By: Engr. Edgar Mana-ay
IN THIS part of the series on water crisis, we will assess where and how much water resources we have in Iloilo and how to find ways to exploit it at a sustainable level to help alleviate our water shortage problem.
The most vaunted water resource is underground water because it cannot be seen and everybody assumes it is there in bountiful quantity and that we can withdraw it at any time we want. Just like love, since it is out of sight most often, its presence is also out of our mind!
That is all myth. For starters, we are not endowed with natural, large underground basins where ancient ground water is stored just like the Great Artesian Basin (1.7 million square km.) beneath Queensland and New South Wales in Australia; or the great fossil underground water basin beneath the dessert of Sirt, Kufra and Hamra in Libya.
For both countries, these are ancient groundwater stored from melted ice age 30 to 100 million years ago. These are confined and layered aquifers with thickness from 50 to 600 meters in Australia and 1,500 meters thick in Libya. In contrast, aquifers in Iloilo, although existing in multi layers, but its average thickness is only 8 meters!
In Libya for example, it is estimated that water abstraction from confined aquifers in the underground basins for the next 50 years (for the country’s use) is at 130 cubic kilometers, or ONLY 0.3 PERCENT of total groundwater resource under the dessert!
Unlike in Australian aquifers where there is a recharge point, there is none in Libya so extraction of its water is technically termed as mining of water. However, it will take Libya 10,000 years to consume its vast underground water resource without recharge! Presently they have now more than 300 wells at 400 to 500 meters deep, (the deepest well here in Iloilo is 400 feet), with each well producing 8,000 cubic meters per day (cmd) or a total of 2 million cmd of ready to drink water. An average deep well in Iloilo produces 500 to 600 cmd.
Here in Iloilo, we are not endowed with ancient water stored underground and so we have to depend on current rainfall that will be stored underground for our use. The amount of rainfall that can be stored underground will depend on:
-the vegetative or tree cover in the mountains that provides the cushioning effect for rain to fall slowly to the ground and allow for more penetration rather than over land flow;
-ground surface both in the mountain and in the plain that is not compacted by man and animals; and
-the absence of impermeable surfaces such as roads, basketball courts, rooftops of buildings and houses.
There is no in depth technical study of how much of our rainfall is stored underground at the present devastated watershed condition. There were technical companies hired by MIWD and other agencies before such as Sweco in the 1990s for watershed assessment but the scope were superficial and general, maybe because of budget constraint or the hiring agency is not an expert in preparing the scope of work.
A true hydro-geological study to assess how much underground water we have would involve a macro concept of a basin or catchment water balance where all known water inflows and outflows are accurately accounted for. Meaning, it must be an accurate measurement of the area covered by the Maasin watershed, for example of precipitation, transpiration, evaporation, stream flow and ground water recharge among others.
There is also the need to identify the location of underground aquifers so that citizens will be guided where to drill deep wells and abstract water content. By the way, an aquifer is that strata under the ground where water stays as it slowly seeps towards the sea at the rate of 1 km. in 7 years, hence that is where drillers would want to hit so that it can transmit water to the wells. Some laymen call it an underground river which is nonsense. Not all areas underground can make water stay. It has to be a section composed of sand and/or gravel or even sandy clay because these are types of “ground” that are porous where water can stay and move. If there are known thick and confined aquifers (in excess of 50 meters) with lateral extent of more than a kilometer, it is worth spending to conduct a study on its transmissivity and storativity.
Unfortunately, even the National Water Resource Board (NWRB) does not have any study and/or data on confined aquifer locations in the country. Drillers and companies planning to drill a water well have to use their own initiative and experience (mostly hit or miss) in guessing at what depth underground is a confined aquifer located.
Money and science is required to look for water underground on a regional basis, say exploring the entire Iloilo Province. The basic theory is that underground water flows from inland towards the sea because of difference in elevation. It has to start with remote sensing imagery. With the use of a satellite imaging, groundwater can be traced and detected. Remote sensing result is just a general indication of presence of water flow underground, it will have to be confirmed by resistivity survey at the potential location initially identified by satellite remote sensing imagery.
In the next article we will present the different deep wells drilled beyond 100 ft. deep so we can have an appreciation of the locations of the various “mini” aquifers we have in Iloilo.
Note: The author is a Professional Member of the Geological Society of America (GSA) and the National Ground Water Association of the U.S. (NGWA).