The objective of the Aquatic Ecosystems Research Program of the CARB was to determine the effects of acidic deposition onwater quality and biological populations in high elevation lakes and watersheds of the Sierra Nevada. Toward this end, the CARB funded ten years of research on the hydrochemistry and biology of high elevation sites in the Sierra Nevada. The resulting information is contained in 32 final reports and ca. 65 publications. Until now, an overall synthesis of this large body of survey, monitoring and experimental data was lacking. In this report, we integrate the findings of the Aquatic Ecosystems Research Program in order to evaluate (1) the chemistry of snow and rain that currently falls in the Sierra Nevada, (2) the current chemical status of high elevation surface waters, (3) long term trends in chemical and biological features of high elevation lakes, (4) the mechanisms of ANC generation and consumption in high elevation catchments, (5) the sources and fates of solutes over the course of the hydrological year, (6) the role of infrequent events on the hydrochemistry and biota of high elevation watersheds, (7) the potential use of bio-indicators in the Sierra Nevada and (8) the use of models to predict the hydrochemistry of Sierra Nevada surface water. Annual loading rates for hydrogen, sulfate, nitrate, ammonium and calcium are low in the Sierra Nevada relative to the country as a whole. For most sites, hydrogen was the most concentrated ionic species in snow in the Sierra Nevada, and ammonium and nitrate were the second-ranked and third-ranked ions in snow, respectively. Nitrate and ammonium were the two most concentrated species in rain at all of the high elevation monitoring sites. Third and fourth ranked ions in rain were sulfate or hydrogen at most sites. In summer rainfall, NH4+:H+ is always > 1 and NH4+ is strongly correlated with N03- and SO4-2. Thus in the summer, NH4+ is an important neutralizer of the strong acid anions N03-, SO4-2, and Cl-. In the absence of NH4+, [H+] in rainfall would potentially be 1 l-fold higher. In most respects, the chemical composition of 89 lakes surveyed under CARB support was similar to that of the Sierra Nevada lakes sampled during the EPA's Western Lake Survey (WLS) of 1985. Results of the CARB lake surveys indicate a somewhat higher sensitivity to acidification for high elevation lakes of the Sierra Nevada than was indicated by the WLS. For example, 65 percent of Sierra Nevada lakes in the WLS had ANC 5 100 PEq L-1, whereas 74 percent of CARB sampled lakes had ANC I 100 PEq L-l. Although none of the Sierra Nevada lakes sampled in the WLS had ANC IO, 4.5 percent of the CARB sampled lakes had ANC I 0. In the WLS only one lake was sampled with pH c 6.0; in the CARB survey, 10 lakes had pH < 6.0. These differences are partly due to the inclusion in the CARB surveys of a number of naturally acidic lakes in the Mt. Pinchot area of Kings Canyon National Park in the southern Sierra Nevada, the presence of which has been tentatively ascribed to the oxidation of pyrite. In addition, generally higher concentrations of strong acid anions (nitrate, sulfate and chloride) were measured in the CARB survey lakes than in the WLS lakes. No inter-annual trends in the pH or ANC of lakewater or outflow streamwater were found during the period 1983 through 1994. Surface waters in high elevation regions of the Sierra Nevada have not undergone measurable acidification since 1983. The majority of [H+] stored in the snowpack of Sierra Nevada watersheds is currently neutralized before reaching outflow streams. Buffering by formate and acetate in snow, reactions with particulate clay and dust from dry deposition, and neutralization by cation exchange in soils and talus may all contribute to this neutralization. All of the high elevation Sierra Nevada watersheds studied produced sufficient ANC to neutralize much of the acidity of precipitation and to be net exporters of ANC and base cations. Sierra Nevada watersheds are effective at retaining dissolved inorganic nitrogen delivered in wet deposition. Ammonium was almost completely retained by the headwater catchments studied. Net annual retention of nitrate was almost always observed, although the percentages of nitrate consumed by watershed processes were lower than for ammonium. Ammonium consumption appears to occur along the pathway of meltwater to the lakes, rather than in the lakes. Rain in the Sierra Nevada in summer is acidic. Most of the annual deposition of nitrogen, sulfate and organic acids occurs during the non-winter months. Although the quantity of non-winter precipitation is much smaller than snowfall, solute concentrations in rain are much greater than in snow. Large summertime rainstorms have been observed to cause a drop in the pH and ANC of Emerald Lake. The chemistry of rain in the Sierra Nevada is greatly changed by passage through foliage (e.g. chinquapin, western white pine, and willow). Nitrate is almost doubled, and ammonium is almost completely retained, by foliage, thus lowering the ANC of precipitation.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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