By Hydrogeology and Environmental Geology Section
The third 2022 1:50:000 Scale Groundwater Resource and Vulnerability Assessment (GRVA) of MGB, RO-IX Geosciences Division will commence on June 28, 2022, in the Municipality of Siay, Zamboanga Sibugay. 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 will cover the entire Municipality of Siay, composed of twenty-nine (29) barangays and their primary water sources.
The municipality occupies portions of the Sibuguey, Siay, Buayan, and Kabasalan 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 Sibuguey 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 completed the fieldwork from June 28 to July 24, 2022, and assessed all the barangays of the municipality that includes Barangays Bagongsilang, Balingasan, Balucanan, Bataan, Batu, Buyogan, Camanga, Coloran, Kimos, Labasan, Lagting, Laih, Logpond, Magsaysay, Mahayahay, Maligaya, Maniha, Minsulao, Miranga, Monching, Paruk, Poblacion, Princesa Sumama, Salinding, San Isidro, Sibuguey, Siloh, and Villagracia. There are no vertical electrical sounding (VES) points established in the municipality through the conduct of georesistivity surveys due to a non-operational equipment.
Water Point Inventory
The groundwater-related source inventory included sixty-one (61) sites that includes deep well, shallow/dug well, and springs (Figures 08 and 09). Communities are generally dominated by shallow wells, which are present in almost all barangays and account for 42% or 26 sites of all water sources surveyed. Spring sources account for only 38% of the inventoried sites. A total of twelve (12) deep wells were identified in the barangays of Coloran, Dacanay, Kimos, Labasan, Logpond, Mirangan, Monching, Poblacion, Salinding, and Sibuguey. The deep wells account for 20% of the total surveyed sites. In terms of usage, a total of 34 sites are utilized for domestic use and 27 sites are for drinking, with 1 deep well for commercial distribution.
Water point inventory also collects data on discharge from springs and static water level (SWL) from shallow/dug and drilled deep wells whenever available. If available, this data can also be obtained from the records of the local authority responsible for the water system. For shallow and dug wells, at least 25 sites have SWL measurements ranging from 0.1 m to 15.5 m. For drilled deep wells, SWL ranges from 4.5 m to 34.2 m depth. Measurements from springs Flow rates ranged from a weak 0.06 liters per second (lps) to 3.53 lps. This groundwater discharge represents the natural withdrawal of groundwater from the underlying reservoir.
In-Situ Test Results
The summary of results presented here describes the physical properties of water obtained from the multi-probe handheld device used during the program. Water pH and total dissolved solids (TDS) results are compared to the Philippine National Standards for Drinking Water (PNSWD) of the Department of Health (DOH) under Executive Order (A.O.) No. 10, Series 2017. Salinity and conductivity values were compared to standards published in selected literature.
The concentration of the physical properties of the surveyed water sources are summarized in the table below and each parameter is discussed in the sub-section.
Table 1. Summary of the physical test conducted on the inventoried water sources.
A pH is a measure of hydrogen ion activity, which means that it tells how acidic or basic the water is. pH is not a pollutant, but it is a chemical master variable (https://www.worldbank.org/). The power of hydrogen (pH) is a measure of the water’s acidity or basicity that goes from zero (0) to fourteen (14), with seven (7) being neutral. A pH of less than 7 indicates acidity, whereas a pH of greater than 7 indicates a base. The pH determines the solubility of substances, like chemical constituents such as nutrients and heavy metals, in the water.
The permissible pH value of drinking water specified by the National Standard for Drinking Water is 6.5 to 8.5. Based on field test results of inventoried water sources, pH values were between 5.3 and 7.6, with most sources below the acceptable NSDW standard for drinking water (Figure 14). This pH range can be considered slightly acidic to alkaline. Although unacceptable for drinking, these water sources can still be utilized for domestic usage as mostly noted during the assessment. A local spring source at Brgy. Villagracia returned the lowest pH of 5.3 which is used for drinking water while the deep well in Brgy. Coloran has the highest pH value at 7.6. Eleven (11) sites comprised of springs and deep wells consumed for drinking are below the specified standard. The sites are located in the barangays of Buyogan, Paruk, Mahayahay, Villagracia, Magsaysay, Mirangan, Labasan, San Isidro, Monching, and Dacanay. As shown in Figure 15, pH above 6.5 are randomly distributed throughout the municipality while a large portion has been delineated to have pH values outside the permissible level.
It should be noted that the field inventory was conducted during a rainy season in the municipality which could potentially affect the pH results of the groundwater sources. pH can also fluctuate with precipitation (especially acid rain) (https://www.fondriest.com/environmental-measurements/parameters/water-quality/ph/).
Total Dissolve Solids (TDS)
The Total Dissolved Solids (TDS) concentration is the sum of the cations and anions in the water (Oram, B., https://www.water-research.net). The test provides a qualitative measure of the quantity of dissolved ions but does not tell the nature or ion relationships (Oram, B.; https://www.water-research.net).
The National Standard for Drinking Water in the Philippines for TDS is set at a maximum permissible level of 600 ppm. TDS results from the water sources investigated ranged from 17 ppm up to 1001 ppm with one site that went beyond the detection limit of the handheld equipment (Figure 16). Two deep wells in the southern part of the municipality returned elevated values above the permissible level. The deep wells in Brgy. Salinding (SIA-12) and a shallow well in Laih Miranda (SIA-42), with a TDS value of 1001 ppm and beyond the detection limit can be classified as brackish water based on the type of groundwater classification by Freeze and Cherry, 1979 (Table 02). The drinking water from a deep well SIA-49 in Brgy. Logpond is still considered freshwater based on this classification despite its TDS at 696 ppm. It should be noted that these sites are located in the southern part of the municipality that forms part of the deltaic environment of the Sibuguey river.
