On Hillsborough County's Water Supply Situation - August 8th, 1974

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On Hillsborough County's Water Supply Situation - August 8th, 1974

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On Hillsborough County's Water Supply Situation - August 8th, 1974
Parker, Garald G. (Garald Gordon), 1905-2000
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Box 2


Subjects / Keywords:
Aquifers -- Hydrogeology -- Florida ( lcsh )
Hydrology -- Florida -- Biscayne Aquifer (Fla.) ( lcsh )

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University of South Florida
Holding Location:
University of South Florida
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The University of South Florida Libraries believes that the Item is in the Public Domain under the laws of the United States, but a determination was not made as to its copyright status under the copyright laws of other countries. The Item may not be in the Public Domain under the laws of other countries.
Resource Identifier:
032968560 ( ALEPH )
891343127 ( OCLC )
G16-00682 ( USFLDC DOI )
g16.682 ( USFLDC Handle )

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. . the safety of our water supplies, as is indicated by numerous recent inquiries received here. The water-supply shortages of l~st sp•ring and summer are still fresh in their minds; also, it is oovious that lake levels are low --many, even in areas of no obvious pumpage and .sotne lakes are approaching all-time low water levels with some already having established new record lows. Streams, too, are generally low, both in stage and in quantity of flow. Reports of many wells "going salt" in the coastal areas along the Gulf,-particularly in northern Pinellas County and to the north in Pasco and Hernando Counties, add further to citizen concern. What is the situation in Hillsborough County? Ik> we have enough water? Will our coastal wells also become lost to salt-water encroachment? What can we do to protect and conserve our water resources? These and other related questions are asked us daily, therefore, we thought it would be well to discuss this problem in the present issue of the Hydroscope. Regarding the quantity of water available to us now and in.the future, the best way to find a useful answer is by ~eans of a water-budget analysis. This will be explained in the following paragraphs. WATER BUDGET ANALYSIS The water budget is based on continuity which, in its sim-;_ . plest form is: P (precipitation)= Et (evap ration)+ R (runoff). Other elements, including ground-water and surface-water inflow and outflow, and changes in groundwater and surface-water storage must also be included if they are of such magnitude as to be of consequence.• By choosing an area that is a hydrologic unit, that is, one that is surrounded by water divides across which no flow occurs, inflow and outflow factors can be ignored. And, if the budgeted period is long enough to


begin and end at the same time of the year, normally the changes in storage c an also be ignored because a complete cycle of wet and dry seasons will have been completed and the water balance at the end of the period is the same as at the beginning. QUANTITY OF WATER AVAILABLE FOR USE Hillsborough County is not a hydrologic unit. Both ground-water and surfacewater inflow take place across its northern, eastern and southern borders from, respectively, adjacent parts of Pasco, Polk and Manatee Counties. But by draw-ing hydrologic boundaries on the potentiometric surface map to coincide with ground-water and, incidentally and fortunately with surface-water divides, a hydrologic unit can be drawn that is not greatly larger than the county. Actually, it is this larger area from which Hillsborough County draws its entire water supply, derived solely from precipitation on its surface, and therefore is the area we must consider in this study. It is an area encompassing about 1,633 square miles, as compared with the county's land area of 1,038 square miles. The Florida Geological Survey, in 1961, published a report (R.I. No. 25) entitled "Water Resources of Hillsborough County, Florida," prepared by C. G. Menke, E.W. Meredith and w. s. Wetterhall of the U.S. Geological Survey. It was one of the earlier water-budget studies made in Florida, preceded only by that of the Kissinnnee River Basin, reported in U. s. Geological Survey WaterSupply Paper 1255, by Garald G. Parker and others, and published in 1955. The present report derives essentially the same water-budget values as that of Menke and others but reaches a vastly different conclusion. Menke and others state (p.17) that "an average of 1,400 mgd (million gallons a day) is potentially available." 'Ibis is enough water to supply 1,250,000 persons if all the flood waters could be stored for use (the italics are mine). And this is the fly in the ointment! All the flood waters cannot be stored, in fact very little can be saved for later use. Practically, we cannot expect to harvest a water crop exceeding one-third of this 1,400 mgd or, based on Menke and others, about 467 mgd. And, -2-


