Distribution of nitrate and related nitrogen species in the unsaturated zone, Redlands and vicinity, San Bernardino County, California

Nitrogen in the unsaturated soil zone in the vicinity of Redlands in San Bernardino County, Calif., has been suspected as the source of nitrate in water from wells. Plans have been made to recharge the aquifer with imported surface water. If this occurs, the rising water table will intercept soluble nitrate in the unsaturated zone. This study was made to quantify and delineate the distribution of the nitrate in the unsaturated zone. Thirteen test holes were drilled in areas of various land uses, and samples of the unsaturated-zone materials were obtained with depth. In 10 of the test holes, the maximum nitrate-nitrogen (N03--N) concentrations occurred within 10 feet of land surface suggesting N03--N sources at the surface. Ranking the test holes according to near-surface N03--N concentrations showed that, of the top six, only two are in irrigated citrus areas. One is in an abandoned feedlot. Of the lowest six, four are in irrigated citrus areas and one is in the Santa Ana River channel. The control test hole ranked fourth suggesting that relatively high N03--N concentrations can occur in near-surface soils even in the absence of human habitation. Analysis according to one conceptual model suggests that raising the water-table altitude by aquifer recharge will intercept N03--N now in the unsaturated zone causing increases in N03--N concentrations in water from wells of up to 48 milligrams per liter. The largest increases will occur in the vicinity of irrigated citrus areas. (Woodard-USGS).