The Search for Groundwater
in a Geologically Complex Area: The Romblon Island Group, Central
Philippines
L.T.
Armada1, C.B. Dimalanta1, E.G. L. Ramos1,
B.D. Payot2, R.A. Tamayo Jr.1, G.P. Yumul Jr.1*,
K.L. Queaño3, E.J. Marquez4
and J.A.S. Gabo1
1Tectonics
and Geodynamics Group, National Institute of Geological Sciences,
University of the Philippines, Diliman, Quezon City, Philippines
2Kanazawa University,
Kakuma-machi, Kanazama City, Ishikawa 920-1192, Japan
3Mines and Geosciences Bureau,
Department of Environment and Natural Resources,
North Avenue, Diliman, Quezon City, Philippines
4Department of Physical Sciences and Mathematics,
University of the Philippines, Padre Faura St., Metro Manila,
Philippines
*also with the Department of Science and Technology,
Bicutan, Taguig, Metro Manila, Philippines
Abstract
The study
area is situated in a geologically complex region, an arc-continent
collision zone between the Palawan Microcontinental Block and the
Philippine Mobile Belt. In such areas, the search for groundwater is
difficult, compounded by intense deformation and the crystalline
nature of the underlying rocks. These crystalline rocks,
specifically igneous and metamorphic rocks, are not good aquifers.
However, these rocks develop secondary porosity as a consequence of
intense fracturing brought about by tectonism. This porosity is
further enhanced by weathering due to percolation of meteoric waters
on these fissures on crystalline rocks.
To
address the water needs of communities in these hard rock terranes,
site investigations in the Romblon Island Group were carried out.
The study areas were selected based on their locations with respect
to observed and mapped fracturing and faulting in crystalline rocks.
These areas are in Magdiwang and San Fernando, Sibuyan; Bagacay and
Sawang in Romblon; and Anahao and Poctoy in Tablas. The extensive
sedimentary and alluvial deposits were also surveyed. Forty-two (42)
vertical electrical soundings (VES) were carried out in selected
sites in the Romblon Island Group in west central Philippines.
Schlumberger arrays with a maximum spread of 300 meters provided
vertical images of electrical resistivity changes in depths of up to
60 meters. VES results show significantly lower resistivity values
for the regolith (i.e. regolith: ~10 to 250 ohm-meters) than those
of the parent units (i.e. ultramafic rocks: ~800 ohm-meters and
metamorphic rocks: 1000 to 2000 ohm-meters) as observed in study
areas in Magdiwang and Bagacay, Romblon. A wide range of values (7
to 2500 ohm-meters) characterizes the resistivity of the sedimentary
rocks and alluvium, particularly in the areas of San Fernando,
Anahao and Poctoy. This variation is a function of clast size and
water saturation of the deposits. Georesistivity sections generated
from the VES data suggest the following: 1) low resistivity value
distribution in the subsurface is attributed to faulted and
fractured rocks; 2) thick regoliths are associated with the
identified faults; 3) the water-bearing layers are the regolith and
the recent alluvial deposits mantling the underlying rocks; and 4)
the secondary porosity developed in the parent units is not
sufficient for significant groundwater accumulation as compared to
the regolith. |