EVALUATION OF WELL-FIELD SITE PRODUCTIVITY The allowable rate of pumping from any particular well field site within the tri-county area is dependent upon a number of natural and man-made constraints. The fundamental constraint is a hydrologic one, and simply reflects the capability of the ground-water system to supply water to wells on a dependable basis, without regard to any adverse impacts that might be caused to surface waters, the ground water on adjacent properties or even on a regional basis (e.g. agricultural pumping in Manatee _ , Hardee & DeSoto Counties, or phosphate withdrawals on Polk, Hillsborough, and Manatee Counties) or the biologic environment. The man-made constraints are those that do take into account these other kinds of impacts which are mostly reflected in the regulations on withdrawals promulgated by SWFWMD. In the following sections, each of the natural and man-made constraints is examined separately, in order to show how they apply in determining the productivity of any particular well field. Hydrologic Constraints The Floridan Aquifer, from the viewpoint of its water yielding characteristics, is one of the best aquifers in the entire United States. As a consequence~ it is entirely feasible to pump from the aquifer many tens of millions of
gallons per day from practically any inland well-field site without causing damage to the hydrologic system itself. Damage, in the hydrologic sense, . generally pertains to such responses as encroachment of saline water, lowering of water levels to such a degree that land subsidence or the formation of sink holes takes place, and long-term depletion or reductions in the overall availability of the ground-water. In the tri-county area, where the rate of natural recharge to the Floridan Aquifer varies more or less in direct proportion to the amount of water being pumped from the aquifer, it might theoretically be possible to extract say 100 mgd or even 200 mgd from most inland well-field sites without adversely harming the ground-water system itself, however, this is not a recommended source to follow. It would not be desirable, however, to lower the potentiometric surface below the bottom of the confining bed that separates the water-table aquifer from the Floridan Aquifer, because storage depletiori of the limestone would begin to occur. In a few inland parts of the tri-county area, this allowable amount of decline to the potentiometric surf&ce might be as great as 40 feet. At well fields located many miles inland from the interface between fresh and salty ground water in the Floridan . , quifer, . the threat of lateral salt-water encroachment is probably of small concern. Although a large lowering of fa
the potentiometric surface in an inland well field will theoretically cause a small lowering of the potentiometric surface along the coast, the resultant effect on the rate of salt-water encroachment is not very great, and many years or decades might elapse before any significant inland movement of the salt water could be detected. Thus, a trade-off between the large benefit of a high rate of withdrawal and a small amount of encroachment might prove acceptable to all concerned. Upward coning of saline or poor quality water from deep zones in the Floridan Aquifer is still another _ potential hydrologic constraint on well-field productivity. However, the historical record of large-scale withdrawals in the region suggests that this is a relatively minor threat in the inland parts of the tri-county area, and in any event, probably would not take place for many decades, if at all. Thus, ignoring all other kinds of environmental implications, any potential well-field site in the tri-county area that has been selected on the basis of a scientific hydrologic evaluation probably could produce as much as several hundreds of millions of gallons daily without causing severe damage to the aquifer system itself. This statement, however, does not take into account threats of sinkhole formation or land subsidence, which would call for a separate detailed investigation to show whether or not a particular well-field site could experience such kinds of damage.
Ecologic Constraints At all well-field sites in the tri-county area, heavy pumping always results in a lowering of the water table, which in turn may adversely impact surface-water features, ground-water levels on adjacent properties and biologic communities in the vicinity of the well-field site. In the tri-c6unty area, and in all parts of Florida for that matter, the . se potential damages are of major concern to the residents and to the regulatory agencies, and have historically constituted a very real constraint on how much water is to be .allowed to be withdrawn from a particular well-field site. Experience at several existing well fields in the tri-county area has shown, for example, that even relatively modest withdrawals of water from the Floridan Aquifer have caused a lowering of lake levels, . some reductions in surface-water ' flows, the killing off of certain varieties of plant life in the areas surrounding the well-field sites and the drying up or reduction of supplies from shallow wells. Lowered water levels mean increased costs to well-owners on affected lands and in some instances have necessitated the drilling of +arger and deeper wells with bigger, deep-well pumps. Because these effects depress land values and cause harm to the natural environment, the general public and the regulatory agencies almost universally have agreed that withdrawals from such well fields should be kept within certain limits so as to
minimize the damages. The problem of developing a sci~ntif ic rationale for damage control at any particular place within the tri-county area has been one of the major problems facing regulatory agencies, as explained in the following section. Regulatory Constraints As explained previously in this report , SWFWMD is charged by State law as the agency responsible for the management of the water resources of the tri-county area. Under this law, SWFWMD has been given the difficult assignment of assessing potential threats to the hydrologic and ecologic systems and of developing rules and regulations to keep rates of ground-water withdrawal in harmony with the need for protecting man, his investments in his property and use of the property as well as his environment. Because the task of defining acceptable limits on withdrawal rates is difficult and expensive, SWFWMD has, as a first step, developed a set of regulatory constraints that are broadly applied throughout the approximately 10,000 square miles of the District, including the entire tri-county area. The lines of reasoning in these regulations can be classified broadly under the headings of: (1) the water-crop concept; (2) the 5/3/1 approach; and (3) the so-called concept "B". Each of these has been applied to some extent in regulating well-
field withdrawals within the district, and to date constitute the principal basis for estimating how much water can safely be extracted without causing an unacceptable level of impact to man and his environment. Water-Crop Concept The concept of a water-crop limitation, as developed by SWFWMD, relates to the estimated amount of water naturally available in any particular locality after the natural requirements for evapotranspiration have been satisfied. Technically, this amount is defined as the difference between the input of wat~r from precipitation and the output of water through evapotranspiration and the amount, on a regional basis is a measure of the runoff of the streams, ca nals and drainage ditches that carry off the excess water. On a regional basis, using average values for rates of precipitation and estimates of average evapotranspiration losses, the water crop is . assumed to be 365,000 gallons per year per acre, or approximately 1000 gallons per day per acre. SWFWMD recognizes that the water-crop figure of 1000 gallons per day per acre does not necessarily reflect the true situation on any particular tract of land. Nevertheless, because the district has not yet been able to compile the hugh volume of scientific information that would be needed
to define the local water crop in all parts of the district, the regional figure of 1000 gallons per day per acre continues to be the figure adhered to in the district's regulations. As an example of how the water crop is applied, an applicant owning 1000 acres of land would be restricted to a maximum withdrawal rate of one million gallons per day. If he were to own twice this acreage, the allowable withdrawal rate would be two million gallons per day. 5/3/1 Approach The 5/3/1 approach in the district's regulations pertains to maximum allowable amounts of lowering of the water table, (5 feet) , of the potentiometric surface, (3 feet) ; and of the water levels of lakes near a particular well field (one foot). The regulations state that the withdrawal must not cause the level of the potentiometric surface under lands not owned or otherwise controlled by the applicant to be lowered by more than five feet. The withdrawal also must not cause the level of the water table under lands not owned or otherwise controlled by the app+icant to be lowered more than three feet. And finally, the withdrawal must not cause the level of the water surface in any lake or other impoundment to be lowered more than one foot unless the lake or impoundrnent is wholly owned or otherwise controlled by the applicant.
