For much of northwest Florida, the Floridan Aquifer is the primary source of ground water. Wells tap the limestones that make up the aquifer. However, as you move west to about Okaloosa County the Floridan Aquifer gradually dips deeper into the subsurface and it becomes thinner and less productive. The Floridan Aquifer underlying Escambia County contains concentrations of dissolved minerals that are not suitable for most water supplies. Here the unconsolidated sediments overlying the Floridan Aquifer become thicker. The uppermost portion of the sediments becomes a very productive ground water source (Figure 1). Consequently, Santa Rosa and Escambia counties rely solely on the shallow aquifer system for their drinking water supplies.
Beneath the land surface in Escambia County lies a thick sequence of sands, gravels, silts, and clays. The sequence that is important for its water-bearing characteristics ranges in thickness from about 250 to nearly 500 feet. This waterbearing section of the subsurface is referred to as the Sand-and-Gravel Aquifer System. The sediments making up this aquifer are exposed at land surface throughout Escambia County.
|Ground water movement within the changing sediment types can be compared to the electrical conductivity of different materials. Electricity is readily passed through a copper wire. Likewise, water is transmitted easily through the pore space of a section of sandy sediments. In contrast, wood will not conduct electricity and similarly clay does not readily transmit water. The ability of sediments to transmit water is referred to as hydraulic conductivity. Thus, where various mixtures of sand and clay exist in combination, the sequence which has a higher percentage of sand than clay will have the higher hydraulic conductivity. Those with a higher percentage of clay will have a low hydraulic conductivity.|
Underlying the surficial zone is the second
layer which is referred to as the low permeability zone. This zone
averages about 50 to 100 feet thick and is primarily composed of
clay, silt, and sand. It characteristically has a limited capacity
to transmit water and functions somewhat as a confining bed. As a
whole, however, this zone functions as a very leaky confining bed.
The leaky nature is attributed to the high variability of the types of
sediments. When the sediments are more sandy than clayey, they form
a vertical connection to the lower segments of the aquifer and leak more
water. Predominantly lateral downgradient movement along the top
of a clay/silt bed exists when the zone contains low permeability sediments.
Movement may remain lateral or if a sandy section is encountered the movement
may become more vertical.
The main producing zone itself has variable
water-yielding characteristics but of the three zones it is always the
best zone for developing ground water supplies. Water availability
for the main producing zone decreases in the southwest portion and the
northern half of Escambia County. However, the central and Pensacola
areas yield supplies as great as 2,000 gallons per minute from a single
Ground water in Escambia County is originally
derived from rainfall within the county. Thus, the Sand-and-Gravel
Aquifer is replenished by the rainfall infiltrating through the soil zone.
Only a small portion of the ground water in the northernmost section of
the county comes from rainfall in Alabama. It is important to point
out that rainfall within the boundaries of the county accounts for nearly
all the available ground water.
Movement of ground water within the main producing
zone is primarily toward the larger rivers such as Escambia and Perdido
rivers and toward the bays and the Gulf of Mexico. Thus, the flow direction
can be either east, west, or south depending on the part of the county.
Since a majority of the wells tap this zone, ground water withdrawals also
affect ground water movements within the aquifer. Locally, when a
well is pumping, water movement in the aquifer is toward the well.
If there are many wells pumping close together, the flow will be in the
direction of the pumping centers. Therefore, the effects become more
widespread and larger areas of the aquifer flow system are influenced by
The extent of contamination within the aquifer
is dependent on: 1) the quantity of the contaminant; 2) the density
of the contaminant (some will float on the water surface while others will
settle and sink deeper into the aquifer); and 3) the contaminant's ability
to be tied up into the sediment itself (that is, some contaminants bind
chemically to the sediment particles which would limit or slow the movement
within the aquifer). More importantly, all contaminants have different
behavior patterns within the subsurface environment. It is for these
reasons that determining the source and extent of contamination often is
costly and time consuming. Likewise, because the sediments are extremely
variable in types, determining the rate of movement and other time factors
is difficult. The same factors which control the movement of the
contamination into the subsurface also influence the clean-up of an aquifer.
Thus, returning the aquifer to a safe drinking status is also costly and
| In summary, recharge to the ground water for
the Sand-and-Gravel Aquifer occurs by rainfall within Escambia County.
The flow system for the aquifer is somewhat localized and is bounded by
the Perdido River and Perdido Bay in the west, the Escambia River and Escambia
Bay to the east, and Pensacola Bay and the Gulf of Mexico to the south
The Sand-and-Gravel Aquifer is made up of three distinct layers referred to as the surficial zone, the low permeability zone and the main producing zone. The main producing zone is the most important water-bearing zone for ground water supplies. The movement of ground water within the aquifer is such that all zones are interconnected to varying degrees. Because of the interconnection between land surface activities and the subsurface, the Sand-and-Gravel Aquifer is highly susceptible to contamination. As the aquifer is the sole source of drinking water supplies, any contamination of the ground water will have a lasting impact on the Sand-and-Gravel Aquifer.
Aquifer- A sequence
of sediments which is hydraulically connected, saturated, and sufficiently
permeable to yield water.
- A sequence of sediments which consists primarily of material with low
permeability such as clays and silts and functions as a barrier to vertical migration of water.
A substance or substances or quantity of substances which has a detrimental
impact to the receiving water. Its occurrence is not natural and it renders the receiving water
unfit or undesirable.
Discharge - That
portion of ground water which is expelled to the surface. Natural
consists of ground water which is released by springs or into rivers which cut into the aquifer.
Discharge also occurs by well withdrawals. Discharge areas are those areas in which ground
water flow escapes to the surface.
- The ability to transmit water from one portion of the sedimentary sequence
another. Sands generally have a high permeability, clays have a low permeability, while limestones
range from low to high.
Recharge - That
portion of the total rainfall which infiltrates to replenish an aquifer.
areas are those areas which readily allow movement of water into the aquifer.
Water Table -
The water level surface of an unconfined aquifer. It can be measured
wells into the saturated zone and then measuring the water level in the wells.
Source: Northwest Florida Water Management District, Public Information Bulletin 87-2, April 1987. This material was based upon work supported by the U.S. Department of Agriculture, Extension Service - July, 1992.
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