Republic of the Philippines Davao City Water District
FINAL REPORT Volu Volume me I. I. Main Main Rep Repor ortt
Groundwater Assessment and Development of G roundwater Flow Flow M odel for Davao Da vao Cit C ityy Aquif Aqu ifers ers APRIL 2016
EXECUTIVE SUMMARY
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
Executive Summary 1.
Introduction
Background Davao City mainly relies on groundwater for the requirements of its 1,449,296 population. Currently, the Davao City Water District (DCWD), the largest water service provider in the City, derives 99 percent of about 250,000m 3 of water it distributed daily to nearly 200,000 customers from 59 deep wells. Barangays outside the DCWD service area also obtain water from the City’s aquifer through wells. The trend towards urbanization-industrialization of the City attracts in-migration that resulted in, among others, rapid population growth and environmental degradation, such as fresh water pollution and aquifer stress. Hence, it is prudent for DCWD to assess the City’s groundwater for its sustainable use and find new sources to augment its well sources to meet the water demands of a growing population and its concomitant socio-economic development Objective The study on Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers is one of the initiatives of the DCWD in the management of groundwater of the City. The general objective of the undertaking is to investigate and evaluate the groundwater resources of the Talomo-Lipadas-Sibulan and Davao-Bunawan-Lasang Basins, where majority of the existing wells are constructed, including its impact of current extraction and vulnerability assessment. A numerical model developed under the study will be utilized as a tool to manage sustainable extraction, prevent incursion of seawater and anthropogenic pollution of groundwater, and mainstreaming climate change adaptation measures. Study Methodology The services carried out for this study include data collection, review and analysis of related documents and literatures, field investigations, surveys data management, procurement of computer hardware and software and field equipment, and consultation with stakeholders. Most of the secondary data and information were provided by DCWD while others were taken from several government agencies such as PAGASA, BRS, MGB Regional Office XI and NAMRIA. Primary data collected for this study include: i) well location; ii) hydraulic parameters; and iii) water quality. Data obtained were stored in Microsoft Excel platform. For maps acquired in various digital formats (e.g. jpeg, png, bmp), creation of shape file by digitizing using ArcGIS was made. Elevation contour lines were used to create a digital elevation model (DEM). The main software used for data handling and analysis are the following: HydroGeo Analyst (HGA), AquaChem, Aquifer Test Pro, and ArcGIS. HGA handles the acquired well data from DCWD and private entities. It aids in the development of a conceptual hydrogeologic model of Davao City in order to design a numerical groundwater flow model. AquaChem was intended for handling and manipulating water quality monitoring data. Pumping test data were handled by AquiferTest Pro software. ArcGIS, an application-oriented spatial information system, was used to (i) facilitate digitization; (ii) data conversion; (iii) integration of different spatial data and creation of thematic maps to help analyze and interpret the prevailing hydrogeologic condition; (iv) DEM needed for development of conceptual and numerical groundwater flow models; and (v) data visualization. The general procedures in the development of the groundwater flow model for Davao City aquifers followed the accepted standard methodologies used in numerical groundwater flow model for MODFLOW, a three-dimensional model which uses block-centered finite difference scheme for saturated zone. Specifically, the Visual MODFLOW Flex 2014 was the modelling software used for this project.
ES-1 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
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F I NA L R E P O R T
April 2016
Review of Past Studies
The following are the major observations and comments on the available data and literature:
3.
Hydrogeological modeling by PCEEM using Visual MODFLOW did not consider recharge as part of the input. The water resources or aquifers are found only in the volcaniclastic and pyroclastic rocks derived from the volcanic centers. Only a relatively small amount of water is obtained from Miocene-Pliocene sedimentary rocks. The PNRI study concluded that there are no confining layers in the Talomo-Lipadas well fields. This conclusion is not compatible with the existence of free-flowing wells in the well fields. Water quality data on the groundwater from DCWD wells did not include the Sodium content which is most relevant in the chemical characterization of the natural groundwater. Recent pumping tests of DCWD wells did not include static water levels on which the piezometric surface maps should be based. DCWD does not perform regular monitoring works that would show the changes in static water levels in the wells. Previous tritium dating is rather uncertain owing the low precision of the measurements and the low measured tritium values. Dating based on CFC content is restricted to ages exceeding 50 years but is more credible than tritium dating. Previous studies suggest that recharge for deep coastal aquifers originate from areas at elevations between 500 and 600mamsl. The aquifers supplying the inland wells are replenished mainly by rivers at elevation from 200 to 300mamsl. Hydraulic connection between the deep coastal aquifers and the recharge areas has not been established. Description of the Study Area
