Introduction

Estimating hydraulic properties from tidal attenuation in the Northern Guam Lens Aquifer, territory of Guam, USA

Estimating hydraulic properties from tidal attenuation in the Northern Guam Lens Aquifer, territory of Guam, USA

CP-2013-06
Estimating hydraulic properties from tidal attenuation in the Northern Guam Lens Aquifer, territory of Guam, USA

Kolja Rotzoll & Stephen B. Gingerich & John W. Jenson & Aly I. El-Kadi

Tidal-signal attenuations are analyzed to compute hydraulic diffusivities and estimate regional hydraulic conductivities of the Northern Guam Lens Aquifer, Territory of Guam (Pacific Ocean), USA. The results indicate a signi?cant tidal-damping effect at the coastal boundary. Hydraulic diffusivities computed using a simple analytical solution for well responses to tidal forcings near the periphery of the island are two orders of magnitude lower than for wells in the islands interior. Based on assigned specific yields of ~0.010.4, estimated hydraulic conductivities are ~20800 m/day for peripheral wells, and ~2,00090,000 m/day for interior wells. The lower conductivity of the peripheral rocks relative to the interior rocks may best be explained by the effects of karst evolution: (1) dissolutional enhancement of horizontal hydraulic conductivity in the interior; (2) case-hardening and concurrent reduction of local hydraulic conductivity in the cliffs and steeply inclined rocks of the periphery; and (3) the stronger in?uence of higher-conductivity regional-scale features in the interior relative to the periphery. A simple numerical model calibrated with measured water levels and tidal response estimates values for hydraulic conductivity and storage parameters consistent with the analytical solution. The study demonstrates how simple techniques can be useful for characterizing regional aquifer properties.