**Use of Multiple Tracers and Geochemical Modeling to Assess
Vulnerability of a Public Supply Well in the Karstic Upper Floridan
Aquifer**

**Brian G. Katz^1^, Christy A. Crandall^1^, W. Scott McBride^2^, Marian
P. Berndt^1^**

1 U.S. Geological Survey, 2010 Levy Avenue, Tallahassee, FL 32310

2 U.S. Geological Survey, 10500 University Center Drive, Suite 210,
Tampa, FL 33612

*2006 National Monitoring Conference: Concurrent Session F: F3-Assessing
Ground Water Vulnerability Through Mechanistic Methods II, May 7-11,
2006, San Jose, CA.*

**Biographical Sketches of Authors**

Brian G. Katz, PhD, is a research hydrologist with the U.S. Geological
Survey in Tallahassee, Florida. During his career with the USGS, he has
conducted numerous geochemical studies of processes that control the
composition of natural and contaminated waters. His current research
involves the use of isotopic and other chemical tracers to quantify
hydrochemical interactions between ground water and surface water,
determine sources and chronology of contamination in karst aquifers, and
assess flow-system dynamics in complex aquifer systems.

Christy A. Crandall is a hydrologist with the U.S. Geological Survey\'s
National Water Quality Assessment Program with training and experience
in hydrology and water quality. Since 2002, she has served as project
manager and ground-water modeler for the Transport of Anthropogenic and
Natural Contaminants to Supply Wells project from its planning phase
into full implementation. Christy moved to the U.S. Geological Survey in
1993 from the National Park Service and has worked in a variety of water
programs over the last 20 years.

W. Scott McBride is a hydrologist with the U.S. Geological Survey in
Tampa, Florida. He has 17 years of experience in surface and groundwater
investigations, water quality sampling and analysis, technical equipment
procurement and use, specialized sampling techniques, and project
management. He has coordinated and implemented several groundwater
studies for the Georgia-Florida Coastal Plain Study Unit of the National
Water-Quality Assessment Program, and is currently project chief of a
watershed study in central Florida.

Marian P. Berndt is a hydrologist with the U.S. Geological Survey in
Tallahassee, Florida. She is currently the project chief for the
Georgia-Florida Coastal Plain study of the National Water Quality
Assessment program. She has worked on a variety of water quality and
ground water geochemistry studies and is currently working on studies on
the geochemistry and occurrence of contaminants in the surficial and
Floridan aquifer systems in the southeastern U.S.

**Abstract**

As part of a USGS national study of contaminants in public supply wells,
multiple chemical indicators and geochemical modeling techniques were
used to assess contaminant movement in a karstic groundwater system in
Tampa, Florida. Six volatile organic compounds and four pesticides were
detected in trace concentrations in water from a public supply well
(PSW) tapping the carbonate Upper Floridan aquifer (UFA) with an open
interval from 119-174 ft below land surface. These contaminants were
detected more frequently in water samples from 11 monitoring wells in
the overlying clastic surficial aquifer system (SAS) than in water from
13 monitoring wells in the UFA in the study area. Likewise, nitrate-N
concentrations in the PSW (0.72-1.4 mg/L) were more similar to median
concentrations in the oxic SAS (2.1 mg/L) than the anoxic
(sulfate-reducing) UFA (0.06 mg/L). Elevated concentrations of ^222^Rn
and U in the PSW appear to originate from water moving downward through
sands and discontinuous clay lenses that overlie the UFA. SF~6~ and
^3^H/^3^He concentrations in water samples from the SAS were consistent
with recent recharge (4-year median age), but water from the UFA
indicated binary mixtures of recent recharge with tracer-free waters
(\>50 years). Tracer concentrations in the PSW also indicated the
presence of some young water (\< 7 years), and SF~6~ apparent ages for
water from 140-160 ft depth in the PSW were slightly younger (1-2 years)
than ages of water from shallower depths in samples collected during
pumping and non-pumping conditions. Additional chemical and geophysical
data indicated that movement of young water into the 140-160 ft zone is
enhanced by solution features that likely are directly connected to the
surface and SAS. Geochemical mass-balance models for the PSW indicate
mixing scenarios that include 50-80% contribution of water from the SAS,
along with dissolution of minor amounts of calcite, gypsum, and dolomite
from the UFA.