Geographical Setting Located at the southeastern part of Mindanao, Davao City is a 1st class urbanized city based on income classification. It has its coordinates at 06° 58’ and 07° 35’ north latitude and 125° 14’ and 125° 41’ east longitude. It is bordered on the north and northeast by Davao Del Norte, on the south by the Municipality of Sta. Cruz in Davao Del Sur and on the west by North Cotabato. It’s the largest city in the country with a land area of 244,000 ha. The City is divided into 3 congressional districts, which are then divided into 11 sub-districts that are broken down into 182 barangays. Physical Characteristics The landscape of Davao City is the result of geodynamic and geomorphic processes. Thrusting of oceanic crust, faulting, volcanic activities among other processes caused it to have its present form. Since a large part of Davao City is within the eastern limb of the Mindanao Central Cordillera and the Apo Volcanic Complex, a substantial part of the city is mountainous with Mt. Apo and Mt. Talomo as the prominent peaks. From the volcanic promontories on the west boundary towards the Davao Gulf on the east and southeast, the ground becomes moderately steep, rolling to relatively flat with slopes ranging from 0 to 3 percent. The eastern lowland belt is interspersed by low hills and knobs. Davao City has three distinct rock groups based on lithologic character. These are ophiolitic, sedimentary basin, and volcanic arc. The northern portion of the City is comprised of Cretaceous to Pleistocene rocks comprising the basement complex (ophiolites) and Tertiary to Quaternary sedimentary basins. The southwest part, is part of the Central Mindanao Volcanic Zone – Pliocene to Pleistocene Apo Volcanic Complex. The rest of the City is within the Davao-Agusan Basin, which is made up of Late Miocene to Pleistocene rocks, mostly sediments. ES-2 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
Davao City has five main river systems that drain into the Davao Gulf. These are Lipadas,Talomo, Matina, Davao and Lasang. Smaller river systems draining the City include Sibulan River (drains a small portion of the southern part of the City), Sirawan River, Inawayan River, Panacan River and Bunawan River. All these main and smaller natural waterways empty into the Davao Gulf. Meteorological Characteristics The City of Davao is classified under Type IV of PAGASA’s Modified Coronas Classification of Philippine Climate characterized by rainfall more or less evenly distributed throughout the year. It has average annual rainfall of 1,801.5mm. Highest monthly mean rainfall is 190.7 mm in June while March is relatively dry with mean rainfall of 97.2 mm. The mean annual temperature is about 27.9°C Existing Land Use Davao City has a total land area of 244,000 hectares categorized into various uses. Nearly half of the City’s land area is categorized as grass or pasture land. Almost one-third of the City is utilized for agricultural purposes. The dominant use of the urban core is for residential, which makes up 3.44 percent of the City’s total area. Key Socio-Economic Features Based on NSO 2010 census, Davao City registered a total population of 1,449,296. The City’s average annual growth rate from 2007 to 2010 is 2.06 percent. In general, the annual growth rate is decreasing. In terms of economy, Davao City serves as the main trade, commerce, industry and financial hubs of Mindanao. With more investments coming in to complement the needs of existing industries, there is an influx of population. This propels the growth of local real estate sector as well as the construction sector. Although the City is fast becoming urbanized, agriculture remains the largest driver of the local economy. Related Infrastructures Water supply is being served by DCWD to 106 of the 182 barangays comprising the City. The remaining 76 barangays are served by Levels II and III systems either ran or managed by the barangay LGU or a Barangay Water and Sanitation Association. Water supplies of these water systems are sourced from springs, deep wells and shallow wells. Although the City is highl y urbanized, it has no existin g wastewater treatment facility. E ven the market and slaughter house have no operational treatment facility. Common wastewater treatment scheme employed are septic tank systems. Effluent from the septic tanks goes to the drainage system, natural water bodies or directly into the ground. 4.
Meteorological and Hydrological Analyses
Based on the baseline study of surface water resources in Davao City River basin the following conclusions can be drawn:
Davao station had experienced extreme dryness for the year 1967 and severely dry in 1992 while Malaybalay City station, the next nearest station with long record, had experienced severely dry periods in 1958 and in 1998. Based on Standardized Precipitation Index, the recurrence of moderate dryness in Davao City and Malabalay City is 1 in every 10 years. For Talomo-Lipadas-Sibulan basin, the potential evapotranspiration (PE) exceeds the precipitation (P) for only 1 month (March) and the total deficit during the period comes to nil. For the rest of the period the value of precipitation exceeds PE by 507.4 mm. Out of this amount 682.3 mm appears as water surplus and the remaining 25.2 mm adds to soil moisture as recharge. Out of 12 months, seven are water surplus months and none are water deficit months. On an annual basis the basin has a water need of 1,319.08 mm which is less than the rainfall amount of 1,801.46 mm. Runoff in the basin is about 27 percent of the precipitation.
ES-3 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
For Davao-Bunawan-Lasang basin, the potential evapotranspiration (PE) exceeds the precipitation (P) for 1 month (March) and the total deficit during that period comes to zero. For the rest of the period the value of precipitation exceeds PE by 657.2 mm. Out of 12 months, ten are water surplus months and none water deficit months. On an annual basis the basin has a water need of 1,336.89 mm which is less than the rainfall amount of 1,994.05. Runoff in the basin is about 33 percent of the precipitation. The corresponding change in the climatic water balance of the Talomo-Lipadas-Sibulan basin using the Davao City synoptic station and the studies of PAGASA based on medium-range scenario emission would be an increase in precipitation as well as evapotranspiration due to an increase in temperature. However, there is decrease in the amount of water surplus. The same for Davao-Bunawan-Lasang basin using the Malaybalay City and Davao City synoptic stations.
Recommendations
5.
1.
There are potential surface water sources in Davao River basins which have adequate water to supply the requirement of Davao City. Per NWRB guidelines the potential maximum extraction is equal to 90 percent of the 80 percent of the time the flow is equaled or exceeded. Of the gauged rivers in the study area, the Tamugan River exhibited high specific discharge values and can be tapped as possible source for water supply of Davao City.
2.
