Chemical character and origin of organic acids in streams and seepage lakes of central Maine

Organic acids and inorganic chemistry were examined in seventeen seepage lakes, seven streams, and one seep in central Maine. The objectives of this analysis were to determine the quantity and quality of dissolved organic carbon (DOC), and to assess the relationship between organic and inorganic surface water chemistry. Lakes and streams sampled were dilute (average conductivity of 20.3 μS cm^?1) with a wide range of DOC (125–2593 μmol C L^?1). Organic acids in DOC were evaluated by:

DOC fractionation (hydrophobic acids and neutrals, and hydrophilic acids, bases, and neutrals);

DOC isolation followed by FT-IR, base titration, and chemical analyses;

adsorption on solid phase extraction columns; and charge balance studies.

All lakes and streams were dominated by hydrophobic and hydrophilic acids (60 to 92% of DOC). Lakes and streams with low DOC had low hydrophobic to hydrophilic acid ratios (ca 1.2–1.3), regardless of pH and acid neutralizing capacities (ANC), compared to lakes and streams with moderate to high DOC concentrations (ca 1.9–2.4). Based on FT-IR spectroscopy and chemical analysis, organic acids were found to be dominated by a strong carboxylic character. Titration data of isolated DOC allowed accurate prediction of organic anions, which were strongly pH dependent (organic anions ranged from 14 to 198 μeq L^?1). Exchange acidity averaged 11.3, 13.6, and 8.7 μeq mg C^?1 for lake hydrophobic acids, lake hydrophilic acids, and stream hydrophobic acids, respectively. Overall evidence suggested that DOC and organic acid characteristics were related to their carboxylic functional group content and that the nature of these constituents was similar despite the source of origin (upland soils, wetlands, or Sphagnum deposits). Also, contact of soil leachates with B horizons seemed to be a controlling factor in DOC quantity and quality in the lakes and streams studied.