Geologic nitrogen and the occurrence of high nitrate soils in the western San Joaquin Valley, California

Abstract

The content of total, nitrate, fixed, exchangeable, and organic nitrogen in Cretaceous and Tertiary formations in two representative drainage basins of the east central Diablo Range was determined. The results of these analyses showed that high concentrations of organic nitrogen (up to 1,200 ?g/g) occur predominantly in the Cretaceous formations while high concentrations of nitrate (up to 4,800 ?g/g) occur predominantly in the younger Tertiary sediments. The concentrations of fixed and exchangeable ammonium were variable and generally low. Available geomorphic data for the western side of the San Joaquin Valley were analyzed to seek a possible cause for the observed large accumulations of nitrates in west side soils. It was concluded that the occurrence of mudflows and intermediate flows in the alluvial fans bordering the Diablo Range on its east side, in conjunction with a semi-arid climate, is the principal factor responsible for the retention of soluble nitrogen. A method for predicting high nitrate soils in the western San Joaquin Valley was developed by assigning a score to each basin and associated fan based on the occurrence of high nitrate formations, the percentage of fine-grained sediments, and the predominance of mudflow deposits.