FROM THE DESK OF THE CHIEF HYDROLOGIST: THE ROLE OF ARTIFICIAL RECHARGE IN MANAGEMENT OF THE WATER RE~OURCES OF SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT Our District claims one of the fastest growing populations in the country, and with this rapid increase in numbers of people there is an even more accelerated demand for fresh water. As the demand grows so do supply shortages. Moreover, our water supplies are finite; therefore we must develop a thorough knowledge of the water resources and devise, by good management techniques, the capability of husbanding our supplies carefully and to the greatest benefit of the citizenry. This will require, as a basic for effective management, a kno~ledge of HOW MUCH OF WHAT KIND OF WATER IS WHERE AND HOW IT VARIES IN PLACE AND TIME. Next, we need to practice means of increasing and extending our usable water crop. Nature supplies us with just so much water and by water-budget methods we can determine wh~t these limits are. But we have it within our power to increase nature' s normal water supply. We can, for instance, clean water up after its having been previously used, and use it over and over, again and again --water never wears out! We can eliminate much waste of water. For instance, irrigate a crop only to the extent actually needed. We can prevent fresh water from being ruined by salt-water encroachment, both laterally from the oceans, bays and estuaries and vertically from salt-water bodies that, in Florida, underlie the fresh water at some depth everywhere. We can also augment our natural water crop by relatively inexpensive desalination processes, not using the very salty ocean water but the almost limitless quantities of brackish aquifer water that underlies our coastal areas everywhere at shallow depths. And finally, by capturing flood waters and storing them for later use, instead of allowing them to waste to the ocean, we can greatly extend our water crop. However, here in southern Florida there are no wide and deep valleys that can be dammed to create large and capacious reservoirs. We will, instead, have to rely upon storage available in our aquifers, the Floridan Aquifer, particularly. The Floridan Aquifer provides an enormous storage potential of vast economic worth. A rapid calculation of fresh water stored in the upper 1,000 feet of the Floridan Aquifer under the 10,000 square miles of our District gives the astonishing quantity of 697 trillion gallons! This is more than the quantity of water stored totally in the five Great Lakes, about 662 trillion gallons. Most of the time and over most of the area this great, natural, underground storage reservoir is filled to overflowing. It supplies the enormous discharge of all the great springs of the District, such as Silver, Rainbow, Crystal River, Homosassa, Chassahowitzka, Weeki Wachee and host of smaller ones such as Crystal Springs, which is about 20 miles northeast of Tampa and is the principal source of flow in the Hillsborough River during drought times. Likewise in some areas the aquifer supplies, by upward artesian leakage into the overlying water-table aquifer and into some lakes and streams, much of the water these sources receive. For instance Gourd Springs in Lake Apopka discharges into the lake at rate of about 18 millions of gallons a day. Additionally, the Floridan Aquifer is the chief source of all waters withdrawn for human needs in our District more than 90 per cent of all our supplies are developed from artesian wells, both flowing and pumped, from this tremendous underground reservoir. With an average annual recharge to the aquifer of 700,000
Page Two gallons per day per square mile, a well pumping 1,000 gpm would require 2.05 square miles, or 1,232 acres, to produce this much water without taking any . water out of storage. Likewise a well producing 5,000 gpm would require 10.1 square miles of recharge area, or 6,160 acres. When large numbers of big wells are more or less clustered in an area, as a major municipal well field or an industrial plant or thermal-electrical generator plant, each of which might well require 50 million gallons of water a day, the only way they could obtain their water without taking it from storage and thus causing a big drawdown of water levels, would be to spread the pumping (for 50 mgd) over 71.4 square miles, or 45,696 acres. Obviously this is impractical, so each such large user takes his water from a relatively small area, such as a square mile, or 640 acres. The result is the development of a relatively wide and deep cone of depression around the pumped site. And, where many large wells occur_ within an area, such as the upper Peace River Basin with its numerous large supply wells for the mining and processing of phosphate ores and the irrigation and processing of citrus products, a tremendously big cone-of-depression develops. It is this concentration of groundwater withdrawal that produced, from 1949 to 1969, a central area of about 365 square miles in which water levels fell from 40 to 60 feet and an area of about 890 square miles surrounding it in which water levels fell 20 to 40 feet. Since 1969, with increasing withdrawal of water in this