POSITIVE AND NEGATIVE IMPACTS OF LONGWALL MINE SUBSIDENCE ON A SANDSTONE AQUIFER

Subsidence due to longwall underground coal mining changes the hydraul ic properties, heads, yields, and in some cases the groundwater chemis try of overlying bedrock aquifers. A 7-year study of a sandstone aquif er overlying an active longwall mine in Illinois has supported a compr ehensive model of these impacts. Subsidence caused increases in permea bility and storativity over the longwall panel. These changes initiall y caused a major decline in water levels in the sandstone, but the aqu ifer recovered slightly within a few months and fully within several y ears after mining. The enhanced hydraulic properties combined with pot entiometric recovery resulted in a zone of greater well yield. However , at sites with very poor transmissivity and inadequate recharge pathw ays, recovery may not occur. Also, at the study site, the physical enh ancement was accompanied by a deterioration in groundwater quality fro m slightly brackish, sodium bicarbonate water to more brackish water w ith increased sulfate levels.