Tracing the decomposition of dissolved organic carbon in artificial groundwater recharge using carbon isotope ratios |
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| Affiliation: | 1. Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland;2. Department of Geology, P.O. Box 64, FIN-00014, University of Helsinki, Finland;1. Civil Engineering, The Catholic University of America, Washington, DC 20064, USA;2. Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA;3. Deltares, Unit Soil and Groundwater Systems, Utrecht, The Netherlands;4. TNO, Geological Survey of the Netherlands, Utrecht, The Netherlands;5. VU University, Critical Zone Hydrology Group, Amsterdam, The Netherlands;1. University of Oulu, Faculty of Technology, Water Resources and Environmental Engineering Research Group, P.O. Box 4300, FI-90014 University of Oulu, Finland;2. University of Oulu, Department of Ecology, P.O. Box 3000, FI-90014 University of Oulu, Finland;3. Finnish Environment Institute, Natural Environment Centre, P.O. Box 413, FI-90014 University of Oulu, Finland |
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| Abstract: | ![]()
Reducing the concentration of dissolved organic C (DOC) in water is one of the main challenges in the process of artificial groundwater recharge. At the Tuusula waterworks in southern Finland, surface water is artificially recharged into an esker by pond infiltration and an equal amount of groundwater is daily pumped from the aquifer. This groundwater study was conducted to consider the role of redox processes in the decomposition of DOC. The isotopic composition of dissolved inorganic C (δ13CDIC) in the recharged water was used as a tracer for redox reactions. The isotopic composition of O and H in water was determined in order to calculate mixing ratios between the local groundwater and the infiltrated surface water. Three distinct processes in the reduction of the DOC content were traced using isotopic methods and concentration analyses of DIC and DOC: (1) the decomposition of DOC, (2) adsorption of DOC on mineral matter, and (3) the dilution of artificially recharged water by mixing with local groundwater. The largest decrease (44%) in the DOC content occurred during the early stage of subsurface flow, within 350 m of the infiltration ponds. The reduction of DOC was accompanied by an equal increase in DIC and a significant drop in δ13CDIC. This change is attributed to the oxidative decomposition of DOC. A further 23% decrease in DOC is attributed to adsorption and a final drop of 14% to dilution with local groundwater. |
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