QUANTIFYING THE SHORT-TERM HYDROGEOLOGIC RESPONSES OF HIGHWAY CONSTRUCTION IN HOT SPRINGS, AR

In 2020, construction and excavation along Highway 5 in Hot Springs, AR resulted in seepage discharging from newly exposed road cut faces. A nearby observation well in the Stanley Shale and other wells in the unconfined aquifer recorded a 19-foot hydraulic head decline that has remained ~20 ft below pre-construction levels in the time since. We quantify how this change in groundwater levels affected total unconfined aquifer storage across a portion of the Hot Springs recharge zone using a transient cross-sectional MODFLOW 6 groundwater model. Construction scenarios were simulated with a drain positioned 30.5-40.5 ft below the ground surface, which we determined to be between 16 to 26 ft below the pre-excavation water table. Simulation outputs captured the observed immediate hydrologic responses due to seepage from the excavated drain, resulting in modeled decreases in mean annual groundwater storage ranging from 5,686 ft3 to 8,918 ft3 in the unconfined portions of the groundwater flow system. During the wettest months, up to 9,000 ft³ of model storage was lost above the previously determined critical elevation necessary to support the springs in Hot Springs National Park. Additionally, annual mean head was lowered by 0.1 ft across ~5,000 ft of the model domain for all scenarios after the construction simulation began, indicating a widespread water table response to the highway construction. This affected portion includes model cells located across a topographic divide from the simulated highway excavation. Furthermore, we used the model to test if drought-related reductions in recharge could account for the observed water table drop instead of the excavation. A reduction of recharge by 30% resulted in a similar long-term groundwater level drop at the nearby monitoring well, but hydroclimatic measurements did not support this level of drought. Our simulations indicate that the excavation along Highway 5 and resulting seepage caused the observed groundwater level declines and could negatively affect spring flow at Hot Springs National Park. Excavation that could intercept groundwater and cause seepage in even small areas of sensitive recharge zones requires careful hydrogeologic analyses beforehand and monitoring long-term to characterize the potential for unwanted hydrologic consequences.