Impact of an extreme rainfall event on solute and sediment dynamics in a mineralised river system

Lead Research Organisation: Newcastle University
Department Name: Civil Engineering and Geosciences

Abstract

Over the weekend of 5 - 6 December 2015 the most intense rainfall ever recorded in the UK fell over parts of Cumbria, peaking at 341.4 mm over a 24 hour period at Honister Pass, resulting in widespread flooding. It is now widely acknowledged (e.g. statement to the media in December 2015 by Rory Stewart MP) that such 'flash flooding from intense rainfall' (FFIR) events are likely to become more frequent in the UK due to the effects of climate change. Prior to the floods of 5-6 December 2015, Newcastle University researchers spent several years measuring the effects of varying flow conditions on the amount of polluting metal and sediment discharged from the Coledale Beck, some 10 km due north of Honister Pass. These metals include zinc which is toxic to fish along with cadmium and lead that are toxic to most flora and fauna. At the head of the Coledale Beck lies the abandoned Force Crag metal mine (a rain gauge at the mine showed that 223.8 mm of rain fell in a 24 hour period during the weekend of 5-6 December 2015). A novel treatment system, completed in April 2014, treats some of the polluted water that emerges from the mine. Nevertheless, although the treatment system works efficiently, we know that some pollution of the Coledale Beck persists, mainly as a result of metals discharged to the river from other locations around the mine site. These sources arise primarily from sediments in the abandoned spoil heaps and in the river itself. The flash flooding of 5-6 December resulted in wholesale changes to the geomorphology of the Coledale Beck. Large volumes of sediment were carried down the river, whilst new sediments were exposed due to landslides and erosion. Because we already hold baseline data relating to the export of metals and sediments under different hydrological conditions from the Coledale Beck, we are uniquely positioned to now investigate how such extreme rainfall events change the whole-system dynamics of metal and sediment cycling in such upland river catchments. In particular, due to the geomorphic impacts of the December 2015 floods on the Coledale Beck system it will be possible to evaluate how exposure of new sediments influences the behaviour of metals. This is important because the form of metals in rivers (their 'partitioning') determines the readiness with which they may be carried downstream, and also influences their toxicity to the ecology of such rivers. These issues in turn have implications for users of water further downstream (e.g. fisheries, utility companies). Because hydrological extremes are likely to increase in the future as a result of climate change, we need to understand how these events change the physical and chemical characteristics of rivers, so that it is possible to plan adaptive strategies to better deal with such events. The research will be undertaken by collecting samples of waters and sediment along the length of the Coledale Beck on 10 separate occasions, thereby ensuring that samples are collected across a range of flow conditions. The results will be compared with baseline data previously collected, to determine what changes in patterns of metal and sediment movement are caused by extreme events. These results will be complemented by laboratory tests and geochemical modelling to help us assess the likely toxicity of the sediments and metals in the river to aquatic life. The intended outcome of this research will allow transfer of the results of the work to other river systems. There are 450 such rivers in England and Wales negatively impacted by abandoned mine pollution. The research is urgent because rivers are dynamic systems with key changes being driven by high magnitude events. However to properly understand the effects of flash flooding events such as that of 5-6 December 2015, it is critical to undertake sampling and analysis as soon as possible after the event itself.

