This work represents a practical and novel approach to the determination of a key hydrogeological parameter (the storage parameter needed for simulation and calculation of transient unconfined ground water flow), at a large spatial scale (a common scale for water resource modeling), for a highly transmissive limestone aquifer (in which execution of a traditional pump test would be impractical and would likely yield ambiguous results). 4 Typical design for a three-layer multirate test (PL production logging). Specifically, a low constant pumping rate is selected, which is increased in progressive steps of higher pumping rates, with each step conducted over an equal interval (from 30 minutes to 2 hours). Jacob Contents Acknowledgement 64 Introduction. A step-drawdown test monitors pumping well performance by varying the pumping rate in a controlled manner. 4 ), a rigorous solution to deriving the dynamic reservoir parameters can be obtained for each layer. RATIONAL DRAWDOWN TEST OF ARTESIAN WELL ELUCIDATES SPECIFIC CAPACITY C. We found good agreement between observed and predicted heads, indicating our estimate of specific yield is representative of the large portion of the Biscayne Aquifer studied here. By using simultaneous bottomhole pressure and flow rate measurements and designing the drawdown test as a succession of step-rate tests ( Fig. We incorporated our best estimate of specific yield into a large-scale two-dimensional numerical MODFLOW-based ground water flow model and made predictions of head during a 183-day period at four wells located 337 to 2546 m from the canal.
A value of 0.15 (our best estimate) was computed based on all drawdown data from all piezometers. Specific yield values computed from drawdown at individual piezometers ranged from 0.050 to 0.57, most likely indicating heterogeneity of specific yield within the aquifer (small-scale variation in hydraulic conductivity may also have contributed to the differences in sy among piezometers). A more general solution to the step-drawdown test, called the General Well Function (GWF), is proposed to replace the classic Jacobs step-drawdown equation.
Specific yield was determined by analyzing data from the unsteady portion of the drawdown test using an analytical stream-aquifer interaction model (Zlotnik and Huang 1999). The drawdown test involved dropping the water level in a canal by about 30 cm and monitoring the response of hydraulic head in the surrounding aquifer. Data from a large-scale canal-drawdown test were used to estimate the specific yield (sy) of the Biscayne Aquifer, an unconfined limestone aquifer in southeast Florida.
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