area, the cone-of-depression has widened and deepened even more. Obviously such a condition (which hydrologists call "mining of water") cannot be permitted to persist in this area. Given a long enough time, it is reasonable to assume that, in this area of deep drawdown, nature will attempt to re-establish a balance between the fresh water above and the salt water below; this will result in a slow but gradual upward migration of salt water and the water supply of the area could be ruined. There are several ways to combat this problem as was mentioned earlier. But one of the best ways would be to store excess flood waters in the aquifer and use its tremendous storage capability much as we might use a huge and capacious surface reservoir, if such a facility could be developed here (it can't). There are several ways of doing this but all either require the use of recharge wells or of land-spreading to replenish the shallow water-table aquifer in areas where, by leakage to the underlying Floridan Aquifer, the water would eventually be stored. The techniques of getting water to the wells will vary, depending upon the sites available for temporary surface storage of flood waters or the availability of areas in which connector recharge well fields could be utilized. Here in our Di.strict, recharge wells work much better than in most other parts of the world. This is because the wells are "open hole" wells, ending in cavernous or highly permeable limestones containing large solution ~avities. Unlike sand or sand-and-gravel aquifers elsewhere where the wells must be screened or gravelpacked, our limestones will not suffer the clogging problems that are so troublesome in most other parts of the world. Our main problem will be to ascertain that the recharge water is chemically and biologically clean enough for recharging the aquifer. We cannot afford to pollute it beyond redemption in our efforts to refill over-drawn parts of the aquifer --such as the upper Peace River Basin or the huge well fields in northwest Hillsborough, northeastern Pinellas or southern Pasco Counties.
Page Three Water is available for recharge, the techniques are known, and already a successful start has been made in the upper Peace River Basin. There the International Minerals Company has, in cooperation with the District and the U.S. Geological Survey, installed a cluster of connector-type recharge wells which are reported to be putting about 2 mgd of recharge water into the drawndown area of the Floridan Aquifer. The water being recharged is from the shallow water-table aquifer and is being removed by gravity in a process that simply speeds up the natural leakage that normally takes place from the water-table aquifer to the Floridan. International Minerals Company is doing this not on~y to recharge the Floridan but to dewater an area scheduled for future mining. Previously, in accord with standard mining practice, the company would have dewatered the mine area by use of large pumps and the water would have been wasted to a surface stream eventually to flow into the ocean. Additionally, the District, in cooperation with the U. s. Geological Survey, is experimenting with different types and kinds of recharge wells to determine the most effective ways of their utilization, not only by the District as a watermanagement technique but also for use by municipalities, counties, the State Road Department and others. Currently, also, the City of St. Petersburg and the County of Pinellas, in cooperation with the State Department of Natural Resources, the u. s. Geological Survey and the District, are preparing for the use of recharge wells in Pinellas County to handle both excess flood waters and highly-treated sewage effluent. These . waters, clarified and chlorinated, will be recharged into the salty water of the Floridan Aquifer. The fresh water, being lighter and less dense than the salt water, will produce a "bubble" of fresh water of a rough cylindrical or oval shape. This Hbubble'1, if it does not mcive away from the recharge s ite by being carried along with flow through the aquifer, should remain more or less intact. Where previously studied by the U. S. Geological Survey elsewhere, from 10 to 80 per cent of the injected fresh water has been recoverable. It is planned to test the recoverability of the injected water at both the St. Petersburg and the Pinellas County project sites. If it works, and it seems reasonable that it will, anotla: useful means of extending the water crop will have been found. Many technical problems still remain to be solved in water management techniques, particularly in connection with artificial recharge. However, the various economic, legal, political and sociological barriers may prove far more difficult to overcome than the hydrologic, geologic, biologic, chemical and engineering problems. We think that we're well on the way to solving many of our problems here in the District and that, within another few years, we will be right up there among the leaders in modern, scientific, effective water-management techniques including artificial recharge. Garald G. Parker, C.P.G. Chief Hydrologist and Senior Scientist September 26, 1972