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