Waterloo Centre for Groundwater Research and Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada. S. E. TRUMBORE l. Institut fiir ...
WATER RESOURCES
RESEARCH,
VOL. 26, NO. 12, PAGES 2949-2957, DECEMBER
1990
Dissolved Organic Carbon Cycling in Forested Watersheds' A Carbon Isotope Approach S. L. SCHIFF
AND R. ARAVENA
Waterloo Centre for Groundwater Research and Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada
S. E. TRUMBORE l Institut fiir Mittelenergiephysik, ETH, Zurich, Switzerland P. J. DILLON
Dorset Research Centre, Ontario Ministry of the Environment, Dorset, Ontario, Canada
Dissolvedorganiccarbon (DOC) is important in the acid-basechemistryof acid-sensitivefreshwater systems;in the complexation, mobility, persistence, and toxicity of metals and other pollutants; and
in lakecarbonmetabolism. Carbonisotopes (13Cand•4C)areusedto studytheorigin,transport, and fate of DOC in a softwater catchment in central Ontario. Precipitation, soil percolates, groundwaters, stream, beaver pond, and lake waters, and lake sedimentpore water were characterized chemically and isotopically. In addition to total DOC, isotopic measurementswere made on the humic and fulvic DOC fractions. The lake is a net sink for DOC. A •4C resultsindicate that the turnover time of most of the DOC in streams, lakes, and wetlands is fast, less than 40 years, and on the same time scale as changesin acidic deposition. DOC in groundwatersis composedof older carbon than surface waters, indicating extensive cycling of DOC in the upper soil zone or aquifer.
INTRODUCTION
1985; Dillon et al., 1987; Devito et al., 1990] have yielded insight into the importance and possible sources of DOC. However, studies of DOC concentrations yield no information on the turnover time of DOC produced in the various carbon reservoirs or if the turnover times are influenced by increases or decreases in the deposition of anthropogenic
The importance of dissolved organic carbon (DOC) in the acid-base ch'emistry of acid-sensitive freshwater systems and in the mobility, persistence, and toxicity of metals and other contaminants in natural waters is now widely recognized. Organic acid anions can contribute up to 20% of the acids. Lake water concentrations of DOC decrease as lakes total acid buffering capacity of lake waters of some acidbecome progressively acidified [e.g., Davis et al., 1985; sensitive lakes [Herczeg eta!., 1985]. Because of the low Dillon et al., 1987], but the reason is unclear. Predictive pK a of some of the organic acids (3.5-5.5; [e.g., Eshleman models of acid precipitation effects on aquatic systems are and Hemohd, 1985])theseacidscan alsohave an acidifying hampered by the lack of information on the sources and effect on natural water bodies [Kortelainen and Mannio, response of organic acids to changes in the deposition of 1987; Eshleman and Hemond, 1985; LaZerte and Dillon, anthropogenicacids [e.g., Herczeg et al., 1985]. 1984].Dissolved organiccarbonis a strongcomplexingagent Naturally occurring DOC is a complex composite of for many geochemicallyimportant and/or toxic metals such molecules and colloids exhibiting a continuous spectrum of as iron, copper, aluminum, zinc, and mercury. Organically sizesand chemical characteristics.To simplify the complexcomplexedaluminumhasbeenfound to be lesstoxic to aquatic ity, natural DOC has been further divided into various organismsthan inorganicforms of aluminum[e.g., Baker and fractions (humic and fulvic acids, hydrophobic neutrals and Schofield, 1982]. DOC can increase the weathering rate of hydrophilic acids, bases, and neutrals) based on chemical minerals[Drever, 1988]and increasethe solubilityand thus the reactivity [Thurman, 1984; Bourbonniere, 1989]. Changesin mobility and transport of many metals and organic contami- the distribution of the DOC fractions due to differences in nants of concern [Stumm and Morgan, 1981]. relative turnover times will affect the acid-base balance, the Despite the importanceof DOC in the acid-basechemistry complexing ability, and thus the inherent toxicity of these of freshwater systems and in aquatic toxicity studies, very waters to biological organisms. In an alternative scenario for little is known about the production and turnover of DOC acidification of freshwater systems, Krug and Frink [1983] within natural watersheds.Investigationsof the transportof suggest thatas sulfatedepositio n is decreased,the releaseof DOC in soils and DOC budgetsin watersheds [e.g., Wallis et humic acids from the watershed will increase, resulting in al., 1981; LaZerte and Dillon, 1984; Eshleman and Hemond, little change in the pH of the receiving water. This hypothesisimpliesthat acid precipitationaffectsthe turnover times I Now at Centerfor Accelerator MassSpectrometry, Lawrence of the fractions of DOC. Livermore National Laboratories, Livermore, California.
Our approach involvesthe useof 13Cand 14Cmeasure-
Copyright 1990 by the American Geophysical Union.
ments to examine the origins and turnover times of DOC in
Paper number 90WR01516.
various
0043-1397/90/90WR-01516505.00
lake. Until recently,•4C measurements of DOC were not 2949
carbon reservoirs
in the watershed
of a soft water
2950
SCHIFFET AL.' ORGANICCARBONCYCLINGIN WATERSHEDS
streams
Vadose
Zone
Precipitalion
o,c
Soil CO2
DOC •'
........ DISC" •-- -,•, t•or beaverpond wetland [Solids •,•,.D•-,•
soils
"• DOC •
So•DOC ) groundwater -r groundwater • •
streams
DIC
gas exchange epilirnnion
DIC DOC
be
groundwat"••'""'•• --,,•page Iso,dsl
sedimentation
.
'=•--'•"'"'
hypolirnnion
ø0%
DIC •
sediment flux
DIC DOC
•
[Solids [•
DOC
DIC
lake sediments
Fig.1. Carbon cycle ina watershed thatdoes notcontain carbonate minerals. DICisdissolved inorganic carbon andDOCisdissolved organic carbon. Theprocess DIC-• DOCisreferred toascarbon fixation andtheprocess DOC
-• DIC is referredto as mineralization or decomposition.
possibledue to the extremelylow Dec concentrations in
intensive investigation aspartof theOntarioMinistryof the groundwaters andnondystrophic lakes(typicallylessthan10 Environment's acidicprecipitation researchprogram[Dillon mg/L). Recent developmentsin tandem acceleratormass et al., 1980,1982,1987;Seipet al., 1985;Devitoet al., 1990; spectrometry(TAMS) now easilypermit the measurementof
Dillon and Molot, 1990]. The catchment has an area of 506
•nCactivityon samples containing 1 mgof carbon. This ha and is almostentirely coveredby mixed deciduous-
novel approach has now been used in a few studiesconcern-
ing Dec cycling in groundwater,rivers, and seawater [Williamset al., 1969;Thurman,1985;Hedgeset al., 1986; WilliamsandDruffel, 1987;Murphyet al., 1989a,b; Wassenaaret al., 1990]but not to studyDec cyclingin lake watersheds.
Our aim was to investigatethe sourcesandtransformation of Dec, assessthe importanceof Dec in lake carbon
coniferforestconsisting mainlyof Acer saccharum (sugar maple),Fagus grandifolia(beech),Betula alleghaniensis (yellowbirch),Pinusstobulas(whitepine), and Populus tremuloides (aspen).Theunderlying bedrockcomposition is predominantly biotite and horneblendegneiss(68% of the area),with amphibolite andschistin the westernportionof thebasin.Theoverburden consist of a minortill plain(> 1 m thick;50%of the watershed area)andthin till (
