Progress Report: 1:10,000 Scale Groundwater Resource and Vulnerability Assessment in the Municipality of Godod, Zamboang del Norte

By Hydrogeology and Environmental Geology Section

The second 2022 1:50:000 Scale Groundwater Resource and Vulnerability Assessment (GRVA) of MGB, RO-IX Geosciences Division commenced on May 10, 2022, in the Municipality of Godod, Zamboanga del Norte. This project is part of the groundwater resource study of the entire Zamboanga Peninsula for 2022, covering three (3) municipalities: Bayog, Zamboanga del Sur, Godod, Zamboanga del Norte, and Siya, Zamboanga Sibugay. The program’s primary objective is to identify the groundwater availability of the municipality by determining its various water sources, whether surface water or groundwater. The data gathered will update (1) the 1:250,000 scale Hydrogeological and Groundwater Availability Map to a 1:50,000 scale; and (2) the database of various water resources in the municipality. Such information can help manage and develop water resources, land-use planning, and appraising land-use classification and allocation. The program covered the entire Municipality of Godod, composed of seventeen (17) barangays and their primary water sources.

The municipality occupies portions of the Salug, Mucas, Palandok, and Lipakan River Basins. Therefore, the data collected during the municipal water sources inventory, including the water’s physical characteristics, will be utilized for the groundwater resources vulnerability assessment of the entire Salug River Basin. This assessment aims to characterize the river basin’s groundwater resource vulnerability to surface pollutants derived from various anthropogenic and geological processes, including the possible impact of climate change.

The GRAVA Team scheduled the fieldwork from May 10 to June 6, 2022, and assessed Barangays Baluno, Mauswagon, Lomogom, Bunawan, Sianan, Dipopor, Sioran, Raba, Limbonga, Rambon, Guisapong, Banuangan, Miampic, Sarawagan, Dilocot, Poblacion, and San Pedro. There are no vertical electrical sounding (VES) points established in the municipality through the conduct of georesistivity surveys due to a non-operational equipment.

Figure 1. A map showing the lithologic units that underlie the Municipality of Godod, Zamboanga del Norte.

INITIAL FINDINGS

Figure 2. Water point inventory distribution in the Municipality of Godod, Zamboanga del Norte.

The inventory of groundwater-related sources totaled (49) forty-nine sites. Generally, the municipality is dominated by spring sources that are present in almost all barangays and accounted for 86% or 42 sites of the total water sources surveyed. Shallow/dug wells only accounted for 6% that is commonly found in flat-lying barangays. Four deep wells were identified in the barangays of Baluno, Limbonga, Miampic, and Raba that constitutes 8% of the total inventoried sites.

Water point inventory also collects discharge rate data of springs whenever available and from records in the municipal government handling the water distribution system. Based on records obtained from spring sources and intake boxes, discharge rates of small springs vary from 0.3 liters per second (lps) to as high as 0.9 lps. This groundwater discharge represents the natural withdrawal of groundwater from the underlying reservoir.

Only a handful of inventoried wells have static water level measurements and information. Two shallow wells in Brgy. Limbonga registered a static water level of 3m and 7.8 m from GDD-29 and GDD-31 respectively. These wells are for domestic use and are potentially hosted in a sedimentary package that’s is common within and around the area. A deep well in Brgy. Raba has an SWL of 32m and a well depth of 57.6m.

Figure 3. Water source inventory distribution and usage.

Potential Aquifers of the Study Area

The classification hydrogeologic units present in the study area is shown in the Hydrogeologic Map of the municipality. In addition to the types of hydrogeological units, the inventoried water sources in every barangay are also reflected on the map. Based on the assessment, at least three main aquifer types can be inferred in the study area, the limestone unit of the Sibuguey Formation, the volcaniclastics and volcanic units of the Zamboanga Volcanic Complex, and the sedimentary package of Zamboanga Formation. Although considered as an aquifer, the Ophiolite Complex and minor Chert in the north-western part is still an important hydrogeological unit if shallow groundwater resources are considered. There were no identified water sources underlain by this hydrogeological unit.

Figure 4. Hydrogeologic Map of the Municipality of Godod, Zamboanga del Norte.

