Nitrogen in the unsaturated soil zone in the Highland-East Highlands area of San Bernardino County, Calif., has been suspected as the source of nitrate in water from wells. Plans to recharge the local aquifers with imported surface water would raise the water table and intercept that nitrogen. This study was made to describe the distribution of inorganic nitrogen and other chemical constituents and nitrogen-using bacteria in the unsaturated zone, to relate nitrogen occurrences, in a general way, to present and historical land use, and to attempt to predict nitrogen concentrations in ground water after recharge. Some generalized correlations between nitrogen occurrence and land use were observed. In 11 of 13 test holes, the maximum nitrate-nitrogen (NO3--N) concentrations occurred within 10 feet of the surface, suggesting that the major source of nitrogen is from the surface at these sites. Test holes were ranked according to maximum NO3--N in the top 10 feet, total NO3--N in the top 10 feet, and total NO3--N in the top 40 feet. In all three rankings, the top seven test holes were the same--five in or near present or historical agricultural areas (primarily citrus groves), one in a feedlot, and one adjacent to an abandoned sewage-treatment plant. Two test holes in historically uninhabited areas ranked lowest. The control test hole in an uninhabited area ranked high in geometric mean of ammonium-nitrogen concentration (NH4+-N), suggesting that present in freshly weathered granite. The geometric means of NH4+-N concentrations in six of eight citrus-related test holes were significantly lower than in the control hole, suggesting that irrigation in citrus groves may have created conditions favoring nitrification of the primary NH4+-N. Rank correlation analyses between various measurements in test holes showed that high NO3--N concentrations tend to occur with high specific conductance and chloride concentrations in soil extracts. If recharge is carried out as planned, assuming complete mixing of the recharge water and interstitial pore water in the top 20 feet of the saturated zone, NO3--N concentrations in water at the top of the saturated zone may exceed 10 milligrams per liter in seven areas studied. Concentrations could reach as high as 66 milligrams per liter in the worst case projected. The highest concentrations may be found in three areas that are now, or were recently, citrus groves.
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USGS Numbered Series
Distribution of nitrate in the unsaturated zone, Highland-East Highlands area, San Bernardino County, California