Geological, Geohazard and Hydrogeological Assessment as Part of Environmental Baseline Study Prior to Mining: A Vital Input for Safe and Environmental-Friendly Mineral Development

Juanito A. Manzano, Jr.¹, Liza S. Manzano² and Edward S. Lucero^3,*1

Chief Geologist and ³Senior Geologist
Geosciences Division^*
email address: cguwapo@yahoo.com
²Supervising Science Research Specialist
Mining, Environment and Safety Division
Mines and Geosciences Bureau-Reg. X

Abstract

This study investigates the geological and geohazard potentials of the MPSA applied area of Holcim Philippines Lugait-Plant at Brgy. Tawan-Tawan, Initao, Misamis Oriental. This activity was part of the baseline study on the environmental setting of the area prior to the actual mining operation. The MPSA applied area of Holcim Philippines, which is about four blocks, is equivalent to 324 hectares and bounded by coordinates 124°20’00 and 124°21’00 east longitude and 08°28’00 and 08°29’00 north latitude. Based on geological mapping survey, spatial and temporal distribution of different rock units in the area can be delineated. The resulting geological data was correlated with groundwater data so as to provide inputs on the hydrogeology including the water resources. Likewise, based on this geological data, preliminary assessment on the total reserve deposits in the said area can be achieved. The output of this study will serve as bases for planning of a safe and environmental-friendly mineral resources development in the applied MPSA area.

The MPSA area is blanketed by two rock formations, namely, in order of decreasing age, Tood Formation and Opol Formation, respectively. Exposed outcrop of Tood Formation can be encountered on the fourth quadrant of the area while Opol Formation comprised the rest of the quadrants. This study proposed three distinct members of Opol Formation, namely, LOWER MEMBER (conglomerate-sandstone sequence and agglomerate); MIDDLE MEMBER (interbeds of tuff and tuffaceous clastics); and the UPPER MEMBER (probably of dacitic tuff).

Interestingly, the source rock of the low silica shale, which will be used as raw material for a highly specialized type of export cement, is the middle member of Opol Formation. This rock/lithological unit is mostly encountered at the western and northeastern part of the area. Based on preliminary field survey, this member exhibits a much thinner deposit on surface. Economically, this implied lesser reserve of silica shale deposits, lesser productivity.

In terms of groundwater resources, the lower member of Opol Formation appears to be the water-bearing layer. This is conformably overlain by the middle member of the said formation and acts as confining layer. This aquifer is somewhat considered to have a fairly good amount of water yield in the area. Groundwater data shows significant total coliform count. This is most likely attributed to groundwater contamination, which may be contaminated by surface water. In addition, this study concludes that the water in the vicinity is ‘inadequate’ and derived primarily from surface water. This finding is consistent with the previous study of Manzano, Jr. (2005).

In terms of geological hazard, which poses threat to humans (and/or properties), mass movement appears to have greatest impact in the applied MPSA area and its immediate vicinities. Five types of mass movement can be encountered and these are; Wedge, Toppling and Circular Failures, Creep and Combination of Rock and Debris Slides. Remarkably, these types of mass movement (slope instabilities) can be seen in Opol Formation.

Lastly, this study proposes a semi to detailed geological mapping survey in the applied MPSA area. This future undertaking will better evaluate/assess the said area in terms of geology, possible measures on the impact of geohazard potentials, hydrogeology and the resources (i.e., total reserve deposits). Similarly, recommendations on proper location of drainages for quarry operation, location of mitigating measures (i.e., siltation pond), and design depth of pits so as not to contaminate the groundwater system in the area will be possible. Consequently, this project proposal serves as enormous challenge to Holcim Philippines.