Average δ15N Values Research Articles

Ground‐Water Contamination and Well Construction in Southeast Nebraska

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 mi2 area 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%0 for 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 well contained alachlor.

Nitrogen-Isotope Ratios of Nitrate in Ground Water Under Fertilized Fields, Long Island, New York

Ground-water samples from two heavily fertilized sites in Suffolk County, New York, were collected through the 1978 growing season and analyzed for nitrate-N concentrations and nitrogen-isotope ratios. Six wells were at a potato farm; six were on a golf course. The purpose of this study was to determine whether the 15N/14N ratios (δ15N values) of fertilizer are increased during transit from land surface to ground water to an extent which would preclude use of this ratio to distinguish agricultural from animal sources of nitrate in ground water. Ground water at both sites contained a greater proportion of 15N than the fertilizers being applied. At the potato farm, the average δ15N value of the fertilizers was 0.2%0; the average δ15N value of the ground-water nitrate was 6.2 %0. At the golf course, the average δ15N value of the fertilizers was -5.9%0, and that of ground-water nitrate was 6.5%0. The higher δ15N values of ground-water nitrate are probably caused by isotopic fractionation during the volatile loss of ammonia from nitrogen applied in reduced forms (NH+4 and organic-N). The δ15N values of most ground-water samples from both areas were less than 10%0, the upper limit of the range characteristic of agricultural sources of nitrate; these sources include both fertilizer nitrate and nitrate derived from increased mineralization of soil nitrogen through cultivation. Previous studies have shown that the S15N values of nitrate derived from human or animal waste generally exceed 10%0. The nitrogen-isotope ratios of fertilizer-derived nitrate were not altered to an extent that would make them indistinguishable from animal-waste-derived nitrates in ground water.

Nitrogen isotope ratio of atmospheric N2O as a key to the global cycle of N2O.

An analytical system to separate N2O in the air was developed to measure the δ15N value and concentration of N2O. The system was designed so that δ15N value and concentration of ca. 1 μmol N2O from ca. 60 l air could be measured with the accuracy of ±0.2‰ and ±10 ppbv, respectively. Air samples were collected at three individual sites in Japan. The average values of δ15N and concentration of N2O for 32 samples with the standard deviation were +8.1 ± 1.0‰ and 316 ± 14 ppbv, respectively. The diversity of the observed data was attributed to the change in the mixing ratio of air masses of different characteristics. A discussion was made on the global cycle of N2O. Bacterial reduction of N2O was proposed to be the major consuming process of N2O on the Earths surface.