Geologic nitrogen a potential geochemical hazard in the san joaquin valley california usa
Strathouse, S.M.; Sposito, G.; Sullivan, P.J.; Lund, L.J.
Journal of Environmental Quality 9(1): 54-60
N geochemistry was studied in 2 selected drainage basins of the Diablo Range, adjacent to the San Joaquin Valley in California [USA]. Cantua Creek Basin drains into alluvial soils which contained up to 2000 mg/l indigenous nitrate N in the soil solution. It represents a fairly complete stratigraphic section of Upper Cretaceous 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 imcomplete 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 .mu.g/g to nearly 4800 .mu.g/g. Organic N was the major species in the Cretaceous sediments, reaching a maximum concentration approaching 1200 .mu.g/g. Nitrate N concentrations were always < 100 .mu.g/g in the Cretaceous sediments but approached 4800 .mu.g/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 .mu.g/g to 360 .mu.g/g. Organic N concentrations in the Ortigalita Creek Basin geologic sediments attained a maximum value of 500 .mu.g/g and nitrate N concentrations there reached a maximum of 600 .mu.g/g. The highest total N concentration of 2000 .mu.g/g occurred as 75% fixed and soluble-exchangeable ammonium. The contrast in concentrations and chemical species of N between the 2 basins was explained partially by sediment type. Organic matter was associated with fine-grained sediments and Cantua Creek sediments contained a higher portion of organic matter and a more available geologic source of N. 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 N in rocks from that basin was fixed ammonium bound to layer silicates.