Search

Nitrogen isotope ratios of ammonium and nitrate ions from soil and water samples can be analyzed reproducibly with an experimental error of approximately ±1⁰/₀₀. Two isotopic ranges of soil nitrate are found in the soils of southern Runnels County, Texas. Nitrate from the decomposition of animal waste nitrogen has a δN¹⁵ of +10⁰/₀₀ to +22⁰/₀₀. The isotopic ratio is controlled by the volatilization of isotopically light ammonia gas during the decomposition of urea in urine. Nitrate derived from the mineralization of organic nitrogen in cultivated soils has a δN¹⁵ of +2⁰/₀₀ to +8⁰/₀₀. In southern Runnels County the major source of nitrate in groundwater is natural soil nitrate. The isotopic composition of groundwater nitrate beneath cultivated 15 fields corresponds with δN¹⁵ of natural soil nitrate. Groundwaters beneath farmhouse-barnyard complexes have a higher average δN¹⁵, indicating the addition of animal waste nitrate. Eleven samples of groundwater from Macon County, Missouri, have δN¹⁵ of +10⁰/₀₀ to +19⁰/₀₀ indicating that the waters are contaminated with nitrate from animal wastes. Nitrates in groundwaters from the Upper Glacial aquifer in Queens County, New York, appear to be from an animal waste source, whereas nitrates in groundwaters from the Magothy aquifer in Nassau County, New York, appear to be from either natural soil nitrogen or artificial fertilizer.

Nitrate concentrations in the groundwater in southern Runnels County, Texas, ranged from less than 0.5 mg/l to 3,580 mg/l. Only ten percent of the water samples analyzed contained less than the U. S. Public Health Service recommended limit for nitrate in drinking water (45 mg/l). The major sources of nitrate contamination are soils beneath or near barnyards or septic tanks. Average total nitrate concentration in barnyard soils was 26,000 pounds of nitrate per 15 acre-feet, whereas the average total nitrate (NO₃) concentration in cultivated fields and pastures was 4,100 lb. NO₃ / 15 acre-feet and 3,900 lb. NO₃ / 15 acre-feet, respectively. Nitrates appear to be added to the groundwater by two mechanisms. First, large volumes of water from cattle excrement can enter the aquifers by easy drainage down poorly cased water wells. Second, extensive terracing has caused an appreciable rise in the potentiometric surface with subsequent dissolution of nitrate caliches from the soils by groundwater. Groundwater flow is restricted to solution cavities and fractures in the limestones. Aquifer tests indicate transmissivities on the order of 10,000 gpd/ft, and coefficients of storage on the order of 10⁻⁵. Numerous poorly cased water wells, unplugged seismic shot holes and abandoned oil wells have interconnected the thin limestone aquifers and have permitted extensive contamination of the aquifer system. Early improvement of future groundwater quality cannot be expected because of the vast quantities of nitrate still in the barnyard soils. Water importation or desalination may prove economically feasible for human consumption, but not for agricultural needs.