EFFECTS OF AGRICULTURAL PRACTICES AND VADOSE ZONE STRATIGRAPHY ON NITRATE CONCENTRATION IN-GROUND WATER IN KANSAS, USA

Differences in nitrate-N concentrations in ground water in Kansas can be explained by variations in agricultural practices and vadose-zone s tratigraphy. In northwestern Kansas, past use of a local stream for ta ilwater runoff from irrigation and high fertilizer applications for su gar-beet farming resulted in high nitrate-N concentrations (12-60 mg L (-1); in both soil and ground water. Nitrogen isotope values from the soil and ground water range from +4 to +80 parts per thousand, which i s typical for a fertilizer source. In parts of south-central Kansas, t he use of crop rotation and the presence of both continuous fine-textu red layers and a reducing ground-water chemistry resulted in ground-wa ter nitrate-N values of < 3 mg L(-1);. The effects of denitrification in the vadose zone and ground water are indicated by enriched delta(15 )N values of +10 to +15 parts per thousand At a site study, irrigated continuous corn was grown on sandy soils with discontinuous fine-textu red layers. Here, nitrate-N concentrations were often > 10 mg L(-1); i n both soil and grounwater. Nitrogen isotope values of +3 to +7 parts per thousand indicate a fertilizer source. Crop rotation decreased nit rate-N values in the shallow ground water (9 m). However, deeper groun d water showed increasing nitrate-N concentrations as a result of past farming practices. Copyright (C) 1996 IAWQ.