The dynamic balance between organic acids and circumneutral groundwater in a large boreal peat basin D.I. Siegel a, * , P.H. Glaser b , J. So c , D.R. Janecky d a Department Earth Sciences, Syracuse University, Syracuse, NY 13244, USA b Department of Geology and Geophysics, University of MN, Minneapolis, MN 55455, USA c Drycleaner Environmental Response Trust Fund of IL, Bensenville, IL 60106, USA d Los Alamos National Laboratory, Los Alamos, NM 87545, USA Received 10 May 2005; revised 23 May 2005 Abstract Large raised bogs and patterned fens cover 56% of the landscape in the Glacial Lake Agassiz region of northern Minnesota (USA). Organic acids supply most of the acidity in the surface water of the bogs, but groundwater upwelling from the underlying glacial deposits neutralizes these organic acids within deep peat. Substantial concentrations of organic acids also occur in the surface waters of fens mixed with variable amounts of inorganic solutes contributed by groundwater discharge. We used a triprotic analog model to determine the extent to which organic acids in fen and bog waters behave as strong or weak acids. The modeling approach optimized charge balance by calibrating estimates of mole site density in the DOC (dissolved organic carbon) of surface and pore waters with estimates of triprotic acid dissociation constants. Before the calibration process, all of the bog waters and 76% of the fen waters had more than C20% imbalance in charge balance. After calibration, more than 75% of all waters were electrochemically balanced within 20%. In the best calibration, the mole site denisty of bog DOC was estimated as w0.05 mmol/mmol C., approximately six times smaller than that estimated for fen DOC or the DOC in the fen deeper fen peats that underlie all bog landforms. The three modeled de-protonation constants were; pK a1 Zw3.0, pK a2 Zw4.5 and pK a3 Zw7.0 for the bog DOC, and; pK a1 Zw5.2, pK a2 Zw 6.5 and pK a3 Zw7.0 for the fen DOC. Bog DOC, behaves as a strong acid despite its small mole site density. The DOC in bog runoff can therefore theoretically acidify the surface waters in adjacent fens wherever these waters do not receive sufficient buffering alkalinity from active groundwater seepage. q 2005 Elsevier B.V. All rights reserved. Keywords: Peatland; Organic acids; Acids; Bogs; Fens 1. Introduction Thick peat deposits accumulate wherever the local hydrogeologic setting maintains a waterlogged and anoxic soil profile. As these organic deposits thicken and spread their dynamics become more closely linked to the flow of water from external sources. The acid–base equilibria in peatlands, for example are strongly influenced by the influx of water from 3 different sources: (1) atmospheric precipitation, (2) surface runoff, and (3) groundwater flow. Journal of Hydrology 320 (2006) 421–431 www.elsevier.com/locate/jhydrol 0022-1694/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.07.046 * Corresponding author. Fax: 001 315 443 3363. E-mail address: disiegel@mailbox.syr.edu (D.I. Siegel).