Assessing potential saline intrusion in a coastal aquifer

Client: Environment Agency                                                                            Location: County Durham

Challenge

We were asked by the Environment Agency to work with a manufacturing organisation in the Hartlepool area who rely on groundwater, to establish whether their abstraction boreholes could be causing saline intrusion into the Magnesian Limestone aquifer. The aquifer is classified as part of the Water Framework Directive (WFD) as being at ‘poor status’, due in part to saline intrusion.

Groundwater Abstraction in Hartlepool: Assessing the Potential for Saline Intrusion
Baseline understanding of the freshwater-saline interface (assuming no abstraction from the aquifer)

The coastal location of the aquifer combined with a history of pumping through dewatering of mine water within the deeper Coal Measures, and historically high levels of public supply abstraction in the area, make it vulnerable to saline intrusion. This can lead to long changes in groundwater chemistry and deterioration in water quality. Historical dewatering from the deeper Coal Measures during mining activities in the South Durham Coalfield to the west may have reversed the natural groundwater gradient. As mining ceased and water levels began to recover, public water supplies commenced, making it difficult to separate the signal of effects on any water level or chemistry changes.

This project aimed to determine the degree to which historical activities within the aquifer are responsible for the ‘poor groundwater status’, and whether current groundwater abstraction for manufacturing purposes in the Hartlepool area could be causing saline intrusion.

Solution

Groundwater levels at Red Lion and Hart Reservoir
Groundwater levels at Red Lion and Hart Reservoir

To develop a good understanding of the historical and current pressures facing the aquifer we reviewed a range of information which included: historical water level data to determine how groundwater movement in the aquifer has changed over time; groundwater chemistry data provided by the Environment Agency to determine spatial and temporal trends; and details of operational processes at the manufacturing site to understand typical trends in groundwater usage and enable design of borehole pumping tests.

Constant operational usage of the abstraction boreholes and site infrastructure posed a challenge to carrying out an industry standard pumping test. However, through working closely with the manufacturing facility team a pumping test was designed utilising their boreholes for monitoring the draw-down and subsequent recovery of water levels, and avoiding disruption to day to day activities on the site. Pumping tests were undertaken at boreholes on separate manufacturing sites to determine aquifer properties and monitor groundwater chemistry through electrical conductivity profiling to assess salinity.

Benefit

Hydrogeological conceptual model of the study area during the dewatering of the Coal Measures

Analysis of pumping test data allowed us to estimate the zone of contribution of the abstraction boreholes which suggests that saline intrusion from the coast may be possible. However, electrical conductivity profiling during the pumping test did not provide any evidence of saline water as a result of pumping.

Overall, our analysis showed that there is no clear evidence that abstraction from the boreholes is currently causing saline intrusion. For the manufacturer, who relies on the borehole supplies as a source of water, there is no requirement on them to make changes to their current abstraction licence. We were also able to help improve the current efficiency of the borehole operations, giving them greater confidence in managing potential future impacts to the aquifer.

Our study also provided key baseline information on which the Environment Agency as regulator can expand their studies into wider aquifer water quality issues, in order to address WFD failures for this important water body.

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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