Planned Impact

The research team has an exemplary track record of undertaking high impact research, previously catalysing government investment in addressing pollution from abandoned mines and shaping the Environment Agency's approach to monitoring in abandoned mine catchments in England and Wales. The proposed research will have not only academic impacts, but also wider benefits to society and the economy. Defra and the Environment Agency will be the most immediate beneficiaries of the research, as the outcomes of the project will have a direct bearing on policy and practice in the management of freshwater pollution in the face of the increasing frequency of hydrological extremes associated with wider climate change. Specifically, the Environment Agency - a Project Partner for this research - will benefit from better explanation and understanding of the changes to metal and sediment dynamics that result from flash flooding events. In turn this will inform both more effective ways to monitor such occurrences in the future, and also strategies for heightening the resilience of downstream infrastructure, directly benefiting Government Departments tasked with managing floods and their impacts. The research team will ensure these benefits occur by engaging closely with Defra and the Environment Agency from the inception of the project. If deemed appropriate by these agencies, the research team will prepare non-technical summaries of the research for publication by and / or circulation within Defra, the Environment Agency and other stakeholder organisations. Project outcomes will also be available on the websites of Newcastle and Hull Universities. Similar strategies for ensuring impact will be adopted with the National Trust and Coal Authority (both formal Project Partners), and any other organisations with which they work (e.g. National Parks Authority, engineering consultants). Beyond traditional conference and symposia outputs, international academics and stakeholders outside academia will benefit from the work by results being shared directly with international associations such as the International Mine Water Association, and our existing network of NGOs (e.g. Rivers Trusts and CAFOD, the Catholic Agency for Overseas Development, with which we work closely in Peru).

Publications

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Description The overall objective of the research was to understand the effects of a major storm event (Storm Desmond, December 2015) on the distribution of metals and sediments in a river system (the Coledale Beck, Cumbria) that is polluted with metals due to an abandoned mine near the top of the river system. A secondary objective was to understand whether this extreme rainfall event had any adverse impact on a water treatment system designed to remove metals from the polluted water from the mine. Key findings are that:
1. The introduction of fresh sediments to the river (due to landslips and other erosion) results in more rapid attenuation of metals in the river system during lower flow conditions, which we believe is due to increased potential for adsorption of the metals to these fine sediments on the river bed (zinc in particular). Even 12 months after the storm event there is evidence that this is occurring.
2. During higher flow conditions though these fresh sediments, and the metals associated with them, may be mobilised, and therefore the metal flux (mass per unit time) down the river in such conditions has the potential to be greater than before the storm event.
3. Storm Desmond did not appear to have any adverse effect on the mine water treatment system at the top of the river catchment, which we believe may relate to the large volume of water held within the treatment system even under normal operating conditions (and therefore the relatively minor contribution of even this extreme rainfall event).
Exploitation Route This was a short duration research project (< 12 months) that has only just finished, and we are therefore still analysing the results. However, we believe that the results can be taken forward to assist in understanding how flash flooding events affect water quality in rivers, and therefore how best to deal with changes in water quality, and metal and sediment flux, to prevent problems for downstream water users. The water treatment system on the Coledale Beck was not adversely affected by the storm event, and we believe that lessons may be learned from this in terms of developing flood resilient infrastructure.
Sectors Environment

 
Description The research has generated additional interest in diffuse pollution in abandoned mine catchments, and in particular the potential consequences of hydrological extremes on metal export from such catchments. The results of the work have been discussed at two conferences in Europe (2016 and 2017), and have been a catalyst behind the funding of further research in this area by the UK Government (research due to commence in April 2018). The research has provided insights into the hydrological and geochemical changes in mining-impacted river systems resulting from storm events, and has demonstrated the resilience of some types of mine water treatment systems to flash flooding from intense rainfall events. This is of particular importance given ongoing investment in construction of mine water treatment systems in the UK.
First Year Of Impact 2016
Sector Environment
Impact Types Economic,Policy & public services

 
Description Legacy wastes in the coastal zone: Environmental risks and management futures
Amount £695,414 (GBP)
Funding ID NE/T003286/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 01/2020 
End 01/2024
 
Description Project Partner meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact A presentation of results of the investigation to date with individuals from the Environment Agency, Coal Authority and National Trust, together with a representative from a local mining enthusiasts group.
Year(s) Of Engagement Activity 2016
 
Description Site visit with Cumbrian Geological Society 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Approximately 20 members of the Cumbrian Geological Society visited the Force Crag mine with the National Trust, and the tour included a talk and questions about the impacts of flooding on the water quality of the local river.
Year(s) Of Engagement Activity 2017