A GEOCHEMICAL AND HYDROLOGIC ASSESSMENT OF COAL MINE DRAINAGE IN GLACIATED EASTERN OHIO

The historic Cherry Valley Coke Ovens are located near the Village of Leetonia in the glaciated region of eastern Ohio. Between the 1860's and 1930's the Middle Kittanning coal seam was mined there via a slope entry. The mine entry now discharges water with elevated levels of sulfate and metals typical of coal mine drainage but it is alkaline with a circumneutral pH rather than acidic. This study characterizes the hydrologic and geochemical properties of the mine drainage over the course of one year (May 2011-May 2012) to understand what factors control discharge chemistry.

Subsidence features that provide a direct route for recharge into the mine void were identified on the surface above the mine. The mine behaves like a karst system where approximately 20% of the discharge was quickflow attributed to subsidence. Recharge primarily influences the magnitude of discharge by raising the hydraulic head in the mine. The majority of water that enters the mine has a sufficiently long residence time with which to reach chemical equilibrium with oxidation reactions within the partially flooded mine. Therefore only a small dilution effect is seen in the sulfate concentration as the discharge increases during the recharge period. Large runoff events that are captured by subsidence features can produce a rapid increase in discharge with a corresponding dilution of oxidation products. These quickflow events are evidence that some of the water entering the mine through fractures has a much shorter residence time. The δ¹⁸O of the mine water exhibits minimal variability due to the predominantly baseflow composition of mine discharge. Due to the application of road salt the shallow groundwater within the glacial till has a high chloride concentration compared to deeper groundwater sources. Chloride analyses show that the water within the main entry tunnel of the mine originates entirely within the glacial till at the surface and is brought to the mine via fractures. Alkalinity imparted to the water from the till is sufficient to buffer the acidity produced by oxidation reactions within the main entry. Chloride analyses also provide evidence that there is a deeper groundwater component to the discharge.