Environmental Engineering Reference
In-Depth Information
Site-specific risk-based criteria (RBC) for both human health and ecologi-
cal protection were developed for each of the contaminants of concern. As
the Damplands property has no onsite workers with little to no likelihood
for using groundwater for irrigation purposes, the most relevant remedia-
tion criteria for TCE was the ecological criteria (330 μg/L) for the protection
of the Helena River.
The two source areas are located in an area referred to as the SIA, which
is north (upgradient) of the area referred to as the Damplands, a low-lying
wetland area bordering the Helena River. The Damplands and the SIA are
separated by an escarpment with topographical relief of approximately
10 m. The Helena River is the main drainage feature of the area and it forms
the southern boundary of the study area. It is located approximately 300 m
south-southwest of the WCS and approximately 120 m from the base of the
river valley escarpment where the PRB system was installed.
A drain located in the northeast of the Damplands collects stormwater
from the SIA and directs it southwest into the wetland depression. It is com-
mon during the winter months (May to October) for the Damplands area to
be inundated and form a seasonal pond, which, when full, overflows into the
Helena River. Standing water infiltrates into the alluvial aquifer and contrib-
utes base flow to the Helena River. This infiltration may dilute the concen-
trations of contaminants and prevent advancement of the plume during the
high-rainfall autumn and winter months.
The surficial geologic unit beneath the WCS and the SIA is the Guildford
Formation, which is part of the Quaternary Swan Coastal Plain sediments and
extends to about 20 m depth. The Guildford Formation comprises interbed-
ded layers and lenses of sand, silt, and clay of fluvial origin. The base of this
unit is often iron-stained and/or iron-cemented and is present in the study
area at about 0 to −5 m Australian Height Datum (AHD). Below the Guildford
Formation is the Leederville Formation. Sediments of the Leederville
Formation are alluvial in origin, and are mostly clays and siltstones.
Investigation boreholes along the base of the escarpment and a few meters
upgradient of the PRB location initially encountered sand, silty sand, and
clayey sand deposits. These sediments could not be readily attributed to a
specific unit; rather, this was interpreted to be a transitional zone that may
include a combination of colluvial deposited Guildford Formation from the
escarpment interbedded with more recent deposits of the alluvial deposits
from the Helena River. This transitional zone (Figure 7.2) is located where the
Helena River and its predecessors have cut a valley into the Guildford and
Leederville Formations. This valley has, in turn, been in-filled with younger
silts, sands, and clays; the full thickness of these in-filled alluvial sediments
has not been investigated.
In these same boreholes, at a depth of approximately 1 to 0 m AHD, sands
and silty sands had an orange-brown color, which was visually consistent
with sediments found near the base of the Guildford at other locations in
the study area. The top of the Leederville Formation was encountered at
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