and BTEX not being detected in the infiltration
tration of sodium bromide at the point of mixing
gallery monitoring wells during operation tells
was 4 ppm (mg/kg). Injection was stopped on 1
us that aromatic hydrocarbons were being
August 1992. Frequent monitoring of the bro-
removed by the infiltration gallery or by micro-
mide concentrations in 15 wells and monitoring
bial degradation.
points surrounding the gallery continued until
Before the operation began in 1992, benzene
13 August 1992.
and BTEX levels were found at concentrations
The second tracer study introduced 1000 L of a
similar to those measured prior to operation in
330 ppm (mg/kg) solution of water and sodium
1991(1 ppb [g/kg]) in the monitoring wells sur-
fluoride into an injection well that was hydro-
rounding the gallery. Benzene was not detected
logically up-gradient of the infiltration gallery.
and 1.8 ppb (g/kg) BTEX was detected in the
The tracer was injected during a 1-hour, 39-minute
pumping well before operation in 1992. Similar
period on 8 July 1992. Fluoride concentration
to the 1991 season, benzene and BTEX were gen-
was monitored in wells next to the point of injec-
erally below detectable limits in the six monitor-
tion for several weeks after.
ing wells surrounding the infiltration gallery dur-
ing the nearly 100 days of operation in 1992.
Groundwater sampling
Benzene was detected at less than 2 ppb (g/kg)
The wells mentioned in the previous para-
and BTEX levels were less than 5 ppb (g/kg) for
graph were constructed with PVC pipe. The B,
water extracted from the pumping well during
IG, DEC, and TR series wells were constructed
1992, which is significantly lower than the 1991
with flush-threaded PVC pipe. The screened inter-
levels.
val of the wells consists of machine-cut slots in
The size of the microbial population before
the PVC pipe, with silica sand used as the out-
startup of the infiltration gallery in 1991 was
side packing. The upper portion of the TR series
higher (counts of hydrocarbon-degrading micro-
wells is cased in a 1-ft-diam. (30-cm) pipe that
organisms in groundwater samples collected next
extends above the ground surface, terminating in
to the infiltration gallery) than in samples from
a lockable sampling shelter. The B, IG, and DEC
other monitoring wells on and off the site. A
series wells are sealed with bentonite pellets, and
similar trend was observed in the mineralization
capped at the surface with a cementbentonite
potentials. After startup, our estimates of micro-
slurry seal. Construction details of the PTAN well
bial population numbers and mineralization
are not readily available. We assumed the con-
potential declined significantly at the monitoring
struction of this well to be similar to that of the
wells near the infiltration gallery.
IG series wells.
The observed rapid transport of nitrate away
from the gallery and the decline of microbial
Results
population and activity levels showed us that the
During July 1989, Shannon & Wilson sampled
water being pumped from deeper in the aquifer
the B series wells for purgeable aromatics and
flows across the surface of the groundwater table
purgeable halogens. From fall 1990 to the present,
some distance from the gallery. As a result, the
personnel from the Department of Natural
microbial population possibly was being moved
Resources (DNR), Division of Water, sampled
from the site faster than it could regenerate. To
groundwater at the site. Initially, only wells
quantify the hydrological influence of the infil-
PTAN, B1, B2, and B4 were sampled. Well B3 was
tration gallery in terms of flow rates, radius of
added during spring 1991. Monitoring at the IG
influence, and dilution factors, groundwater
and DEC series wells commenced during August
tracer studies were developed for the 1992 oper-
1991 in conjunction with the startup of the infil-
ating season.
tration gallery.
Two groundwater tracer studies, conducted in
Samples collected by DNR were analyzed by
conjunction with the infiltration gallery, deter-
the Alaska Division of Water, Water Quality Lab-
mined the flow pattern and flow rate of the
oratory in Fairbanks, Alaska, and by Northern
nutrient-enriched water as it moved away from
Testing Laboratories (NTL), also located in
the infiltration gallery and was drawn towards
Fairbanks. Parameters measured by DNR
the groundwater pumping well. The primary
included field measurements of conductivity,
tracer study introduced sodium bromide into
dissolved oxygen, temperature, and pH. Analyses
the stream of water flowing to the infiltration
by the Water Quality Laboratory include the con-
centrations of Cl, NO3, PO4, SO4, Ca, Mg, Na, K,
gallery, beginning on 21 July 1992. The concen-
14
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