| Abstract |
Landsvirkjun, the National Power Company of Iceland, owns and operates a 5 MWe geothermal power plant at Bjarnarflag in NE Iceland, targeting the western part of the Námafjall high-temperature geothermal reservoir. As part of the operational design of a proposed new power plant at Bjarnarflag, Vatnaskil provided Landsvirkjun with consultation relating to the disposal of geothermal wastewater from the plant with emphasis on minimizing environmental impacts on the shallow groundwater aquifer and nearby Lake Mývatn nature reserve. A numerical model was utilized to simulate several disposal scenarios, injecting the geothermal wastewater below the shallow groundwater aquifer to depths of 300-600 meters. An existing regional watershed model of NE Iceland, developed by Vatnaskil and utilized as a tool for environmental related projects in the region, was used as a foundation for the modelling work. The watershed model covers an area of roughly 16,000 km2 extending from the northern coastline of Iceland up to the Vatnajökull glacier and consists of a surface runoff model and a two-dimensional groundwater model simulating the shallow groundwater aquifer (upper 100 m of the groundwater system). In order to simulate the proposed disposal scenarios, it was necessary to expand the groundwater model vertically to take into account the deeper groundwater system, including the upper section of the Námafjall geothermal reservoir. A conceptual model of the local groundwater system and underlying reservoir was formulated utilizing data collected over decades of groundwater monitoring and geothermal exploration in the area. The conceptual model was then used to construct an integrated three-dimensional numerical model of a 20x30 km area centered around Bjarnarflag, extending to a depth of 900 meters, using iTOUGH2 modelling software in conjunction with the regional groundwater model. The integrated three-dimensional model was used to simulate several disposal scenarios to analyse the sensitivity of key model input parameters as well as the effect of variations in the injection parameters. Model results revealed that enhanced permeability along vertical fractures is a major control on groundwater flow paths connecting the shallow groundwater aquifer with deeper sections of the groundwater system. In all scenarios, a significant amount of the injected geothermal wastewater migrates upward through the Krummaskarð fracture and into the overlying shallow groundwater aquifer. From there, the geothermal wastewater plume is carried southwest in the direction of regional groundwater flow. The Grjótagjá fracture acts as a hydrologic barrier, preventing further plume migration west towards Lake Mývatn. Enhanced groundwater flow in the vicinity of the Grjótagjá fracture significantly dilutes and cools the geothermal wastewater, forcing much of it downward through the fracture into deeper sections of the groundwater system. The integrated modelling approach utilized in the project proved effective for simulating groundwater flow and transport processes within a complicated geological environment. The approach is transferable to other geothermal fields where characterization of environmental impacts from wastewater disposal is required. An integrated three-dimensional model is a valuable management tool for use by geothermal developers during all phases of field development, including determining optimal disposal methods in the planning phase and mitigating existing impacts during the production phase. |