The effects of liming an Adirondack lake watershed on downstream water chemistry

Abstract

Calcite treatment of chronically acidic lakes has improved fish habitat, but the effects on downstream water quality have not previously been examined. In this study, the spatial and temporal effects of watershed CaCO3 treatment on the chemistry of a lake outlet stream in the Adirondack Mountains of New York were examined. Before CaCO3 treatment, the stream was chronically acidic. During spring snowmelt before treatment, pH and acidneutralizing capacity (ANC) in the outlet stream declined, and NO3 and inorganic monomeric aluminum (AliM) concentrations increased sharply. During that summer, S02and NO3 concentrations decreased downstream, and dissolved organic carbon (DOC) concentrations and ANC increased, in association with the seasonal increase in decomposition of organic matter and the attendant SO2--reduction process. A charge-balance ANC calculation closely matched measured downstream changes in ANC in the summer and indicated that SO2reduction was the major process contributing to summer increases in ANC. Increases in Ca2+ concentration and ANC began immediately after CaCO3 application, and within 3 months, exceeded their pretreatment values by more than 130 peq/L. Within 2 months after treatment, downstream decreases in Ca2+ concentration, ANC, and pH, were noted. Stream mass balances between the lake and the sampling site 1.5 km downstream revealed that the transport of all chemical constituents was dominated by conservative mixing with tributaries and ground water; however, non-conservative processes resulted in significant Ca2+ losses during the 13month period after CaCO3 treatment. Comparison of substrate samples from the buffered outlet stream with those from its untreated tributaries showed that the percentage of cation-exchange sites occupied by Ca2+, as well as non-exchangeable Ca, were higher in the outlet-stream substrate than in tributary-stream substrate. Mass-balance data for Ca2+, H+, A1IM, and DOC revealed net downstream losses of these constituents and indicated that a reasonable set of hypothesized reactions involving AliM, HCO3, Ca2+, SO2-, NO3, and DOC could have caused the measured changes in stream acid/base chemistry. In the summer, the sharp decrease in ANC continued despite significant downstream decreases in SO24concentrations. After CaCO3 treatment, reduction of SO2was only a minor contributor to ANC changes relative to those caused by Ca2+ dilution from acidic tributaries and acidic ground water, and Ca2+ interactions with stream substrate.