Using metal isotopic compositions to track near surface hydrologic cycling of anthropogenic inputs in North East England

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.

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Image Captions

North East England River


Field Locations:
1-The engineered waste water network of Northumbrian Water up to a waste water treatment plant
2-The Wear catchment: the Wear is a 60 mile river, with the headwaters located in the North Pennines, impacted by mining legacy.
3-A pristine, unimpacted catchment to be used as comparison site.

Repeated longitudinal sampling of the different networks, including tributaries.
Chemical separation and measurement via multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) of Zn and potentially other metals of interest.

Additional analysis of routine data from the Environmental Agency and Northumbrian Water (concentration data only, no isotopes).

Statistical data analysis (end-member mixing analysis, clustering, PCA, etc.) to identify sources and their varying contributions over time.

Project Timeline

Year 1

Review of literature, compilation of existing data supplied by Northumbrian. Training in isotope analysis. Field sampling in the Wear catchment.

Year 2

Sample processing and data analysis. Field sampling in the Wear catchment and at the comparison site.

Year 3

Synthesis of data. Conference attendance. Writing of thesis and publications.

Year 3.5

Finalize thesis and publications for submission.

& Skills

This project would suit a student with a degree in Earth or Environmental Sciences (or a related field) and strong interests in environmental, trace metal and isotope geochemistry and hydrology.

Excellent time management skills coupled with strong numerical, verbal and written communication are important. Previous analytical experience would be an advantage but is not essential. Training will cover field sampling and cutting-edge geochemical methods.

The student will join the vibrant Durham Isotope Group, which includes research students and postdocs from the Earth Science, Geography and Archaeology departments. S/he will attend national and international conferences, networking events and outreach activities, developing an important network for feedback and future employment.

The project will be carried out in close collaboration with Northumbrian Water, with the opportunity to spend several months at the company’s Head Office in Durham, and appropriate sewage treatment works discharging into the river catchment, such as the one at Washington. Northumbrian Water is committed to improving the quality of the region’s rivers and strongly supports fundamental research seeking to understand issues in need of address. As such the researcher will be able to work alongside the company’s operational and technical teams and build their understanding of a leading water company.

References & further reading

Chen, J., Gaillardet, J., Louvat, P. and Huon, S., 2009. Zn isotopes in the suspended load of the Seine River, France: Isotopic variations and source determination. Geochimica et Cosmochimica Acta, 73(14), pp.4060-4076.

Sivry, Y., Riotte, J., Sonke, J.E., Audry, S., Schäfer, J., Viers, J., Blanc, G., Freydier, R. and Dupré, B., 2008. Zn isotopes as tracers of anthropogenic pollution from Zn-ore smelters The Riou Mort–Lot River system. Chemical Geology, 255(3-4), pp.295-304.

Vance, D., Matthews, A., Keech, A., Archer, C., Hudson, G., Pett-Ridge, J., Chadwick, O.A. 2016. The behavior of Cu and Zn isotopes during soil development: controls on the dissolved load of rivers. Chemical Geology, 445, 36-53.

Zeng, J. and Han, G., 2020. Tracing zinc sources with Zn isotope of fluvial suspended particulate matter in Zhujiang River, southwest China. Ecological Indicators, 118, p.106723.

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