USE OF ENVIRONMENTAL ISOTOPES TO ASSESS THE AGE OF WATER, SOURCES OF NITROGEN AND FLOW PATHWAYS OF SHALLOW GROUND WATER IN AN AGRICULTURAL AREA IN SOUTH-CENTRAL GEORGIA

By Christy A. Crandall

ABSTRACT

Environmental isotopes (tritium, delta-15N, delta- 18O, and delta-D) were used to determine the age of water and investigate sources of nitrogen and flow pathways in a row-crop agricultural area in south-central Georgia where high nitrate concentrations were found in shallow ground water. This study was part of the Georgia-Florida Coastal Plain National Water-Quality Assessment program. Preliminary analysis of the data from 23 wells reveals that most of the water is probably less than 20 years old (50 percent of the tritium concentrations ranged between 7.0 and 9.5 tritium units (TU)). In 3 samples, the concentration of tritium was less than 1.0 TU. These samples were from the 2 deepest wells in the study (70 and 65 feet deep) and a well with an extremely low yield. The age of water in these 3 wells was probably greater than 40 years. The maximum nitrate concentration in water which had tritium concentrations less then 1.0 TU was 0.36 mg/L as nitrate as N, whereas the median nitrate concentration was 7.8 mg/L in younger waters.

Nitrogen isotope ratios (delta-15N) were used to identify the probable sources of the nitrogen in ground water. Seventy-five percent of samples from wells where the depth of the screened interval below the water table was less than 17 feet had delta-15N values of less than 3.0 per mil, indicating that inorganic fertilizer is the probable nitrogen source. Values of delta-18O and delta-D were compared to the Global Meteoric Water Line (GMWL) (Craig, 1961). Twenty-one samples from shallow ground water were depleted in delta-18O and delta-D compared to GMWL (probably due to evaporation before recharge occurred). Values of delta-D were negatively correlated with concentrations of nitrate whereas values of delta-18O were not significantly correlated with nitrate concentrations. This suggests that some mechanism(s) occurred to deplete the water of delta-18O since values of delta-18O and delta-D are generally highly correlated. Values of delta-18O and delta-D were generally higher when the depth to water was greater. This may indicate differing or mixed sources of water or water that is moving laterally rather than downward as a result of the presence of a clay impeding layer.

Craig, H, Science, 133, 1702-1703, 1961.


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