HYDROGEOLOGY, WATER QUALITY, AND LAND MANAGEMENT IN THE BIG SPRING BASIN, CLAYTON COUNTY, IOWA
G.R Hallberg, B.E. Hoyer, E. A. Bettis III, R.D. Libra
Iowa Department of Natural Resources, Geological Survey Bureau,
Open File Report 83-3, 1983, 255 p.
The Big Spring study is the second phase of an assessment of groundwater quality in the karst-carbonate aquifers in NE Iowa. The project is jointly funded and conducted by the Iowa Geological Survey (IGS), Iowa Department of Environmental Quality (DEQ Contract No. 85-5500-02), and the U.S.D.A.-Soil Conservation Service (SCS), with assistance from the Iowa Conservation Commission (ICC), University Hygienic Laboratory (UHL), and U.S. Geological Survey. Staff from other institutions have participated in a consultative role, including personnel from Iowa State University, the Cooperative Extension Service, the U.S.D.A.-Agricultural Stabilization and Conservation Service, and the Iowa Department of Soil Conservation.
The Big Spring study was designed to provide a controlled assessment of a karst groundwater basin. This allows a more thorough understanding of the groundwater quality, the processes resulting in groundwater degradation, and evaluation of possible control measures or management practices. The Big Spring area was chosen because: 1) prior knowledge of the groundwater system in the Galena aquifer existed; 2) ICC and SCS had specific concerns with water-quality and landuse in the area; and 3) the ICC Fish Hatchery at Big Spring allowed the direct measurement of groundwater discharge from the basin, which is generally impossible in other areas.
The study was initiated with a basin-wide inventory. Over 320 rural residences were visited and 271 wells were inventoried. About 125 wells were sampled for water-quality analyses. The geology, soils, sinkhole locations, landuse, and piezometric surface of the Galena aquifer were mapped. SCS Staff conducted surveys and inventories of ag-chemical use and application rates, as well as land-treatment practices used in the basin. The boundaries of the groundwater basin were defined from the piezometric mapping and dye-trace studies. As defined, the Big Spring basin is about 103 square miles in area; about 11% of the area drains entirely into sinkholes. Water discharge and quality were monitored at Big Spring from 11/81 through 12/82. Water quality also was monitored at selected wells, streams, springs, and tile lines. Of prime concern are the water-quality data on nitrates, pesticides, bacteria and turbidity because of their possible effects on public health.
Results of monitoring groundwater in this study confirm many conclusions of the first phase of this study, which assessed regional water-quality problems in these karst-carbonate aquifers. During an original basin-wide water sampling inventory, the median nitrate concentration from the Galena aquifer was 35 mg/L, with individual analyses as high as 280 mg/L. However, where the Galena aquifer is protected from significant surficial infiltration or sinkhole "run-in" by a cover of Maquoketa shale, nitrates are not detected (<5 mg/L) in the groundwater. For the water year, the mean nitrate concentration in groundwater discharging at Big Spring was 40 mg/L, approaching the U.S.E.P.A. drinking water standard for nitrate (45 mg/L). This is in marked contrast to water-quality analyses from Big Spring from 1951 and 1968 which had a mean nitrate concentration of 13 mg/L. Comparison of these values suggests a 230% increase in nitrate concentrations in groundwater since the late 1960's. During this period, corn acreage increased about 40% and the application rate of fertilizer-N increased about 80%. As the corn acreage and application rate are "additive," the total fertilizer-N applied in the basin increased by about 250% during this same period. Other potential sources of increased nitrate were negligible by comparison. The primary reason for increased nitrate concentrations in the Galena aquifer clearly seems to be the dramatic increase in nitrogen fertilization.
The total discharge of nitrogen (as nitrate-N) from the Big Spring basin for the water year was 905 tons; 527 tons in groundwater and 378 tons in stream-flow. This amounts to 27 lbs-N/acre for the entire basin, or, 47 lbs-N/acre for the row-crop area of the basin. As a matter of perspective, the total N lost from the basin was equivalent to 33% of the total fertilizer-N applied in 1982. This is not to imply that all the N lost was 1982-applied N. Monitoring of the Turkey River indicates that such substantial N-losses occur regionally and constitute an economic as well as environmental loss.
Pesticides were not detected in groundwater during the winter and early spring of 1981-1982. The herbicide, atrazine, appeared in detectable amounts in groundwater at Big Spring and monitored wells within two weeks of application. At Big Spring atrazine persisted throughout the remainder of 1982, but it dropped below detection limits in most wells. Concentrations of atrazine in groundwater ranged 0.04 to 2.5 µg/L. Three other herbicides, Bladex, Lasso, and Dual were also detected in groundwater, but only during May and June. The pesticide concentrations measured are all very low, well below toxic levels and estimated-safe-average-daily-intake levels. The discharge of pesticides in groundwater during the water year amounted to only about 14 lbs, approximately 0.04% of that applied in the basin. The total loss of pesticides in groundwater and stream-flow estimated at 0.4% of the amount applied.
Bacterial contamination of the aquifer was found in association with peak runoff periods. Turbidity and associated problems, such as sediment, soil-attached pesticides (especially dieldrin), and other organics, are also related to peak runoff. Persistent bacteria problems are not necessarily related to the karst-groundwater system, but may be associated with faulty domestic water systems. Cisterns, in particular, were found to be a common source of bacterial contamination in rural drinking water supplies.
Groundwater discharge was separated into two principle components: 1) a "base-flow" or "infiltration" component; and 2) a "peak conduit flow" or "run-in" component, related to surfacewater run-in to sinkholes. The "infiltration" component delivers to groundwater: 1) the highest concentrations and largest mass (94%) of nitrate (and other soluble nutrients); 2) the largest mass (84%) of soluble pesticides, but in very low concentrations; and 3) generally little sediment, turbidity, organic, or bacteria problems. The "runin" component delivers to groundwater: 1) peak pesticide loadings, with concentrations 10 to 100 times greater than the "infiltration" component; 2) peak turbidity and sediment problems; 3) peak bacteria problems; and 4) generally lower concentrations of nitrates, compared to the "infiltration" component. The respective contributions of these components must be considered in any planning of control measures or management practices.