Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 406

aquitard, at depths of 180-225 ft. The average thickness throughout the remediation area is 70 ft.
The upper zone consists of unconsolidated, highly permeable sand and gravel layers interbedded
with layers of clayey, impermeable sediments. The stratigraphy is similar to that of the unsaturated
zone. Water within the individual sand and gravel beds occurs under unconi ned to semiconi ned
conditions. In some parts of the area, the individual sand and gravel beds appear to be hydraulically
isolated from beds above and below at the same location.
Locally, the upper zone of the regional aquifer is the most productive zone. The upper zone will
support sustainable pumping rates of 50-150 gpm in the southeast part of the remediation area and
as much as 350 gpm in the northwest (where more coarse-grained, permeable beds predominate).
The upper zone also contains the bulk of the solvent contamination. An elongated contaminant
plume formed as a result of a continuing migration from source areas (below former disposal sites)
combined with relatively high groundwater l ow rates in the upper zone. Regional groundwater l ow
in the upper zone is northward to northwestward across the remediation area.
The lower zone of the regional aquifer begins at about 300-350 ft bgs at the base of the coni n-
ing clay and continues to an unknown depth. The lower zone contains minor levels of solvent
contamination. To the northwest, the upper zone and lower zone combine to become the undi-
vided regional aquifer, which is the principal aquifer for large volumes of drinking water for the
City of Tucson.
8.7.3 C HLORINATED S OLVENT U SE , R ELEASE M ECHANISMS , AND D ISCOVERY
Historical industrial processes conducted at AFP 44, including the production, maintenance, and
modii cation of weapons systems, have resulted in wastewater and general industrial waste includ-
ing solvents, paint sludge, and thinners. Chromium and chlorinated solvents, including TCE and
methyl chloroform, were the principal materials used at the facility. Releases occurred primarily
from sludge-drying beds, lagoons, degreasers, and landi lls. Chlorinated solvents associated with
AFP 44 were identii ed in off-site groundwater to the northwest, as were similar compounds released
within other project areas at the TIAA Superfund Site ( Figure 8.11 ). Chlorinated compounds were
present in both shallow and deep soil; shallow soils were contaminated primarily by chromium and
other inorganic constituents.
TCE was used at AFP 44 from the 1950s until it was replaced by methyl chloroform in 1974.
Methyl chloroform was used from 1974 until the mid-1990s although the last decade of usage was
at a much lower rate owing to process changes and efforts to minimize waste. The chlorinated
solvents identii ed in the groundwater consist of TCE with lesser amounts of DCE present. 1,1-
DCE was not used as a solvent on-site, but resulted from the breakdown of methyl chloroform.
Other chlorinated daughter products are present on the site at insignii cant levels. The primary
sources of solvents that contaminated the groundwater, in the order of greatest probable contribu-
tion, are Site 2—FACO Landi ll, Site 3—Inactive Drainage Channel Disposal Pits, Site 5—Sludge
Drying Beds/Building 801, and Site 14—Shallow Groundwater Zone.
The chromium plume was much smaller than the solvent plume. At the start of remediation in
1987, it covered 190 acres and was almost entirely on AFP 44 property. The sources of chromium
in the groundwater were plating wastes placed in unlined sludge-drying beds at Site 5, plating
wastes placed in disposal pits at Sites 3 and 4, and wastes accumulated in associated drainage chan-
nels at Site 6. The chromium plume extent was reduced dramatically once these source areas were
excavated in the early to mid-1990s. Chromium is only present above standards in a few wells at the
site, which are in areas of known disposal or release. Hexavalent chromium is the more soluble and
more toxic oxidation state for chromium; however, at AFP 44, natural aquifer conditions are condu-
cive to the reduction of hexavalent chromium to trivalent chromium, which is the less soluble and
less toxic oxidation state.