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Katz, B.G., Berndt, M.P., Bullen, T.D., and Hansard, Paul, 1999,
Factors controlling elevated lead concentrations in water samples from aquifer system
in Florida: U.S. Geological Survey Water-Resources Investigations Report 99-4020,
Concentrations of total lead (Pb) and dissolved Pb exceeded the U.S. Environmental
Protection Agency action level of 15 micrograms per liter (mg/L) in approximately
19 percent and 1.3 percent, respectively, of ground-water samples collected during
1991-96 from a statewide network of monitoring wells designed to delineate background
water quality of Florida's major aquifer systems. Differences in total Pb concentrations
among aquifer systems reflect the combined influence of anthropogenic sources and chemical
conditions in each system. A highly significant (p<0.001) difference in median total Pb
concentrations was found for water samples from wells with water-level recording devices
that contain Pb-counterweights (14 mg/L) compared to non-recorder wells (2 mg/L).
Differences between total Pb concentrations for recorder and non-recorder wells are even
more pronounced when compared for each aquifer system. The largest differences for
recorder status are found for the surficial aquifer system, where median total Pb
concentrations are 44 and 2.4 mg/L for recorder wells and non-recorder wells, respectively.
Leaching of Pb from metal casing materials is another potential source of Pb in ground water
samples. Median total Pb concentrations in water samples from the surficial, intermediate,
and Floridan aquifer systems are higher from recorder wells cased with black iron than for
recorder wells with steel and PVC casing material.
Stable isotopes of Pb were used in this study to distinguish between anthropogenic and
natural sources of Pb in ground water, as Pb retains the isotopic signature of the source
from which it is derived. Based on similarities between slopes and intercepts of trend
lines for various sample types (plots of 206Pb/204Pb versus 208Pb/204Pb and 207Pb/204Pb
versus 208Pb/204Pb) the predominant source of total Pb in water samples from the surficial
aquifer system is corrosion of Pb counterweights. It is likely that only ground-water
samples, not the aquifer, were contaminated with elevated Pb concentations. Pb-isotopic
ratios of water from the Floridan aquifer system plot between trend lines connecting the
isotopic composition of Pb counterweights and the composition of acid leachates of material
from the Floridan aquifer system, indicating that Pb in these waters most likely is a
mixture of Pb derived from aquifer material and corrosion of Pb counterweights.