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Temporal and Spatial Trends in Streamwater Nitrate Concentrations in the San Bernardino Mountains, Southern California

USDA Forest Service, Pacific Southwest Res. Stn., Forest Fire Lab., 4955 Canyon Crest Dr., Riverside, CA 92507.

^* Corresponding author (mfenn{at}deltanet.com).

ABSTRACT

We report streamwater nitrate (NO^–₃) concentrations for December 1995 to September 1998 from 19 sampling sites across a N deposition gradient in the San Bernardino Mountains. Streamwater NO₃ concentrations in Devil Canyon (DC), a high-pollution area, and in previously reported data from the San Gabriel Mountains 40 km northeast of Los Angeles, are the highest values reported in North America for undisturbed forest or shrub land watersheds. Concentrations in the primary stream draining western DC peaked at 350 µmol L^–1 in December 1997 and minimum base flow NO^–₃ concentrations were nearly always 80 µmol L^–1. In the San Gorgonio Wilderness (SGW), average NO^–₃ concentrations in four streams along the southern transect (moderate N deposition), ranged from 10 to 37 µmol L^–1, while average NO^–₃ concentrations were 0.7 µmol L^–1 in seven streams along the northern transect (low N deposition). Peak NO^–₃ concentrations in DC and in the SGW occurred after large winter storms, and a large spike in NO^–₃ concentrations (10–370 µmol L^–1) in SGW Streams 1 to 5 was observed after thundershower activity in July 1997. Streamwater export of NO₃-N from Devil Canyon ranged from 3.6 to 11.6 kg ha^–1 yr^–1 during water years 1995 to 1998. This study further indicates that N emissions from fossil fuels and agriculture impact not only air quality, but also water quality from watersheds that are recipients of atmospheric N deposition.

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