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

Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 33, 1995, 107 p.


The Big Spring basin is a 103 sq. mi. 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 basin since 1981. This report summarizes the results of monitoring at sub-basin surface-water sites BOOGD (1.15 sq. mi.), L23S (4.39 sq. mi.) and RC02 (70.7 sq. mi.) during water years (WYs) 1986 through 1992. During the monitoring 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 1981. Annual precipitation for the basin increased from 22.94 inches in WY 1988 and 24.32 inches in WY 1989 to 37.87 inches during WY 1990 and 47.27 inches during WY 1991. The precipitation total for WY 1992 was 35.74 inches. The WY 1990 and 1991 totals were 115% and 143% of the long-term average precipitation of 32.97 inches. The increased precipitation generated both runoff and infiltration recharge. During WY 1991, annual surface-water discharge totaled 1,020 acre feet (ac-ft) at BOOGD and 33,443 ac-ft at RC02. These were the highest annual discharges during WYs 1986-1992. The highest annual discharge from L23S, 3,980 ac-ft, occurred during WY 1992. The lowest annual surface-water discharges occurred in WY 1989, and were 57 ac-ft at BOOGD; 552 ac-ft at L23S; and 3,160 ac-ft at RC02.

The lowest annual flow-weighted (fw) mean nitrate-N concentrations and nitrate-N loads during WYs 1986-1992 occurred in WY 1989. Annual fw means and loads were 2.0 mg/L and 317 pounds at BOOGD; 2.0 mg/L and 2,998 pounds at L23S; and 2.0 mg/L and 17,393 pounds at RC02. During the WY 1982-1992 monitoring period at Big Spring, the lowest annual fw mean nitrate-N concentration and smallest nitrate-N load, 5.7 mg/L and 195,000 pounds, also occurred in WY 1989. The highest annual fw mean nitrate-N concentrations and greatest nitrate-N loads occurred during WY 1991. The fw mean and load from Big Spring for WY 1991 were 12.5 mg/L and 1,446,000 pounds. The fw mean and load from BOOGD were 16.0 mg/L and 44,336 pounds and from RC02 the mean and load were 11.3 mg/L and 1,032,119 pounds during WY 1991. At L23S, the highest annual fw mean nitrate-N concentration, 12.0 mg/L, occurred during WY 1991, but the greatest annual nitrate-N load, 123,530 pounds, was discharged during WY 1992. The annual fw mean nitrate-N concentrations and loads from the Turkey River were also lowest during WY 1989 and highest during WY 1991. Means and loads were 2.6 mg/L and 1.6 million pounds in WY 1989 and 9.9 mg/L and 29.6 million pounds in WY 1991.

Atrazine is the most consistently detected herbicide in Big Spring groundwater. It was detected in 96% of the samples from Big Spring that were analyzed for pesticides during WYs 1982-1992. During WYs 1986-1992 atrazine was detected in 93% of the samples analyzed from BOOGD; 76% of the samples from L23S; and 94% of the samples from RC02. The highest annual fw mean atrazine concentrations and greatest atrazine loads generally occurred during WY 1991. The lowest means and loads were recorded during WYs 1987 and 1988. The annual fw mean atrazine concentrations and loads from Big Spring ranged from 0.13 g/L and 9.2 pounds during WY 1988 to 1.17 g/L and 135 pounds in WY 1991. At BOOGD fw means and loads varied from 0.17 g/L and 0.1 pounds in WY 1987 to 3.32 g/L and 9.2 pounds in WY 1991. At L23S the lowest fw mean, 0.12 g/L, and smallest load, 0.4 pounds, occurred in WY 1987. The highest fw mean from L23S, 6.75 g/L, occurred in WY 1989 and the greatest load, 22.5 pounds, occurred in WY 1991. At RC02, the lowest fw mean atrazine concentration, 0.24 g/L, and smallest load, 6.7 pounds, were recorded in WY 1988, and the highest fw mean, 7.20 g/L, and greatest load, 655 pounds, were registered in WY 1991. During WYs 1982-1992 the annual fw mean atrazine concentrations and loads from the Turkey River varied from 0.34 g/L and 407 pounds in WY 1988, to 1.11 g/L and 3,330 pounds in WY 1991. Alachlor, cyanazine, and metolachlor were also detected at most monitoring sites within the Big Spring basin and at the Turkey River during WYs 1986 through 1992.

Analysis of annual data from Big Spring and the Turkey River for WYs 1982-1992, and annual data from BOOGD, L23S and RC02 for WYs 1986-1992 indicates that while fw mean nitrate concentrations and loads generally parallel changes in discharge, fw mean atrazine concentrations and loads do not. Relatively high concentrations and loads of atrazine have occurred during years with low 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. The climatic variations and resulting hydrologic conditions exhibited in the Big Spring basin during WYs 1982-1992 have led to variations in discharge rates and contaminant concentrations and loads by factors ranging from two to ten during the period of record. Extreme climatic variations complicate the interpretation of changes in water quality and illustrate the need for detailed, long-term monitoring of nonpoint-source contamination.

The similarity of seasonal trends and pronounced short- and long-term changes in nitrate and atrazine concentrations seen at monitoring sites throughout the Big Spring basin demonstrate the effectiveness of the nested monitoring network design. The pronounced short-term changes in nitrate and atrazine concentrations are responses to significant recharge events. The concentration changes at the larger watershed scales are not as great or immediate as changes at smaller scale monitoring sites, although they clearly occur. The nested design allows chemical responses to recharge events to be tracked through the hydrologic system, from the soil zone beneath individual fields to the basin water outlets. The design also allows integration and comparison of water-quality responses at different scales to assess effects of landuse and landscape-ecosystem processes. The water quality of the Big Spring and Turkey River basins is an integration of the management practices on all the individual parcels of land they contain.