GROUND WATER IN ESCAMBIA COUNTY, FLORIDA

    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.



...the Sand-and Gravel Aquifer is replenished
by the rainfall infiltrating through the soil zone.

     The Sand-and-Gravel Aquifer as the name implies consists primarily of quartz sand ranging in size from very fine to very coarse with beds of gravel interspersed in some sections of the aquifer.  Layers of silt, clay and various combinations of all the sediment types occur within the aquifer.  Often these layers of clay or silt will be of non-uniform thickness and have a very limited lateral extent -- occurring only for short distances.  The variability of the sediment types within the subsurface is important in affecting the way ground water moves within the aquifer.  This variability results in many different flow patterns for ground water over fairly short distances both laterally and vertically.  Figure 2 shows a cross section of the aquifer and an example of its various sediments.
 
     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. 
Commonly, the variations in sediment type result in porosity and hydraulic conductivity variations which cause temporary isolation of water movement from one part of the aquifer to another.  As a result, the water levels in deeper wells are generally lower.
...rainfall within the boundaries of the county accounts
for nearly all the available ground water.

     Although the hydrogeology for the Sand-and-Gravel Aquifer is highly variable and complex, the basic framework of the system is represented by three major layers that comprise the aquifer.  Figure 3 shows a generalized profile through Escambia County displaying the three layers making up the Sand-and-Gravel Aquifer.  The uppermost layer is referred to as the surficial zone.  It generally averages less than 100 feet thick and is primarily composed of fine sands and clayey or silty sands.  This zone contains the watertable surface.  The water table is not a flat surface but generally reflects the irregularities of the topography.  The upper portion of this zone is above the water table and thus is not saturated with water.  The thickness of the unsaturated zone is variable and depends greatly on the topography.  In general, the higher areas have a larger unsaturated thickness, whereas the lowlands usually have a water table near land surface and a much smaller unsaturated section.  The surficial zone has limited ground water availability, but is sometimes used for small quantities (10 gallons per minute or less).

     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.

Water availability for the main producing zone decreases in
the southwest portion and the northern half of Escambia County....
the central and Pensacola areas yield supplies as great as 2,000 gallons
per minute from a single large-diameter well.

      The most important zone of the Sand-and-Gravel Aquifer from the standpoint of water availability is the third layer and is known as the main producing zone.  This zone of the aquifer contains a larger percentage of the coarse sand and gravel beds and is usually more than 100 feet in thickness.  A majority of all water wells in the county tap this zone for ground water.  All major water users for pubic, industrial, and agricultural supplies withdraw water from the main producing zone.

     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 large-diameter well.

Contaminants will be introduced into the aquifer along
with the same waters that....replenish the supply.

     Ground water flow is contained within the three layers because at a depth of about 250 to 500 feet below the land surface a thick mostly clay bed with a very low hydraulic conductivity is present and underlies the aquifer.  This clay bed acts as a barrier which prevents water from moving downward.

     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.

...any disposal of waste products or misuse of toxic chemicals on
 the land surface, whether accidently or on purpose, has a high
probability for impacting the ground water....

     Although Escambia County receives nearly 60 inches of rain each year, not all of the rain becomes part of the ground water.  In fact, only a small fraction of the rainfall reaches the water table.  Most of the rain is lost to runoff to streams or is returned to the atmosphere by evaporation and transpiration by plants.  Rainfall recharges the Sand-and-Gravel Aquifer as it moves through the soils and unsaturated portion of its surficial zone to the water table (Figure 4).  Water within this zone moves primarily laterally from high areas to low areas.  Where the low areas intersect streams or ponds the ground water seeps out of the aquifer and is discharged to the surface waters.  A majority of the volume of water occurring during low flows of the rivers and tributaries in Escambia County comes from the ground water discharge.

...all contaminants have different behavior
patterns within the subsurface environment.

    That portion of the ground water flow which is not discharged to streams or held in storage within the zone, gradually leaks through the low permeability zone.  The flow within this zone is primarily lateral and water is discharged where the topography intersects this zone.  Thus, small quantities of ground water seep from the hillsides or discharge directly into surface waters.  The ground water that moves vertically in this zone evidently replenishes the main producing zone.

     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 pumping.

...returning the aquifer to a safe drinking
status is also costly and time consuming.

     From the previous discussion of the flow and make-up of the Sand-and-Gravel Aquifer, it is readily apparent that any disposal of waste products or misuse of toxic chemicals on the land surface, whether accidentally or on purpose, has a high probability for impacting the ground water both shallow and deep.  Contaminants will be introduced into the aquifer along with the same waters that infiltrate to recharge or replenish the supply.  In very general terms, a contaminant will follow the same flow paths as that of the natural system.

     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 time consuming.

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.
     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 (Figure 5).

     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.

 


  Glossary

    Aquifer- A sequence of sediments which is hydraulically connected, saturated, and sufficiently
    permeable to yield water.

    Confining Unit - 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.

    Contaminant - 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 discharge
    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.

    Permeability - The ability to transmit water from one portion of the sedimentary sequence to
    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.  Recharge
    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 by installing
    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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