Unconfined or Water Table Aquifers

Unconfined and confined aquifersAn aquifer in an unconfined state has entirely different storage properties than an aquifer in the confined or artesian state.

For a groundwater reservoir to be classified as unconfined, it must be shown that it is not confined by impermeable material (relatively speaking) and, furthermore, its water table cannot be confined from the effects of atmospheric pressure.  Horizontal permeability in sedimentary rocks and sediments is commonly greater than the permeability at right angles to the bedding planes in these materials.  Thus, it is common to have a reduction in vertical permeability above an aquifer creating a degree of confinement, which in most areas varies widely from place to place above the water table of the groundwater, reservoir, caused to some degree by the weight of the atmosphere.  Water in unconfined aquifers is subject to losses due to plant uptake and evaporation.

When a well is constructed into an unconfined aquifer, the water level in the well remains, temporarily, at the same altitude at which it was first found in drilling.  This level can fluctuate later due to changes in many factors.

The water level in the well defines the top or surface of the zone of saturation; this surface has a pressure that is everywhere the same as atmospheric.

Immediately above this level in the adjacent geologic materials is a zone that is completely saturated because of capillary action.  This zone varies in thickness depending upon the grain size of the material.  Generally, it is thicker in fine-grained materials than in coarse-grained sediment.  It may be nonexistent in a clear coarse gravel or pebble zone.

Above the zone of capillary saturation are two zones that are only partly saturated; together they define the zone of aeration.  The lowermost zone is characterized by semicontinuous capillary saturation while the uppermost zone is characterized by discontinuous capillary saturation.  In both of the latter zones, the materials will yield no water to wells.

Pumping an unconfined aquifer

Pumping a well in an unconfined aquifer causes actual dewatering of the material within an inverted, roughly cone-shaped volume, called the cone of depr​ession or the cone of influence.  Dewatering occurs by simple gravity drainage toward the lowest point at the apex of the cone, the well.  The widest part of the cone, at the top, is called the area of influence.  When pumping ceases, the cone gradually fills up with water.

The ratio of the volume of water that drains from this cone under the influence of gravity to the volume of the cone is called the specific yield, and is generally expressed as a percentage or decimal fraction.  Of course, it is not possible for all the water to drain from the geologic materials that were initially saturated within the cone.  Part of this water is attracted to the rock particles by a force that is stronger than gravity — the molecular attraction of the water molecules for the surface of the geologic materials or adhesion.  Thus, the specific yield is equal to the total volume of pore space in the rock, expressed as a percentage of the total volume of the rock, minus the specific retention — the amount of water held or retained by molecular attraction to the rock particles.

In most unconfined aquifers, the specific yield ranges from 10 percent to 30 percent.  In other words, of the water held by an aquifer, 10 percent to 30 percent can be given up to pumping or other discharge.  A coarse-grained aquifer will have a higher specific yield than a fine-grained one.  Specific yield is not to be confused with maximum yield, which is affected by the size of the aquifer.

Cone of influence in unconfined aquifers

When a well is pumped, the water drawn into it leaves behind a dewatered area, the cone of depression or influence.  The pumped well is always located at the apex of this cone.  The shape of the cone and the rate at which it expands across the top depend on the coefficients of transmissivity and storage of the aquifer and on the rate of pumping.  The first water to be pumped by the well is derived from the pores in the immediate area of the well.  However, as pumping continues, the cone enlarges and continues to do so until it intercepts a source of recharge (replenishment) that will produce all of the water demanded by the pump.  In unconfined or water table aquifers, the cone of influence expands initially at rates ranging from less than 100 meters to, in some cases, more than 1,000 meters per day.

Sources of water to unconfined aquifers

As the cone enlarges, it continues to dewater the strata that are engulfed by its margin.  If the cone of influence intercepts a stream or a lake, it will induce infiltration in such a way as to keep up with the demands of the pumped well.      Once these demands are satisfied, the cone of influence will cease to grow.  The shape of the cone will be modified in the immediate vicinity of sources of direct recharge (lakes or streams).  It modifies itself by bulging away from the axis of the surface source.  In some cases, instead of intercepting a lake or stream, the cone captures springs that may cease to flow.  The general rule is that the cone will continue to grow until it intercepts sufficient base area to satisfy the demands of the pumped well at the prevailing rates of groundwater recharge.

Some cones are so large that they advance beyond the original divides of the groundwater reservoir and induce water from drainage basins that are situated on either side of the basin containing the pumped well.  The cone will modify itself in response to any changing influence of recharge and discharge within the reservoir.  For example, during periods of precipitation, when the aquifer is recharged, the cone shrinks to a size that is dependent on the amount of recharge it receives.  Conversely, when the area goes through long periods of drought, the cone deepens and expands so as to withdraw the additional water from storage it needs to continue to satisfy the demands of the pumped well.

Mutual interference of cones of influence

Occasionally, two or more wells have developed their cones of influence in such a way that they interfere with one another.  This situation requires that the wells be relatively close and developed in the same aquifer.  There is always a chance this will occur in any intensive development of the same groundwater reservoir.  Simply stated, the cone of influence of one well overlaps the cone of a neighboring well.  A part of the cone of influence that fed one well must now satisfy another well also.  The amount and areal extent of the interference is directly related to the rate of pumping of each well.  Other factors of no less importance are the spacing between the wells and the hydrologic characteristics of the groundwater reservoir furnishing the water to the two wells.

If more wells are developed in the same area, the chance for interference increases.  The cones of influence of the initial wells expand and deepen in order to satisfy their pumps with each development of another well and its subsequent cone of influence.  The cones must always establish a hydraulic gradient just sufficient to supply the amount of water required by the pumped well.  If this water is not available in the area of the initial cone because some groundwater is diverted into another cone, the initial cone simply enlarges to an outlying area where sufficient replenishment can be derived.

The development of deep-well turbine pumps enables water to be pumped from great depths and from geologic strata of low head.  The characteristics of this type of pump enable it to operate at peak capacity until the water level declines to one or two meters below the pump bowls.  The pumping capacity will decline sharply at this point and thereafter until the pump breaks suction.  The importance of this point is that it is possible for the well yield to be sustained for a long period of time even though the rate of water level decline is constantly increasing because of mutual well interference.  There is no warning that the well is close to failing.  It might be advantageous to periodically check the water level to avoid the consequences of a declining pumping level.

The above information is excerpted in large part from Chapter 14 of the 1999 NGWA Press publication, Ground Water Hydrology for Water Well Contractors.