Predictions and Reality: Generation of Strongly Net-Acidic Mine Waters Through Flooding of Underground Coal Mine Workings With Limestone Roof Strata, Blenkinsopp Colliery (Northumberland, Uk)

Abstract

Blenkinsopp Colliery (Northumberland, UK) was the second-last underground coal mine in production in the former Great Northern Coalfield of England. The immediate roof beds of the single worked seam included a thick limestone bed; an unusual occurrence in UK coal mines. Hydrogeochemical investigations of the water encountered underground during the working of the mine revealed that specific qualities of water were logically related to details of the flowpaths the waters were inferred to have taken to reach the accessible sampling points. Working from this information, and taking into account observations of mineralogical and mining engineering aspects of the workings, a conceptual model was developed which assisted in predicting the likely response of the colliery to abandonment and flooding. In particular, it was predicted that, despite the presence of limestone in the roof and in goaf materials, the early outflows from the mine would be sufficiently acidic that they would require active treatment. Flooding of the mine is now complete, and post hoc analyses have largely vindicated earlier predictions of the time it would take for the mine to flood to surface, and the approximate post-flooding flow rates. The prediction that the water would be strongly net-acidic has also been borne out by observations: it is clear that the presence of limestone in a mined sequence is no guarantor of a neutral-pH discharge. However, the extremely high iron concentrations encountered post-flooding (≤ 1000 mg/l Fe +2) have exceeded expectations by a factor of around 3.3. Improvement in the understanding of likely quality, perhaps by means of tracer tests in recently flooded workings, could help reduce risks related to treatment scheme design, thus enabling more robust design and budgeting to be carried out as early as possible in the postclosure management period.

A simple, low-cost approach to predicting the hydrogeological consequences of coalfield closure as a basis for best practice in long-term management

“…This is because total sulphur content is a good proxy for total pyrite content, at least at higher total sulphur contents where the correlation matters most (Casagrande 1987). This is good news, as typical Master Seams will seldom be high-sulphur (though in some smaller coalfields they certainly are; see, for instance, Younger and Thorn 2006). Nevertheless, accurate prediction of a wide range of parameters is beyond present capabilities, and the only simplified prediction protocol produced to date is for total iron (Younger 2000).…”

Approaches of Applying Non-Darcian Flow Equations to Predicting Groundwater Rebound in Response to Coal Mine Closure

“…Such massive mine closure is not limited to China but has also been experienced in many other countries including the Unites States, United Kingdom, and South Africa (Aljoe 1994;Younger 2000;Younger et al 2006;Wolkersdorfer 2006;Li et al 2014). Where mining of coal extends below the groundwater table or under potentiometric pressure, sustained dewatering is necessary throughout the life of the mine to prevent flooding of active parts of the workings (Younger & Thorn 2006). As a consequence, the natural hydro geological conditions are disrupted, and the local and regional hydro geological regime is controlled to a large extent by the dewatering.…”