To sustain the quantity and quality of the water in the river, the watershed should be regularly monitored by concerned provincial/city governments. Deforestation should be banned and a regular program on reforestation should be implemented. Environmental practices that will prevent slope and bank erosion should also be encouraged such as planting of trees and plants in the upper reaches of Tamugan River. Hydrogeology
Inventory and Groundwater Utilization The inventory made on groundwater sources realized a total of 694 wells and 33 springs in the entire City. The water sources accounted are either for public (382) or private (345) use. Of the 694 wells, 345 are deep wells, 90 shallow wells, 72 dug wells and 187 unclassified since depth information are missing (these are either deep well or shallow well). Majority of wells considered in the groundwater flow model is from DCWD. Of the inventoried well, only 53 were utilized, mainly for simulating current pumping condition. These wells have discharge and depth information. Groundwater Occurrence Hydraulic Properties of Various Rock Units in Davao City. An aquifer is a geologic material that is capable of holding and transmitting water. It needs to be porous and permeable to hold and transmit substantial amount of water. Clay, silt, claystone, mudstone, shale and siltstone are highly porous but less permeable or impermeable. Gravel, sand, conglomerate, sandstone and coarse pyroclastics are less porous but are more permeable. Clean and well-sorted gravels or sands have higher hydraulic conductivity. Basaltic lavas, such as the Apo Volcanic Complex make more porous materials than lava flows (extrusive igneous rocks) and intrusive igneous rocks which are massive, hard and impervious. The rocks comprising the different lithological units in Davao City have varying features in terms of water conductivity. In general, for areas falling within the Davao Basin, majority of the formation have hydraulic properties which can be described as pervious, which yield considerable amount of water to well. For areas under the Mindanao Central Cordillera and Apo Volcanic Complex, rock formations have varying hydraulic properties from generally impervious to pervious which can yield modest to considerable amount of water to well.
ES-4 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
Water Level. Generation of map showing the spatial variation of ground water level with respect to mean sea level was accomplished. From the groundwater maps, high hydraulic conductivity characterized the coastal zone and the immediate upland area of Davao City as depicted by wide contour interval. Further, it can be noted that there is deepening water level in the coastal area of Toril Sub-district to Talomo Sub-district and the immediate upland area of Tugbok Sub-district. These areas are built-up and industrialized zones. Likewise, majority of the production wells of DCWD are located in these areas. Hydraulic Properties of Aquifers Pumping tests were performed in majority of the DCWD wells to determine aquifer transmissivity. By definition, the hydraulic conductivity is the ratio of the transmissivity to the saturated thickness of the aquifer. For modelling, the hydraulic conductivity was used instead of the transmissivity. Since the saturated thickness of the multi-layered aquifers is difficult to delineate, owing to the differences in the lithologic descriptions by well drillers, the cumulative length of the well screens was assumed as the saturated thickness of the aquifer. Groundwater Movement The groundwater generally flows parallel to Talomo River. The water flows towards the east from the mountain slopes but shifts to the south when it reaches the Riverside area. In the Lipadas side, ground water moves easterly towards Toril and the coast of Davao Gulf. Water Quality There are several chemical groundwater types observed in Davao City. But, two are most dominant type, the Ca-Mg-HCO 3 and Mg-Ca-HCO3. These groundwater types are noticeable mostly in the inland areas, from Marilog and Paquibato districts to Tugbok-Toril-Talomo and Bunawan districts (Panacan-Tibungco area). Other chemical types found in the above areas are Ca-HCO3 and MgHCO3 dominant waters. Of the various chemical types, there is no clear distinction on the water chemistry of well and spring waters. This suggests connection between spring and deep groundwater. Ca, Mg, Na, HCO3, Cl waters are found in wells and springs in the intermediate upland and coastal areas. Na-Mg-HCO3 and Na-Ca-HCO3 waters are most dominant in the coastal areas and intermediate upland areas. The above waters are interpreted as a result of ion exchange between CaMg and Na ions. Climate Change Implication Considering the rate of travel of groundwater in Davao City based on the previous isotope study, there is no immediate impact of the climate change on the existing DCWD wells. The use of groundwater has particular relevance to the availability of many potable water supplies because groundwater has a capacity to balance large swings in precipitation and associated increased demands during drought and when surface water resources reach the limits of sustainability. During extended droughts the utilization of groundwater for irrigation is expected to increase, including the intensified use of non- renewable groundwater resources, which may impact the sustainability of the resource. However, increased utilization of groundwater is highly probable as surface water users, particularly the agricultural sector, will resort to groundwater for their irrigation requirement. 6.
Conceptual Groundwater Flow Model
The conceptual model describes how water enters an aquifer system, flows through the aquifer system and leaves the aquifer system of Davao City. The tentative three-dimensional conceptual groundwater flow model of the Davao City aquifers includes shallow and deep groundwater systems subdivided by a confining layer; 1) The Talomo-Lipadas-Sibulan aquifers consist of volcaniclastic rocks that were thrown out the volcanic centers to the east of Mt. Apo, Mt. Talomo and others. These rocks are composed of agglomerates and volcanic ash. Angular volcanic rock fragments cemented with fine volcanic ash constitute the agglomerates. Finer to sandy rock fragments compose the tuffaceous sandstone which often serves as the main aquifer in the city. Two types of aquifers evolved from the depositional pattern on the slopes of the volcanoes; the rocks that rest on generally compact or ES-5 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
hard rocks created unconfined or water table aquifers while pyroclastic rocks or tuffaceous sandstone that had been deposited earlier became the confined aquifers. 2) The Cabantian-Indangan-Panacan aquifers present a different provenance. The sedimentary rocks were deposited earlier than the volcaniclastic aquifers in Talomo and Lipadas. The aquifers consist of thinly to thickly bedded sandstone, minor conglomerate and occasional limestone beds. The important aquifer system is concentrated on a north-trending syncline that extends from Cabantian to Indangan. It should be noted that these rocks were laid under marine environment. 3) Confining layers in a sedimentary environment usually consist of thick and impermeable clay layers above the confined aquifers. This is true in the Cabantian-Indangan aquifers. However, in the Talomo-Lipadas-Sibulan aquifers, the driller’s logs failed to identify any distinct clay layers that could be considered as confining layers. For the present, suffice it to say that some of the more recent massive volcanic flows and agglomerates occur within the volcaniclastic rocks and serve as confining layers. Model Limitations It must be recognized that to understand and interpret the modeling results properly, its limitations should also be understood, as enumerated below.
