Spatial variability of specific yield and vertical hydraulic conductivity in a highly permeable alluvial aquifer

Lower-than-expected specific yield values of an unconfined aquifer estimated from pumping test data have been a concern in the groundwater community over the last several decades. Delayed yield above the declining water table during pumping has been considered the primary cause of this uncertainty in the specific yield. The drainage process above the declining water table is a dominantly downward flow. Thus, the vertical hydraulic conductivity (K_v) must have an effect on the drainage process. However, careful examination of K_v variation has not been done. The spatial variation of S_y has not been well evaluated either. This paper examines the heterogeneity of the specific yield and the vertical hydraulic conductivity of an alluvial aquifer. A drainage experiment and permeameter tests using more than 60 sediment cores were conducted to determine specific yield and the vertical hydraulic conductivity. Drainage experiments indicated that more than 50% of drainable water was released from the permeable sediment cores in less than 2 h, and nearly 100% of drainable water was released from most of the cores within 24 h. Thus, the appreciable effect of delayed yield in a permeable aquifer is unlikely to last longer than 24 h. Both S_y and K_v indicated a strong tendency of variation between one and another sediment core, and the two variables have some correlation. Because K_v is heterogeneous and the two parameters are correlated, holding a constant K_v value for different observation wells in the analysis of pumping test data can introduce errors to the derived S_y values.