Marker species for identifying urban groundwater recharge sources: A review and case study in Nottingham, UK

Urban environments significantly alter the nature of recharge to underlying aquifers. Direct precipitation is reduced, but additional recharge may result from storm water runoff, mains supply leakage and sewer leakage. If urban aquifers are to be effectively and sustainably managed, it is vital that these recharge sources should be identified and quantified. A sound theoretical approach is the use of marker species for identifying the three principal sources of urban recharge (precipitation, mains and sewers). The ideal marker species should be unique to a particular recharge source (irrespective of geographic location), and easily identifiable in the groundwater system, enabling quantification of that source. A review of potential markers and a detailed study of the aquifer beneath the city of Nottingham, UK, was unable to find suitable markers for precipitation and mains leakage. Trihalomethanes, which are chlorination by-products, and so a potential marker of mains water, were hardly detected in either mains or groundwater. More potential markers are available for sewage, including d-limonene, which is a new ingredient in some detergents. For shallow groundwater, the most effective means of identifying sewage recharge was a combination of stable nitrogen isotopes and microbiological indicators; effectively a sewage `fingerprint'. This study confirms the need for a multi-component approach rather than using individual marker species. Additionally it demonstrates that the impact of sewer leakage on groundwater quality beneath Nottingham is generally not high.