TRENDS IN PRECIPITATION AND STREAM CHEMISTRY IN A PRISTINE OLD-GROWTHFOREST WATERSHED, OLYMPIC NATIONAL-PARK, WASHINGTON

Human induced long-term changes in precipitation and stream chemistry have been observed in eastern North America and Europe, but few long-t erm studies have been conducted in coastal western North America. The objectives of this research were to determine: (1) time trends in prec ipitation and stream chemistry in a pristine old-growth forest watersh ed, and (2) seasonal patterns in precipitation and stream chemistry. I t was conducted in 58 ha West Twin Creek Watershed, Hoh River Valley, Olympic National Park, Washington from 1984 to 1993. Vegetation consis ts of old-growth forest, with western hemlock, Douglas-fir, western re dcedar, Pacific silver fir, and Sitka spruce being the dominant tree s pecies. Annual precipitation varied from 2336 to 4518 mm during the st udy period with the majority of the rain falling between October and M ay. Chemistry of precipitation was strongly dominated by oceanic influ ences with Na and Cl being the dominant ions. The chemistry of the str eam was influenced by bedrock weathering and was dominated by Ca, HCO3 , and SO4 and was not strongly related to precipitation chemistry. The pH of precipitation averaged 5.3 over time and ranged from 4.3 to 7.1 , while the stream pH averaged 7.5 and ranged from 5.5 to 9.0. There w ere few long-term trends in the chemical constituents of bulk precipit ation or stream water with the exception of a slight decrease in NO3 i n precipitation and an increase of SO4 in stream water. A trend of dec reasing concentrations of Ca, Mg and Na in precipitation also occurred . There were no significant seasonal patterns in precipitation althoug h the highest SO4 concentrations usually occurred in late spring and s ummer perhaps due marine algal activity. Strong seasonal trends occurr ed in concentrations of HCO3, SO4, Ca, Mg, and Na in stream water resu lting from weathering and stream flow patterns, with highest ion conce ntrations occurring just before the onset of the rainy season. Pulses of NO3 in the stream were observed during fall and early winter result ing from the release of NO3 which had accumulated in soils or sediment s.