WATER QUALITY MONITORING OF THE NISHNABOTNA RIVER ALLUVIAL SYSTEM

C.A. Thompson and P.E. VanDorpe


Iowa Department of Natural Resources, Geological Survey Bureau,
Open File Report 88-1, 1988, 60 p.

ABSTRACT


Water quality, with particular attention to nitrate and pesticides, was monitored in the Nishnabotna alluvial aquifer during 1987. With respect to major ions, water quality is good except for iron, which tends to be high. Average regional nitrate concentration was 13.2 mg/L nitrate (2.9 mg/l nitrate-N) for data collected during this study. This is lower than has been observed in most other alluvial aquifers in the state. Higher average concentrations of nitrate occur in wells located near the valley edge. These wells may be intercepting shallow groundwater flow from the uplands which may have a higher concentration of nitrate. Wells located where the valley is narrow and the aquifer thins also have higher concentrations of nitrate. Vertical stratification of nitrate was not observed in the Nishnabotna alluvial aquifer. Rainfall events did not appear to correlate with increases in nitrate concentration, which may be related to the thick cover of fine-grained alluvium. Nitrate concentrations in surface water were always higher than in groundwater and have increased four-fold over the last ten years.

Pesticides were found at two of six municipal wells during this study. No pesticides were detected in any of the monitoring wells. Examination of all available pesticide data collected between 1985 and 1987 shows pesticide detections at seven of sixteen towns (13 of 60 samples). Most concentration detected are low; alachlor (Lasso) has exceeded the lower limit of the proposed health standard. Atrazine was the most frequently detected compound in groundwater. Pesticide occurrence did not correlate with nitrate concentration.

Pesticides were detected in all surface water samples and at higher concentrations than in groundwater. Surface water concentrations of both alachlor and atrazine exceeded the proposed standards. Atrazine and cyanazine (Bladex) were detected most frequently in surface water. Metribuzin (Sencor) was detected only in surface water.