Pyroclastic deposits of the Mt. Pinatubo eruption in 1991 have been transported as Lahar (Mud flow) by rainfall and inflicted damage to the area surrounding the Mt. Pinatubo. changes in the quality and quantity of groundwater have affected the regional community, which mainly depends on underground water for water resources.
The purpose of this study is to understand groundwater flow and recharge in the Lahar disaster area around Pasig-Potrero and Porac river basins, Pampanga province, Philippines.
Resistivity sounding was applied to measure underground water level, and a water analysis was conducted to understand the regional distribution of ion concentration.
A landform classification was carried out to consider the influence of landforms on underground water flow and regional distribution of water quality. The former landforms (before 1991 eruption) were classified into mountain, alluvial fan at volcanic foot, alluvial fan (Porac, Pasig, Abacan), delta, and flood plain. Underground water flow and regional distribution of water quality were prescribed by these former landforms, except in the area surrounding Sand-Pocket. From the fact that the deposition of lahar in Sand-Pocket has raised the bed of Pasig-Potrero River, it is considered that river water has intended to infiltrate into the underground water in the area surrounding Sand-Pocket. The influences offormer landform and artificial landform (Mega Dike) were pointed out as factors regulating characteristic underground water flow and distribution of water quality in this area.
Spatial distribution and seasonal changes of underground water flow and water quality were obtained from observed results of resistivity soundings and analytical results of water quality. Underground water flow, which had small seasonal changes, was classified into 3 regional groups (IIII). The existence of a peculiar regulating factor for underground water flow was suggested. In addition, monthly changes of water level indicate a translocation of the recharge area. The propagation speed of recharge area was related to the true speed of the translocation of recharge area, which was estimated from the results of a pumping test and from calculations of the apparent velocity of underground water flow. Furthermore, the distribution of SO42-
concentration showed a good correspondence with underground water flow. In particular, the high concentration area of SO42-
was distributed inside the Mega-Dike. From the results obtained, a model was suggested to explain the formation of water quality in this area characterized by the underground water flow from Pasig-Potrero River.
From above results, groundwater flow and recharge in Lahar disaster area around Pasig-Potrero and Porac river basins were understood.
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