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Evaluating the hydrologic properties of water-bearing materialsThe classic report, "Outline of methods for estimating ground-water supplies" (1932, USGS Water-Supply Paper 638-C), by
Oscar E. Meinzer, the "father" of groundwater hydrology in the United States, emphasizes that the
greatest difficulty in applying quantitative methods to the evaluation of the hydrologic properties of an aquifer is the
great variability in the texture of geologic 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 materialsThe 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.