December 13, 1978 File No.
TO: B.R. Laseter, Director, Department of Operations 9
THRU: M.D. Courser, Supervisor, Environmental Section
C.H. Miller, Director, Resource Development Division
FROM: K.C. Jones, Hydrologist, Environmental Section eAcJ-
T.F. Rochow, Biologist, Environmental Section f/ j.
RE: Investigation of Dry Conditions at Mr. S.C. Bexley's Property,
2.5 miles North of South Pasco Wellfield
On Friday, December 1, a field investigation of Mr. S.C. Bexley's property
was conducted to observe the extremely dry conditions in some of his
cypress areas. The property is located approximately 2.5 miles north of
the South Pasco Wellfield and within six miles of other surrounding
wellfields. The objective was to determine if the pumpage from the.
South Pasco Wellfield has had significant effects on his cypress areas
and to determine What other factors may contribute to the low water
table conditions presently experienced on his property.
Although our field observations were somewhat clouded by a 1.5 Inch
rainfall occurrence the day before the investigation, drought conditions
were still present at the time of the Investigation. In addition to the
field observations, several other days were spent computing mathematically
the wellfield effects on the area of investigation, compiling rainfall
and evapotranspiration (ET) data, reviewing previous investigations in
the area,-and preparing the.memo.
Three major factors have been found to contribute to the low water table
conditions that presently exist on Mr. Bexley's property. They are<--.
described as follows:
Mr. Bexley's property lies approximately 2.5 miles from the center of
pumpage of the South Pasco Wellfield and within six miles of the Cypress
Creek, Starkey, and Eldridge-Wilde wellfields. In addition, there are
several private Irrigation well systems.in the area which are in use
periodically. A value for the total groundwater withdrawal is beyond
the scope of this small investigation but should be calculated if future ,,
studies are to be done. The main concern of this study was to dete V)dsri d
the effects of the South Pasco Wellfield, the wellfield of closest /. e
proximity and greatest drawdown effects on the area. .'
,f ,. -,% .
Memorandum December 13, 1978
B.R. Laseter Page 2
Re: Dry Conditions- S.C. Bexley's Property
To compute the drawdown of the South Pasco Wellfield on Mr. Bexley's
property, several assumptions had to be made: 1) the Floridan Aquifer
has the same transmissivity and storage coefficient between Mr. Bexley's
property and the wellfield; 2) the surficial aquifer has the same trans-
missivity between Mr. Bexley's property and the wellfield; 3) the confining
bed is homogeneous and has the same leakage coefficient between Mr.
Bexley's property and the wellfield; 4) wellfield pumpage is an average
of 16 mgd; 5) yearly water table fluctuation is 6 feet annually; 6) the
average ET rate Is 37.5 inches per year. The equation used was developed
for the Starkey Wellfield project (Motz; pp. 1061-1074), however, all of
the aquifer properties were changed to define the conditions at the
South Pasco Wellfield as follows:
Transmissivity of Floridan Aquifer 350,000 gpd/ft
Transmissivity of Shallow Aquifer 1,500 gpd/ft
Leakance of Confining Bed 1.3 x 10" gpd/ft3
(Guyton; pg. 20)
Assuming the above conditions are true, the drawdown in the water table
in Mr. Bexley's property due to the pumpage at South Pasco is only 5.9
inches. The drawdown in the potentiometric level is 57.7 inches (see
work sheet). This, however, is only the effect from the closest wellfield.
Other wells and wellfields in the area would probably double these
The lack of rainfall in the area has had a significant effect on the
lowering of water table levels in the past several years. The average
expected rainfall over the last ten years has been estimated at 50 -
inches per year at the South Pasco wellfield (Courser; pg. 20) for a
total expected rainfall amount of 250 inches over a 5-year period. The
actual rainfall amount at the wellfield for 5 years since pumping first
began (March 1973-February 1978) is 236.94 inches, creating a deficit of
more than 13 inches. More recently, in the last four months we have
experienced extreme drought conditions. In the period of August 1-
November 30, 1978 only 10.35 inches of rainfall occurred over the wellfield.