Figure 17 shows the distribution of TDS values in the municipality. TDS values in the most municipality are significantly below the set limit. This is attributed to its underlying geology compared to the southern part of the municipality which is mainly underlain by a recent deposit.
Table 2. Groundwater classification based on TDS (Freeze and Cherry, 1979)
The salinity standard reported here follows the general freshwater salinity value which is expressed as parts per thousand (ppt). Salinity is a measure of dissolved salts in water that is affected by changes in weather patterns (e.g., droughts or storms) or an increase in urban runoff and sewer discharge (https://sfyl.ifas.ufl.edu/). As the concentration of dissolved salts in water increases, their quality tends to decrease. Freshwater has a salinity of 0.5 ppt or less (https://sfyl.ifas.ufl.edu/).
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). During a radio show Tinig Klima (August 2022), Pagpasok ng Tubig Alat: Panganib sa Seguridad ng Tubig, of DENR Climate Change Service and RMN DZXL 558 Manila, Engr. Susan Abano, Chief, Policy and Program Division, NWRB-DENR pointed out that the salinity limit for drinking water particularly for groundwater is 900 ppm or 0.9 ppt.
The salinity values for the inspected water sources range from 0.01 to 2.04 ppt or 10 to 2037.84 ppm (Figure 18). The shallow well in Brgy. Laih Miranda (SIA-42) which also registered 4000 micro-Siemens and beyond the detection limit in TDS is considered brackish groundwater. Elevated salinity value is also noted in the deep well SIA-12, a deep well in Brgy. Salinding with a conductivity value of 1380 micro-Siemens and a salinity value of 0.57 ppt or 570 ppm. Another deep well, SIA-49, is used for drinking in Brgy. Logpond exhibit an elevated salinity value of 0.42 ppt, this site also returned a TDS above the specified drinking water standards. These two deep wells can be monitoring sites to indicate if brackish groundwater up-coning is already present around their locality.
As shown in Figure 19, brackish groundwater sites are located in the southern section of the municipality which could potentially indicate a landward movement of seawater as these areas are either developed into an aquaculture farm or adjacent to an estuary and mangrove sites.
Table 3. Suitability of Groundwater based on Conductivity and Total Dissolved Solids (Richards, 1954).
Conductivity is a measure of water’s capability to pass electrical flow and this is directly related to the concentration of ions in the water. These conductive ions come from dissolved salts and inorganic materials such as alkalis, chlorides, sulfides, and carbonate compounds (https://www.fondriest.com/environmental-measurements/parameters/water-quality). Such a parameter gives an indication of groundwater quality on salinity that can in turn be used 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.
The values for the conductivity of the examined water sources range from 10 to 4000 μS (Figure 20). The SIA-42 site that has the highest conductivity value is also among the sites with the lowest pH and also the peaked value for TDS and salinity considering that it is directly related to these parameters. As such, conductivity values in the southern part of the municipality can be monitored to ascertain the landward progression of seawater. Elevated values are mostly noted in this part of the municipality as shown in Figure 21, similar to other parameters that are directly related or correlate to conductivity.
As shown in the contour map of conductivity, <250 μS generally prevails in the municipality. Elevated to extremely anomalous values were measured in the southwestern part of the study area.
Temperature is an important water quality parameter as it can influenced also other physicochemical characteristics 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 field data measurements, the temperature at the source within the municipality varied from 26.10C to 33.70C with an average temperature of 28.6.50C (Figure 20 and Figure 21).
The maximum temperature measured from SIA-12 in Brgy. Salinding is 33.70C which also has the highest TDS and elevated conductivity values. At 30-350C temperature can lead to an increased release rate of metal concentration (Li, H., et. al, 2013). A hot spring was also noted in Purok 7, Brgy. Siloh (SIA-62) with a temperature of 700C. Its TDS also returned outside the limit of detection and a significantly high conductivity at 7,830 μS despite having a near neutral pH of 7.2. Such a site was not included in the discussion of TDS, conductivity, and salinity as the nature of the groundwater is distinct. The temperature of the water most likely affected the other physicochemical parameters investigated on-site.
Potential Aquifers of the Study Area
At least three main aquifer types can be inferred in the study area, the highly productive aquifers of alluvium or recent deposits and the sedimentary sequence, and the moderate to highly productive volcanic sequence. Two (2) Type-I hydrogeologic units were identified in the area, the Alluvium and the Lumbog Formation that host multiple shallow wells and deep wells in the southern part of the municipality. The rock units of the Zamboanga Volcanic Complex are categorized as Type II-A hydrogeological units composed generally of andesitic and volcanogenic derived sediments. This aquifer type can be found in the following barangays hilly to mountainous barangays of Siay where spring water sources abound.
The predominating hydrogeological rock units in the Municipality of Siay are volcanic and volcanogenic rock units of the Zamboanga Volcanic Complex, the sedimentary package composed of sandstone and mudstone layers of the Lumbog Formation, and the vast recent deposits that covered the sedimentary package. The majority of the inventoried water sources were hosted in the sedimentary package and the recent deposits particularly deep wells and shallow wells. Springs were mostly confined in the elevated section of the municipality that is dominantly underlain by the volcanics and volcaniclastics.