Shallow groundwater temperature response to urbanization and climate change in the Twin Cities Metropolitan Area: Analysis of vertical heat convection effects from the ground surface

Groundwater temperatures, especially in shallow (quaternary) aquifers depend on ground surface temperatures which in turn depend on climate and land use. A key heat transfer processes that contributes to shallow groundwater temperatures is conduction from the soil surface into the groundwater. Groundwater temperatures therefore respond to ground surface temperatures. Ground surface temperatures have seasonal and diurnal cycles, and are modified by urban development, and climate change. In Minnesota seasonal temperature cycles penetrate the ground to depths on the order of 10 to 15m. In this report, we explore concepts and basic relationships by which groundwater temperatures are induced by conduction from the ground surface. Our analysis indicates that a fully urbanized downtown area at the latitude of Minneapolis/St. Paul is likely to have a groundwater temperature that is nearly 3°C warmer than an undeveloped/ agricultural area. Pavements are the main cause of this change. Data collected by the MPCA in the St. Cloud, MN area confirm that land use influences groundwater temperatures. Global warming will also result in a rise of ground surface temperatures and hence groundwater temperatures. In the extreme case of a 2xCO2 climate scenario groundwater temperatures would be expected to rise by up to 4°C. Compounding urbanization and climate change, by applying the extreme 2xCO2 climate scenario to a land use change from “undeveloped” to “fully urbanized”, is expected to raise groundwater temperatures by about 5°C at the latitude of Minneapolis/St. Paul. A mean annual groundwater temperature rise of 5°C would likely have a very adverse affect on trout habitat in coldwater streams in summer.