INFLUENCE OF AGRICULTURAL PRACTICES ON WATER QUALITY IN NEBRASKA: A SURVEY OF STREAMS,GROUNDWATER, AND PRECIPITATION1

ABSTRACT Rainfall, stream flow and groundwater have been sampled systematically throughout Nebraska since 1970 and analyzed for mineral N and P and the character of any sediments contained. Fallout N and P in rainfall ranges from 5–14 pounds N and 1 pounds P/A/yr, increasing from west to east across the state with increasing rainfall. The amount of NH4-N is essentially double that of NO₃-N. The mean concentration of 2ppm N in rainfall is four times the mean N concentration of streams, demonstrating a substantial depolluting action of soils and growing crops. Where nutrient levels of streams are elevated, cause can usually be traced especially to industrial, sewage or livestock waste intrusion and not to crop production practices. The only significant quantity of nutrient N and P induced by cultivation is that accompanying sediments from eroded fields. The P content of Nebraska groundwater has remained essentially constant during the past 10 years while average NO₃-N has increased slightly, a period during which farmer fertilizer use quadrupled. During the same time, irrigation acreage has increased by 50%, livestock numbers by 30%, with corresponding growth in human population and attendant industries. Indications are that irrigation practice has contributed more than any other factor to the small increase in groundwater NO3-N recorded. Individual cases do exist where groundwater NO₃-N has increased substantially, especially in areas of intensive irrigation agriculture on very sandy soils and elsewhere with irrigation development in the proximity of ancient NO₃-N deposits in mantlerock above the water table.