GROUNDWATER MONITORING IN THE BIG SPRING BASIN 1990-1991: A SUMMARY REVIEW
R.D. Rowden, R.D. Libra, G.R. Hallberg, B. Nations
Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 27, 1993, 36 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) 1990 and 1991. This period was characterized by above average precipitation, and followed the driest consecutive two-year period in the state's history. Precipitation totals 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 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. Groundwater discharge from the basin to the Turkey River totaled 17,500 acre-feet in WY 1990 and 42,500 acre-feet in WY 1991, which was the highest annual discharge measured during the WY 1982 through 1991 period of record at Big Spring. Discharge rates generally increased slowly across the period, with a minimum monthly average of 11 cfs occurring in December 1989 and a maximum monthly average of 141 cfs in June 1991. Annual discharge of the Turkey River at Garber was 631,900 and 1,103,000 acre-feet, for WYs 1990 and 1991, equivalent to 92% and 160% of the long-term average discharge.
Annual flow-weighted (fw) mean nitrate concentrations at Big Spring were 37 mg/L in WY 1990 and 56 mg/L in WY 1991. The total load of nitrate-nitrogen discharged by the groundwater system was 388,500 pounds in WY 1990 and 1,445,500 pounds in WY 1991. Annual fw mean nitrate concentrations and loads for WY 1991 were the highest recorded since monitoring began at Big Spring. Nitrate concentrations increased slowly across the period, in a manner similar to the rate of discharge. For the Turkey River, fw mean nitrate concentrations were 31 mg/L and 44 mg/L, in WYs 1990 and 1991. Total nitrate nitrogen discharged by the river was about 11.7 million pounds in WY 1990 and 29.6 million pounds in WY 1991.
Atrazine is the most consistently detected herbicide in Big Spring groundwater. It was detected in all samples analyzed for pesticides during WYs 1990 and 1991. WY 1991 registered the highest fw mean atrazine concentration (1.17 µg/L), and the highest annual atrazine load (135 pounds), during the period of record. WY 1990 recorded the second highest fw mean atrazine concentration (1.10 µg/L) and the second highest total atrazine load (50.0 pounds) during the period of record. Alachlor, cyanazine, and metolachlor were also detected at Big Spring and the Turkey River during WYs 1990 and 1991. The Turkey River discharged about 3,260 pounds of atrazine at a fw mean concentration of 1.90 µg/L during WY 1990, and about 3,330 pounds of atrazine at a fw mean concentration of 1.11 µg/L during WY 1991.
Analysis of annual data for WYs 1982 through 1991 indicates that while fw mean nitrate concentrations and loads generally parallel discharge, fw mean atrazine concentrations and loads do not. Relatively high concentrations and loads of atrazine have occurred during years with low groundwater discharge. The climatic variations and resulting hydrologic conditions exhibited in the Big Spring basin during WYs 1982 through 1991 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.