2003-02-07_HYDROLOGY - M2001085 - Laserfiche WebLink

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01/31/2003 17:09 2799186 DOTY & ASSOCIATES PAGE 04 Mr. Christopher C. LaRue Page 4 January 28, 2003 Doty & Associates yield. It is generally accepted (e.g., Kruseman and de Ridder, 1991) that a delayed yield response consists of three phases: a. an early period before delayed yield effects are measurable and the response follows a Theis prediction (perhaps as short as the first few minutes of pumping); b. an intermediate period dominated by drainage effects (after a few minutes and extending to a few hours); and c. a late period (when water levels are changing slowly) during which the response again is well described by the Theis equation. In addition, as drawdowns propagate away from the well in the formation, the saturated thickness which is assumed constant actually decreases. This is often handled by adjusting the observed drawdown in a pumping test by the Jacob correction (Kruseman and de Ridder, 1981). The correction reduces the observed drawdown by the drawdown squared divided by twice the original saturated thickness. The correction would range from 0.02 feet for a one foot drawdown to 1.14 feet for an 8 foot drawdown, which is the maximum drawdown possible given the 8 foot penetration of the well. Because we are developing a theoretical drawdown (the reverse of analyzing a pumping test), the actual drawdown will be larger by a factor similar to the Jacob correction. Thus, the effects of non-confinement are that the early and intermediate drawdown predictions are unreliable and the late time predictions are likely to under-estimate the actual drawdowns (however, the magnitude of the under- estimation is probably no more than one foot). In summary, the early and intermediate time drawdowns predicted using the Theis expression are too large (because of the finite well diameter and delayed yield), the predicted drawdowns for all times are too small near the well (because of partial penetration), and the predicted drawdowns for all times are slightly too small (because of the thinned saturated thickness). Thus, only the late time predictions (times greater than a few hours) can be used and all late time predictions under-estimate the actual drawdowns. The predictions for points at large distances from the well (on the order of 80 feet) are likely to be only slight under-estimations. Current Drawdowns in the Well Recognizing that drawdowns predicted using the Theis equation are probably under- estimations at points close to the well, a series of analyses were performed at a distance of 2 feet from the center of the well (the edge of the 4 foot diameter well). After 180 days (a late time) of pumping at 500 gallons per minute, drawdowns are greater than the 8 foot saturated interval In the well (the well is dry) if the hydraulic