The average expected rainfall over the same period for previous years is
estimated to be 17.7 inches, creating a deficit of 7.35 inches in the
last four months alone. This amounts to a drought intensity frequency
of approximately one in twenty years (J. Whalen; personal communication).
Land use is also a large factor contributing to the lowering of water
table levels in the area of study. Much of the natural vegetation has
been cleared and replaced with improved pasture. The amount of ET loss
due to converting natural vegetation to improved pasture has been estimated
by the Institute of Food and Agricultural Services (I.F.A.S.) to be 17.6
inches per year greater than the ET values for natural vegetation (Wehle,
1971). The ET values for natural vegetation in the area of study is
approximately 37.5 inches annually. The total ET value for improved
pasture would therefore be 55.1 inches per year, a value 5 inches greater
than the average annual rainfall.
Bemorandum December 13, 1978
B.R. Laseter Page 3
Re: Dry Conditions S.C. Bexley's Property
Extensive drainage and irrigation ditching has also occurred, mainly
along the southern portions of the property. This will reduce the
amount of runoff into the cypress areas during rainfall events by diverting
runoff water into the ditches. -It will also drain the water table-
aquifer at times when the static water level is above the level of the
base of the ditch. A value for the amount of water lost due to the
extensive ditching is beyond the scope of this investigation but should
be calculated if future study is warranted.
Water levels on Mr. Bexley's property are affected by many factors.
Wells and wellfields surrounding the property lower both the Floridan
and surficial aquifer levels. The South Pasco Wellfield alone contributed
5.9 inches of drawdown in the surficial aquifer and 57.7 inches of drawdown
in the Floridan Aquifer. Rainfall deficits have contributed to lower
water levels. Since the wellfield was first put into operation, there
has been a rainfall deficit of at least 20 inches. Finally, land use
practices also influence water levels through lower runoff rates and
higher ET values. For Instance, 17.7 inches of additional water is lost
annually by ET from the surficial aquifer when pasture grass is substituted
for natural vegetation. More water is lost when ditching at times
drains wetland areas, reduces runoff into wetland areas, and drains the
water table aquifer.
Mr Bexley's agricultural water management systems created optimum conditions
for agricultural use at the time they were installed. However, the
recent droughts and wellfields have made this system overeffective
If further study seems necessary at a later date, the following suggestions
might serve as guidelines to follow: 1) determine the total drawdown
effects in the Floridan Aquifer and surficial aquifer from all wells and
wellfields in the 6-mile radius; 2) determine the amount of water loss
due to ditching on Mr. Bexley's property; 3) continue with a monitoring
program as discussed in the previous memo on the subject, dated May 24,
Attachments: References Cited
Courser, W.D. & P.A. Hernandez. April, 1977. Third Annual Report of the
St. Petersburg-South Pasco'Wellfield Study, Environmental Section
Technical Report #1977-1. Southwest Fla. Water Management District,
Guyton, William F. and Assoc. South Pasco Wellfield, regulation and
effects of pumping prepared for the Board of County Commissioners,
Pasco County. Florida. TD 407. W5. C2.
Motz, Louis H. "Steady-State Drawdowns in Coupled Aquifers."
the Hydraulics Division, ASCE, Vol. 104, No. HY7, Proc.
13887, July, 1978. pp .1061-1074,
Schmidt, Ronald G.,.Richard W, Davis. PertinenhtHydrologic Data and
Design Criteria, Pasco County Wellfield. April, 1971. TD 407. S356.
Wehle, John R. and H:arry'F.-Oleson. Determination of the Consumptive-
Use of Water by Agricultural Practices, 1971, also described in
SWFWMD Technical Memoranda, Available Use Codes, Consumptive Use
Practices, February 21, 1978; -
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