Monitoring Mobile Metals: A Pb-Zn Isotopic Approach to Trace Metal Pollutant Sources in Rivers

Dissolved heavy metals in river systems reflect contributions from a range of natural and anthropogenic inputs. These inputs can vary spatially and temporally. It is well documented that dissolved heavy metal concentrations show pronounced changes in response to discharge in both polluted and pristine catchments.

Environmental legislation (such as the Water Framework Directive) sets recommended limits on heavy metal concentrations in freshwater systems, which many rivers exceed. To effectively remediate pollutant sources, it is vital to identify and quantify the specific contributions from different endmembers. Concentration data alone are insufficient to apportionment sources whilst accounting for hydrological controls and within-catchment processes. To overcome this shortcoming, we combine concentration measurements with metal isotope signatures, which provide a powerful tool for distinguishing and quantifying individual source contributions under varying flow conditions.

We utilise a novel multi-tracer approach combining trace element, major cation and dissolved organic carbon (DOC) concentrations, as well as stable Zn and radiogenic Pb isotopes to identify and differentiate sources of dissolved heavy metal pollutants. We apply this approach in the River Wear catchment - a historic Pb-Zn mining region in northeast England, UK. In this catchment, legacy mine wastes dominate dissolved metal loads in the headwaters, while downstream reaches show increasing influence from agricultural activities and urban sources (e.g. wastewater effluent and road runoff).

We present data from catchment-wide transect sampling completed under three contrasting hydrological conditions: high, medium, and low flow. We show a resolvable and significant change in Zn isotope composition under these different flow conditions. Combining Zn and Pb isotope measurements with supporting chemical tracers provides enhanced resolution for distinguishing between legacy mining contributions, diffuse agricultural inputs, and urban sources, as well as for identifying key catchment processes such as mixing, dilution, and hydrologically mediated mobilisation. This integrated framework offers a powerful tool for source apportionment and can assist the development of targeted remediation strategies in historically contaminated river systems.