads tend to parallel groundwater flux, as inferred by discharge, while annual fw mean atrazine concentrations and loads do not. Relatively great concentrations and loads of atrazine have occurred during some years with low groundwater discharge, and relatively small concentrations and loads have occurred during some years with high groundwater discharge. Annual climatic variations and resulting hydrologic conditions, along with variations in discharge, loading and fw means for NO3-N and atrazine by factors ranging from two, to more than ten during the period of record complicate interpretation of changes in water quality and illustrate the need for detailed, long-term monitoring of nonpoint-source contamination. Climatic effects during the monitoring period make it difficult to ascertain relationships between landuse changes within the sub-basin and water quality responses at the monitoring sites. Incremental reductions in the application of nitrogen fertilizer and herbicides such as atrazine resulting from improved management practices may not result in pronounced short-term water-quality changes, but they will be detectable over time. Within both the Big Spring and Bugenhagen watersheds, many landuse and management practices are integrated, and water-quality responses are dampened and complicated by climatic variations, storage effects, and biochemical processing in both surface- and groundwater systems. Therefore, before declines in fw mean NO3-N and atrazine concentrations and loads can be attributed to improved agricultural management and source reduction, overall system variations must be considered.