RECONSIDERING HAWAII'S HYDROGEOLOGIC CONDITIONS IN LIGHT OF RECENT EXPLORATION RESULTS

Some recent, and not so recent, results of exploratory drilling in Hawaii have suggested that Hawaii’s hydrogeology may be more complex than has been generally recognized. Instead of a more-or-less homogeneous pile of highly permeable eruptive basalts that are intermittently punctuated by volcanic dikes confined to calderas and rift zones, we are finding that dike compartmentalization is occurring outside of recognized rift zones, leading to significantly higher volumes of stored groundwater within the island. Further, sub-horizontal variations in permeability, associated with significant changes in eruptive character (e.g. explosive vs effusive activity) are acting as significant perching and confining bodies over significant aerial extents. Although these latter features have not yet been fully characterized, the observations made so far suggest that they also contribute to increased storage of recharge. However, they also appear to significantly impact subsurface groundwater flow in ways that are not accounted for in traditional methods of computing sustainable yields for near shore aquifers: where buried confining formations extend to depths well below sea level, higher elevation recharge is being intercepted and diverted to deep submarine groundwater discharge well below depths that are typically investigated or quantified. Whereas this diversion of high elevation recharge can be perceived as a threat to water-starved coastal aquifers, it may also offer an alternative to basal groundwater wells for coastal communities while having minimal impacts on nutrients supplied to near shore marine ecosystems whose survival are dependent on shoreline groundwater discharge.

The results of some recent drilling programs will be presented along with a discussion of the implications of those findings for understanding Hawaii’s groundwater hydrology.