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    Dilution and the Elusive Baseline

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    Knowledge of baseline conditions is critical for evaluating quantitatively the effect of human activities on environmental conditions, such as the impact of acid deposition. Efforts to restore ecosystems to prior, “pristine” condition require restoration targets, often based on some presumed or unknown baseline condition. Here, we show that rapid and relentless dilution of surface water chemistry is occurring in the White Mountains of New Hampshire, following decades of acid deposition. Extrapolating measured linear trends using a unique data set of up to 47 years, suggest that both precipitation and streamwater chemistry (<i>r</i><sup>2</sup> >0.84 since 1985) in the Hubbard Brook Experimental Forest (HBEF) will approximate demineralized water within one to three decades. Because such dilute chemistry is unrealistic for surface waters, theoretical baseline compositions have been calculated for precipitation and streamwater: electrical conductivity of 3 and 5 μS/cm, base cation concentrations of 7 and 39 μeq/liter, acid-neutralizing capacity values of <1 and 14 μeq/liter, respectively; and pH 5.5 for both. Significantly large and rapid dilution of surface waters to values even more dilute than proposed for Pre-Industrial Revolution (PIR) conditions has important ecological, biogeochemical and water resource management implications, such as for the success of early reproductive stages of aquatic organisms