Draft Internal Memorandum
D R A F T
Internal Memorandum
21 January 2004
Page 3
GeoSyntec Consultants
turbidity is not necessarily indicative of poor construction and development; however, it
will certainly have an influence on metals concentrations.
ARSENIC EXCEEDANCE SUMMARY
Sheet 3 (Arsenic concentrations > 10 µg/L in UF and WACS databases) summarizes
sites with detections between 10 and 50 µg/L and greater than 50 µg/L. A total of 39
sites have at least one arsenic detection greater than 10 µg/L. While a total of seven
sites have reported arsenic concentrations greater than 50 µg/L. The sites with the
highest arsenic detections and most detections of arsenic higher than 50 µg/L include
the following:
Realco-Picketville;
Nine Mile Road; and
Country Road 672.
The WACS exceedance Summary (Sheet 4) identifies 13 facilities with one or more
exceedances of the 50 µg/L arsenic MCL and/or 100 µg/L chromium MCL.
SITE-SPECIFIC ASSESSMENT
Realco-Picketville (Sheet 5) compares arsenic concentrations over time in a
“background” and “compliance “well, and includes a time series graph. Both of these
wells, background included, have arsenic concentrations greater than 300 µg/L. In
many cases, the reported arsenic level in the background well is actually higher than the
compliance wells for most sampling events.
County Road 672 (Sheet 6) compares arsenic levels over time in ten wells. Most of the
problems (arsenic > 50 µg/L) are associated with a single well (MW-3), identified as
both a compliance and a detection well. Dade Recycling (Sheet 7) compares arsenic
levels in 17 wells over time. Nine Mile Road (Sheet 9) compares arsenic levels with
time in nine wells. There is one arsenic detection over 50 µg/L.
ARSENIC AND CHROMIUM CHEMISTRY
Arsenic and chromium groundwater solubility are going to be controlled by redox state,
and to a lesser extent pH. Arsenic is generally more soluble under reducing conditions,
while chromium generally more soluble under oxidizing conditions. Under reducing
conditions, chromium is reduced to the Cr 3+ species which, in most cases, will sorb
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