The description and characterization of the physical conditions of the subsurface, such as drilling or lithologic logs, are seldom entirely reliable and accurate. Also, the data is often not representative of the whole groundwater system as water resources development is usually concentrated only in highly populated areas or in industrial areas. The model is based on various assumptions regarding the real groundwater system that is being studied. These assumptions imply typical or ideal conditions that are hardly ever encountered in actual conditions. They include homogeneity of the aquifers in terms of textural composition and saturated thickness, and laminar and radial flow to wells. Recharge to the system is often assumed to be global, although they may be extremely diverse because of topography, vegetation and human intervention. Hydrogeologic and hydrologic inputs in the model are almost always approximations of the real parameters which can never be determined absolutely.
Conceptual Model Development The development of the conceptual model requires a thorough understanding of the hydrogeology, hydrology and flow dynamics of the aquifer system being studied. Thus, the conceptual model includes the following elements:
General characteristics of the aquifers;
Characteristics of the confining layer;
Aquifer boundaries; Direction of groundwater flow; Aquifer geometry; and Hydraulic properties of the aquifers.
The Conceptual Groundwater Model Using available data, a groundwater flow model may be developed in order to evaluate and assess aquifer response to stresses imposed by the operation of newer wells. This assessment helps water resource planners decide where new developments should be implemented in order to avoid or mitigate adverse effects on the hydrogeologic system. It can also forewarn the well owners about potential problems that may arise from over-pumping.
ES-6 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
The conceptual model for the Talomo-Lipadas-Sibulan area is a three layer model consisting of the following:
An unconfined (water table) aquifer that occurs at the m iddle slope of the mountains; A confining layer of indeterminate thickness below the unconfined aquifer and extends from the middle slopes to the coastal area; and A confined aquifer that occurs below the confining layer and the aquifer base (Mandug Formation).
In the modelling activities, the average hydraulic conductivity shall be main input as to the hydraulic property of the aquifers. The hydraulic gradient will be derived from the piezometric surface map. This procedure was applied to both the Talomo-Lipadas-Sibulan aquifers and the CabantianIndangan aquifers (Davao-Bunawan-Lasang area). 7.
Numerical Groundwater Flow Model
The development of conceptual hydrogeological and preliminary 3-D numerical groundwater flow models of the Davao City aquifers used DCWD and privately-owned well data. Numerical modeling was completed using the commercially available finite-difference code Visual MODFLOW Flex version 2014.2 (by SWS). A three-dimensional numerical groundwater flow model of the Davao City aquifers was constructed by:
Incorporating 3-D stratigraphical and geological domain models; Assigning hydraulic parameters of major hydrogeological units and inner and outer boundary conditions; Calibrating the model to measure water levels under steady-state conditions; and Incorporating the DCWD proposed wells into the model, for simulation of future groundwater withdrawal rates.
The preliminary groundwater flow model was used to predict and evaluate:
The effect of groundwater withdrawals on the present groundwater flow dynamics;
The progression of drawdowns during proposed operation and pumping of new wells;
Potential impact of pumping on existing wells, including water levels and springs; and
Sensitivity of predicted parameters to the hydraulic parameters of major hydrogeological units.
Modeling Objectives and Scope The main goal of the project is to develop a groundwater flow model for the Davao City aquifers. More specifically, the modeling project intends to determine the maximum sustainable extraction from an aquifer. The model outcomes should be able to assist DCWD determine sustainable extraction rates. The regional model developed for the project encompasses parts of the watersheds of TalomoLipadas-Sibulan basin, where the wells of DCWD are presently in operation, and Davao-BunawanLasang basin, specifically the Cabantian-Indangan area. Modelling Properties Model Domain. The maximum extents of the active model area are approximately 20.5 kms from northwest to northeast and 25.5 kms from northwest to southeast. Layer Definition. The model is simplified into a single layer aquifer to represent the vertical extent of the model. The layers generally coincide with hydrostratigraphic units (where present) identified.
