GROUNDWATER MONITORING IN THE BIG SPRING BASIN 1992-1993: A SUMMARY REVIEW
R.D. Rowden, R.D. Libra, G.R. Hallberg, B.K. Nations
Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 34, 1995, 43 p.
The Big Spring basin is a 103 sq. mi. groundwater basin in Clayton County, Iowa. Precipitation, 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 Big Spring and the Turkey River during water years (WYs) 1992 and 1993. The cumulative annual discharge, and discharge as a percentage of precipitation during WYs 1992 and 1993 were the greatest recorded since monitoring began at Big Spring. While annual precipitation totals were similar to totals during WYs 1990 and 1991, annual discharge varied significantly. Antecedent conditions, including the volume of groundwater in storage within the basin, and increased groundwater levels and soil moisture, were factors causing greater recharge-discharge rates during WYs 1992 and 1993. Water years 1990 and 1991 followed the driest consecutive two-year period in the state's history, but WYs 1990 and 1991 were the two wettest consecutive years since monitoring began at Big Spring, providing very different antecedent conditions for WYs 1992 and 1993. Precipitation totals for the basin were 37.87 inches in WY 1990, 47.27 inches in WY 1991, 35.74 inches in WY 1992, and 46.47 inches in WY 1993. The WY 1992 and WY 1993 totals were 108% and 141% of the long-term average precipitation of 32.97 inches. The increased precipitation generated both runoff and infiltration recharge. Groundwater discharge from the basin to the Turkey River totaled 17,500 acre-feet (ac-ft) in WY 1990, 42,500 ac-ft in WY 1991, 37,278 ac-ft in WY 1992, and 58,186 ac-ft in WY 1993, which was the highest annual discharge measured during the WY 1982 through WY 1993 period of record at Big Spring. Discharge rates generally increased across the period, with a minimum monthly average of 31 cfs occurring in October 1991 and a maximum monthly average of 137 cfs in July 1993. Annual discharge of the Turkey River at Garber was 631,900 and 1,103,000 ac-ft, for WYs 1990 and 1991, and 1,101,000 and 2,103,000 ac-ft for WYs 1992 and 1993. Annual discharge from the Turkey River during WYs 1992 and 1993 was equivalent to 159% and 295% of the long-term average.
The annual flow-weighted (fw) mean nitrate concentration at Big Spring decreased from 56 mg/L in WY 1991, which was the highest recorded during the period of record, to 54 mg/L in WY 1992 and 51 mg/L in WY 1993. The total load of nitrate-nitrogen discharged by the groundwater system increased from 388,500 pounds in WY 1990 to 1,445,500 pounds in WY 1991, then decreased to 1,220,099 pounds in WY 1992. The annual nitrate-nitrogen discharge during WY 1993, 1,796,013 pounds, was the highest recorded since monitoring began at Big Spring. Nitrate concentrations were relatively stable across the period, remaining between 35 and 70 mg/L during WY 1992 and 15 and 70 mg/L during WY 1993. For the Turkey River, fw mean nitrate concentrations were 41 mg/L and 26 mg/L in WYs 1992 and 1993, respectively. Total nitrate-nitrogen discharged by the river was about 27.2 million pounds in WY 1992 and 32.4 million pounds in WY 1993.
Atrazine is the most consistently detected herbicide in Big Spring groundwater. It was detected in all samples analyzed for pesticides during WY 1992, and in 96% of the samples analyzed in WY 1993. The WY 1991 annual fw mean atrazine concentration of 1.17 µg/L and annual atrazine load of 135 pounds were the greatest recorded during the period of monitoring at Big Spring. During WY 1992 the annual fw mean atrazine concentration and load decreased to 0.22 µg/L and 22.5 pounds, and during WY 1993 the fw mean and load increased to 0.27 µg/L and 42.0 pounds. Alachlor, cyanazine, and metolachlor were also detected at Big Spring and the Turkey River during the growing seasons of WYs 1992 and 1993. Significant decreases in annual fw mean atrazine concentrations and loads also occurred at the Turkey River from WY 1991 to WY 1992. The Turkey River discharged about 3,330 pounds of atrazine at a fw mean concentration of 1.11 µg/L during WY 1991, and about 739 pounds of atrazine at a fw mean concentration of 0.25 µg/L in WY 1992. During WY 1993 the annual fw mean atrazine concentration and load increased to 0.59 µg/L and 3,386 pounds.
Analysis of annual data for WYs 1982 through 1993 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 some years with low groundwater discharge and low concentrations and loads of atrazine have occurred during some years with high groundwater discharge. The climatic variations and resulting hydrologic conditions exhibited in the Big Spring basin during WYs 1982 through 1993 have led to variations in discharge rates and contaminant concentrations and loads ranging by factors from two to ten during the period of record. Extreme climatic variations complicate the interpretation of changes in water quality related to landuse and management changes and illustrate the need for detailed, long-term monitoring of nonpoint-source contamination.