ABSTRACT: Atoll Hydrogeology
Frank L. Peterson
On small oceanic islands often the single most critical resource is potable water. Surface catchment constitutes the atoll islands. However, many of the atoll islands experience a long dry season each year, and during this time groundwater, if available, must be developed for potable supplies.
During the past decade a considerable amount of geologic and hydrologic field work and mathematical modeling of atoll hydrogeology has substantially changed our thinking on how atoll groundwater systems behave. For many years hydrogeologists treated atoll islands essentially as single-layer homogeneous aquifers that were subjected to laterally propagated tidal stresses. Based on recent detailed geologic, groundwater salinity profile, and groundwater tidal response data, we now believe that a two-layer anisotropic, heterogeneous aquifer system subjected to primarily vertically propagated tidal stresses is more realistic. In this system a thin, moderately permeable Holocene aquifer composed of unconsolidated calcareous sediments overlies a Pleistocene aquifer composed of calcareous sedi ents that have undergone diagenetic changes which produced very high effective permeabilities. The tidal signal is rapidly propagated laterally through the highly permeable Pleistocene aquifer and then moves more slowly vertically into the overlying Holocene aquifer.
This paper will summarize recent advances in our knowledge of atoll hydrogeology and present results, based on recent field hydrogeologic and numerical modeling work, of the relative effects of various geologic and hydrologic parameters on atoll lens dynamics and groundwater development.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990