ABSTRACTNitrate‐nitrogen (NO3‐N) concentrations and total (TC) and fecal coliform (FC) densities were determined in ground water from 268 household and stock wells in an 1100 mi2area of southeast Nebraska. Seventy‐one percent of the samples had NO3‐N concentrations above 10 mg/1 and/or TC densities ≥ 1/100 ml. Most of the NO3‐N contamination is from the dissolution of manure in barnyards and corrals. The average δ15N value was + 11.9%0for the 115 samples with NO3‐N levels in excess of 8.5 mg/1.Only 10% of the wells met Nebraska's criteria for private well construction. NO3‐N levels exceeded 10 mg/1 in 4% of these wells while 30% contained one or more TC/100 ml. The highest incidence of NO3‐N and coliform contamination occurred in dug or augered wells with open‐jointed casing. The rates were 47% and 80%, respectively.Low levels of atrazine were detected only in the poorly constructed wells. One wel