for once-through-the-mill uses, we will be lucky to capture this much of our potential water crop. We have no means of storing much of our flood waters, as is connnonly done elsewhere by use of surface reservoirs, because in our flatlands region no large and deep valleys exist in which capacious and economic reservoirs can be built to hold such flood waters. 1Likewise, in non-flood times we cannot withdraw all, or even much, of the streamflow. We must leave most of it to keep the streams flowing, to prevent the marshes_ and swamps from drying out, to provide for recreational water uses such as swimmi11:g, boating, and fishing, to provide water to dilute and carry off industrial_ , agricultural and municipal wastes, to benefit fish and wildlife, and last but not least, to help maintain a beauty of the landscape that we all treasure. However, we do have at our disposal a natural subterranean reservoir of vast potential for storage of billions of gallons of excess water, but to date we are only beginning to investigate its uses. By means of artifical recharge some of our otherwise wasted flood waters and our cleaned-up and reclaimed previously used waters could be stored underground for subsequent reuse. Such recycling and reuse of waters is both practical and needed. We need to get on with this method of extending and augmenting Nature's water crop as soon as the experimental tests indicate the best ways to do it •. As an example, if our entire water crop of 448 mgd (see following paragraph) were used over only once the water crop would be doubled --to 976 mgd. This is a goal to be sought. To estimate the quantity of water available for use, based on long-term hydrologic data averages, we go about it like this: One inch of runoff (R) from one square mile during one year amounts to 17.4 mgy (million gallons a year), and the Hillsborough County water-catchment area covers 1633 square miles. Precipitation (P) over this area averages about 53.8 inches per year, of which 35 inches, or 65 percen.t,,;.-are lost shortly after falling on the land surface. This leaves a potential water crop of 18.8 inches of which, as we've previously said, we would be lucky to capture more than one-third. One-third of 18.8 inches is 6.27 inches -3-


per year per square mile. Multiply 6.27 inches by 17.4 mgy and we find the water yield to be 109.1 mgy per square mile. Multiply this by the total number of square miles in the water-contributory area to Hillsborough County and we obtain 178,160 mgy or about 488 mgd. However, about 60 mgd of this is currently being exported to St. Petersburg and Pi._nellas County, thus reducing the available and harvestable water crop for in-County use in .Hillsborough County to 428 mgd. QUANTITY OF WATER NEEDED Until better data are available on consumptive use of water in Hillsborough County, our best means of deriving a reasonable estimate is to base our values on selected per capita use figures. Reliable, comprehensive data are not available to sum up water withdrawals by the following major uses: (1) agriculture, particularly citrus irrigation; (2) self-supplied industry, particularly phosphate and citrus; (3) nrunicipal (although this is fairly well documented); (4) commercial; and (S) self-supplied hotels, motels and dwellings plus lawn-watering supplies. Here in Hillsborough County we do not h~ve the large uses of the industrial East or the agricultural West, but industry, agriculture and connnerce in Hillsborough County are large enough to have caused the U. s. Geological Survey (in the Menke and others report) to estimate the per capita use then (1960) to be 1,100 gpcd. Based on the u. S. Geological Survey canvass of water use in Hillsborough County during their IO-year recurring national water-use study, the Survey now estimates the Hillsborough County per capita use at 600 gpcd for 1970. For our current values I am using two enveloping curves as shown on the following illustration. The higher, maximum-use curve A is based on a per capita use of 800 gpcd and the lower, minimum-use curve Con 500 gpcd. Between these two is the estimated actual-use curve B of 600 gpcd based on the U. s. Geological Survey water-use inventory of 1970. -4-


19 60 196s 1~70 1975 _1q80 19ss 1990 HILLSBOROUGH COUNTY WATER CROP? DEMAND CURVES, 1960-r99o Next, to estimate current use and to project future water demands, population forecast data as develope~ by the Tampa Bay Regional Planning C.Ouncil were utilized to develop the following table: Year Population Water Use at 500 Water Use at 600 gpcd Water Use at . 800 gpcd (Minimum) (1970 Value, USGS Data) gpcd (Maximum) 1960 397,788 a/ 198.9 mgd 238.7 mgd 318.2 mgd 1965 453,000 b/ 222.3 mgd 266.9 mgd 355.9 mgd 1970 490~265 a/ 245.2 mgd 294.2 mgd 392.2 mgd 1975 536,294 !.I 268.2 mgd 321.8 mgd 429.0 mgd 1980 590,855 295.5 mgd 354.9 mgd 472. 7 mgd 1985 654,936 s./ 327.5 mgd 392.9 mgd 532.9 mgd 1990 724,416 362.2 mgd 434.6 mgd 579.9 mgd a/ from u. s. Census b/ from curve, figure 4 from TBRPC Cohort -survival projection -5-


