CONCEPTUALIZING GROUNDWATER-FLOW SYSTEMS: USING WATER-LEVEL MODELS TO UNDERSTAND STRESSES AFFECTING WATER LEVELS

Groundwater studies require an initial conceptualization of the flow system that is followed by more rigorous testing of the conceptualization. One way to conceptualize a groundwater-flow system is to analyze water-level trends in well hydrographs. A water-level trend reflects the summation of all natural and anthropogenic stresses acting on the aquifer at the well location. Common stresses include recharge, evapotranspiration, barometric pressure, earth tides, and pumping. A water-level model (WLM) can be used as a tool to distinguish which stresses are affecting the trend and quantify the impact of each stress on the trend. A WLM is an analytical model that fits measured water levels to a synthetic curve representing the sum of one or more time-series components that can explain the water-level trend. Time-series components include recharge pulses simulated with Gamma transforms of precipitation; moving averages of water levels in background wells to simulate barometric pressure and earth tides; background water levels dominated by evapotranspiration; pumping signals simulated with Theis models; and Bouwer and Rice transforms to simulate slow recovery following well construction and development. WLMs characterizing 20 years of water-level data from Nevada’s Pahute Mesa-Oasis Valley groundwater basin will be presented and used to show how results can conceptualize the groundwater system. WLMs show most water levels are affected by recharge, where the thick unsaturated zone (> 1,000 ft) causes ‘fast’ and ‘slow’ responses indicative of a dual-porosity system, and 9 of 86 wells were affected by nearby pumping, which was masked by recharge, evapotranspiration, or wellbore storage.