INFLUENCE OF DAIRY AND POULTRY OPERATIONS ON NITRATE CONCENTRATIONS IN THE UPPER FLORIDAN AQUIFER IN NORTH-CENTRAL FLORIDA

By Hilda H. Hatzell

ABSTRACT

The number of poultry and dairy operations in north-central Florida has increased during the past 10 to 15 years. These land-use activities could be increasing nitrate concentrations in ground water in the Upper Floridan aquifer. In water from 148 background monitoring wells that were selected to avoid known areas of ground-water contamination, the median nitrate-as-nitrogen concentration in the mid 1980's was 0.05 milligrams per liter (mg/L) which is equal to the method detection limit. The median nitrate concentration of water in 83 private drinking water wells in the same area and time period was 0.96 mg/L. To determine whether poultry and dairy farms in Suwannee and Lafayette Counties are affecting nitrate concentrations in the Upper Floridan aquifer, one hundred private drinking-water wells in north central Florida were sampled for nitrate concentrations and nitrogen isotope ratios from July through September 1991. The nitrate-as-nitrogen concentrations and the delta values of the nitrogen isotope ratio (delta15N) for nitrate were kriged to produce grids of predicted concentrations and delta values. In addition, delta15N values less than or equal to +2 per mil were classified as derived from fertilizer sources. The delta15N values greater than +2 per mil were classified as derived from non-fertilizer sources, such as animal manure, or from a mixture of fertilizer and non-fertilizer sources. Indicator kriging was then used to produce a grid of predicted probabilities that the delta15N was less than or equal to +2 per mil.

Dairy and poultry operations appear to be influencing nitrate concentrations more in Lafayette County than in Suwannee County. The median of predicted nitrate concentrations of 388 grid locations for the overall study area is 1.04 mg/L with a range of 0.35 - 3.57 mg/L. The median of predicted nitrate concentrations of 195 actual poultry and dairy locations in Suwannee and Lafayette Counties is 1.81 mg/L with a range of 0.54 - 3.56 mg/L. A contour map made from the 388 predicted nitrate concentrations indicates that the highest nitrate concentrations occur in western Suwannee County and north-central Lafayette County. A contour map made from the 388 predicted probabilities indicates that the probability that the delta15N is less than or equal to +2 per mill is greater than 60 percent in western Suwannee County but is less than 40 percent in north-central Lafayette County. The higher percentage implies that nitrates in the ground water in western Suwannee County are more likely to be derived from fertilizer sources than non-fertilizer sources. Conversely, the smaller percentage implies that nitrates in ground water in north-central Lafayette County are more likely to be derived from mixed or non-fertilizer sources. Differences in the nitrate sources between Suwannee and Lafayette Counties could be attributed to such factors as different hydrogeological conditions, the shift from row-crop agriculture to animal operations, and the density of dairy and poultry operations per unit area.


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