Geology Reference
In-Depth Information
SELECTION OF TEST SITE
The factors to be considered before selecting a test site could be: (a)
topography, (b) geology of the area, (c) porosity and permeability, (d) joints
and faults and folds, (e) proximity to tanks, rivers, springs, lakes, unlined
channels and (f) existing pumping wells.
As the aquifer parametres have to be utilized for effective planning and
management of the groundwater in an area, it is very essential that the wells
be so selected that they do not fall near contact zones of formations. The
movement of water in hard rocks is through joints, fractures and cracks
which are interconnected. Wells that tap the highly jointed rocks or along
fault plane produce very high yield in comparison to surrounding areas.
When the rocks are folded into anticlines and synclines, the synclines are
favourable spots for groundwater storage in the pervious layers, while if a
well taps the anticlines, it may not yield, since the crest forms a groundwater
divide. Thus a judicious selection for a pumping test site should be made.
The water levels in confined aquifers are subject to immediate changes in
pressure heads caused due to heavy traffic passing nearby the test sites, such
as railway lines or heavy road trucks.
In addition to the above factors, adequate subsurface investigations prior
to performance of an aquifer test are to be determined for ascertaining the
lithological character, thickness of aquifer, location of aquifer boundaries,
character of bed overlying and underlying the aquifer, nature of barrier or
recharge boundaries if any, direction of groundwater flow, water table gradients
and regional water level trend. The thickness of aquifer, and the features of
the overlying and underlying beds can be ascertained during the drilling of
the bore-well. The depth vs. yield of the various formations could be collected
from the data recorded during drilling. If the main aquifer is bounded by any
partially permeable formations, then adequate care has to be exercised for
sealing of the inflows by inserting a suitable packer assembly. It is also very
important to dispose off the pumped water preferably through pipeline to a
distant place so that it does not return back and cause interference with the
water level changes in wells. Gradient of the water table should be low and,
preferably, the boundary conditions of the aquifer should be clearly defined
and simple.
Abstraction Well (Pumping Well - PW)
The diameter of the well should be able to accommodate pumping equipment
and sufficient length of casing must be inserted to avoid collapsing of loose
material if any. The length of the casing of the screen should at least be 70%
of the aquifer thickness, so that it does not become a partially penetrating
well.
