Background
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.
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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