Water Quality (In-Situ Test Results)
pH

The permissible pH value of drinking water set by the National Standard for Drinking Water is at 6.5 to 8.5 pH. Based on the in-situ test results of the inventoried water sources, pH values range from 5.9 to 6.8 thus majority of the water sources are mostly below, the permissible NSDW standard values . Most of these water sources are being utilized for potable use. GDD-40 returned the lowest pH value at 5.9, a localized source located in Brgy.Banuangan. The pH of this spring is likely affected by the surface run-off that affects the water in the small intake box. Turbidity and color are also affected every time a heavy downpour occurs around the area. Nineteen (19) of the samples are within the permissible value for drinking water with the highest value at 6.8 pH. The shallow wells and deep well in Raba and Limbonga are within the permissible value at pH 6.7 although the shallow wells are exploited for domestic consumption. Spring sources in Rambon, Bunawan, Raba, Dipopor, Limbonga, Guisapong, Sarawagan, Dilocot returned pH values within the acceptable range. This is shown in the thematic map as an extensive cluster of pH values above 6.5. Patchy distribution can be also noted in the southern and eastern area.

Variation plot of pH values.
Figure 5. Contour map of pH values

Total Dissolve Solids (TDS)

The national standards for drinking water in the Philippines is set at the maximum allowable value of 600 ppm. The TDS results from the surveyed water sources range from 15 ppm to a maximum value of 350 ppm that is below the limit of TDS set by PNSWD. the distribution of TDS values in the municipality can be clustered into <150 and above 150 ppm. The eastern half of the municipality is dominated by TDS values < 150 ppm while on the western portion, clusters of >150 ppm TDS dominates. Values greater than 300 ppm can be also noted on the western side of Godod albeit patchily.

Variation plot of Total Dissolve Solids (TDS)
Figure 6. Contour map of TDS values of inventoried water sources.

Salinity

Salinity values were calculated based on the measured temperature and conductivity of the water sample. An excel template format for conversion developed by James Douglas is used here. It is based on the conversion factors published in the 1983 Technical Paper from UNESCO- “Algorithms for computation of fundamental properties of seawater” (http://jamesgdouglass.blogspot.com/). The reference conductivity is 42,900 micro siemens per cm2. HANNA instrument measures conductivity as milli-siemens thus the values are converted into micro siemens before calculating the salinity. Calculated salinity is measured in PSU (denoting practical salinity unit), a unit practically related to the seawater properties. PSU is equivalent to parts per thousand (ppt).

The salinity values for the surveyed sources range from 0.01 to 0.22 ppt. The peak value of 0.22 ppt is from GDD-025 that also corresponds to the highest TDS and conductivity values among the surveyed water sources. GDD-025 is the main water source in Upper Brgy. Raba that is located below the stream channel. It is suspected that the springs source can be affected by surface run-off during prolonged rain.

Variation plot of salinity values.
Figure 7. Contour map of the salinity values within the Municipality of Godod.

Conductivity

The values for the conductivity of the surveyed water sources range from 10 to 480μS (Figure 19). Based on the values obtained during the survey, most water sources returned a conductivity value below 250 μS. Such can be considered as moderate saline as categorized by Richards (1954). Conductivity and Total Dissolved Solids (TDS) can be used for assessing the quality of water for domestic use and irrigation purposes as categorized by Richards (1954) in Agricultural Handbook 60 of USDA. Based on the suitability of water for irrigation by Richards, 1954, the values below 750 can be considered as low to moderate saline and is safe for irrigation purpose.

Based on the contour map for conductivity, a clustering of <250 μS and above 250 μS values can be deduced in the area. Values below 250 μS dominates in the eastern part of Godod while a cluster of >250 μS values are noted on the north western side of the municipality particulalry in the adjoining barnagays of Sarawagan, Guisapong, Rambon, Mauswagon, Raba and Limbunga.

Conductivity values variation plot
Figure 8. Contour map of conductivity values of surveyed water sources.

Temperature

Temperature is an important water quality parameter as it can influenced also other physico-chemical characteristic of groundwater. The measurement of the groundwater sources was only conducted once thus there is no current reference on the fluctuation of the water temperature over time. Based on the measurements, temperatures of water within the municipality range from 250C to 31.50C with an average temperature of 27.50C

Variation plot of temperature values.
Figure 9. Thematic map of temperature values.
Photo 1. GDD-37 (8° 2’ 4.1”N, 122° 46’ 19.8”E) is a spring in Purok 4 utilized for domestic purposes. It is a level 1 source and was developed via Kalahi Project. The spring has a good discharge but water get murky during rainy season.
Photo 2. GDD-38 (8° 2’ 13.6”N, 122° 46’ 7.9”E) is a domestic-used spring in Purok 6 owned by Mr. Dodoy Makahis. It was developed with intake box but can still be affected by flooding.
Photo 3. GDD-32 (8° 1’ 28.5”N, 122°47’ 11”E) is one of the inventoried spring in the barangay. It is a level 1 water source and it flows through the bedding plane.
Photo 4. GDD-02 (8° 2’ 59.4”N, 122° 51’ 10.8”E) is a potable spring in Purok 3 managed by the barangay. The spring is situated within a rolling terrain and serves as main water source for Purok 7.

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