ES-7 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
Model Boundaries. Boundary conditions define the character of water movement at the limits of the active model and at certain points within the model domain where water is either added or removed. The boundary conditions used in the current mode includes: 1) pumping wells, 2) no flow (impermeable area/zone, topographic divides, and streamlines), 3) constant head, 4) river, and 5) recharge. Hydraulic Parameters and State Variables. The horizontal hydraulic conductivity assigned to the model was based on data from the pumping tests, specific capacity tests of DCWD wells and other wells supplemented by the results of pumping test on selected DCWD wells during the time of the study. Values of vertical hydraulic conductivity were assigned based on the rule-of-thumb set to 10 percent of the horizontal hydraulic conductivity. Time Basis. The tentative model is constructed to provide a steady state simulation of the groundwater flow system (no change in ground storage). Model Calibration The Davao City groundwater flow model was calibrated when the model has converged. Then, an assessment of the model’s ability to calculate a head distribution that matches actual site head data based on NWRB 1980 water level map was initiated. To quantitatively assess the degree of calibration of the model, the calculated head distribution was plotted against the observed head distribution using the observation point data of NWRB in 1980. Model Verification and Validation The calculated model mass balance at each calculation interval is an important output that is often used to illustrate key model predictions. The mass balance includes a summation of all fluxes into and out of the model, including those defined as model inputs and those calculated by the model. The final steady-state model produced a mass balance error of 0 %. The ideal percent discrepancy of a model should be less than 1 percent (Anderson, 1993). The final transient-state model produced a mass balance error of nearly zero percent. The ideal percent discrepancy of a model should be less than one percent (Anderson, 1993). Sensitivity Analysis During trial-and-error calibration, sensitivity analysis involves changing a model parameter by a small amount to establish how model predictions are affected by that change. Manual sensitivity analysis requires changing a single model parameter, re-running the model to obtain a new set of predicted heads and fluxes and observing the effect of the change, either by eye or numerically by differencing. In this context, a true sensitivity (derivative) is never calculated. The emphasis is on determining how sensitive the model is to each parameter, using a non-technical interpretation of sensitive‘. This procedure applies to the project model. Summary of Model Inputs Modeling Software
Visual Modflow Flex 2015.1 Built 3.0.507.4 - Waterloo Hydrogeologic MODFLOW – 2005 for Flow solution STRUCTURE AND TIME DISCRETIZATION Model Domain 20.5 km x 25.5 km Grid Spacing 77 rows x 70 columns. Approximately 300 m x 300 m Active layers Layer 1 Layer Type All layers are set as MODFLOW type 3 Stress Periods Steady state and transient state simulation Simulation start date: 1970 Time period represented Simulation end date: 2020 Boundaries, Fluxes and Initial Conditions Talomo Bay coastline Constant head boundary set at 0 meter Net average annual recharge based on water balance assigned over the entire Recharge model domain. Set to 200 mm/year Initial heads Set at 100 meters Rivers represented None Modeling Software
ES-8 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
Drains None General heads None DCWD Pumping wells 63 700 Other wells Hydraulic and Physical Parameters Horizontal hydraulic Based on the analysis of the DCWD wells pumping test data. conductivity (Kx, Ky) Layer was assigned a value of 0.0000195 Vertical hydraulic Set to one to two order of magnitude lower than horizontal conductivity conductivity (Kz) Specific storage (Ss) Set to 0.0001 based on literature. Specific yield (Sy) Set to 0.30 based on literature. Effective porosity Set to 0.25 based on literature. Variable assignment based on literature values; not required for flow simulations. Total porosity Set to 0.30 based on literature. Solution Approach PCG2 Solver Pre-conditioned conjugate solver
Archiving of the Modelling Project A model archive was created that will allow the model results to be produced exactly, while at the same time act as a repository for data and knowledge of the groundwater flow system. The archive contains all data used to create the model. Some of the data were stored in a spreadsheet, a database, a GIS or even in a groundwater modeling software GUI (graphical user interface) will allow easy access to the large amount of data and model results stored in a format that is supported by the commercial version of the software when reusing or revisiting earlier modeling project. Predictive Modelling The general objective of the predictive simulation is to have a general insight into the effect of well extraction on the Davao City groundwater system and the identification of the contributing zone. Predictive scenarios are formulated to be able to quantify groundwater behavior in absolute terms and in relative terms. In the latter, the particular model outcome is obtained by comparing one model result from another (null scenario) result. 1) Aquifer Response to Pumping Groundwater extraction from DCWD pumping wells and other existing wells were imposed into the groundwater model as an input dataset to determine temporally and spatially the distribution of groundwater heads within the DCWD jurisdiction and watershed limits. Groundwater extraction was modeled as point sinks. Steady State Scenario The response of the aquifer under steady state was simulated. The contour head distribution map shows the model simulated heads without any imposed stress or extraction wells. Hence, this model result may be set as a null-scenario. The hydraulic head across the model area ranges from 231 m to 10 m above the reference level. The steady state model would be particularly useful to predict long-term groundwater responses given an adequate representation of stresses and boundary conditions as being constant with time. Steady state models are often useful to long-term impact assessment of sustainability of various assumed levels of extraction. One advantage of running steady state predictive scenarios are the short model run times and the outcomes not being clouded by seasonal fluctuations or trends leading up to steady state realization. Transient State Scenario Transient predictive models are used assuming that groundwater trends with time are an important model outcome or the stresses in the model are not constant in time. Under this scenario the model simulation time period started in year 1970 and ends at year 2020.
