San Ildefonso Water District (SIWAD) holds COC No. 273 and is classified as a Category C or Average sized water district.
Under the leadership of Mr. Galo Antonio V. Violago, the water district experiences growth and hence demand for additional water sources.
On September 27, 2018, SIWAD engaged the services of Cresencio A. Sosa (Geo-i Consultancy), a Hydrogeologist, to help conduct a georesistivity evaluation of its service area for the purpose finding suitable sites for water well drilling.
A total of 12 vertical electrical sounding (VES) stations were surveyed employing Schlumberger Array. The total wire spread used was 900m with an effective depth of investigation of 180 – 200m.
Georesistivity, geoelectrical and electrical resistivity, are terms used interchangeably, that pertain to non-invasive geophysical method which involves the introduction of direct current into the ground. Many arrays are used but for groundwater investigation, the Schlumberger Array is the most preferred method due to deeper depth of investigation and ease of field execution. Older literatures state that the depth of investigation of Schlumberger Array is about 25% to 30% of the total electrode wire length (AB). But newer literatures point to a depth of investigation to about 20%, which is more conservative and realistic. Shown in Figure 2 is the latest model of georesistivity instrument used in this survey.
Schlumberger Array (Figure 3) uses four stainless electrodes driven into the ground in straight line, two outside electrodes (AB or C1C2) are used to inject current from an external battery source and two electrodes (MN or P1P2) measure the potential drop. The resistance (R) of the ground for each reading can be computed from the current (V) and potential drop (I), which is then multiplied to the geometric factor (K) corresponding to the particular reading. The product of the resistance multiplied by the geometric factor is the apparent resistivity ρa; apparent because as the depth of investigation goes deeper, variability in resistivity of different layers of lithologic materials is encountered, and the georesistivity averages this upper to lower layers and reads off the apparent resistivity.
After the survey is completed the apparent resistivity (y-axis) is then plotted on the log-log paper against AB/2 spacing (x-axis). After plotting, interpretation follows. Two methods of interpretations are employed, the manual interpretation by using the Orellana and Mooney georesistivity master curves, and the automatic interpretation using computer softwares (which is very convenient and fast, and there are many commercial available on the market).
Fig. 2 Georesistivity Instrument
Figure 3. Schlumberger Array
Georesistivity points were located strategically throughout the entire San Ildefonso service area. Shown in Figure 4 is the map of the VES Points and the corresponding coordinates. Another criterion in the location of the sounding points were in reference to the pumping stations, in order to establish fill in data to the existing lithologic and aquifer characteristics data already obtained from the drilling of the existing pumping stations.
The field georesistivity survey data are given in Annex 1. Interpretation of individual sounding points (VES), including recommendation for drilling depths are given in Annex 2.
Photographs of field georesistivity survey in action can be found in Annex 3.
Based from the results of georesistivity and from the information that can be extracted from Figure 5 which is the transmissivity and specific capacity map, new additional well drilling can now be safely located. From all the data gathered the following recommendations are hereby given.
a. Priority Areas for Drilling No. 1 Ves-04, 05, 06, 07, and 11
b. Priority Areas for Drilling No. 2 VES-02, 08, 09 and 10
c. Priority Areas for Drilling No. 3 VES–03 and 12
Site VES-01 is not recommended for drilling, except if the drilling will be for data or information gathering purposes only. Recommendations on the depth of drilling are given for each VES stations at Annex 2 – Interpretation and Recommendation.
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