GWD3_HO_Case study Groundwater model

WASH Cluster - Groundwater Development and Drilling
CASE STUDY 1: Developing a conceptual model of Groundwater
Occurrence, Eastern Sierra Leone
For a small village water supply, one hand dug well had been installed and more
were being considered. A hydrogeological conceptual model was interpreted
based on the existing information. This included observing outcrops of granite
bedrock, observing the soil profile in successful and unsuccessful hand dug wells
in the village. A simple geophysical investigation (resistivity traverse) was
conducted to identify subsurface soil properties and confirm the approximate
depth of the granite. (The resistivity traverse measures variation in electrical
properties subsurface and can give an indication of different soil zones as well as
the presence of groundwater. It is a useful tool applied worldwide in groundwater
investigations, requiring specialist equipment, software and is best done by
experienced hydrogeologist or geophysicists)
The information suggested that for this site, water table rested on the granite and
therefore helped identify a target for hand dug well construction to supplement
the existing well
Example: Diagrammatic cross section - A geological model of Makpedu, Kailahun
region, Sierra Leone
It is also important that when dealing with a shallow system such as this, it is
preferable to do the investigation in the dry season when water levels are lowest
and give a better appreciation of the longer term availability of groundwater.
Geophysical investigations are commonly used in conjunction with geological
mapping to try to more accurately locate possible sites for wells. This is really a
WASH Cluster - Groundwater Development and Drilling
specialist area and needs well maintained equipment, as well as software to
process data.
The other important thing in this case study is to understand the potential volume
that may be available for the required population. In this particular location,
there may be adequate groundwater for a small village population, but not
necessarily sufficient for a large refugee or IDP population. Such a conflict is
noted later
Setting up resistivity traverse
In selecting sites for wells it is fundamental to understand the geology (ie the
likely aquifer) and the water table location and orientation.