Influence of organic acids on the pH and acid‐neutralizing capacity of Adirondack Lakes

Abstract

Past approaches for evaluating the effects of organic acids on the acid‐base characteristics of surface waters have typically treated them solely as weak acids. Analysis of data collected by the Adirondack Lakes Survey Corporation (ALSC) from 1469 lakes throughout the Adirondack region shows that this approach is not valid. While the data indicate that natural organics contain a continuum of acid functional groups, many of which display weak acid characteristics, a significant fraction of the organic acid is strong (pKa < 3). Dissolved organic carbon (DOC) contributes 4.5–5 μeq/mg DOC of strong acid to solution. The associated anions make a negative contribution to Gran acid‐neutralizing capacity (ANC). Because organic anions can produce negative Gran ANC values, the common practice of considering negative values of Gran ANC evidence of acidification solely by mineral acids is not valid. The strength of organic acids also influences the observed deviation between Gran ANC values and ANC values calculated as the difference between base cation and mineral acid anion concentrations (CB – CA). Ninety percent of the deviation is due to the presence of strong organics while the remaining 10% is due to DOC‐induced curvature in the F1 Gran function. Organic acids can also strongly influence pH. Their largest effects were found in the 0–50 μeq/L Gran ANC range where they depressed pH by up to 1.5 units. In addition, a method for predicting changes in pH in response to changes in mineral acidity, DOC, or both without having to rely on inferred thermodynamic constants and the uncertainties associated with them has been developed. Using the predictive method, the response of representative lakes from four sensitive lake classes to a 15‐μeq/L decrease in mineral acidity ranged from +0.17 to +0.38 pH units. If concurrent increases in DOC are considered, the pH changes would be even smaller.