 
Seawater intrusion and its related effects with saline groundwater were observed in Cebu City’s coastal aquifer for the
past three decades. All water samples for these studies were collected by pumping groundwater out of the observation wells.
Therefore, the original densities at various depths were all mixed due to the pumping. It was not possible to determine
the distribution of saline groundwater inside the well, caused by the different density of fresh- and saltwater. To get
more information about the three-dimensional distribution of the saltwater intrusion it was necessary to get depth-dependent
data. |
A depth-dependent measurement of the saltwater distribution provides more details and information about the fresh-saltwater
interface, and the three-dimensional distribution of saltwater intrusion in the coastal aquifer than pumped samples. Based
on the existing observation wells in Metro Cebu, it was the objective of Phase 2 of this project to determine the
three-dimensional distribution of chloride in the coastal aquifer. This was the first study offering information on the three-dimensional
distribution of the chloride content in the coastal aquifer of Metro Cebu.In preparation of the field work, basic data were gathered on production and observation wells in Metro Cebu. The purpose of the gathering was to receive information about the construction of the wells and the groundwater conditions (measured water level and salinity) in the past. This was necessary to determine which wells were appropriate for the study. |
There are two common methods used in gathering depth-dependent data of the chloride content in a well:
The first is to get water samples with a special sampler for definitive heights for laboratory analysis.
For chloride analysis it is possible to use the titration with silver nitrate solution or to use an electrical
conductivity cell. However, this method requires more time to gather water samples and to do a laboratory analysis.
The second method is to lower a conductivity cell directly inside the observation well. It allows to read the results
immediately from a recorder. This method can be used on-site and is much faster than the first method. An added
advantage is that the immediate results give more flexibility in choosing, adding, and adjusting measuring points.For purposes of this study a Microprocessor conductivity meter with a 100 m long cable was used for on-site measurement. Both temperature and electrical conductivity are displayed at the same time. With this equipment, measurements inside wells up to 100-meter depth was possible. |
Compared to the usual sampling method, the depth-dependent measurement of the electrical conductivity shows
much more detailed information about the distribution of the chloride content.
Apart from showing less detail, the usual method of taking samples by pumping is also not very reliable
compared to the depth-dependent measurement. The pumped groundwater does not represent the groundwater of a
specific depth because it is already mixed.Based on the depth-dependent measurements it was possible to show wells with a lower electrical conductivity at the upper part of the screen and a higher electrical conductivity at the lower part of the screen. This effect could be due to saltwater intrusion or overpumping (saltwater upconing). The results of the measurements show that among the districts covered, Mandaue City has the highest level of chloride content, possible due to upconing from overpumping. This is an industrial city and the findings are not surprising. A previous study by Kampax-Krüger Lahmeyer International in 1980 revealed, that the groundwater surface in Mandaue City was several meters below sea level, and consequently, salinization of the aquifer occurred. |
Many thanks to everybody who supported this study.
