On water-resource conditions in the vicinity of Pinellas county's Eldrige-Wilde Well Field - June 5th, 1973


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On water-resource conditions in the vicinity of Pinellas county's Eldrige-Wilde Well Field - June 5th, 1973

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Title:
On water-resource conditions in the vicinity of Pinellas county's Eldrige-Wilde Well Field - June 5th, 1973
Creator:
Parker, Garald G. (Garald Gordon)
Publication Date:
Language:
English
Physical Location:
Box 2

Subjects

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

Record Information

Source Institution:
University of South Florida
Holding Location:
University of South Florida
Rights Management:
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 )
G16-00699 ( USFLDC DOI )
g16.699 ( USFLDC Handle )

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Book

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PAGE 1

FROM THE DESK OF THE CHIEF HYDROLOGI,ST: On Water-Resource Conditions in the Vicinity of Pinellas County's Eldridge-Wilde Well Field. Pinellas County's Eldridge-Wilde Well Field comprises about 2.8 square miles of land lying in extreme northeastern Pinellas County and extreme northwestern Hillsborough County (Fig. 1). First put into service in April 1956, its average daily pumpage then was only 5.7 mgd (million gallons a day) taken from ten 12-inch wells. Since then the system has expanded as demands for water to serve all of Pinellas County outside of St. Petersburg have increased. Currently the system consists of 58 producing wells with a total design capacity of 75.43 mgd and an average daily pumpage in May 1973, of 43.56 mgd. The system now serves about 50,200 connections. Largest customers are the Cities of Clearwater, Tarpon Springs, Largo, Safety Harbor and Pinellas Park plus a number of large industrial and coIDinercial users. Peak deliveries from the system have ranged from about 54 mgd in 1970 to 86 mgd in May of 1973. However, these peaks were partly supplied from storage; at no time in recent months, which have experienced the greatest demand, has direct pumpage from the well field exceeded 50.25 mgd and this pumpage was reached on May 21. The previous day, May 20, pumpage was 50.20 mgd. None-the-less, 50.2 mgd is a lot of water to take out of an area of 2.8 square miles. With an average direct recharge to the aquifer of about 650 mgd/mi 2 (million gallons a day per square mile), the direct recharge to Pinellas County's well field is only 1.82 mgd. The recent pumping averages of 43.56 mgd thus require about 65.5 mi 2 of recharge area. Or, saying it another way, it requires 65.5 mi 2 of land to supply the current, average-day's pumping from this well field; this is an area almost 23 times larger than the existing field. This pumping has created a widespread cone-of-depression around the well field and has lowered water levels in the center of the well field about 30 feet below pre-pumping levels. Currently the deepest part of the cone-of-depression of the potentiometric surface (artesian water levels) in the well field hovers

PAGE 2

around sea level --some days a few feet below msl, on other days of lesser demand, a few feet above (Fig. 2). Not only has this wide and deep cone-of-depression developed in the potentiometric surface but a related, shallower and smaller drawdown has developed in the water table of the overlying shallow aquifer. Deepest parts of this cone are 15 to 20 feet lower than pre-pumping levels (Fig. 3). This, with other impacts caused by drouth and numerous coastal-zone drainage canals, has caused regional water levels to drop, and in late May, for the first time in history, lake Dan went dry. Drought effects are a principal cause of lowered water levels. Not only is direct recharge to the aquifers decreased but landholders, to maintain lawns and shrubbery, gardens and citrus groves in good health, must irrigate. Somewhere between 40 and 60 percent of all the water pumped from Pinellas County's and St. Petersburg's well fields is consumed by irrigation. During periods of normal precipitation irrigation uses are minimal but during drought times they are maximal. Thus, at current pumping rates of about 43 mgd from the Eldridge-Wilde field, about 25 mgd is being used for irrigation. The current drought is the worst in the history of the Tampa weather station, which has the longest record of any station in the region. The drought began in 1961 and is still continuing (Fig. 4). In 10 of the last twelve years there has been less rainfall than is normally received. This deficiency shows up as bars below the normal line; only in 1964 and 1969 were there anyexcesses above average and these show as bars above the normal line. Totally, 1961-1972 inclusive, there has been a departure from normal of 102.3 inches. This is a rainfall deficiency of about 8.5 feet! Had all of this expected rainfall occurred, our water-level situation would not be nearly so alarming as it is. Other precipitation stations in the region generally record somewhat higher rates of rainfall t han doe s the Tampa station, however none of them have as long a record. To show t h e general trend of recorded precipitation t h e following -2-

