3 August 1998
GROUND WATER SUPPLIES
5-1. GENERAL. Ground water is subsurface water occupying the zone of saturation. A water
bearing geologic formation which is composed of permeable rock, gravel, sand, earth, etc.,
and yields water in sufficient quantity to be economical is called an aquifer. Unconfined water
is found in aquifers above the first impervious layer of soil or rock. This zone is often referred
to as the water table. Water infiltrates by downward percolation through the air-filled pore
spaces of the overlying soil material. The water table is subjected to atmospheric and climatic
conditions, falling during periods of drought or rising in response to precipitation and infiltration.
A confined aquifer is defined as the aquifer underlying an impervious bed. Areas of infiltration
and recharge are often some distance away from the point of discharge. This water is often
referred to as being under artesian conditions. When a well is installed into an artesian
aquifer, the water in the well will rise in response to atmospheric pressure in the well. The
level to which water rises above the top of the aquifer represents the confining pressure
exerted on the aquifer. Materials with interconnecting pore spaces such as unconsolidated
formations of loose sand and gravel may yield large quantities of water and, therefore, are the
primary target for location of wells. Dense rocks such as granite from poor aquifers and wells
constructed in them do not yield large quantities of water. However, wells placed in fractured
rock formations may yield sufficient water for many purposes.
a. Economy. The economy of ground water versus surface water supplies needs to be
carefully examined. The study should include an appraisal of operating and maintenance
costs as well as capital costs. No absolute rules can be given for choosing between ground
and surface water sources. Where water requirements are within the capacity of an aquifer,
ground water is nearly always more economical than surface water. The available yield of an
aquifer dictates the number of wells required and thus the capital costs of well construction.
System operating and maintenance costs will depend upon the number of wells. In general,
groundwater capital costs include the wells, disinfection, pumping, and storage with a minimum
of other treatment. Surface water supply costs include intake structures, sedimentation,
filtration, disinfection, pumping, and storage. Annual operating costs include the costs of
chemicals for treatment, power supply, utilities, and maintenance. Each situation must be
examined on its merits with due consideration for all factors involved.
b. Coordination with State and Local Authorities. Some States require that a
representative of the state witness the grouting of the casing and collect an uncontaminated
biological sample before the well is used as a public water supply. Some States require a
permit to withdraw water from the well and limit the amount of water that can be withdrawn. All
wells and well fields must be located and designed in accordance with State Well Head
Protection Programs and the Safe Drinking Water Act.
c. Arctic well considerations. Construction of wells in arctic and subarctic areas
requires special considerations. The water must be protected from freezing and the
permafrost must be maintained in a frozen state. The special details and methods described
in TM 5-852-5/AFM 88-19, chapter 5 should be followed.