DEFORMATION-INDUCED CHANGES IN HYDRAULIC-HEAD DURING GROUNDWATER WITHDRAWAL

Ground-water withdrawal from a confined or semiconfined aquifer causes three-dimensional deformation in the pumped aquifer and in adjacent l ayers (overlying and underlying aquifers and aquitards). In response t o the deformation, hydraulic head in the adjacent layers could rise or fall almost immediately after the start of pumping. This deformation- induced effect is analyzed by a linear poroelasticity model using prop erties typical of unconsolidated sedimentary materials. Model simulati ons suggest that an adjacent layer undergoes horizontal compression an d vertical extension when pumping begins. Hydraulic head initially dro ps in a region near the well and close to the pumped aquifer, but rise s outside this region, Magnitude of head change varies from a few cent imeters to more than 10 centimeters. Factors that influence the develo pment of deformation-induced effects includes matrix rigidity (shear m odulus), the arrangement of aquifer and aquitards, their thicknesses, and proximity to land surface. Induced rise in hydraulic head is promi nent in an aquitard that extends from land surface to a shallow pumped aquifer. Induced drop in hydraulic head is likely observed close to t he well In an aquifer that is separated from the pumped aquifer by a r elatively thin aquitard. Induced effects might last for hours in an aq uifer, but could persist for many days in an aquitard. Induced effects are eventually dissipated by fluid flow from regions of higher head t o regions of lower head, and by propagation of drawdown from the pumpe d aquifer into adjacent layers.