WRRC SeminarNovember 7, 3:00pm - 4:15pm
Mānoa Campus, POST 126
Island Hydrology: Time to take a closer (and broader) look at confining structures
Don Thomas, Director, The Center for the Study of Active Volcanoes (CSAV), University of Hawaii at Hilo.
The recent completion of a 1760 m deep core hole, from a surface elevation of 1946 mamsl, in the Humu'ula Saddle region of the Big Island has recovered a continuous stratigraphic core sequence from the surface to its final depth. The geologic and hydrologic conditions encountered were strikingly different from those expected from our conventional view of Hawaii's island hydrology. The stratigraphic column showed quite high levels of fracture permeability near the surface but a strong depth dependence down section: the deeper formations showed a much higher degree of compaction and secondary mineralization than we typically consider in assessing deep fluid flows. The hydrology encountered in the borehole proved equally divergent from our expected conditions: perched aquifers were found at elevations of 1794 mamsl, 1733 mamsl, 1550 mamsl, 1540 mamsl and 1515 mamsl; and the "basal" water table was encountered at an elevation of 1396 mamsl. Coring recovered the perching formations that, although numerous, were individually quite thin and made up only a small fraction of the entire section. The perching layers contain a higher proportion of clay-rich fines and were as thin as a few 10's of cm indicating that these impermeable layers play a much larger role in subsurface groundwater infiltration and flow than their relatively small contribution to the stratigraphic section would indicate.
Geophysical surveys conducted prior to siting the test hole now suggest that the deeper aquifers encountered may extend beneath the entire Saddle region. The abundance of "basal" groundwater in a region of limited rainfall (300 mm to 600 mm per year), was initially puzzling; however recently published analyses of the gravity field around the state (Flinders, et al., 2013) postulate that much of the Saddle region is underlain by a broad dike complex and, hence, the "basal" groundwater inferred from the geophysical data is best characterized as a collection of dike-impounded aquifers rather than a mobile basal groundwater aquifer.
The Flinders et al. gravity field analyses are interpreted to show a very extensive dike complex extending from Kilauea to Kohala volcano on the Big Island suggesting that groundwater storage and flow within the island's interior is more dependent on dike distribution than has generally been recognized. Follow-on magnetotelluric surveys, that will be coupled with additional gravity work, is now underway for the Hawaii island in effort to better characterize the extent of these postulated dike-impounded systems.
Water Resources Research Center, Mānoa Campus
Philip Moravcik, 808-956-3097, email@example.com