Water-Bearing Materials
Evaluating the hydrologic
properties of water-bearing materials
.
Texture refers to
the appearance seen on a smooth
surface of a homogeneous (uniform composition)
rock and the size of the grains (granularity).
Sands and sandstone aquifers must be interpreted
with great care. Their porosities can be computed and used in determining the
safe yield of the area.
It is only by the carefully documented reports of
water well drilling contractors that the water resources of many parts of the
nation can be documented, especially in the initial or reconnaissance phase of
the investigation of the groundwater, of an area.
Because each well will pump a small part of the
aquifer and add to the cumulative discharge, it is again important to keep thorough logs.
Meinzer indicates that a study of the relations of
the water
levels to the amount of pumpage is likely to give more reliable information as
to the safe yield than can be obtained by any other method of
studying an undeveloped reservoir.
For example, if the water levels in the wells
remain virtually stationary during a considerable period of pumping, it can he
concluded that the rate of recharge has been about equal to the rate of
discharge.
If, at the end of any period of pumping, the water
level does not return approximately to the position that it had at the
beginning of the period, the safe yield has been exceeded and the rate of
pumping is greater than the maximum pumping rate the aquifer can possibly
sustain.
Reservoir
capacity of fine-grained materials
The significance of storage or reservoir volume
provided by fine-grained materials is commonly overlooked in developing
groundwater resources.
This is probably due to the fact that these beds
are overshadowed by the importance attached to the search for coarse-grained
materials in which the well screen is to be placed or the borehole bottomed.
Although it is advisable to place the well screen
in the coarsest material available to reduce entrance losses at the face of the
well, the fine-grained
material should not be overlooked.
There is an especially strong temptation to develop
only the isolated lenses of coarse material, even in an aquifer made up
predominantly of fine-grained material.
Developing only thin lenses of gravel in this type
of geologic sequence will not markedly improve the performance of the well
(compared to a well simply developed in the finer material).
The withdrawal of groundwater for any prolonged
period is possible only to the extent of the regional or overall capacity of
the water-bearing formation.
This is little influenced by isolated lenses of
coarse material, except as the materials represent a small fraction of the
total reservoir volume.
Again, the texture of the sands and finer-grained
material is very important.
Fine silt might not be capable of being developed
whereas fine-grained
sand should be if it represents the principal texture of the water-bearing
materials.
The above information is
excerpted in large part from Chapter 19 of the 1999 NGWA Press publication, Ground Water Hydrology for Water Well Contractors.
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