water, but will prevent salt-water encroachment both in the dannned-off section of the canals and streams and in the aquifer at depths directly related to the height to which fresh-water head can be held above msl each foot of fresh-water above msl depresses encroaching salt water by about 40 feet. By holding fresh-water to 2% feet above msl, salt-water would thus be held to -100 feet msl in the aquifer. b. Reduce Et (evapotranspiration) losses. This can best be accomplished by lowering the water table in swampy and marshy places below the reach of water-wasting plants. Choices of areas will have to be made to decide what areas can be utilized and what ones not used. Some areas must be saved from lowering the water level in order to preserve natural forest and swamp environments for esthetics as well as sanctuaries for wildlife. Our large well-fields_are prime examples of how efficiently this works. c. Reduce waste of water: 1. Increase charges for water, particularly for large users, so as to obtain the joint benefits of augmenting income (needed to pay for increased costs of-water supply and management) and causing water users to be concerned with wasting. The more costly the water, the less the people are likely to waste it. 2. Insist on reuse of water for industrial and those agricultural uses that permit reuse. Once-through-the-mill and then discharge to the Bay should not be tolerated. 3. Many irrigators now put far too much water on their crops. Educate irrigators to crop needs and allow only what is really required. To avoid excessive_irrigation losses due to evaporation, spray irrigation should be done at night, preferably in the pre-dawn hours. -7-


4. Hundreds of abandoned artesian wells are now flowing to waste in Hillsborough County; particularly in the Ruskin area, depleting the aquifers and causing salt-water intrusion. Each of these wells should be plugged securely from bottom to top. d. Augment present supplies: 1. Recycling sewage wastes is one of our biggest source of "new" water. Most municipal sewage is 99% reusable water. Being run through "tertiary" (extended secondary} treatment to reduce impurities of all kinds to be!!_ least~ good as water naturally available in the aquifers and . streams of the area, would make such water available for reuse and essentially make this region's water supply self-sufficient for the next thirty years or so. This can be done, but at a cost. It is a cost that, eventually, we must pay. The question isn't if~ should do it, the question .is only when shall we do it? 2. Capture as much of flood flow as we can and iriject it into the only large storage reservoir we have --the Floridan Aquifer. This can be accomplished best by developing flood retention r _eservoirs with discharge channels and works leading to those parts of the Southwest Florida Water Management District where large drawdowns of water level have created billions of gallons of available storage volume. Some such storage capacity exists in the areas of pumping influence. from every large well field in.the TBR, but the largest potential storage is in the areas of. large drawdown around the phosphate production and irrigational areas, mostly in Polk, eastern Hillsborough and eastern Manatee Counties, where over hundreds of square miles the potentiometric surface of the Floridan Aquifer has been lowered 60 feet or more since 1949. -8-


3. Locate and operate well fields and recharge facilities so as to. manage withdrawals and replacements (recharge) scientifically. The well fields and other dispersed well-supply sources should all be components of a regional water-supply system, hooked together much as the electrical industry has regionalized their electrical capacity. 4. In the shore-zone region which has been invaded by salt-water encroachment, almost unlimited supplies of brackish water are available. This water ranges from nearly as salty as ocean waters to only slightly more salty than normal ground water. M>st of it extends a mile or so inland from the Gulf of Mexico, is only mildly saline, and can be e~onomically reclai~ed for use. This will be more costly than use of fresh water (if it were locally available), but has recently become comparable to the cost of transporting fresh water from distant well fields. The new reverse osmosis (RO) method now in use at the 500,000 gpd Rotunda West water-treatment plarcin southwestern Sarasota C.ounty was installed at a total cost of $385,000 in the sunnner of 1972 and is expecteG to produce fresh water at about fifty cents a thousand gallons. More such plants are needed in our coastal areas. 5. Import ~ater from great distances, such as from Weeki Wachee Springs, Chassahowitzka Springs, Homosassa Springs and others. But .this will be extremely costly, probably much more costly than other means previously mentioned. Engineering studies will need to be made to evaluate just how much these alternatives will cost us. Then, with such knowledge, the taxpayers will be in a position to make the necessary choices. B. Mine the aquifer: The Floridan Aquifer and its associated overlying shallow system of water--9-


table aquifers contains far more ground-water in storage than all the Great Lakes combined. In the TBR, for example, the upper 1,000 feet is generally filled with fresh water inland from the 25-foot contour on the potentiometric surface, and the upper 2,000 feet inland from the 40-foot contour. However, salt-water underlies this aquifer everywhere and bounds it on the west all along the shore. If the aquifer is ove~pumped, salt-water encroachment follows. Tampa and St. Petersburg, to name only two large users, lost their downtown well fields to saltwater encroachment in the late 1920's. And thousands of private wells in the shore-zone that extends generally inland to about the 10 foot contour on the potentiometric surface all along the Bay and our Gulf. Coast either have been lost to salt-water encroachment or are in imminent danger of becoming lost. Great care must be taken that the aquifer not be mined of its fresh water with-resultant salt-water encroachment • . Detailed research must be made to develop better knowledge of the aquifer's hydrologic characteristics so that realistic, •effective management decisions can be reached. Right now we have some usable generalized information and hydrologic understandings that will serve to guide .us until better and more detailed data are available. We can make do, then, for a while. But, we can't afford to dally. The situation is upon us now. Garald G. Parker, c.P.G. January 08, 1973 -10-


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