CARBON ISOTOPES AS TRACERS OF GROUNDWATER AND SOIL WATER CONTRIBUTIONS TO STREAMFLOW IN A FORESTED HEADWATER CATCHMENT

Estimating soil water contributions to streamflow is of primary concern to hydrologists wishing to improve rainfall-runoff models, and quantification of carbon export from watersheds is of importance for understanding of global carbon fluxes. To aid in these efforts, we are measuring dissolved inorganic and organic carbon (DIC and DOC) concentrations and their bulk d¹³C values as tracers of soil water contributions to streams during runoff events. We present high-resolution time series measurements of DIC and DOC and their d¹³C values from samples of stream water, groundwater, and soil water collected at Sleepers River Research Watershed in Vermont, USA. 

DOC concentrations in groundwater are low (<2 ppm C), while soil water concentrations are higher (up to 8 ppm C). Groundwater DOC show distinct d¹³C values that are up to 6‰ more positive than those of soil water DOC. Stream DOC concentrations consistently peak at the highest stream flows, with d¹³C values 2-3‰ lower than those during low flow, indicating a change in primary source from groundwater to soil water during runoff events.

DIC concentrations in groundwater are high (>10 ppm C) due to weathering of carbonate minerals in the watershed. In contrast, soils are carbonate-depleted and DIC concentrations in soil water are more than twice as low as those of groundwater (<5 ppm C). d¹³C of DIC values in groundwater show variable contributions from soil CO₂ and dissolution of carbonate minerals, while soil water d¹³C of DIC values reflect soil CO₂ alone. DIC concentrations in the stream drop during peak flow, as does d¹³C of DIC, again indicating greater soil water contributions to the stream at peak flow. However, d¹³C of DIC is affected by other processes occurring in the stream in addition to mixing between different source waters. For example, rapid outgassing of CO₂ at the stream headwaters may cause the d¹³C of DIC to become higher than that predicted by a conservative mixing model, particularly during the growing season when soil PCO₂ is highest. We present theoretical model calculations illustrating how CO₂ outgassing may influence the d¹³C of stream water DIC. Field data will be discussed in the context of the model predictions as well as other possible interpretations of our observations with regard to the hydrological processes occurring in this watershed.