Lake
Grady, Tampa, Florida
Source Number:
2.3.1
Source Description
Cardinale, Tom. Groundwater Contamination from Lake Grady Sinkhole #2.
Environmental Protection Commission of Hillsborough County, Florida.
Water Management Division. December 2000.
Hillsborough County
Health Department, Environmental Health Services Narrative Report Waterborne
Illness Outbreak Investigation #290060. "An Outbreak of Gastrointestinal
Illness Related to Groundwater Contaminated by Surface Water through
a Sinkhole in Hillsborough County, Florida." November 7, 2000.
Key Words
sinkhole; fecal coliform; total coliform, bacteria, Lake Grady, Florida,
tracer dye, contamination; water transport; groundwater; surface water
Executive Summary
In March 2000, residential drinking water wells east of Lake Grady,
near Riverview, Florida, exhibited poor water quality. Subsequent bacteriological
analysis of approximately 200 residential wells indicates that many
of the wells were contaminated with total and fecal coliform bacteria.
In response to a reported increase in the groundwater contamination
over the next 3 to 4 months, the Southwest Florida Water Management
District (SWFWMD), the Hillsborough County Health Department (HD), and
the Environmental Protection Commission (EPC) of Hillsborough County
conducted a tracer dye study to ascertain that a sinkhole approximately
200 feet from the southeast corner of Lake Grady was the source for
contaminated surface water transport to the groundwater. A colored dye
was placed into the sinkhole, followed by well water monitoring and
dye analysis. Tracer dye test results confirmed the sinkhole/well water
contamination relationship, and plugging of the sinkhole proceeded.
It was estimated that approximately 50 million gallons of surface water
flowed into the shallow aquifer through the new sinkhole. This event
replicated a similar event at the same site in 1974.
Well Monitoring
Before Sinkhole Plugging
Well monitoring and water analyses were conducted each by SWFWMD, HD,
and EPC. SWFWMD sampling concluded elevated turbidity, color, and total
organic carbon (TOC). HD tested for the presence or absence of total
and fecal coliform and Esherichia (E) coli using Standard Method 9223
B to determine the extent to which the private homeowner wells were
contaminated by surface water. Results of that test are provided in
Table 5.2 of Narrative Report #290060 (EPC, 2000).
Well Monitoring
After Sinkhole Plugging
EPC testing performed before and after sinkhole plugging indicates a
decrease in total and fecal bacteria count and E. coli. EPC well monitoring
data indicate that the fecal coliform bacteria in two of the private
owner wells sampled returned to zero or near zero 14 to 22 days after
the sinkhole was plugged, as shown in Tables 5.4 and 5.5 and on Graphs
5.1 and 5.3. These wells had experienced the greatest degree of contamination
after the sinkhole formation, and were located closest to the sinkhole.
Source Water
Quality of Lake Grady
Table 6.1 indicates the range of microbiota concentrations in the lake
water that was contaminating the wells, based on data collected over
a six-month period. The dye studies indicate a dye presence in the Schlarbaum's
well within 18 hours and in the Morley's well within 29 hours of application.
The Schlarbaum well is 4 inches in diameter with a well casing depth
of 78 feet, and overall depth of 215 feet. These well construction characteristics
are fairly representative of known well construction characteristics
in the area. No information on the Morley well construction was available.
Location and
Geographic Characteristics
The referenced sinkhole is 3 miles southeast of Riverview in Section
36, Township 30, and Range 20 of Hillsborough County, Florida. The center
of the sinkhole is at Latitude N 27 49.634" and Longitude W 82
15.473," as measured by EPC on 8/11/00 using a GPS instrument.
Sinkhole No. 2 is about 2.5 miles south of the Alafia River on the private
property of Thomas and Jeanne Schlarbaum, 13821 Raulerson Road, Riverview,
Florida 33569.
The lake/stream
system in which the sinkhole lies is a tributary to the Alafia River.
The sinkhole is approximately 200 feet offshore from the normal high
water level of Lake Grady. It is located in a relatively shallow southeast
portion of the lake. This lake area is broad, shallow marsh with a main
flow in the remnants of what was Bell Creek. The sinkhole is to the
east of this remnant creek. All of the affected homes in the area are
within a 3,000-foot radius of the sinkhole.
Graphics and
Tables
The following graphics and tables are provided in Adobe .pdf format:
Table
5.4 - Schlarbaum Well
Table 5.5 - Morley Well
Table 6.1 - Surface
Water Quality in Lake Grady
Graphic 2.2 - Water
Depth and Elevation
Graph 5.1 - Schlarbaum
Fecal Coliform Bacteria
Graph 5.3 - Morley Fecal
Coliform Bacteria
Contact Information
Mr. Tom Cardinale
Hillsborough County Environmental Protection Commission
1900 9th Avenue
Tampa, Florida 33605
Phone: 813-272-5960
Source Documents
Hardcopy source documents are available on request. Source documents
are not available electronically.
