ABSTRACTThe ground waters of Runnels County, Texas, are highly contaminated with nitrate. The average nitrate concentration of 230 water samples was 250 mg/I NO3.The natural variations of the stable nitrogen isotopes N14and N15identified natural soil nitrate as the predominant source. Nitrate from animal wastes was of minor importance. The δN15range of natural soil nitrate was +2 to +8% whereas the δN15range of animal waste nitrate was +10 to +20%‐ (Atmospheric nitrogen was used as a standard for mass spectrometric analysis. Experimental error for sample preparation and isotopic analysis was ±1 %.) More than 66 percent of the ground‐water nitrates analyzed were in the δN15range of natural soil nitrates.Dryland farming since 1900 has caused the oxidation of the organic nitrogen in the soil to nitrate. Minimal fertilizer has been used because of the lack of suitable water for irrigation. During the period 1900‐1950, nitrate was leached below the root zone but not to the water table. Extensive terracing after the drought in the early 1950's has raised the water table approximately 6 meters and has leached the nitrate into the ground water. Tritium dates indicate that the ground water is less than 20