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 acid-neutralizing capacity (ANC) in the outlet stream declined, and NO ⁻₃ and inorganic monomeric aluminum (Al_IM) concentrations increased sharply. During that summer, SO ⁻₄ and NO ⁻₃ 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 SO ⁻₄ -reduction process. A charge-balance ANC calculation closely matched measured downstream changes in ANC in the summer and indicated that SO ⁻₄ reduction 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 μeq/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 13-month 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⁺, Al_IM, and DOC revealed net downstream losses of these constituents and indicated that a reasonable set of hypothesized reactions involving Al_IM, HCO ⁻₃ , Ca²⁺, SO ⁻₄ NO ⁻₃ , 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 SO₄ ²⁻ concentrations. After CaCO₃ treatment, reduction of SO ⁻₄ was 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.