GROUNDWATER QUALITY OBSERVATIONS FROM THE BLUEGRASS WATERSHED AUDUBON COUNTY, IOWA
L.S. Seigley and G.R. Hallberg
Iowa Department of Natural Resources, Geological Survey Bureau
Technical Information Series 20, 1991, 50 p.
The Bluegrass Watershed encompasses 1,024 acres of agricultural land in north-central Audubon County, Iowa. Groundwater from active and inactive private wells, tile lines and surface water from Bluegrass Creek have been monitored for bacteria, nitrate, and pesticides since May 1987 to determine water quality in and around the watershed. The majority of the rural population in this part of Iowa use shallow (20 to 40 feet deep), large-diameter (three to four feet) "seepage" wells located in surficial geologic deposits (loess, glacial till, and alluvium). The groundwater yielded by these wells is susceptible to contamination from land-applied chemicals. Sites were sampled on monthly bases. In addition, the sites were sampled after rainfalls of over one inch to determine what immediate changes in water quality occur during these recharge events.
Most of the sampling results described are from 1988 and 1989, the driest consecutive two-year period in Iowa climatic records. The drought conditions confound interpretation of changes in water quality that may result from changes in farm management in the watershed; further monitoring will clarify these relationships.
Bacteria samples were primarily collected from active wells; coliform bacteria were detected in 90% of the samples from active wells. Total coliform bacteria typically occur in samples from such water-table wells.
Yearly mean nitrate concentrations for active wells, tile lines, and surface water showed a statistically significant decline from 1987 to 1989. Mean annual nitrate concentration decreased from 48.3 to 37.8 mg/L (as nitrate) in active wells, 28.8 to 11.5 mg/L in inactive wells, 65.3 to 50.9 mg/L in tile lines, and 60.0 to 40.8 mg/L in surface water. Statistical analysis of rainfall (event) and normal monthly sampling (non-event) for each site category showed no significant difference in nitrate concentration. One active well (W-1) showed the most significant change in nitrate concentration, declining from over 70 mg/L to <1 mg/L since May 1987. This decline is related to a change in land management; the land surrounding the well was taken out of row-crop production in 1987 and placed in the Conservation Reserve Program (CRP).
Three hundred and fifty-three samples were analyzed for pesticides (herbicides and insecticides). No insecticides were detected. Forty-one percent (145/353) of the herbicide samples had detections of one or more herbicides at concentrations of 0.1 µg/L or greater. Atrazine was detected in 90% of the 145 positive samples, cyanazine in 54%, alachlor in 28%, metochlor in 24%, trifluralin in 4%, and pendimethalin in 3%. Herbicide concentrations ranged from 0.1 to 28.0 µg/L. In general, herbicide concentrations were greater for rainfall samples than normal monthly samples. Eighty-four percent of all herbicide detections occurred during the months of May to September with the highest concentrations of individual herbicides occurring during the months of May, June or July.
The nitrate concentrations in the wells appear related to the geologic materials in which the wells are located, whereas, herbicide occurrence appears to be related primarily to the type of well (active versus inactive).