Geologic Nitrogen: A Potential Geochemical Hazard in the San Joaquin Valley, California

Nitrogen geochemistry was studied in two selected drainage basins of the Diablo Range, which is adjacent to the San Joaquin Valley in California. Cantua Creek Basin drains into alluvial soils which contain up to 2,000 mg/liter indigenous nitrate N in the soil solution. It represents a fairly complete stratigraphic section of Upper Cretaceous to Tertiary geologic sediments, dominated by fine-grained mudrocks and shales. The Ortigalita Creek Basin drains into alluvial soils with low levels of indigenous nitrate. It contains an incomplete stratigraphic section dominated by Upper Cretaceous coarse-clastic rocks with subordinate amounts of mudrocks, capped by Tertiary gravels and marls. Total N concentrations in the Cantua Creek Basin geologic sediments ranged from a few µg/g to nearly 4,800 µg/g. Organic N was the major species in the Cretaceous sediments, reaching a maximum concentration approaching 1,200 µg/g. Nitrate N concentrations were always < 100 µg/g in the Cretaceous sediments, but approached 4,800 µg/g in a rock sample from a recent Tertiary unit. The total N and nitrate N concentrations increased from older to younger geologic sediments, while ammonium N concentrations varied from a few µg/g to 360 µg/g. Organic N concentrations in the Ortigalita Creek Basin geologic sediments attained a maximum value of 500 µg/g and nitrate N concentrations there reached a maximum of 600 µg/g. However, the highest total N concentration of 2,000 µg/g occurred as 75% fixed and soluble-exchangeable ammonium. The contrast in concentrations and chemical species of nitrogen between the two basins was explained partially by sediment type. Organic matter is associated with fine-grained sediments and, consequently, Cantua Creek sediments contain a higher portion of organic matter and a more available geologic source of nitrogen. On the other hand, the presence of high ammonium concentrations and a corresponding high-charge smectite in the Ortigalita Creek Basin suggested that one of the principal sources of nitrogen in rocks from that basin was fixed ammonium bound to layer silicates.