Simulation of coupled groundwater flow and transport of heat in the groundwater system under Vestre Lovénbreen, with the model METROHEAT; a surveying study

Abstract

Svalbard has an arctic climate and glaciers cover about 60% of the land. The almost continuous permafrost layer prevents groundwater recharge in most places on Svalbard. Groundwater recharge takes place under glaciers where the base is at melting point. The Vestre Lovénbreen is a small glacier near Ny-Alesund in the northwestern part of Spitsbergen. The low runoff at the front of the glacier indicates a two-part drainage: an upper area where water drains into a moulin and a lower area where water drains to the front of the glacier. It is assumed that the moulin extends through the whole thickness of the glacier. The bedrock in the area of the Vestre Lovénbreen consists of Precambrian, Carboniferous, Permian, Triassic and Tertiary rocks. A spring is situated at the former entrance of an old coalmine close to Ny-Alesund. The quantity and quality of the water indicate that the water originates from the Vesfe Lovénbreen. From the glacier base the water may infiltrate into the underlying limestones, after which it flows through a sandstone aquifer. In the sandstone the water gets a higher temperature due to geothermal heat. Mining activities in the past have resulted in the opening of artificial groundwater channels that go through the permafrost zone. From the sandstone the water can flow through the permafrost to the surface. The chemical composition indicates a long storage time in sandstones and a short storage time in limestones. The spring discharge has decreased due to a decrease in recharge. To estimate possible consequences for the ecosystem, knowledge of groundwater flow and transport of heat is very important, especially the temperature and velocity distribution, the travel path and the residence time. METROHEAT is a computer program capable to simulate 3-dimensional groundwater flow and transport of heat in a finite element mesh. METROHEAT uses averaged forms of the mass and momentum balances of the total liquid phase and the energy mass and momentum balances. Postprocessing was done with the program Matiab. A simplified hydrogeological model was constructed for the simulation of the groundwater system under Vestre Lovénbreen. The permafrost layer was incorporated into the model in order to simulate the depth of the permafrost. A 3-dimensional discretisation was made of the area and includes the Vestre Lovénbreen and the groundwater system. The element mesh consisted of deformed cubic elements. With the available data and information a base case and 5 variants for sensitivity analysis were made. The variants considered the permeability and porosity in the mining area, the spatial variation of the infiltration, the amount of water infiltrating into the system, the interaction between the groundwater flow, temperature and viscosity and the construction of the mesh. The results of the pressure, temperature and velocity distribution were plotted in surface and contour maps. The simulated depth of the permafrost layer and the simulated temperature of the outflow point are close to the field data. Estimated residence times of the groundwater in the system are long and very debatable, the estimated residence time in the limestones is 0.6-2 years. An increase of the permeability and porosity in the mining area did not decrease the residence time of the groundwater. The influence of the spatial variation of the infiltration under Vestre Lovénbreen on the outflow point is small. With a decrease in infiltration of 50% the Ester Spring did not freeze during the simulations, but in the simulations the groundwater system is saturated and in the physical system unsaturated conditions may occur when the infiltration decreases. The temperature dependence of the viscosity and the deformation of an element mesh should not be underestimated.