WATERRESOURCES RESEARCH, VOL. 30,NO. 2, PAGES 297-306, FEBRUARY 1994 .. Modeling the acid-base chemistry of organic solutes in Adirondack, New York, lakes Charles T. Driscoll and Michael D. Lehtinen Department of Civil and Environmental Engineering, Syracuse University, Syracuse, New York Timothy J. Sullivan E&S Environmental Chemistry, Incorporated, Corvallis, Oi'½gon Abstract. Data from the large and diverse AdirondackLake Survey were used to calibrate four simpleorganicacid analogmodels in an effort to quantifythe influence of naturally occurring organic acids on lake waterp H and acid-neutralizing capacity (ANC). The organic acid analog models were calibrated toobservations of p H, dissolved organi. c carboil(DOC), and organic anion(An-) concentrations from a reduced data set represe.nting 1128individu.al lake samples, expressed as 41 obs'6rvations of mean p H, in intervals of 0.1 p H units from p H 3.9 to 7.0. Of the four organic analog approaches examined, including the Oliver et al. (1983) model, aswell as monopi'otic, diprot•ic, ahd triprotic representations, the triprotic analog model yielded thebest fit (r • = 0.92) to the obseryed data.Moreover, the triprotic model was qualitatively consistent with observed patterns of change in organic solute charge density as a function ofpH. A lowcalibrated value forthe first H + dissociation . constant (pKal -- 2.62) andthe observation that organic anion concentrations were significant evenat very low pH (<4) indicate that naturally occurring organic solutes in these waters have strongly acidicfunctional groups. Inclusion of organic acidityin model calculations resulted in goodagreement between measured and predicted values oflake waterpH and ANC. Assessments to project the response of surface waters to future changes in atmospheric deposition, through theuse of acidification models, will need to include representations of organic acids in model structure to make accurate predictions of p H and ANC. Introduction Theinfluence of dissolved organicsolutes on the p H and acid-neutralizing capacity (ANC) of soft waters is an areaof considerable uncertainty that has received much attention in thescientific literature in recent years. Dissolvedorgahic carbon (DOC) is commonlymeasured as a surrogate for organic acidity,but it includes a variety of organic constit- uents that differ in size, composition, and function [•hur- man, 1985]. It is difficult to characterize the heterogeneous miitureof naturally occurring organic solutes found in freshwater, which contain diversefunctional groups with a range of acid-base cha. racteristics. These functional groups include carboxylic acids, phenols, thiols, andalcohols [Per- due,1985; Thurman, 1985]. Moreover, weak acidic func- tional groups areoften associated withmetallic cations (e.g., A1, Fe), which may dissociate or Undergo hydrolysis. The dissociation of H + from organic matter to water iscontrolled by the number andtypes of functional groups present, as well as ambient waterchemistry. Until recently, organic acids werenot considered impor- tantin regulating the acidity of surfacewaters in areas impacted by atmospheric deposition of strong acids [e.g., Galloway et at., 1983]. Hemond [1980] and Gorham et al. [1985] demonstrated the dominant roleof organic acidity in Copyright 1994 bythe American Geophysical Union. Paper number 93WR02888. 0043-1397/94/93 WR-02888505.00 peatlands, but it was widely assumed th atorganic. solutes played a minor rolein theacid-base status of most freshwa- ters. Although Rosenqvist [1978] and Krug andFrink [1983] put forth hypotheses to thecontrary, theacid-base status of lakes and streams Was generally studied within thefi'ame- work of an ifiorganic carbon pH-buffering system. More recentstudies, however,have clearly demonstrated the importance oforgariic &cids in modifying both the acidity of surface waters and the respons• [o changes in strong inorganic acid inputs [Kramer and Davies, 1988; Lainet al., 1989; Kahl et al., 1989; David andVance, 1991; Wilkinson et al., 1992].Synoptic surveys of lakes in the Adirondack Mountains, New York [Kreiser et al., 1989; Bakeret al., 1990], and Finland [Kortelain. enet al., !989], for example, have illustrated that organic acids may be important in regulating lake water pH. Baker etal. [1990] concluded that organic acids depress the pH ofAdirondack lakes by 0.5-2.5 pH units in the ANCrange of•50 •eq/L and that organic acids could explain over 90% of the H + in 41% ofthe1469 lakes studied. Kortelainen et al. [1989] found the organic acid anion (An-) to be the major anion in small lakes in southern Finland, which hada median total organic carbon concentration of 1000/a. molC/L. The exclusion of incoming acidic deposition atthe Risdalsheia site in southern Norway, aspart of theReversing Acidification in Norway (RAIN) Project, resulted inan increase inrunoff ANC,calculated as the sum ofbasic cations (CB: Ca 2++ Mg 2++ Na+ + K+ + NH•) minus the sum ofstrong inorganic acid anions (CA' 297