The hydrologic relationships that govern the amount of lowering of ground-water levels and lake levels are highly complex and differ widely throughout the tri-county area. It is possible that five-foot lowering of the potentiometric surface in one place, for example, may have absolutely no effect upon the overlying water table or on the levels of nearby lakes, whereas in another location, these hydrologic impacts might be much more pronounced. To determine all of these complex interrelationships calls for a high degree of knowledge bf the local geologic and hydrologic system, which normally cannot be obtained without drilling test wells, conducting various field tests, and making complicated hydrologic evaluations. The Concept "B" Approach The Concept "B" approach is basically a technical method for determining how much the potentiometric surface in a well field can be safely lowered in terms of local fluctuations of the water table, the leakance coefficient of the confining beds, and head differences required to pass the water crop from the water table to the confined aquifer. The deepest allowable level of the potentiometric surface determined in this manner is called the regulatory level for that particular well field. The precise method of determining the regulatory levels is described in Section 16J-0.15 of the District's Regulations.
The equation utilized in Concept "B", when applied to specific well-field sites, definitely favors those sites where the value of the leakance coefficient is low. When applied in places where the leakance coefficient is high, the regulatory level determined from the Concept "B" equation is in many instances so re strictive that no withdrawal at all would be allowed from the well field. Also, in places where the potentiometric surface is naturally very much below the water-table the allowable yield for a well field would be zero when the Concept "B" equation is applied. At present, the district has established regulatory levels for a number of well fields within the tri-county area, based on calculations made under the Concept "B" approach. Unfortunately, the WCRWSA (West Coast Regional Water Supply Authority) thus far has not had an opportunity to review the original data used by SWFWMD in determining the regulatory levels for these particular well fields. Comparision of Regulatory Approaches Table lists calculated allowable yields for several well fields in the tri-county area, based on the different regulations relating to the water crop, the 5/3/1 approach, and Concept "B". It will be noted that each of these regulatory approaches produce somewhat different values for allowable withdrawal rates. Under the water-
crop concept-, for example, the Eldridge Wilde Well Field would have an allowable pumping rate of 3.1 mgd. The same well field, under the 5/3/1 approach, would have an allowable pumping rate of 12 mgd, and under the Concept "B" approach, would have an allowable yield of 25.7 mgd. Under current regulatory levels, the Eldridge Wilde Well Field has an estimated productivity of 35 mgd average and 41.5 mgd peak depending upon antecedent precipitation and seasonal variations in _ the potentiometric surface. Similar differences in allowable withdrawal rates can be calculated for the Section 21 and South Pasco Well Fields, as shown in the table. Thus far, the district had not rigidly applied all of the foregoing three regulatory approaches in determing allowable pumping rates for municipal well fields in the tri-county area. Instead, it has selected the one which, in its opinion, provided the best guarantee of protecting the adjacent land-owners property interests and the natural environment. Nevertheless, because different rates of withdrawal can be calculated for each of the different approaches, a certain amount of confusion still exists over which is the appropriate one to be used in the case of a new well field that is in the preliminary design stage.
TABLE COMPARISON OF WELL FIELD PRODUCTIVITY Eldridge South Lutz Wilde Pasco Section 21 Surface Area 4.8 mi2 1.0 mi2 0.8 mi2 Leakance 2.0 X 10-3 gpd/ft3 1.3 X 10-3 gpd/ft3 l.5x 10-3 gpd/ft3 -~ Estimated Productivity Per: Water Crop Concept 3.1 mgd 0.6 mgd 0.5 mgd 5/3/1 Regulatory Levels 12.0 mgd 9.1 mgd 7.8 mgd Concept 11B11 Levels 25.7 mgd 16.7 mgd 4.9 mgd Present Regulatory Levels . !/ 35.0 mgd ave 16.7 mgd ave 10.6 mgd ave 41. 5 mgd peak 19.0 mgd peak 12.0 mgd peak Notes .!./Estimates based upon records of withdrawal and measurements from regul~tory observation wells over the period of September, 1975 to November, 1976 (West Coast Regional Water Supply Authority).