Comparison of the hydrogeology and water quality of a ground-water augmented lake with two non-augmented lakes in northwest
Hillsborough County, Florida
Dataset Identification:
Resource Abstract:
<p>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 nonaugmented lakes (Dosson Lake and Halfmoon Lake). </p><p>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.</p><p>Ground-water flow patterns around Round Lake were considerably different than the nonaugmented lakes.
For most of the study, groundwater 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. </p><p>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 <i>Nitella</i> sp., and from the deposition of carbonate-rich mollusk shells, such as<i> Planerbella</i> 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 atrributed 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.</p><p>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. </p><p>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 groundwater 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)</p>
Citation
Title Comparison of the hydrogeology and water quality of a ground-water augmented lake with two non-augmented lakes in northwest
Hillsborough County, Florida