GROUNDWATER MONITORING IN THE BIG SPRING BASIN 1988-1989: A SUMMARY REVIEW
R.D.Libra, G.R.Hallberg, J.P. Littke, B.K.Nations, D.J.Quade, R.D. Rowden
Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 21, 1991, 29 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 itself and for the Turkey River during water years (WYs) 1988 and 1989. This period was characterized by extreme drought, and was the driest consecutive two-year period in the state's history. Precipitation totals for the basin were 22.94 inches in (WY) 1988 and 24.32 inches in WY 1989. These totals were 70% and 74% of the long-term average precipitation of 32.97 inches. The drought conditions severely limited groundwater recharge and runoff. Groundwater discharge from the basin to the Turkey River totaled 26,000 acre-feet in WY 1988 and 12,700 acre-feet in WY 1989, the latter figure being the lowest annual discharge measured during the WY 1982 through 1989 period of record at Big Spring. Discharge rates generally declined slowly across the period, with a minimum monthly average of 12 cubic feet/second occurring in July 1989. Average discharge of the Turkey River at Garber was 436,000 and 220,000 acre-feet, respectively, for these water years, equivalent to 65% and 32% of the long-term average discharge.
Flow-weighted mean nitrate concentrations at Big Spring were 43 mg/L in WY 1988 and 25 mg/L in WY 1989. The total load of nitrate-nitrogen discharged by the groundwater system was 672,000 pounds in WY 1988 and 195,000 pounds in WY 1989. The nitrate concentration and load for WY 1989 was the lowest recorded since monitoring began at Big Spring. Nitrate concentrations in general declined slowly across the period, in a manner similar to the rate of discharge. For the Turkey River, flow-weighted mean nitrate concentrations were 23 mg/L and 12 mg/L, respectively, in WYs 1988 and 1989. Total nitrate-nitrogen discharged by the river was about 6 million pounds in WY 1988 and 1.6 million pounds in WY 1989.
Atrazine is the most consistently detected herbicide in Big Spring groundwater. WY 1988 showed the lowest flow-weighted mean atrazine concentration (0.13 µg/L), the lowest total atrazine load (9.2 pounds), and the lowest frequency of atrazine detections (75%) for the WY 1982 through 1989 period of record. During WY 1989 these parameters increased to 0.61 µg/L, 21.2 pounds, and 88%, respectively. Cyanazine was the only other herbicide detected in WY 1988; cyanazine, alachlor, and metolachlor were detected in WY 1989. Concentrations and frequency of detection for herbicides did not follow the generally slow declining trend exhibited by discharge rates and nitrate concentrations. During WY 1988, the Turkey River discharged about 407 pounds of atrazine at a flow-weighted mean concentration of 0.34 µg/L. During WY 1989, the atrazine load was 571 pounds at a flow-weighted mean concentration of 0.95 ug/L.
An overview of groundwater discharge, nitrate, and atrazine data for the WY 1982 through 1989 period indicates that while nitrate concentrations and loads vary in a parallel fashion with discharge, atrazine concentrations and loads do not. Rather, relatively higher concentrations and loads have occurred during years with lower discharges. Discharge rates and contaminant concentrations and loads have varied by factors of two to six across the period of record at Big Spring, underscoring the need for long-term monitoring for the understanding of natural systems.