CONSTRAINTS TO DEEP BASIN GROUND WATER FLOW FROM GROUND WATER TEMPERATURE, HELENA VALLEY, SOUTHWESTERN MONTANA

The Helena Valley Aquifer represents a potable water source for over 40,000 people, comprised of relatively permeable shallow alluvial deposits overlying basin fill materials. The local conceptual model identifies a single downgradient discharge point for both surface and ground. Aquifer recharge occurs from direct infiltration of local precipitation and stream loss from irrigation canals and streams. A water balance from regional numerical models indicates that significant recharge to the aquifer also occurs from subsurface flow between deep, basin bounding bedrock aquifers into the shallow system; especially during winter months. Local water management programs typically reflect local recharge and stream loss for recharge, ignoring the deeper flow component, as quantification is more complex.

Recent studies compiled water temperatures with water quality types, defined by major ions, from multiple wells across the area. Valley ground water temperatures were noted exceeding temperatures expected under normal geothermal gradient conditions. The background geothermal gradient was confirmed as normal in deep bedrock wells upgradient from the valley. For this study, the thermal overprint in the valley wells is considered the difference between actual temperature and that expected from a normal geothermal gradient. Thermal overprints from 2°C to 7°C were noted across the valley. Measured temperatures from well sampling are interpreted to reflect a mixture of surface recharge waters, under a normal geothermal gradient, and deeper thermal waters from the basin upwelling. From this data, the depth necessary to account for the temperature, under a normal geothermal gradient, represents the minimum depth below surface for the waters. To account for monitored temperatures, the upwelling occurs from depths ranging from several hundred feet to more than 1,500 feet below ground surface providing constraints to vertical flow in the valley. The conclusions derived from temperature data are consistent with ground water types, where locally recharged waters are typically Calcium-Magnesium Bicarbonate, and deeper thermal waters are Sodium-Potassium Sulfate-Bicarbonate waters. The data demonstrate the use of ground water type and temperature as a method to evaluate vertical flow and recharge in western basins.