GROUNDWATER MONITORING in the BIG SPRING BASIN 1994-1995: A Summary Review
Huaibao Liu, R.D. Rowden, R.D. Libra
Iowa Department of Natural Resources, Geological Survey
Technical Information Series 37, 1997, 45 p.
The Big Spring basin is a 267 km2 (103 mi2) groundwater basin in Clayton County, northeast Iowa. Precipitation, groundwater and surface-water discharge, and the concentrations and loads of various agricultural 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) 1994 and 1995.
After the wet period of WYs 1990 through 1993, WYs 1994 and 1995 were relatively dry. The annual precipitation totals for the basin were 30.42 inches in WY 1994 and 29.28 inches in WY 1995, about 92% and 89% of the long-term average precipitation of 32.97 inches. The annual groundwater discharge from Big Spring was 31,266 acre-feet (ac-ft), with an average discharge rate of 43.2 cubic feet per second (cfs), in WY 1994, and 30,013 ac-ft, with an average rate of 41.5 cfs, in WY 1995. These annual discharges were about half of the highest discharge value, 58,186 ac-ft, recorded in WY 1993, but were close to the average annual discharge, 31,987 ac-ft, during WYs 1982 through 1995. While annual precipitation totals in the Big Spring basin were lower than normal, the discharge as a percent of precipitation, 18.7% for both WYs 1994 and 1995, was the fifth greatest rate since WY 1983. The non-linear relationship between recharge and discharge at Big Spring is affected by antecedent conditions of groundwater in storage and soil moisture within the basin. Annual discharge of the Turkey River at Garber was 719,000 ac-ft for WY 1994 and 729,000 ac-ft for WY 1995. The annual discharges for WYs 1994 and 1995 were 100.7% and 102.1%, respectively, of the long-term average, 714,200 ac-ft, for WYs 1913 through 1995.
The annual flow-weighted (fw) mean nitrate concentration at Big Spring continued to decrease from 56 mg/L (12.4 mg/L as NO3-N) in WY 1991, which was the highest concentration during the period of record, to 47 mg/L (10.4 mg/L as NO3-N) in WY 1994 and 45 mg/L (10.1 mg/L as NO3-N) in WY 1995. The total load of nitrate-nitrogen discharged by the groundwater system was 888,518 pounds in WY 1994 and 822,569 pounds in WY 1995. These annual nitrate-nitrogen loads were only about half of the total load, 1,796,013 pounds, of WY 1993, which was the highest recorded since monitoring began at Big Spring. For the Turkey River, fw mean nitrate concentrations were 23 mg/L (5.1 mg/L as NO3-N) in WY 1994 and 30 mg/L (6.7 mg/L as NO3-N) in WY 1995. Total nitrate-nitrogen discharged by the river was about 9.9 million pounds and 13.3 million pounds in WYs 1994 and 1995, respectively. These loads were significantly less than the highest annual nitrate-nitrogen load, 32.4 million pounds, recorded at the Turkey River in WY 1993. The decreases in nitrate-nitrogen fw mean concentrations and loads likely resulted from several factors, including lower precipitation and discharge during the two-year period, antecedent hydrologic and climatic conditions, as well as the cumulative effects of reduced nitrogen applications.
Atrazine was detected in 94% and 75% of the samples analyzed for pesticides from Big Spring groundwater in WYs 1994 and 1995, respectively. The annual fw mean atrazine concentration declined from 0.27 µg/L in WY 1993 to 0.21 µg/L in WY 1994, and the annual atrazine load declined from 42.0 pounds in WY 1993 to 17.8 pounds in WY 1994. In WY 1995, the annual fw mean atrazine concentration at Big Spring, 0.12 µg/L, was the lowest recorded since the monitoring project started. The annual atrazine load in WY 1995, 9.8 pounds, was the second lowest to date. The lowest annual atrazine load, 9.2 pounds, was recorded in WY 1988. The highest annual fw mean atrazine concentration, 1.17 µg/L, and the greatest annual atrazine load, 135 pounds, were recorded in WY 1991. The concentrations of other common herbicides detected in Big Spring groundwater also remained at low levels during WYs 1994 and 1995. For the Turkey River, the annual fw mean atrazine concentration for WY 1994 was 0.41 µg/L and the annual atrazine load was 803 pounds. In WY 1995, the fw mean concentration was 0.42 µg/L and the load was 841 pounds. These values were also significantly lower than the highest fw mean atrazine concentration, 1.90 µg/L, recorded in WY 1990, and the greatest annual atrazine load, 3,386 pounds, recorded in WY 1993. Unlike the fluctuations of the annual nitrate concentrations and loads, it has been found that the annual concentrations and loads of atrazine do not correlate to discharge. The reasons for this are not clear. To better understand the transport and degradation mechanisms of agricultural contaminants, and to maintain a record of surface- and groundwater quality, continued monitoring in the Big Spring area is needed.