I suspect that the difference is in the geology of the strata above the coal measures. The magnesian limestone and the underlying sand bed which overly the coal measures in East Durham form an excellent aquifer which is an important source of the public water supply for a fair part of Sunderland, much of East Durham, and all of Hartlepool. This aquifer caused many problems in the sinking of the pits which can can be seen if you read the story of the sinking of Murton Colliery, for example. In later years freezing of the strata had to be resorted to when sinking took place.
Since the closing of the coastal collieries the minewater has risen to such an extent that it is now well up the aquifer in the Horden, Blackhall and Seaham areas and as a result the groundwater used for public supply is threatened. The minewater is of extremely poor quality and is extremelly saline and ferruginous.
For additional information - as a holding measure, minewater pumping is to start at Horden Colliery shaft (I believe that tests may habve already commenced) in order to lower the minewater levels to safe levels, the treated minewater will then be discharged to sea below the low water level. Here the aquifer is about 114.29 metres thick and the minewater has risen about 82 metres from its base. A site for a permanent pumping station and passive treatment facility, similar to that at Wheal Jane, has been suggested to be at Dawdon Colliery shaft with the treated minewater being discharged to sea via the long sea outfall from the fairly close Seaham Sewage Treatment Works.
Alan Vickers.
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