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well volumes usingla`dedicated inertial pump (Waterra®). NAPL was present in the sand aquifer below the average
<br /> f Ground-water samples.were then collected in 40-m1 vials water-table elevation. Outside of the immediate spill area,
<br /> with Teflon®-lined`septa and no headspace. Samples for contaminants occur only in the sand aquifer.Contaminants
<br /> metals and ions were pumped through a 0.45-µm filter prior are not present in the sandy clay since the water table is close
<br /> to collection. _ to the sand-clay contact,and significant ground-water flow
<br /> i Temperature,pH,oxidation-reduction(redox)poten- does not occur in the clayey material.
<br /> tial, and dissolved oxygen were measured in the field with
<br /> standard probes:Redox potentials are reported as measured Ground-Water Monitoring
<br /> (� in the field and have not been corrected for pH. Carbon As the first step in evaluating the monitoring data,an
<br /> dioxide was measured in the field by titrating with NaOH to analysis was conducted to determine if there had been a
<br /> pH 8.3 and is reported as total CO2(carbonate,bicarbonate, statistically significant change in contaminant concentra-
<br /> and free CO2):•W hen dissolved iron exceeded 1 mg/1,CO2 tions over the monitoring period.The concentration of each
<br /> results were corrected for NaOH consumed during the reams individual BTEX component was plotted versus time on a
<br /> tion with iron". BTEX samples were concentrated by purge logarithmic scale for each well. The"best-fit line was then
<br /> ! and trap followed"by analysis on a Perkin Elmer 8500 gas determined by linear regression and evaluated to determine
<br /> chromatograph (GC) equipped with a flame ionization if the slope of the line was significant at the 95%level.Of the
<br /> ' detector.GC-operating conditions and quality control mea- 96 possible combinations examined, 10 had statistically sig-
<br /> sures are described Gomez(1993).Individual compounds nificant slopes. Of these 10, seven had a negative slope
<br /> were quantified. by comparison of detector response and (concentration decreasing), and three had a positive slope
<br /> retention time with known standards. Inorganic nutrients ' (concentration increasing).The largest negative slope identi-
<br /> and ions(NO3 ,-N,H4 ,PO;',SO,-')and metals(Ca,Cd, fied would result in a drop in average concentration of 0.8%Q
<br /> j Zn,Ni, Fe Mn-,i,,*,Na, K,-Cu, S)were analyzed by ion over the monitoring period.This.distribution is very similar
<br /> �] chromatographf 4,hd inductively coupled,plasma-emission to what would be expected from a totally random popula-
<br /> spectroscopy foo6wing standard methods(American Public tion and indicates that the plume is stable with no significant
<br /> Health Association, 1992). increase or decrease in concentration over the monitoring
<br /> period studied.
<br /> Results On the basis of these results,the arithmetic average of
<br /> all concentration measurements was used in subsequent
<br /> Residual Hydrocarbon Distribution
<br /> { _ Tanalyses. Average concentrations of BTEX and indicator
<br /> The observed distribution of total BTEX in a longitu-
<br /> { dinal transect is shown in Figure 3.In the area immediately Parameters are listed in Tables 1 and 2.
<br /> beneath the former tank (0 and 9 ft from well U10), the
<br /> highest BTEX concentrations were present at a depth of 12 Discussion
<br /> f ,
<br /> to 13 ft below grade. The depth to water varies between 4 Background Geochemistry
<br /> and 13 ft with an average of 10 ft(well till). The NAPL Ground water from upgradient wells(U 11 and U9)and
<br /> probably migrated downward into the sand unit when the immediately west(A20)of the dissolved hydrocarbon plume
<br /> water table was low. contains moderate levels of dissolved oxygen (2-3 mg/1),
<br /> -� When the water table rose, the NAPL was trapped nitrate(1-6 mg/l as N), and sulfate(6-9 mg/l as S). Essen-
<br /> below the finer grained,sandy clay.Downgradient from the tidily all of the sulfur in the ground water is present in the
<br /> tank pit-(14 ft),the zone of highest BTEX concentrations is sulfate form.Dissolved iron at these locations is low(<0.5
<br /> slightly deeper. At this location, the sand-clay interface is mg/l).The ground water is acidic(pH<5)with low buffer-
<br /> ing capacity(alkalinity-~6 mg/1 as CaCO3)and low levels of
<br /> below the land.surface.In all locations,the majority of the dissolved CO2 (15-30 mgJl as Q. Background dissolved
<br /> phosphorus levels are low(<0.2 mg/1)and could be a factor
<br /> limiting the rate of biodegradation.
<br /> ------Distance from well u10------------- Variation in BTEX and indicator Parameters in
<br /> O Ft 9 Ft 14 FT Direction of Flow
<br /> 0 The observed variations in BTEX components, elec-
<br /> a trop acceptors,and indicator parameters in a north-South
<br /> s profile along the dissolved hydrocarbon plume centerline
<br /> 10 are shown in Figure 4 and Table 1.Several distinct zones can
<br /> 12 be identified where different oxidation-reduction processes
<br /> ie dominate.
<br /> 18
<br /> p 7, ,--b 00 400 0 200 300 400 0 100 200 300 400 The upgradient edge of the BTEX plume 1S imme-
<br /> Total BTEX(mglft Total BTEX(mgjft Total BTEX(mg*g)
<br /> diately.north of well U 10. Here,nonaqueous phase hydro-
<br /> Fig.3.Vertical distribution of total BTEX in three soil cores in carbon is present in the aquifer material. As clean ground
<br /> the NAPL-contaminated zone. The 0-ft core was collected water enters this region,soluble hydrocarbons partition out
<br /> J immediately adjoining well U10 in the center of the farmer tank
<br /> pit.Other borings are in a line directly downgradient(south) of of the NAPL and into the aqueous phase.A portion of these
<br /> U10. hydrocarbons are immediately degraded using electron
<br /> 1 184
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