ES-9 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
Groundwater extraction started on year 1973. The temporal stress periods or time steps was set to monthly interval The transient simulation results show the progressive retreat of the head contours upward, beginning from the southeast section to the northwest side. This progression indicates the effects of increased groundwater extraction to that direction. 2) Particle Tracking Particle tracking in groundwater flow models was considered in this situation where advection is the dominant process controlling solute movement. Particle tracking and analysis of velocity vectors was set up under the steady-state and transient simulations to determine the location of the zone boundaries where groundwater particle may have originated. Under the steady-state scenario, the point of origin of the groundwater particle emanates from the far north within the vicinity of Talomo River at Tugbok and Mintal area. The particle trace at different travel times is also exhibited for comparative study on the probable origin of each particle. The transient scenario results shorter particle path trace located within the Dumoy well field cone of depression. Using the Model as Management Tools The Davao City groundwater flow model (DCAGWFM) was designed with an expectation that they will be used to predict future behavior. The model parameters that represent aquifer properties and some attributes of boundary conditions are expected to be the same in the future as they were in the past. They are generally time-invariant. DCAGWFM may be used for:
Assessing long term groundwater sustainability;
Assessing potential water quality impacts;
Estimating groundwater-source protection zones; and Assessing climate change impact.
The Groundwater Flow Model: Its Limitations As with all models, the quality of hydrogeologic models depends on the quality of the information that can be gathered for its construction, which in turn depends on the availability of financial resources. As a general rule the conceptual model should be updated, based on insights obtained during the subsequent stages of the modeling. Collection of new data invalidates an original conceptual model. The foundation of a groundwater flow model analysis is the conceptual model. Hydrogeological models and predictions are only hypotheses; they rarely can be proven. Many aspects of the conceptual model are not possible to represent in numerical model, because the hydrogeological systems are very complex (Wagener, et al. 2007). The conceptual model simplifies the issues being examined and organizes the data so the system can be analyzed effectively. Simplification is necessary because a complete reconstruction of the system is impossible. The Panacan-Cabantian Groundwater Flow Model 1) Modelling Description The initial groundwater flow model for Panacan-Cabantian area was developed using AquiferTest Pro 2014.1. The application and use of the AquiferTest Pro model is intended for predictive analysis to determine how the pumping well(s) will affect other wells in the area (e.g. if there are private water wells nearby) and determine how the pumping wells affect the drawdown. A 3D-model using Visual Modflow Flex was developed later for the system out of limited hydrogeological data.
ES-10 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
2) Modelling Assumptions The assumptions include extensive and infinite aquifer system, fully-penetrating wells and isotropic condition. The theory of superposition is applied to calculate the drawdown under the stated aquifer conditions with respect to pumping rates and boundary conditions. 3) Modelling Results The results of the simulation model are presented as drawdown contour maps. The contour maps depict the spatial changes in water level across the model area of Panacan and Cabantian in which pumping wells operate in 24-hours. The average transmissivity of the model is 300 m 2/day and the storativity is 0.001. The drawdown contours are overlaid on the Google map. The simulations made show that in the worst scenario, all existing wells are pumping, the operation of each well influences the surrounding wells. 8.
Groundwater Availability and Sustainability
Concerns about availability of the groundwater have substantially increased in Davao City. This is in view of population growth and urbanization of the City that corresponds to drawing out more from the groundwater sources to meet the needs of the present generation and leads to stressed condition of the local aquifers. Groundwater availability is simply the amount of groundwater that is available for use from an aquifer. Although easy to define in words, it is much more difficult to quantify in r eality. There is no set formula or equation for calculating the same. This is because an estimate of groundwater availability requires the guidance of policy as well as the procedures of science. At present, there is no national policy on how to quantify this. The groundwater flow model cannot absolutely quantify groundwater flow or storage. It has several limitations as provided in the report. Estimates of Groundwater Availability An estimate of the available groundwater was made for two wellfields: Talomo-Lipadas-Sibulan (specifically in Tugbok district) and Davao-Bunawan-Lasang (particularly Cabantian-Panacan) areas. The flow of groundwater is governed by Darcy’s Law, a simple proportional relationship between the instantaneous discharge rate through a porous medium and the pressure drop over a given distance. Using Darcy’s formula, the Talomo-Lipadas-Sibulan area has estimated flow of 700,000m3/day. On the other hand, the Davao-Bunawan-Lasang (specifically Cabantian-Panacan) area has discharge of about 40,000m 3/day. The estimated groundwater flow in Talomo-Lipadas-Sibulan area is more than the current withdrawal rate of about 355,000 m3/day. For Cabantian-Panacan area, the available flow is already exploited. 9.
Aquifer Vulnerability
Groundwater vulnerability is a measure of how ease a pollutant from land surface to reach a productive aquifer. The degree to which aquifers becomes vulnerable to pollution depends on several natural conditions and from anthropogenic (harmful human) activities. For the Davao City aquifers, the major apparent threats come from domestic sewage, leachates in landfills and from agricultural practices, specifically large banana plantations. Sea water intrusion along coastal areas also elicits serious concerns of the groundwater system. A preliminary groundwater vulnerability map was developed for this study which contains three areas, classified according to the level of vulnerability. Area I (Low) - covers the land above the 400-meter contour line where most of the recharge to the deep Talomo-Lipadas aquifers occurs. Most of the large plantations are located in this area and agricultural inputs constitute the potential pollutants to the groundwater system. Factors such as the deep water table (>30 meters) and moderate topography that will cause relatively rapid runoff and further decrease the infiltration of rain water into the subsurface indicate that the groundwater vulnerability of this area is low to moderate.
