Impact of Oil Shale Mining on Flow Regimes in Northeast Estonia

Abstract

In addition to the well-documented effects of aquatic pollution, mining operations can have major impacts on hydrological pathways and flow regime in downstream catchments. This paper documents long term (1923-2008) changes in surface drainage areas and runoff characteristics in two small to medium (100-1,000 km 2 ) rivers draining part of the Ordovician oil shale field of north east Estonia. Through coupling analysis of flow regime with mining records (discharge rates and workings locations) the impact of expansion in oil shale mining through the mid to late twentieth century on downstream flow is assessed. During phases of intense mining, winter and summer baseflow is between 53 and 72% higher than long term average baseflow in the Purtse catchment and between 66 and 92% higher in the smaller Puhajogi catchment where the volumetric significance of mine discharges is greater. The contribution of pumped deeper groundwater to surface run-off is shown to control the largest increases in mean annual run-off. While flow augmentation is the most com- mon hydrological impact of the mining operations, phases of dehydration are also recognised in streams where cross- watershed transfers reduce the effective catchment area. Implications of the changed flow regime on river quality and management options are considered.