PAGE 3

tabular comparison is made: YEAR STATION TAMPA TARPON SPRINGS COSME-ODESSA SECTION 21 1970 38.27 in. 42.89 in. 41.58 in. 45.33 in. 1971 46.33 in. 63. 51 in. 57.58 in. 49.58 in. 1972 37.37 in. 41., 97 in. 43.17 in. 41.81 in. TOTAL: 121.97 in. 148. 37 in. 142~33 in. 136. 72 in. Thus, in the past three years, the Tampa station recorded 26.40 inches of rainfall less than Tarpon Springs, 20.36 inches less than Cosme-Odessa and 14.75 inches less than Section 21. However, none of these other stations are recording up to expected normals --all reflect varying degrees of drought and thus lower-than-normal aquifer recharges. The other main aspect of water-level lowering is brought on by the dredging of both natural streams and man-made canals and ditches. These drainage channels lower the water table and potentiometric surface much more efficiently than the well fields and those that are connected with the gulf or the bay are, in effect, arms of the sea. They not only introduce salt water far inland but by lowering the ground-water levels a mile or so along their courses decrease fresh-water aquifer pressures with resultant salt-water encroachment. Such is the condition all along the Gulf Coast from Hernando to Hillsborough County and beyond. Part of this encroachment is shown in Fig. 5, and points up the potential danger to the Eldridge-Wilde Well Field, which is only about two es est of the inland e ge of the encroaching salt-water wedge. The nature and shape of the encroaching body of salt water into coastal fresh water is shown in Fig. 6A, 6B, 6C and 6D~ adapted fr u. s. Geological Survey. a report by nald C. Reichenbaugh of the The lowered water levels in the region not only create greater pumping lifts and therefore increased energy uses to pump the water but also they create increased monetary costs. Aside from this, and perhaps of more concern, is the fact that -3-

PAGE 4

salt-water encroachment is occurring all along the coastal zone to the west, as previouslymentioned. Lowered fresh-water levels upset Nature's longestablished equilibrium between salt water at depth in the aquifer long the shore and fresh water inland from the shore zone. To restore a balance~salt water pushes inland to replace the depleted fresh water. It comes in as a blunt-nosed wedge, thickest at the shoreline and thinning inland. Eventually a new equilibrium is established, but at some distance inland. As the salt wedge moves inland parts of the aquifer previously fresh become filled with salt water and wells drawing water from this encroached zone now yield only salt water (Fig. 6). It was this same process that destroyed both Tampa's and St. Petersburg's \ well ~ields in the late 1920's and that recently has caused New Port Richey's well field to turn salty. All along the coastal zone the salt water front has been and still is moving inland, destroying literally hundreds of wells in the coastal zones of Pinellas, Pasco, Hernando and Hillsborough Counties. It is apparent that the Pinellas County Eldridge-Wilde Well Field is being overexploited. It's effects of lowering the water table are both widespread and, in the center of the cone-of-depression, the potentiometric surface is commonly deep --close to or below sea level. These lowered levels court, or invite, salt-water encroachment into the well field which would be catastrophic should it occur. The well field, in this instance, would have to be abandoned or arrangements made for desalination of the salt-contaminated water. To avoid such a contingency every effort should be made by the county to restrict use of their water supply and, at the same time, speed up the development of additional supplies. Because of mitigating circumstances it has not been possible to run a comprehensive aquifer test in the well field; neither have needed "outpost" wells yet been drilled between the well field and the encroaching saltwater wedge some two miles to the west that would warn of further eastward movement of the salt-water wedge. Thus it is not possible to determine at this time just how much of a water-production cutb~ck is needed to protect the field. It appears, -4-

PAGE 5

however, based on experience with other well fields in the region, that a safe figure may be in the vicinity of 18 to 20 mgd. By greatly reducing the irrigation uses until replacement supplies can be developed, it should be possible to live th th t d t t d GARALD G. PARKER, C.P.G. Senior Scientist and Chief Hydrologist 06-05-73 -5-


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