North East England has a long heritage of historic coal and metal mining that dominates the landscape and impacts river catchments in the region. The weathering and remobilisation of legacy mining deposits can thus pose a significant risk to water quality. Abandoned mines are point sources of contaminants, whereas buried spoils can act as diffuse sources. Current anthropogenic inputs to the river system also include industrial inputs that typically enter the river system through waste water effluent from sewage treatment plants. However, the ultimate source of metal release into rivers will be a combination of natural and anthropogenic inputs. It is also likely that input sources vary seasonally in addition to short term dependence of metal mobilisation on local meteorological conditions (rain intensity, antecedent conditions, etc.). The mobilisation, transport and concentration of metals from different sources is therefore closely coupled with hydrology both in natural river catchments and engineered waste water networks.
In order to develop mitigation strategies, it is key to understand (1) which metals and oxidation states pose health hazards, (2) whether metal concentrations in rivers and the near surface hydrological system exceed safe levels, and if so, (3) the origin of problematic metal release and their changing contributions over time.
The Water Framework Directive specifies contaminant limits on a European level with the aim of achieving good water quality status of lakes and rivers by identifying and monitoring established and emerging substances of concern for water quality. It provides guidelines assessing the potential health hazards of substances including metals. In the UK, companies responsible for water distribution, quality and management need to adhere to guidelines set out by the Water Framework Directive. For example, Northumbrian Water carries out monitoring of elemental abundances in many natural river catchments, in the effluent of their waste water treatment plants, and throughout their engineered waste water networks, with the data available to the public.
Understanding the sources of metal inputs is key to mitigation strategies that ensure good water quality. Arguably the most challenging aspect of this task is disentangling the types of natural and anthropogenic inputs. To address this issue, isotopic compositions are increasingly used as a powerful source tracer tool. For example, one identified element of concern is zinc (Zn), with the Water Framework Directive recommending that Zn levels do not exceed 10.9 µg/L of bioavailable Zn above the ambient background. Unfortunately, many rivers exceed this value in the UK (e.g., Zn concentrations in the river Wear at Chester-le-Street ranged from 10-50 µg/L over the last three years).
Recent work has successfully applied the isotopic composition of Zn to disentangle natural and anthropogenic Zn sources in rivers. It is also becoming clear that different types of anthropogenic inputs (e.g., coal, smelters) can have distinct Zn isotopic signatures (e.g., Zen & Han, 2020; Chen et al., 2009). The combination of metal isotopes and other tracers can further constrain distinct end members and thus fingerprint sources. For example, combination of Zn and Cu isotopes allows redox and assessment of natural fractionation processes to be better constrained (e.g., Vance et al. 2016).
In collaboration with Northumbrian Water, this project aims to
(1) Evaluate and compare Zn elemental cycling in the near surface and river system of (i) the Wear catchment and (ii) within the wastewater network of the Wear, in response to variable hydrologic parameters.
(2) Use the isotopic composition of Zn and potentially complementary elements (e.g., Fe, Cu, Pb) to disentangle natural and anthropogenic inputs to both the natural river system and the engineered waste water system.
(3) Compare the impacted Wear catchment with a more pristine system to further constrain isotopic fractionation due to natural biogeochemical cycles.
Ultimately, the ability to understand and fingerprint metal release and cycling through the near surface hydrologic system underpins mitigation strategies to ensure good water quality.