ES-11 Executive Summary
F I NA L R E P O R T
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
April 2016
Area II (Medium) - includes most of Mintal and Tugbok where the land elevation varies from 100 meters to 400 masl. The land use includes sparse residential development and small farms. The possible pollutants in this area would be domestic wastes. Industrial development is limited and industrial wastes would probably be minimal. The groundwater vulnerability for the area is low considering water table depth (10-20 meters), the rolling topography, and clayey and sticky soil which do not readily allow infiltration of rainwater into the ground. Area III (High) - includes the more highly developed sections of Davao City along the coast. The industrial activity is concentrated in this area and urbanization created extensive housing developments. Because of these elements, the amount of pollutants would be the highest among the three categories. Nevertheless, the vulnerability of the aquifer would still be low because of the existence of a confining layer. While pollutants can be expected from industrial plants and housing developments, the groundwater vulnerability rating of the confined aquifers would still be fairly low although it would be slightly higher than that of Area I. The Cabantian-Indahag would likewise fall under this area since its geology is primarily limestone which allows rapid infiltration. 10.
Issues and Challenges
As part of the project, some stakeholders were gathered to identify issues and challenges and formulate measures to address the concerns on groundwater. In general, the issues and challenges described sums up to inadequacy policies and weak implementation of existing laws and policies related to water and sanitation as well as environmental protection as a whole. Also, insufficient capacity of the human resources involved and awareness of the populace are critical in the management of the groundwater. 11.
Groundwater Monitoring and Management Plan
Groundwater Monitoring Effective groundwater management requires a monitoring system. Well sites have been selected for groundwater monitoring (water level and quality). The monitoring well network encompasses areas where heavy exploitation is currently occurring. But areas at the fringes, which are relatively less exploited, are also considered to determine baseline data, extent or progression of water level decline and water quality deterioration. Eleven (11) areas, as enumerated below, were selected for monitoring well installation. These sites provide wide distribution of wells across the City based on hydrogeological (physical and chemical) conditions. The sites more or less follow the general flow of surface and groundwater. Table ES-1 Recommended Locations of Observation/Monitoring Wells WELL SIZE Diameter Depth (mm) (m)
Well ID
Location
MW 1
Waan
100 mm
70 m
MW 2
Mintal
100 mm
70 m
MW 3
Dumoy
100 mm
70 m
MW 4
Cabantian
100 mm
70 m
MW 5
Indangan
100 mm
70 m
100 mm
70 m
100 mm
15 m
100 mm
30m
100 mm
70 m
MW 6 MW 7, 7a and 7b MW 8 MW 8a MW 8b
CatalunaPequeno Near Coastline LizadaSirawan
Remarks
For monitoring groundwater level decline and agricultural pollution For monitoring groundwater level decline and ambient water quality For monitoring groundwater level decline and domestic/industrial pollution, and ambient water quality For monitoring groundwater level decline and agricultural pollution For monitoring groundwater level decline and agricultural pollution For monitoring groundwater level decline and ambient water quality For monitoring groundwater level decline and saltwater intrusion For monitoring groundwater level decline and domestic/industrial pollution, and ambient water quality
ES-12 Executive Summary
F I NA L R E P O R T
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
Well ID
Location
MW 9
WELL SIZE Diameter Depth (mm) (m)
100 mm
30 m
100 mm
70 m
Near Cemetery
100 mm
30 m
MW 10a MW 10b
(Barangay 5A)
100 mm
70 m
MW 11
Near Dumpsites (Maa and Magtuod)
100 mm
30 m
Panacan MW 9a MW 9b MW 10
April 2016
Remarks
For monitoring groundwater level decline and domestic/industrial pollution, and ambient water quality
For monitoring groundwater level decline, domestic pollution and ambient water quality
For monitoring leachate from solid wastes, groundwater level decline, and domestic/ industrial pollution, and ambient water quality
The monitoring well should tapped the same aquifers tapped by a production well being monitored. The depth of the monitoring well is not dictated by the depth of the static water level. The depth of the aquifer tapped dictates the depth of the monitoring well. If the aquifer is at 120 to140 meters then the monitoring well should be drilled down to 140 meters. Well Head Protection Area Wellhead protection is paramount to ensure drinking quality of water distributed to DCWD’s consumers. The protection of wellhead can be classified into three zones. Zone I (inner source protection) is located immediately adjacent to the well or source. As a minimum, it has a radius of 50m from the source. Zone II or the outer protection area is larger in extent to provide delay and attenuation of slowly degrading pollutants. Zone III or the source catchment is defined as the area that supports long-term abstraction. The present well head protection areas of DCWD are limited to the immediate surroundings of each existing wells. Concrete seal around the annular space between the borehole and well casing, and perimeter fencing around the well pumping stations are the provisions to protect wellhead. A well head protection area defines the capture zone of the well or well field. Based on the numerical particle tracking, wellfield protection area were delineated for the Talomo-Lipadas-Sibulan and DavaoBunawan-Lasang areas. The defined areas covers both protection and catchment area. Since the Talomo-Lipadas-Sibulan area hosts to almost 80 percent of the DCWD production wells, it is relatively extensive from Dumoy on the south to Calinan on the north and from Talomo on the northeast to Toril on the southwest. The Davao-Bunawan-Lasang is limited to the Cabantian-Indangan-Panacan production area where DCWD wells are situated. To protect the above wellfield areas, the following should be implemented:
Protection against pathogenic bacteria and viruses, and hardly degradable and hazardous chemical (e.g., persistent organic pollutants, which include pesticides and polychlorinated biphenyl or PCB, usually from electrical facilities); Activities such as transport and storage of hazardous goods, industrial sites, waste disposal sites, building, military activities, intensive agriculture and cattle breeding, quarrying, and wastewater disposal sites should not be allowed;
Develop contingency plan for contamination event; and
Develop and implement public information education and communication (IEC) campaign.
