Application field

Sorsogon City groundwater investigaion

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Ⅰ、An introduction


Water supply in Sorsogon City is being catered by Sorsogon City Water  District (SCWD). At present Sorsogon City Water District derives its water  sources from twelve (12) operational pumping stations and five (5) spring  sources. Table 1 below gives the water production by year of Sorsogon City  Water District for the period 2015 up to the third quarter of 2019. 

 According to Engr. Mark Anthony Anonuevo of Primewater, the monthly  water consumption of the water district concessionaires is about 19.5 cubic  meters per service connection at per capita consumption of 120 liters per  second. For comparison, the record of SCWD water production and service  connections was extracted from LWUA Databank and shown in Table 1a. 

Table 1. Water Production of SCWD

Table 1a. Water Production of SCWD and Service Connections in 2015

SCWD Pumping Stations

The operational pumping stations (12 PS) of SCWD produce groundwater  in excess of 10Lps, except PS 9 Pamurayan (2 Lps), PS 10 Guinlajon (9  Lps), PS PS 11 San Pascual (10 Lps), and the newly constructed PS 16  Madan-an (4 Lps). 

Three pumping stations were abandoned because of saltwater intrusion (PS  3 Seabreeze) and low yield (PS 6 Abuyog 1 and PS 7 Abuyog 2). Salt water intrusion occurs when a well is drilled near the coastline and the pumping rate is quite large thus inducing the advance of saltwater inland. 

Figure 1 shows the distribution of the SCWD pumping stations. 

Ⅱ 、Study methodology

The approach done in this study is a combination of research of past waterrelated activities in the province of Sorsogon with emphasis given to  Sorsogon City and fringe municipalities, and actual fieldwork. The  investigation was undertaken through the following activities: 

a. Spring source inventory

b. Groundwater source inventory

c. Surface water source inventory

d. Evaluation of rainfall pattern in Sorsogon City, Province of  Sorsogon 

e. Hydrogeological investigation of geologic rock formation


second stage of the groundwater investigation. A total of sixteen  (16) Vertical Electrical Soundings (VES) were distributed at selected  locations throughout the city.

In the selection of locations, the elevation and geology of the area were  taken into consideration. Areas with high elevation are considered hard  formation, and therefore characterized by difficult groundwater occurrence.  Areas that are gently sloping to low elevation are considered to have good  potential for groundwater development.

Georesistivity is an excellent tool for groundwater exploration because it  can detect the differences in resistivity of various rock formations. Rock  formations with low resistivity in contrast to surrounding and adjoining  formations, both horizontally and vertically, indicates that there may be a  favorable formation saturated with water.  

VES Stations 

To determine the most favorable locations where to drill new wells, sixteen  (16) VES Stations were measured as shown in Figure 6. As can be noted,  the measurements were located in low-lying areas. The low-lying areas are  interpreted as recent alluvial deposits and sedimentary formation. High  elevation areas indicates hard and very competent rocks and therefore are  not likely to host reliable source of groundwater. 

Three (3) VES Points were sited near the high elevation areas like SOR-06  in Bgy. Salvacion, SOR-02 in Bgy. Basud, and SOR-03 in Bgy. Bulabog, but  the results of the interpretation suggest that these areas are not candidates  for deepwell drilling.

All the other thirteen (13) VES Points were scattered from west to east  sections of the city.

After the georesistivity survey, the raw field data were analyzed and  interpreted utilizing a software specifically for the interpretation of the  georesistivity field data. 

We are looking at low resistivity values from about 10 to 100 ohm.m where water-bearing formation may be drilled to extract groundwater. However,  the true discharge rate or flow rate of the well can only be determined after  drilling and pump testing of the well.

Fig. 2. Project Area Showing VES Points


Interpretation of VES Stations

Automatic interpretation of field georesistivity data was employed in the  processing of raw field data.

From the interpretation, the true resistivity and thickness of the various  lithologies (rock formation) in the area were estimated, and this are given  in Annex 2.

The results of the georesistivity interpretation is tabulated below, Table 4.

Figure 7 shows the section lines A-A’, B-B’, C-C’ and D-D’. Figures 7a to 7d  shows the correlation between VES Points along lines A-A”, B-B”, C-C”, and  D-D”, respectively. Here we can see the elevation of the individual VES  Points (vertical scale exaggerated), and the correlation of lithologies along  the section.

Fig. 3. Key to Section Lines A-A’, B-B. C-C’ and D-D


Fig. 3a. Correlation Along Section Line A-A”




Fig. 3b. Correlation Along Section Line B-B”




Fig. 3c. Correlation Along Section Line C-C”




Fig. 3d. Correlation Along Section Line D-D”




Table 4. Summary of VES Interpretation






From the foregoing discussion of the various factors relating to  groundwater, the following conclusion can be offered: groundwater supply  is still the most economical and practical source of water supply for  Sorsogon City. In consideration of all the data gathered and analyzed,  groundwater development by deepwell drilling can still support additional  water requirement for the city. The wells must be located in appropriate  location and must be drilled to the best standard as possible. 

Figure 4 shows the most suitable locations for well drilling. By locating  future drilling in the recommended sites, the failure of drilling unproductive  wells may be avoided. From the delineated recommended areas for  deepwell development, drilling may be done wherever within the map.

Recommended Criteria for Locating New Large Diameter Drill Wells 

Must be within the recommended sites for drilling 

  • Must be at least 1 km from the coastline 

  • Must be located some distance away from existing water district  pumping stations to prevent interference during pumping, and  

  • Must be drilled according to high standard of drilling and thoroughly  completed and developed


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