Karst groundwater exhibits piston-like response to heavy rainfall during winter Storm Franklin in 2022 (Derbyshire, UK)

The hydrology of karst regions which have experienced mining is complex, and the impact of drainage from mines and mine waste can have a strong negative impact on water quality within springs and rivers. This can be diffuse and affect water quality under base flow conditions, but acute events can also occur when highly mineralised groundwater is displaced by high rainfall events. This study reports changes in the chemistry of a karstic conduit during an extreme rainfall event in 2022. The site, Goodluck Mine in Derbyshire (UK), has a long mining history and abundant mine waste underground and on the surface. Water which has equilibrated with this waste could be displaced into the Derwent–Trent river system, and affect water quality downstream. We report a strong displacement of water during a named storm in February 2022, which created a pronounced piston effect where older groundwater was displaced by infiltrating rainwater. The older water has a higher temperature and conductivity than the baseflow water, and so originated from a perched water body in the unsaturated zone. However, whereas acid mine drainage would result in this water having lower pH than the baseflow water, we find that similar events later in the year had higher than baseline pH. Consequently, we conclude that the water displaced had mineralized in contact with country rock, not with mine waste deposits. The approach used here can straightforwardly be transferred to other mined karst regions to identify local histories of piston-flow water.