The tragic slope failure at
the mountain of trash in the Payatas dumpsite last July 10, 2000
focused attention on one obvious geotechnical hazard of
landfills. The catastrophe also prompted this investigation of a
more subtle type of environmental engineering hazard, i.e., the
possible leakage of leachate into the La Mesa reservoir less
than half a kilometer north of Payatas.
Electrical resistivity sounding and profiling
at several strategic points near to as well as far from the
dumpsite showed a very significant difference in apparent
resistivities.
Along the north-south projection of the
landfill of garbage, the electrical sounding points registered
very low values (less than 20 ohm-m). In constrast, the survey
points located away from the longitudinal alignment of the
landfill measures much higher values (greater than 50 ohm-m to
as much as 942 ohm-m). This narrow band of low apparent
resistivities at the northern and southern projections of the
dumpsite strongly indicates groundwater laden with the chemical
by-products of leachates leaking towards the La Mesa reservoir
and the Marikina River.
This dual direction of flow of leachates at
Payatas is due to the reversal of groundwater flow direction
northwards. Leakage towards the north was induced by the rise in
the water table inside the dumpsite from an elevation originally
lower than the La Mesa reservoir to the present water table
higher than the elevation of the dam and reservoir. The
northward flow of leakage is further enhanced by the
northwest-dipping strata of the Guadalupe Formation as well as
the NNW-trending vertical faults between Payatas and the La Mesa
reservoir.
A regional magnetic survey conducted in 1990
also showed a concentration of the highest magnetic intensity
values along the north-south alignment of the Payatas-dumpsite.
This is an evidence of the increase in the metallic elements in
the groundwater borne by the outflowing leachate.
In the early 1990's,
results of geochemical tests of water samples from deep wells of
the MWSS showed abnormally elevated values of critical chemical
elements related to leachates particularly in two water wells
north of the Payatas dumpsite.
Electrical resistivity and magnetic methods
of geophysical surveying are effective in detecting contaminants
in the groundwater around landfills and in establishing the flow
direction of the leachates. Results of this resistivity surveys
can be used in conducting a quick and economical subsurface
investigation to evaluate engineering environmental conditions
around Payatas as a pre-requisite to the designing of
appropriate remedial measures.
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* Chairman, Geotecnica Corp. (The Rock Engineers)
Professorial Lecturer in Engineering Geology,
National Institute of Geological Sciences, U.P. Diliman
** Engineering Geologist, Geotecnica Corp. (The Rock Engineers),
23 Avelino St., Loyola Hts., Quezon City