Assessing the impact of historic coal mining on surface water quality

Client: Environment Agency                                                                                                                      Location: County Durham

Challenge

Skerne catchment showing the WFD-defined sub-catchments
Skerne catchment showing the WFD-defined sub-catchments which form the study area

The River Skerne in County Durham currently faces a range of issues in its upper and middle reaches which contribute to a Water Framework Directive (WFD) classification of Poor to Moderate. This is reflected in several ecological failures and water quality impacts. The project aimed to investigate the degree of connectivity between groundwater and surface water, to identify where this may result in gaining or losing flows in the River Skerne and how this may contribute to ecological and/or water quality impacts.

One aspect which the study sought to address was any potential link between poor surface water quality and historic coal mining activities. Following closure of the last collieries in the South Durham Coalfield groundwater levels have risen in the abandoned mine workings and the overlying Magnesian Limestone Aquifer. Upward flow of mine water from the abandoned mine workings has been thought to affect water quality in the River Skerne, as it is underlain by the Magnesian Limestone and there are reaches where groundwater from the aquifer discharges into the Skerne.

Solution

Conceptual understanding of Skerne river reaches
Conceptual understanding of Skerne river reaches

Our investigation commenced with a detailed review of a wide range of background information including previous hydrogeological reports, mine abandonment plans and water level and chemistry data. This data was used to develop a good understanding of the Skerne catchment and historic mining activities which had taken place in the South Durham Coalfield. More specifically, historic mining activities in the area to the south of the Butterknowle Fault, which acts to separate the local mining block from other areas of the coalfield where mine water levels are currently managed through pumping by the Coal Authority.

We developed a conceptual hydrogeological model to define distinct river reach types and the following tasks were subsequently undertaken:

  • Detailed review of high-resolution geological mapping data to identify the thickness of superficial deposits overlying the aquifer and determining depths to groundwater along the river.
  • Flow gauging across the catchment targeting areas of geological/hydrogeological changes where it was thought that groundwater was discharging into the river.
  • Development of flow accretion profiles and assessment of modelled flows in the Skerne using information obtained from LowFlows2 software.
  • Assessment of the impact of groundwater abstractions from the aquifer and discharge data for sewage treatment works within the Skerne catchment to determine whether artificial influences could be affecting the changing river flows.
  • Detailed review of geochemical testing data to identify a “mine water signature” within the river.

To complete the study, we drew on a range of specialist staff within JBA with the following key skills:

  • Groundwater and geochemistry.
  • Hydrology and hydrometry including flow gauging and water quality sampling.
  • Mapping and Geographic Information Systems.
Factors controlling river flow and water quality
Factors controlling river flow and water quality

Benefit

Final reach conceptualisation showing licensed discharges
Final reach conceptualisation showing licensed discharges

Our study has extended the understanding of surface water quality in the River Skerne and identified key controls on river flow and water quality through the catchment as follows:

  • Five key controls on river flow and water quality in the catchment have been identified, with six reaches identified where there is a distinctive surface water and groundwater interaction signature.
  • Flow gauging was able to confirm that where the Magnesian Limestone aquifer is close to the ground surface then there is the greatest degree of interaction with underlying groundwater.
  • Our study has identified strong evidence for discharge of poor quality mine water along the river network, when groundwater levels in the Magnesian Limestone Aquifer are highest indicating upwards movement of mine water from deeper historic mine workings.

Our work was also able to support the on-going assessment of the catchment dynamics of the River Skerne by the Environment Agency and identify potential future areas of investigation. We were also able to support an on-going study carried out by the British Geological Survey, by identifying key areas along the river for monitoring water quality parameters in the zone just below the river bed in which it is believed that a range of geo-chemical processes may buffer the effects of mine water discharge. The findings of our study have been disseminated at several regional external Environment Agency conferences and presentation events.

Want to know more?

Email Dr Michael McDonald for more information about this project and our services. You can also read more about mine water treatment projects on our webpage.



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