Taken from the Environment Agency’s “Challenges and Choices” consultation paper:

The Swale, Ure, Nidd and Upper Ouse the Wharfe and Lower Ouse catchment has a long legacy of mainly metal mining.

Pollution from abandoned mines comes from two main sources, the underground workings and the waste materials spread on the surface. Minewater accounts for 2% of the reasons waters fail to meet quality standards in our catchments. The vast majority of these failures are related to abandoned mines, which are increasing the levels of zinc in waters and damaging habitats.

Minewater is the groundwater that has naturally entered the mine workings. When the mine was operating, the groundwater was drained or pumped to keep it away from working areas.

After the mine closed and pumping stopped, the mine workings flooded and the water level recovered until it reached a point where it could contaminate groundwater and also drain freely to surface water. Waters leaching from these long abandoned mines can be contaminated with iron, zinc, lead, cadmium, aluminium, manganese and copper and can also be quite acidic. Minewater pollution can have significant effects on the ecology of the area, although in some places with a very long history of contamination, wildlife has adapted to the pollution to some extent.

Metals also leach out of the waste material left at mines and from metal-contaminated sediments in the water. This is a significant source of metals, particularly after it has rained and during times of increased river flow. The presence of dissolved metals, such as lead, zinc, copper or cadmium, causes significant pollution that often stretches for many kilometres downstream.

There is also a risk of rising minewater pollution entering aquifers that are adjacent to mine workings. There is an increased risk of minewater pollution travelling along surface seams from old mine workings and entering high quality surface watercourses or aquifers used for drinking water. Floodplain sediments in the River Swale are affected by high levels of heavy metals as a result of historic lead and zinc mines. The Swale sediments could potentially cause pollution downstream if flooding disturbed them.

The Environment Agency’s main priority to tackle pollution from mines is to make sure that there is no deterioration in the water environment in the future as a result of minewater. It is essential that the issue of minewaters is closely monitored and appropriate action taken.