" April 21, 1977 -1-oj t.h-.
MEMO jALohr-. ALULLJ
RE: Governing Board Committee on Rule Implementation in
Manasota Basin/Meeting of April 20, 1977
The following is a summary of the notes I made at the
meeting of the Governing Board Committee on Rule Implementation
in the Manasota Basin. They consist primarily of notes taken
during the presentation by Dr. John Moore, of the U.S.G.S.
As you will recall, the theme of Dr. Moore's presentation was
that because of geological differences, the ground water
hydrology within the Manasota Basin differs from the ground
water hydrology in northern Hillsborough and Pasco Counties.
Initially, Dr. Moore indicated that the differences in
ground water characteristics were centered on a line running
east and west roughly through the center of Tampa Bay. Later
on in the meeting, he indicated that the Alafia River closely
r approximates the dividing line between "northern geology" and
In Dr. Moore's opinion the two most significant characteristics
of ground water hydrology within the Manasota Basin were:
1. That leakance is very small, relative to northern
Hillsborough and Pasco Counties. This was attributed to the
presence of the "hawthorne formation", a geological limestone
formation running from approximately 100 feet below ground level
approximatelyy 350 feet-below, the.-surfaee Dr. Moore
d"e hawthorne formation Lwas relatively impermeable
~:eery;Ehc;icu ,quiclude or confining layer.
,.'B. -.Jr i of a iege number of seams and fraturs..es
Swit ithe "' an Aquifer", below the htawthorne formation,
iFlbridan Aq.tIfer within .the Manasota Basin has extremely
5 h ttr4nsmissiv ty. Thi ,'of course, means that withdrawals
of waer6ffrom the Floridari Aquifer will be noticed at con-
S--*, siderable distances from the point of withdrawal more quickly
S thanwould be the case ii northern Hillsborough or Pasco Counties.
I "',In; cbntrast to the geology within the Manasota Basin, there
S.. no o athQrne formation in Pasco geology. Over the centuries
it ha's 5'p'rently been eroded away. As a result, the Floridan
S Aquifere ipsmuch closer to the surface, the confining layer or
aquiclude is much thinner and more permeable. As a result, in
Pasc hydrology there is a much more direct connection between
i / */-*'*' .i ** '' (*
Memo to LMB
"' Page 2
April 21, 1977
the Floridan and surficial aquifers. This means in
Dr. Moore's opinion, that well pumping in Pasco County will
have a much more direct impact on stream flow and lake
levels, than would a similar amount of pumping within the
It was at this point in the presentation that Dr. Moore
indicated that as one proceeds north from the Manasota Basin,
the hawthorne formation feathers out and disappears approxi-
mately at the Alafia River.
A second distinction drawn by Dr. Moore between the Manasota
Basin and the Northwest Hillsborough River Basin was that
while the Hillsborough River and others in Pasco County are
principally ground fed (spring fed) the stream flows within
the Manasota Basin are much more dependent upon rainfall levels.
A third principal distinction drawn by Dr. Moore between the
Manasota Basin and the Northwest Hillsborough River Basin was
r that in his opinion withdrawals south of the Alafia River tend
to be more consumptive than withdrawals north of the Alafia
River. This results because of the presence of the thicker
aquiclude south of the river. If water is withdrawn from the
Floridan Aquifer and then deposited in the surficial aquifer
within the Manasota Basin, it will tend to go to sea, rather
than "leak" back into the Floridan Aquifer. Conversely, water
deposited in the surficial aquifer in northern Hillsborough or
Pasco Counties will tend to "leak" back into the Floridan
Aquifer at a much faster rate. (At another point in the
proceedings, Dr. Moore indicated that the water in the surficial
aquifer in northern Hillsborough and Pasco Counties tends to
leak back into the aquifer ten times faster than the water in
the surficial aquifer within the Manasota Basin. (The actual
figures: 1 x 10- for Hillsborough/Pasco; 1 x 1T4 in Manasota)
At the conclusion of his presentation, Dr. Moore listed the
following specific differences/problems in ground water hydrology
between the "south" and the "north". If one assumes the
presence of a well pumping at a constant rate in the south as
well as in the north, the following results would be anticipated:
1. The pumping in the south would have an increased
effect on reducing the potentiometric head within the Floridan
Aquifer. Conversely, the pumping in the north would have less
r effect on reducing the potentiometric head in the aquifer.
2. The pumping in the south would have relatively less
impact on stream flow rates than would the pumping in the north.
Memo to LMB
April 21, 1977
3. The pumping in the south would have less effect
on lowering lake levels than would the pumping in the-north.
4. The pumping in the south would have less impact
on the water level within the shallow or surficial aquifer
than would the pumping in the north.
One common characteristic and problem which Dr. Moore indicated
would exist in both the south and the north is the danger of
salt water intrusion caused by a loss in potentiometric head
within the Floridan and surficial aquifers.
Although no specific rules or regulations were discussed
during the meeting, it seems apparent to me that if what
Dr. Moore suggests is in fact true, many of the District's
rules relating to consumptive use permits need to be reviewed.
Some specific rules which need to be considered, in my
opinion, are as follows:
1. Section 16J-2.11(3), F.A.C.: This, of course, is
the water crop rule. If the aquiclude in the Manasota Basin
is in fact as dense and impermeable as Dr. Moore indicated,
the ability of precipitation to recharge the Floridan Aquifer
is diminished. One would suspect therefore that evapotranspira-
tion and surface water run-off would be relatively greater
within the Manasota Basin. I would suspect that the theoretical
water crop (1,000 gallons per acre per day) should be reduced
somewhat for the Manasota Basin area.
2. Section 16J-2.11(4)(b), F.A.C.: This rule indicates
that the permit will be denied if it is going to cause the
level of the potentiometric surface to decline by more than
five (5) feet. Within the Manasota Basin I would anticipate
that this figure be raised because of the greater transmissivity
within the Floridan Aquifer.
3. Section 16J-2.11(4)(c), F.A.C.: This rule indicates
that a permit will be denied if the withdrawal is going to
cause the level of the water table under adjacent lands to be
lowered more than three (3) feet. I would anticipate that this
figure would be reduced because the presence of a relatively
thicker aquiclude minimizes the impact of a withdrawal from
the Floridan Aquifer upon the surficial aquifer on adjacent
4. Section 16J-2.11(4)(d), F.A.C.: This rule indicates
that a permit will be denied if the withdrawal will cause the
level of the surface of water in any lake or other impoundment
to be lowered more than one (1) foot. Here, again, because of
the presence of a relatively thicker aquiclude I would not be
surprised to see a rule change proposed for the Manasota Basin
Memo to LMB
April 21, 1977
which would deny a permit for less than one (1) foot of
decline in the level of surface waters and lakes in the
vicinity of a large water withdrawal.
I 1 ok forward to your comments.