Seasonal Recharge Components in an Urban/Agricultural Mountain Front Aquifer System Using Noble Gas Thermometry

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Thirteen noble gas samples were collected from eleven wells and two mountain springs in the Treasure Valley, Idaho, USA to derive recharge temperatures using noble gas thermometry. One common assumption with noble gas thermometry is that recharge temperatures are roughly equal to the mean annual surface temperature. When water table depths are shallow or variable, or infiltration is seasonal recharge temperatures may be significantly different from the mean annual surface temperature. Water table depths throughout the study area were used to estimate recharge source temperatures using an infiltration-weighted recharge temperature model which takes into account a time-variable water table. This model was applied to six different seasonally-dependent recharge scenarios. The modeled recharge temperatures for all scenarios showed a strong dependence of recharge temperature on mean annual depth to water. Temperature results from the different recharge scenarios ranged from near the mean annual surface temperature to as much as 6 °C warmer. This compared well to noble gas derived recharge temperatures from the valley wells which ranged from 5 °C below to 7.4 °C above the mean annual surface temperature of the valley. Cooler temperatures suggest an influence of recharge through the adjacent mountain block while warmer temperatures suggest an influence from summer irrigation.