INVESTIGATING THE CONTRIBUTION OF MOUNTAIN BLOCK RECHARGE TO SPRINGS IN THE NORTHEAST SALT LAKE VALLEY USING ENVIRONMENTAL TRACERS AND NOBLE GAS THERMOMETRY

As population and urban development increase along the Wasatch front (Salt Lake City, UT), understanding the sources of groundwater in the Salt Lake Valley (SLV) is critical in the creation of water management and contaminant mitigation plans. For intermountain basin-fill aquifers like the one underlying the SLV, the prevailing conceptual model is that mountain block recharge accounts for the majority of recharge. However, previous investigations have noted the complexity of the SLV groundwater system due to the presence of perched aquifers overlying the deeper, valley-wide aquifer. In this study, the noble-gas recharge temperatures and apparent ³H-³He ages of groundwater from springs, shallow monitoring wells, and deeper monitoring and production wells in the northeast SLV, as well as mountain-front springs and springs at the headwaters of Red Butte Creek in Red Butte Canyon, were determined in order to better understand the sources of this shallow groundwater. Groundwater from the valley springs and shallow monitoring wells has warmer recharge temperatures (~10-17°C) and younger apparent ages (~0-15 years) than expected from mountain block recharge, as evidenced by colder recharge temperatures and older apparent ages for the mountain-front and headwater springs. This suggests that groundwater in the northeast SLV shallow groundwater system is not sourced primarily from mountain block recharge that flows in one continuous, saturated flow-path from recharge to discharge as is likely to occur in the southeast part of the SLV; instead, recharge originating in the valley comprises a significant portion of this groundwater. The relationship between the ages of groundwater discharging from these springs and their distance from Red Butte Creek further suggests that a portion of this water is sourced from losses of Red Butte Creek as it flows through the valley. Additionally, the deeper monitoring and production wells in the northeast SLV have warmer recharge temperatures and younger apparent ages than production wells previously sampled in the southern part of the valley, suggesting that they are not entirely fed by mountain block recharge. These findings have important implications for the northeast SLV, where there is an ongoing Superfund investigation into the movement of a groundwater contaminant plume.