investigate processes causing attenuation of a hexavalent chromium (Cr(VI)) plume in sedimentary

Overview

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Key numbers

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• brown sandstones, siltstones and mudstones (37,39). The water table generally occurs within
• and borehole testing involving (3) use of inflatable packers to isolate discrete intervals for
• to provide a sufficiently low porewater detection limit (PWDL; 45 µg/L) (32). This value
• level (MCL) of 100 µg/L for total Cr. Matrix porewater concentrations were estimated by
• path (Figure 4b–d) in bedrock also show stable to declining trends within the plume. A
• conventional well near the river (EPA-4BR; Figure 4e) shows stable Cr(VI) trends over
• the three sampling events (range 130–160 µg/L). Stable to declining trends for Cr(VI)
• highest Cr(VI) concentration (port 2) shows a generally stable to slightly declining trend,
• while the shallower port (port 1) and three deeper ports (ports 3–5) show generally stable
• (Figure 6) estimated using mean values for ϕm and ρb-dry from 36 measurements on core
• standard deviation (σ) to the mean values (ϕm = 0.102, σ = 0.028; ρb-dry = 2.46 g/cm3, σ
• the highest Cr(VI) groundwater concentrations (3000 to 3370 µg/L), equivalent porewater
• matrix. Within the port 3 interval, the groundwater Cr(VI) concentrations ranged from 182–
• 274 µg/L, while the equivalent porewater concentrations were below the 45 µg/L PWDL
• in this interval. Within the port 4 interval, groundwater Cr(VI) concentrations ranged from
• 80–83 µg/L, while all of the equivalent porewater concentrations were below the PWDL. It
• 5 interval, groundwater Cr(VI) concentrations were low, ranging from 7–31 µg/L, and
• equivalent porewater concentrations were below the PWDL except for one sample (54 µg/L).
• below the MDL (<0.10 µg/L) and all equivalent porewater concentrations in matrix samples
• logs within the interval. The Kb values range from 7.1 × 10−8 to 8.1 × 10−7 m/s for the
• 7 port intervals. The number of fractures in each interval ranges from 2 to 8. Applying
• each interval have the same aperture, estimated hydraulic apertures range from 10ʹs to
• a few 100ʹs of microns and ϕf ranges from 3.3 × 10−5 to 1.7 × 10−4. These estimates
• that >99.8% of the Cr(VI) mass occurs in the porewater, demonstrating the significance of
• fractures. However, the matrix mass estimates are biased low since the PWDL (~45 µg/L)
• is much higher than the MDL for groundwater analyses (<0.1 µg/L). There were detectable
• detections of both Cr forms, the ratio of Cr(III) to Cr(VI) ranged from 0.52 to 22.9 with
• 10%. An orthogonal fracture network was assigned with fracture spacing, lengths and
• intervals. Fracture frequencies recorded in core logs ranged from 0 to 3.6 with an average of
• frequencies measured by ATV ranged from 0 to 2.0, with an average of 0.6 m−1 (average
• an overall average horizontal hydraulic gradient of about 1.2% and vertical gradients varying
• parallel average fracture spacing of 1.5 m is within the range of estimates based on core
• packer test values at EPA-21BR on 8 test intervals (range from 7.1 × 10−8 to 8.1 × 10−7 m/s,
• tests in 6 boreholes on 41 test intervals (range from 3.5 × 10−8 to 6.0×10−5 m/s, geometric
• concentration scale, representing the same range as the field data, for a 30-year simulation
• indicated that >99.8% of the Cr(VI) mass occurs in the rock matrix due to large surface area
• site wells. Red lines show the regional bedrock dip of ~8° and dashed orange line shows the
• Matrix porosity ϕm (−) − − 0.10 lab analyses mean

Methods (brief)

• bedrock samples from the study site, providing low porewater method detection limits
• matrix porewater Cr(VI) distributions with spacing of samples informed by lithology and
• conductivity (EC) and collection of samples for Cr(VI) and a variety of other geochemical
• provide samples representative of formation groundwater conditions, consistent with FLUTe
• The rock core recovered from borehole EPA-21BR was subsampled in detail to allow
• observed fractures in the core logs. The core samples for Cr-distribution (lengths from
• on ice to the laboratory. A total of 400 rock core samples for Cr-distributions were
• collected from the 87.3 m cored interval from 21.0–108.3 m bgs, representing an average
• sample spacing of 0.22 m. An additional 42 intact samples were also retained from the
• cores for physical-chemical properties and diffusion tests (sample lengths from 15 to 30
• in situ subsurface redox conditions. In the laboratory selected samples were analyzed
• for Cr-distributions with emphasis on samples overlapping with intervals of the borehole
• (3.05 m length) provided groundwater samples expected to represent blended mobile Cr(VI)
• and adjacent to fractures. Initially a total of 100 samples were analyzed, with average
• sample spacing of 0.19 to 0.25 m within the plume overlapping with MLS ports 1–5 (<75
• Subsequently, an additional 46 samples were analyzed to fill in gaps between ports within
• Laboratory methods were developed for Cr(VI) extraction/analysis from rock core samples
• were measured on a representative subset of samples using gravimetric methods (48).

Implications

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Wiki pages this source may touch

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• Nickel
• Chromium

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Update history

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