Determination of specific yield for the Biscayne Aquifer with a canal-drawdown test

Data from a large-scale canal-drawdown test were used to estimate the speci fic yield (s(y)) of the Biscayne Aquifer, an unconfined limestone aquifer i n southeast Florida. The drawdown test involved dropping the water level in a canal by about 30 cm and monitoring the response of hydraulic head in th e surrounding aquifer. Specific yield was determined by analyzing data from the unsteady portion of the drawdown test using an analytical stream-aquif er interaction model (Zlotnik and Huang 1999). Specific yield values comput ed 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 s(y) among piezometers). A value of 0.15 (our best estimate ) was computed based on all drawdown data from all piezometers. We incorporated our best estimate of specific yield into a large-scale two- dimensional numerical MODFLOW-based ground water flow model and made predic tions of head during a 183-day period at four wells located 337 to 2546 in from the canal. We found good agreement between observed and predicted head s, indicating our estimate of specific yield is representative of the large portion of the Biscayne Aquifer studied here. This work represents a pract ical and novel approach to the determination of a key hydrogeological param eter (the storage parameter needed for simulation and calculation of transi ent 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 wou ld likely yield ambiguous results). Accurate estimates of specific yield an d other hydrogeological parameters are critical for management of water sup ply, Everglades environmental restoration, flood control, and other issues related to the ground water hydrology of the Biscayne Aquifer.