The role of organic acids in the acidification of surface waters in the Eastern U.S.

There is considerable uncertainty concerning the role of naturally occurring organic solutes in the acidification of surface waters. To provide a preliminary assessment of this process, we evaluated water chemistry data obtained from the US EPA Eastern Lake Survey (ELS). A wide range of dissolved organic carbon (DOC) and organic anion concentrations were evident across acid-sensitive lake districts in the Eastern US. In particular, lakes in Maine, the Upper Midwest and Florida contained high concentrations (greater than 1000 gmol C · L^?1) of DOC. High concentrations of organic anions, estimated by discrepancy in charge balance in several subregions, suggest that dissociation of naturally occurring organic acids could significantly reduce the ANC of dilute surface waters. Unfortunately, analysis of acidification by organic solutes is complicated by uncertainty in H⁺ dissociation characteristics. Input of organic acids with weakly acidic pK _a values do not alter acid neutralizing capacity (ANC), while strongly acidic organic functional groups dissociate completely and decrease ANC. As a first step to assess the acid/base characteristics of naturally occurring organic solutes, we calibrated the Oliver et al. (1983) model, using a reduced version of the ELS data set. This model explained 94% of the observed variability in organic anion concentration in this data set. However, model parameters obtained from the ELS calibration were somewhat different than values provided by Oliver et al. (1983), based on potentiometric titrations of pre-concentrated organic acids. The discrepancy in model parameters has implications for estimating organic anion concentrations in water using the Oliver et al. (1983) model. Finally, data from the ELS indicates that across glaciated regions of the eastern US, concentrations of DOC and organic ions were negatively correlated with SO _inf2? ^p4 . This trend would appear to be consistent with the hypothesis that inputs of strong acids immobilize organic acids, resulting in a shift of surface water acidification by organic acids to strong acids.