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Abstract
The potential fluid exchange between lakes of north central Florida and
the Floridan aquifer and the process by which exchange occurs is of critical
concern to the St. Johns Water Management District. High-resolution seismic
tools with relatively new digital technology were utilized in collecting
geophysical data from Orange, Kingsley, Lowry and Magnolia Lakes, and the
Drayton Island area of St. Johns River. The data collected shows the
application of these techniques in understanding the formation of individual
lakes, thus aiding in the management of these natural resources by identifying
breaches or areas where the confining units are thin or absent between the water
bodies and the Floridan aquifer.
Orange Lake, the primary focus of the study, is a shallow flooded plain
that was formed essentially as an erosional depression in the clayey Hawthorn
formation. The primary karstic features identified in the lake were cover
subsidence, cover collapse and buried sinkholes structures in various sizes and
stages of development. Orange Lake was divided into three areas southeast,
southwest, and north-central.
Karst features within the southeast area of Orange Lake are mostly cover
subsidence sinkholes and associated features. Many of the subsidence features
found are grouped together to form larger composite sinkholes, some greater than
400 m in diameter. The size of these composite sinkholes and the number of
buried subsidence sinkholes distinguish the southeast area from the others. The
potential of lake waters leaking to the aquifer in the southeast area is
probably controlled by the permeability of the cover sediments or by fractures
that penetrate the lake floor.
The lake bottom and subsurface of the north-central areas are relatively
subsidence sinkholes that have no cover sediments overlying them, implying that
the sinks have been actively subsiding with some seepage into the aquifer from
the lake in this area due to the possible presence of the active subsidence and
faulting.
The largest and most important features in the lake are the collapse
sinkholes found along the southwestern shore that provide conduits for exchange
between the lake and subsurface aquifer. There are two basic differences
between the southwest and other areas of the lake: (1) the features found
towards the central part of the lake are smaller in scale (1to 10 m across) and
tend to be singular structures compare to the southwest area where features
combined to form larger sinkholes (>400 m), and; (2) the southwest area is the
only site where collapse dolines were identified. These dolines are located
along the southwestern shoreline adjacent to Heagy-Burry Park.
The comparison of seismic profiles from the several other selected lake
and river sites to the Orange Lake profiles showed that other study areas were
constructed of one or two large subsidences or a combination of sinkholes to
form one large sinkhole. Aside from the difference in scale the basic
characteristics of the subsidence sinkholes were similar.
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