TRACING SOURCES OF CARBON IN URBAN GROUNDWATER USING δ13CTDIC RATIOS

Abstract

Total dissolved inorganic carbon (TDIC) and its stable isotope ratio δ13CTDIC are used to trace the evolution of the carbon system of groundwater in three UK Permo-Triassic sandstone aquifers. Samples were collected from multilevel piezometers, open boreholes and sewer sampling points in the British Midlands (Nottingham, Birmingham and Doncaster) to evaluate both local and regional variations in δ13CTDIC. δ13C samples of matrix and pore water have also been analysed in each aquifer to further constrain the interpretations. Combining δ13CTDIC ratios with measurements of TDIC and pH clearly distinguishes the principal processes underlying the geochemical evolution of groundwater in Triassic sandstone aquifers, where processes can be both natural (e.g. carbonate dissolution) and anthropogenic (sewer-derived recharge). The paper shows that δ13CTDIC resolves ambiguities that arise from the interpretation of TDIC and pH measurements in isolation. Field measurements demonstrate that, under natural conditions, the carbonate system evolves similarly in each aquifer. An open-system evolution during recharge largely saturates the groundwater with carbonate depending upon its availability in the sandstone matrix. The contribution of sewer exfiltration to urban recharge is readily distinguished by lower pH and higher TDIC values without significant changes in δ13CTDIC.