Although the need for background water levels to characterize environmental fluctuations has long been recognized ( Stallman 1971), trends and corrections characterizing environmental fluctuations typically have been estimated qualitatively. A useful background well is one in which water levels are affected by tidal potential, imperfect barometric coupling between the atmosphere and water table, and all other stresses that affect water levels in observation wells excluding pumping. Stage changes of a fully penetrating river can cause daily and event-based fluctuations in local groundwater levels (Criss and Criss 2011).Įnvironmental fluctuations from recharge responses, surface water stage changes, or any other external stress can be modeled explicitly by using water levels from background wells that are affected by these environmental stresses ( Halford 2006 Criss and Criss 2011). Recharge likewise can induce long-term rising trends of more than 1 m/year that affect detection of small pumping signals ( Fenelon 2000 Elliott and Fenelon 2010). Individual recharge events also can cause episodic water-level rises that exceed 0.5 m over a few days ( O'Reilly 1998). Barometric pressure and tidal signals acting on the aquifer system can induce water-level changes of more than 0.3 m during periods of less than a few days ( Fenelon 2000). These fluctuations include short-term, seasonal, and long-term stresses such as barometric pressure, tidal signals, natural and artificial recharge, and surface-water stage changes and diversions. Drawdown detection typically is limited to distances of less than 1 km, because environmental water-level fluctuations frequently exceed the maximum displacement from pumping ( Risser and Bird 2003).Įnvironmental fluctuations in measured water levels are described here as nonpumping (natural or anthropogenic) stresses on the aquifer system. The volume of aquifer system that can be characterized with aquifer tests is controlled by the distance at which drawdown can be detected. Maximum drawdowns of about 0.05 m were analytically estimated from field investigations where environmental fluctuations approached 0.2 m during the analysis period. Pumping-induced changes generated with a numerical model and analytical Theis model agreed (RMS as low as 0.007 m) in cases where pumping signals traveled more than 1 km across confining units and fault structures. This approach closely matched drawdowns simulated with a complex three-dimensional, hypothetical model and reasonably estimated drawdowns from an aquifer test conducted in a complex hydrogeologic system. Pumping signals are generated with Theis models, where the pumping schedule is translated into water-level change with the Theis solution. Drawdown is distinguished by analytically simulating all pumping and nonpumping water-level stresses simultaneously during the period of record. A reliable analytical approach for distinguishing drawdown from nonpumping water-level fluctuations is presented and tested here. However, this detection is often obscured by water level fluctuations such as barometric and tidal effects.
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