Statistical Analysis of Rural Well Contamination and Effects of Well Construction

Abstract

<p>A previous statewide survey showed that 14% of rural wells in Iowa contained detectable concentrations of pesticides. To determine if improved private well construction regulations should be included in Iowa’s State Pesticide Management Plan, a two-year study was undertaken to determine: the effects of well construction on pesticide, nitratenitrogen, and bacterial contamination of wells; and the possible role of point sources of contamination. Eighty-eight rural water supply wells in nine Iowa counties were sampled daily for five weeks during late spring and summer of 1993, and 20% of these were resampled in 1994. Short-term variation in nitrate-nitrogen concentrations was examined as a possible indicator of rapid inflow of shallow groundwater associated with well construction defects. Mean total coliform bacteria, nitrate-nitrogen, chloride, atrazine, alachlor, and metolachlor concentrations were statistically analyzed to determine if they were correlated, and t-tests also were used to determine if these water quality parameters were affected significantly by physical well parameters such as depth, type of casing, grouting, location within frost pits, and proximity to various potential sources of contamination. Study results indicate that: short-term water quality fluctuations, by themselves, were not a reliable indicator of deteriorated or improperly constructed wells; although the magnitude and frequency of positive total coliform test results was noticeably higher in shallower wells, a substantial fraction (21%) of wells greater than 30.5 m (100 ft) deep also had positive coliform results; t-tests and correlation analysis failed to show significant differences in mean atrazine or alachlor concentrations when comparing “shallow” and “deep” wells; increased well depth, by itself, did not ensure water supply protection from chemical or biological contaminants; mean nitrate-nitrogen and mean chloride concentrations had the strongest correlation (R = 0.57, p = 0.0001) among any of the contaminants tested; and mean atrazine and alachlor concentrations correlated moderately well with those for the more highly-mobile nitrate-nitrogen and chloride.</p>