DETECTING EFFECTS OF DISTANT GROUNDWATER DEVELOPMENT

Drawdown from groundwater pumping is of interest in areas where streams, springs, and wetlands may be affected. Pumping typically must depress water levels a foot or more before drawdown can be detected with certainty because groundwater levels also are fluctuating in response to environmental stresses. The most common environmental fluctuations result from barometric changes, evapotranspiration, tidal forces, and climatic responses. Groundwater levels fluctuate during periods of a day to a year in response to tidal forcing, barometric changes, and evaporative discharge. Climatic responses are region-wide responses to infrequent recharge events and prolonged drainage that irregularly perturb the regional groundwater system with decadal frequency.

Pumping-induced changes of less than 0.1 ft have been differentiated reliably from environmental fluctuations with models of continuous water-level records. Environmental fluctuations are simulated by summing time series of barometric pressures, earth and gravity tides, and water levels in background wells. Pumping effects are simulated by superimposing multiple Theis solutions that translate pumping schedules into water-level responses. The water-level modeling approach has been successful for interpreting many aquifer-test results, but the need for continuous water-level records will hamper interpretation of regional effects from groundwater development. This is because decades of record are necessary to define regional effects rather than less than a year to characterize signals from aquifer tests.

Correcting periodic measurements with water-level models is a tractable approach for detecting drawdowns of less than a foot that result from groundwater development. A water-level model is developed for each observation well using a few months to a year of continuous water-level records to simulate and remove environmental fluctuations such as barometric and tidal changes. Once the well-specific, water-level model is established, the periodic measurements can be filtered and adjusted for these fluctuations. Climatic responses and drawdowns from groundwater development can be differentiated with conventional water-level modeling of the filtered, periodic measurements.