ES-13 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
Groundwater Management Plan (GMP) The draft action plan or the GMP crafted by DCWD and other stakeholders as facilitated by the project team considers an integrated water resource management, stepwise approach, adaptive implementation, focused programming and participatory planning. The GMP was initially formulated taking into account that at current rates of use, groundwater users’ withdrawal is not alarming considering that:
rejected recharge still occurs even in the lowland areas such as Sirawan and Binugao;
presence of perennial rivers such as Davao, Tamugan, Lasang, Talomo and Lipadas;
numerous free flowing wells along the coastal zone from south to north of the City;
incursion of seawater has not occurred as freshwater persist in the coastal zone; and
quality of groundwater is still at acceptable level for drinking purposes (water quality problem in Cabantian-Indangan area is more natural and aesthetic).
However, even if current state of groundwater is not critical, it is imperative to conserve and protect the resource, and use it at sustainable level. As part of the project, stakeholders were gathered to identify issues and challenges related to the concerns on groundwater, these are as follows: On Sustainable Groundwater Management
Uncontrolled well construction to allocating available groundwater to competing uses, and inadequate protection of groundwater from sources of pollution. The complexity of most groundwater environment prevents easy, cheap or immediate solutions to groundwater quality and supply problems.
On Policy, Ordinances and Regulations
Weak implementation of existing strategies and policies on groundwater management such as the rules and regulations spelled out in the Water Code. Limited enabling environment, such as local codes/ordinances adopting the Water Code of the Philippines and Clean Water Act to support groundwater management.
On Water Resources Planning
Inadequate effective assessment, planning and management of groundwater resources. Limited integration of climate inputs into groundwater resource planning for sustainable infrastructure (climate proofing).
On Human Capacity
Inadequate number of personnel, capacity and resources to fulfil the mandatory groundwater resource management functions. Specifically, the lack or shortage of trained and experienced hydrogeologist in the Region and at the National levels.
On Institutional Capacity
Limited cooperation and coordination among institutional agencies related to groundwater resource management. Inadequate monitoring of groundwater withdrawal, water level and quality specifically on privately-owned wells.
ES-14 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
On Information Management, and Communication and Awareness
Outdated and incomplete hydrogeological and hydrological databasing and in particular, absence of a formal mechanism for information sharing. Limited public awareness and education on groundwater resource management. Lack of a communication plan focusing on promoting “best practices” on groundwater use and management.
From the above, a matrix summarizing the proposed GMP for Davao city was prepared. The matrix consists of measures to address identified problem, the action and sub-actions to be taken, output/s for each actions/sub-actions, time line and resources. These measures include: Short-Term 2016-2020
Strengthen enforcement of existing laws, policies, guidelines and appropriate implementing rules and regulations relevant to groundwater management, specifically the Water Code.
Updating of groundwater and surface water database.
Regulated extraction of groundwater.
Regular monitoring of groundwater withdrawal, water level and quality of both public and privately-owned wells. Creating an enabling environment through LGUs, such as local legislation/ordinances adopting the Water Code of the Philippines and Clean Water Act to support groundwater management. Enhance technical capacity of personnel of concerned government agencies to fulfil the mandatory groundwater management function. Prepare and implement a communication and awareness plan promoting groundwater management utilizing “best practices” of actual implementation as opportunity to enhance awareness. Harmonization of integrated water resource management (IWRM) approach and climate change adaptation to groundwater resource planning and design for sustainable infrastructure (climate proofing).
Long-Term 2021-2030
Effective assessment planning and management of groundwater resources.
Development of guidelines on groundwater management such as artificial recharge strategy.
12.
Promote awareness concept on “Water Sensitive Cities” (sustainable, productive, resilient and livable place). Capacity Development and Training Program
The Capacity Development (CapDev) and Training Program is a vital element of the Project’s component, which plays a major role in ensuring that the “hard-earned” gains of the Project will be sustained and “owned” by DCWD. The CapDev and training program shall thus support, as well as provide impetus to, the various activities of DCWD. In particular, this means that the training activities shall be an important means for ensuring that the knowledge, skills and behaviour required for the successful implementation of interventions/activities undertaken are adequately reinforced or possessed by all participants of the program. The capacity development and training approach highlights design activities covering orientation by mediated lecture, and interactive activity (coaching and mentoring) through on the-job training (OJT) with the end goal of attaining technology transfer to the DCWD staff. In deciding on the specific ES-15 Executive Summary
Groundwater Assessment and Development of Groundwater Flow Model for Davao City Aquifers
F I NA L R E P O R T
April 2016
methods and techniques to be employed, it was guided mainly by the principle of appropriateness and practicality. This implies that theoretical inputs through lectures are limited, both in terms of duration and complexity) to what is essential for achieving the practical objectives of the training program. Furthermore, theoretical inputs or conceptual foundations are presented in as efficient and interesting manner as possible. The Training Course includes modules considered important to groundwater development and management:
Database Development and Management;
Hydrogeological Surveys;
Water Quality Analysis;
Basic Hydrogeology;
Hydrogeological Analysis;
Well Development and Maintenance;
GID Mapping; and
Groundwater Modelling.
ES-16 Executive Summary