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In Sweden, petroleum products were stored for many years in unlined rock caverns. This type of storage, below the groundwater table with the product floating on a waterbed, led to contamination of the inner surface of the rock caverns as well as contamination of fractures in the rock (Figure 1). The rock-storage facilities are now being decommissioned by the Swedish Geological Survey/National Oil Stockpile Agency; and Kemakta has advised on a number of questions concerning the environmental consequences. An important part of Kemakta's work has been the development of methods to prevent the spreading of the remaining product and thereby avoid contamination of groundwater and surface water recipients.
Figure 1
Principle of hydraulic diversion in unlined rock cavities, where contamination of the surrounding rock has occurred. Hydraulic diversion to a nearby surface water recipient, ensures an inward flow of groundwater of a longer period. This prevents further penetration of the contaminants/product into the bedrock and thus protects the groundwater
Kemakta's comprehensive knowledge of the chemical, physical and biological processes which may be of importance for transport, biodegradation and transformation of the contaminants has formed a basis for the development of ecologically sound, functional solutions. The following moments have been included in the environmental risk assessments carried out by Kemakta in conjunction with the closure of rock-cavern storage facilities:
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Transport calculations for petroleum products in crystalline rock
Kemakta has carried out a large number of model predictions of the transport of petroleum products in the rock surrounding the storage cavity. The transport models used are based on the experience which Kemakta has built up during more than 20 years work with the disposal of nuclear waste and other environmental problems involving hydrogeological modelling. Transport of both free-phase petroleum products and components that are dissolved in the groundwater is considered. Contaminants can be transported in the rock fractures and a number of processes can result in penetration into micro-fractures in the rock .
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Development of remediation methods
Groundwater flow through the decommissioned storage cavities can result in continued spreading of contaminants, as the small amounts of petroleum product remaining in the cavity can be dissolved in the groundwater. With the aim of reducing the environmental effect on the surrounding area, Kemakta, in cooperation with J&W, have developed a remediation technique, which has become the main method used in decommissioning of rock cavities. The method is called hydraulic diversion and constitutes the creation of a hydraulic connection between the cavern and a suitable surface-water recipient. This means that under a longer period of time, groundwater will flow in the direction of the rock cavity. In this way, an uncontrolled transport of contaminants and groundwater is avoided and the surrounding rock is protected from contamination by petroleum products. In the recipient to which the contaminated water is led, dilution, evaporation and other processes counteract the build up of harmful contaminant concentrations. The method therefore minimises negative environmental consequences.
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Modelling and environmental risk assessment of the recipient.
Kemakta's work has included assessment of the environmental effect of the transport/release of contaminated leachate from rock storage cavities to nearby lakes. Where the assessment has shown that concentrations in surface water or sediment may exceed guideline values, the hydraulic diversion can be combined with water treatment, or can be led to an alternative recipient. Guideline values are concentration limits above which negative effects on aquatic organisms can be expected.
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