ABSTRACT:
The
hydrologic effects associated with augmenting a lake with ground water from the
Upper Floridan aquifer were examined in northwest Hillsborough County, Florida,
from June 1996 through May 1999. The hydrogeology, ground-water flow patterns,
water budgets, and water-quality characteristics were compared between a lake
that has been augmented for more than 30 years (Round Lake) and two nearby
non-augmented lakes (Dosson Lake and Halfmoon Lake).
Compared to the
other study lakes, Round Lake is in a more leakage-dominated hydrogeologic
setting. The intermediate confining unit is thin or highly breached, which
increases the potential for vertical ground-water flow. Round Lake has the
least amount of soft, organic lake-bottom sediments and the lake bottom has
been dredged deeper and more extensively than the other study lakes, which
could allow more leakage from the lake bottom. The area around Round Lake has
experienced more sinkhole activity than the other study lakes. During this
study, three sinkholes developed around the perimeter of the lake, which may
have further disrupted the intermediate confining unit.
Ground-water
flow patterns around Round Lake were considerably different than the
non-augmented lakes. For most of the study, ground-water augmentation
artificially raised the level of Round Lake to about 2 to 3 feet higher than
the adjacent water table. As a result, lake water recharged the surficial
aquifer around the entire lake perimeter, except during very wet periods when
ground-water inflow occurred around part of the lake perimeter. The
non-augmented lakes typically had areas of ground-water inflow and areas of
lake leakage around their perimeter, and during wet periods, ground-water
inflow occurred around the entire lake perimeter. Therefore, the area
potentially contributing ground water to the non-augmented lakes is much larger
than for augmented Round Lake. Vertical head loss within the surficial aquifer
was greater at Round Lake than the other study lakes, which is additional
evidence of the limited confinement at Round Lake.
A comparison of
the water quality and lake-bottom sediments at the three lakes indicate that
Round Lake is strongly influenced by the addition of large quantities of
calcium-bicarbonate enriched augmentation water. Round Lake had higher
alkalinity, pH, calcium and dissolved oxygen concentrations, specific
conductance, and water clarity than the two non-augmented lakes. Round Lake was
generally saturated to supersaturated with respect to calcite, but was
undersaturated when augmentation was low and after high rainfall periods.
Calcium carbonate has accumulated in the lake sediments from calcite
precipitation, from macrophytes such as Nitella sp., and from the deposition of
carbonate-rich mollusk shells, such as Planerbella sp., both of which thrive in
the high alkalinity lake water. Lake-bottom sediments and aquatic biota at
Round Lake had some of the highest radium-226 activity levels measured in a
Florida lake. The high radium-226 levels (27 disintegrations per minute per dry
mass) can be attributed to augmenting the lake with ground water from the Upper
Floridan aquifer. Although the ground water has relatively low levels of
radium-226 (5.8 disintegrations per minute per liter), the large volumes of
ground water added to the lake for more than 30 years have caused radium-226 to
accumulate in the sediments and lake biota.
The Round Lake
basin had higher calcium and bicarbonate concentrations in the surficial
aquifer than at the non-augmented lakes, which indicates the lateral leakage of
calcium-bicarbonate enriched lake water into the surficial aquifer. Deuterium
and oxygen-18 data indicated that water in well nests near the lake consists of
as much as 100 percent lake leakage, and water from the augmentation well had a
high percentage of recirculated lake water (between 59 and 73 percent lake
leakage). The ground water surrounding Round Lake was undersaturated with
respect to calcite, indicating that the water is capable of dissolving calcite
in the underlying limestone aquifer.
Annual and
monthly ground-water outflow (lake leakage) was significantly higher at Round
Lake than at the non-augmented lakes for the 3-year study period. Minimum
estimates of the total annual ground-water inflow and outflow were made from
monthly net ground-water flow values. Based on these estimates, total annual
ground-water outflow from Round Lake was more than 10 times higher than for the
non-augmented lakes. Local ground-water pumping, augmentation, and
hydrogeologic factors are responsible for the high net ground-water outflow at
Round Lake. Localized ground-water pumping causes the head difference between
the lake and the Upper Floridan aquifer to increase, which increases lake
leakage and results in lower lake levels. Augmenting the lake further increases
the head difference between the lake, the water table, and the Upper Floridan
aquifer, which results in an increase in lateral and vertical lake leakage. The
lack of confinement or breaches in the intermediate confining unit facilitates
the downward movement of this augmented lake water back into the Upper Floridan
aquifer. The increase in ground-water circulation in the leakage-dominated
hydrogeologic setting at Round Lake has made the basin more susceptible to
karst activity (limestone dissolution, subsidence, and sinkhole formation).
Comment from Peter Schreuder:
ReplyDeleteJennifer;
Thank you so much for the information. John and I are aware of the work done by the local USGS office. As the report states lake augmented water will also recharge the surrounding water table and therefore any augmentation of a leaky lake will not immediately result in a rising level of the surface water until the ground water demands in the surrounding water table aquifer are met.
To minimize direct “recycling” of the ground water to be withdrawn from the Floridan aquifer through production wells to be installed next to Lake Brooklyn to augment the lake as recently proposed by Mr. Van Zandt, John and I suggested a possible location of augmentation wells much farther away for the lakes. We suggested in our plan to locate augmentation wells, if that is the chosen option, near the Southwest Quadrant Lake or on DuPont’s property.
Peter