Groundwater is an important component of many water resource systems supplying water for domestic use, industry, and agriculture. Management of a groundwater system, an aquifer, means making such decisions as annual withdrawal of the total quantity of water, location of wells for pumping and for artificial recharge and their rates, and control conditions at aquifer boundaries. The decisions related to groundwater quality are of no less importance. In fact, the quantity and quality problems cannot be separated. In many parts of the country, with the increased withdrawal of groundwater, the quality of groundwater has been continuously deteriorating. In recent years the attention has been focused on groundwater contamination by mine water. A thorough understanding of the system, and the processes that take place in it are needed for modeling procedure. It is important to identify those parts of the system's behavior that are relevant to the considered problem, while other parts may be neglected. On the basis of this understanding, summarized as a conceptual model of the given problem, a numerical model was constructed. Using the conceptual model the groundwater flow model of the Quaternary aquifer, in the eastern part of oil shale deposit area, in the Vasavere buried valley, was constructed. The infiltration of contaminants grows, and the Quaternary aquifer is polluted by sulphate in a large area of the Vasavere buried valley.
The article describes the hydrogeological problems related to water influx to the mine and water sulphate content association with underground oil shale mining in an environmentally sensitive area of Estonia. The water balance method was used to calculate the ratio of mine water outflow/inflow. Water balance calculations are needed for estimating the groundwater transversal flow into closed mines. Before oil shale mining groundwater geochemical processes are characterised by low sulphate content. Dewatering of oil shale mines lowered groundwater level in the Keila-Kukruse aquifer, and caused an about 50-fold increase in sulphate by intensive oxidation of pyrite of natural origin. The mobile sulphate ion may serve as an indicator of changes occurring in the sulphate content of groundwater stored in the Keila-Kukruse and Lasnama¨e-Kunda aquifers of the Ordovician system in the area of closed and working mines. Using the method of inverse mixing, sulphate ion distribution is a possibility to study the movement of groundwater in lateral and transversal directions.
One of the most important industries of Northeast Estonia is oil shale mining. Ground movements caused by mining reach the ground surface easily due to shallow location of workings. A new, artificial topography is formed on undermined areas, where the ground surface depressions are alternating with rising grounds. When the Quaternary cover contains loamy sediments, the surface water will accumulate in the depressions. The response of usable lands on undermined areas depends on the degree of changes in the relief and water regime. The accumulation of solid residues by oil shale mines and processing plants has resulted in numerous ash hills, which are polluting the environment. The streams are polluted by phenols, oil products and sulphates. The main source of water supply is groundwater in the oil shale basin. The hydrostratigraphic section is represented by three aquifers. Two (Quaternary and Ordovician) of these aquifers are affected by the human activity. Intensive water consumption has caused a fall in the water level in these aquifers. Due to oil shale production the concentration of Ca2+, Mg2+, SO4 2- and Cl- contained in the groundwater from the Ordovician deposits is noticeably higher than in the water with a natural background level. The natural water chemical regime is restored at the cessation of mining.
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