GROUNDWATER MONITORING IN THE BIG SPRING BASIN 1984-1987: A SUMMARY REVIEW
G.R. Hallberg, R.D. Libra, D.J. Quade, J.P. Littke, B.K. Nations
Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 16, 1989, 68 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 chemical contaminants have been monitored at sites within and around the basin since 1981. This reports summarizes the results of monitoring at Big Spring itself and for the Turkey River during water years 1984 through 1987. Annual precipitation during the period varied between 32 inches in water year (WY) 1987 and 37 inches in WY 1986, compared to the long-term average of 33 inches. Groundwater discharge from the basin to the Turkey River ranged from 32,700 acre-feet in WY 1984 to approximately 25,000 acre-feet in WYs 1985 and 1987. Discharge from the Turkey River varied between 555,000 acre-feet in WY 1985 to over 1,000,000 acre-feet in WY 1984. After WY 1985 the timing and intensity of rainfall, while adequate for crop production, was not conducive to produce significant groundwater recharge.
Flow-weighted mean nitrate concentrations at Big Spring varied from 43 mg/L in WYs 1984 and 1986, to a low of 31 mg/L in WY 1985. The total load of nitrate-nitrogen discharged by the groundwater system varied from 843,000 pounds in WY 1984 to 477,000 pounds in WY 1985. On an annual basis, both nitrate concentrations and loads vary with total groundwater discharge. For the Turkey River, flow-weighted nitrate means varied from 16 mg/L in WY 1985 to 28 mg/L in WY 1986. Nitrate-nitrogen loads were lowest in WY 1985, 5,341,000 pounds, and greatest in WY 1984 at 15,175,000 pounds.
Atrazine is the most consistently detected herbicide in the groundwater discharging from Big Spring. Flow-weighted mean atrazine concentrations at Big Spring varied from a high of 0.7 ug/L in WY 1985 to 0.3 ug/L in WY 1987. These years also had the highest and lowest total loads of atrazine discharged from the basin, 47.6 and 17.6 pounds, respectively. The annual mean atrazine concentrations and loads do not vary with discharge in the manner of nitrate. Rather, both annual concentrations and loads increased through WY 1985, and then decreased through the remainder of the period. Atrazine data from the Turkey River are available for WYs 1986 and 1987. Flow-weighted atrazine means were 0.60 and 0.47 ug/L, respectively, for these years, while total loads were 1,407 and 891 pounds.
Maximum concentrations of nitrate in Big Spring groundwater occur during periods of significant infiltration recharge. Maximum concentrations of pesticides occur during periods when the discharge at the Spring is dominated by sinkhole-captured surface runoff, particularly in the months following spring application of these chemicals. Since monitoring at Big Spring began in 1981, 98% of all samples analyzed for pesticides have contained detectable atrazine. The majority of detections of other pesticides occur during the months after application. Cyanazine, alachlor, and metolachlor have been detected in 15%, 15%, and 5% of the analyzed samples, respectively. Maximum annual concentrations of all pesticides follow the trend of annual flow-weighted atrazine concentrations, increasing through WY 1985, and then decreasing through the remainder of the period.
Nitrate concentrations at Big Spring during WY 1985 were distinctly lower than other years of similar discharge and appear related to decreased fertilizer applications during the PIK program of 1983. This suggests a two-year lag time for water quality response to changes in nitrogen inputs. Similar changes in pesticide concentrations, or frequency of detections, did not occur in WY 1985. This may suggest a longer lag time for these chemicals. Atrazine concentrations declined in WYs 1986 and 1987. From 1984 to 1987 there was a significant decrease in atrazine usage. The drought conditions that hampered crop production in the area during 1988 began much earlier from a hydrologic standpoint, with little significant groundwater recharge since snowmelt of 1986. Hence, definitive interpretations of these trends are hampered by complex climatic variations, which underscores the need for long-term monitoring of environmental systems.