STATISTICAL-ANALYSIS OF RURAL WELL CONTAMINATION AND EFFECTS OF WELL CONSTRUCTION

A previous statewide survey showed that 14% of rural wells in Iowa con tained detectable concentrations of pesticides. To determine if improv ed private well construction regulations should be included in Iowa's Stare Pesticide Management Plan, a two-year study was undertaken to de termine: the effects of well construction on pesticide, nitrate-nitrog en, and bacterial contamination of wells; and the possible role of poi nt sources of contamination. Eighty-eight rural water supply wells in nine Iowa counties were sampled daily for five weeks during late sprin g and summer of 1993, and 20% of these were resampled in 1994. Short-t erm variation in nitrate-nitrogen concentrations was examined as a pos sible indicator of rapid inflow of shallow groundwater associated with well construction defects. Mean total coliform bacteria, nitrate-nitr ogen, chloride, atrazine, alachlor, and metolachlor concentrations wer e statistically analyzed to determine if they were correlated, and t-t ests also were used to determine if these water quality parameters wer e affected significantly by physical well parameters such as depth, ty pe of casing, grouting, location within frost pits, and proximity to v arious potential sources of contamination. Study results indicate that : short-term water quality fluctuations, by themselves, were not a rel iable indicator of deteriorated or improperly constructed wells; altho ugh the magnitude and frequency of positive total coliform test result s was noticeably higher in shallower wells, a substantial fraction (21 %) of wells greater than 30.5 m (100 fr) deep also had positive colifo rm results; t-tests and correlation analysis failed to show significan t differences in mean atrazine or alachlor concentrations when compari ng ''shallow'' and ''deep'' wells; increased well depth, by itself did not ensure water supply protection from chemical or biological contam inants; mean nitrate-nitrogen and mean chloride concentrations had the strongest correlation (R = 0.57, p = 0.0001) among any of the contami nants tested; and mean atrazine and alachlor concentrations correlated moderately well with those for the more highly-mobile nitrate-nitroge n and chloride.