In Situ Biological Treatment of Hexavalent Chromium at the Selma
Pressure Treating Superfund Site.

***Daniel P. Leigh***, David Cacciatore, Larry Hudson, (Shaw E&I,
Concord, CA)

Tarek Ladaa (Shaw Environmental, Knoxville, TN)

Charnjit Bhullar (USEPA Region IX, San Francisco, CA.)

Waleed Shaheen (US Army Corp of Engineers, Omaha, NE )

Soil and groundwater at many sites, including wood pressure treating
facilities, have been affected by discharges of hexavalent chromium
(Cr^6+^). At many Cr^6+^ contamination sites, plume migration is
controlled by pump and treat (P&T) systems. Although potentially
effective for plume control, P&T systems are generally ineffective as a
stand-alone technology for complete site remediation. Therefore, new
treatment techniques are being developed for remediation of Cr^6+^. An
innovative technique for treatment of Cr^6+^ is being evaluated at the
Selma Pressure Treating Superfund Site (Selma) located in Selma,
California.

Soil and groundwater at Selma have been affected by historical
discharges of Cr^6+^. A pilot test was conducted in 2004 and 2005 in an
approximately 25-foot thick treatment zone in a 40,000 square foot
portion of the unconfined aquifer. Cr^6+^ has been detected in the
groundwater in this area at concentrations in excess 80,000 µg/L.
Groundwater at the site is presently aerobic and sulfate concentration
are in the range of 100 mg/L.

Bench tests were conducted to evaluate potential substrates for
degradation of the Cr^6+^. The bench tests demonstrated each substrate
was effective at degrading the Cr^6+^. During the tests Cr^6+^ was
converted to Cr^3+^ prior to nitrate reduction, indicating that
establishment of highly reducing conditions are not required for
degradation of the contaminant. Arsenic was not mobilized during the
bench tests. In addition, conversion of Cr^3+^ back to Cr^6+^ upon
return to oxidizing conditions did not occur.

Previous treatment of groundwater conducted by high pressure injection
required the application of excessive amounts of substrate to assure
adequate distribution. The excessive amount of substrate resulted in the
establishment of substantially more reducing conditions than necessary
for the reduction of the Cr^6+^. These conditions resulted in the
mobilization of manganese, iron and the production of methane.

In order to more adequately control the distribution of substrate,
minimize establishment of excessively reducing conditions and to provide
a mechanism for distribution of substrate into deeper portions of the
aquifer not reachable by direct push technologies a groundwater
recirculation system was designed, installed and operated. The
recirculation system distributed substrate throughout the treatment area
by extraction of groundwater from radially arranged extraction wells and
injection of water into centrally located injection wells. Substrate was
delivered to the recirculation system by regularly timed pulsed
injections. Prior to implementation of the full-scale system field tests
were conducted to determine the optimal amount of substrate required for
the reduction of Cr^6+^. During full-scale system operation, the amount
of substrate was controlled to provide only sufficient substrate to
reduce the Cr^6+^ and thereby minimize the establishment of excessively
reducing conditions.

In Situ Biological Treatment of Hexavalent Chromium at the Selma
Pressure Treating Superfund Site.

Corresponding Author

+-----------------------------------------------------------------------+
| **Daniel Leigh P.G., R.G., C.HG.**                                    |
|                                                                       |
| Shaw Environmental & Infrastructure, Inc.                             |
|                                                                       |
| 4005 Port Chicago Highway                                             |
|                                                                       |
| Concord, CA 94520                                                     |
|                                                                       |
| Telephone925.288.2193                                                 |
|                                                                       |
| E-mail: <Daniel.Leigh@shawgrp.com>                                    |
|                                                                       |
| Fax: 925.288.0888                                                     |
+-----------------------------------------------------------------------+

Co-Authors

+-----------------------------------------------------------------------+
| Larry Hudson                                                          |
|                                                                       |
| Shaw Environmental & Infrastructure, Inc.                             |
|                                                                       |
| Telephone: 925.288.2193                                               |
|                                                                       |
| 4005 Port Chicago Highway                                             |
|                                                                       |
| E-mail: <Larry.Hudson@shawgrp.com>                                    |
|                                                                       |
| Concord, CA 94520                                                     |
|                                                                       |
| Fax: 925.288.0888                                                     |
+-----------------------------------------------------------------------+

+-----------------------------------------------------------------------+
| David Cacciatore                                                      |
|                                                                       |
| Shaw Environmental & Infrastructure, Inc.                             |
|                                                                       |
| 925.288.2037                                                          |
|                                                                       |
| 4005 Port Chicago Highway                                             |
|                                                                       |
| E-mail:cleet.carlton@shawgrp.com                                      |
|                                                                       |
| Concord, CA 94520                                                     |
|                                                                       |
| Fax: 925.288.0888                                                     |
+-----------------------------------------------------------------------+

+-----------------------------------------------------------------------+
| Tarek Ladaa                                                           |
|                                                                       |
| Shaw Environmental & Infrastructure, Inc.                             |
|                                                                       |
| (865)690-3211                                                         |
|                                                                       |
| 312 Directors Drive                                                   |
|                                                                       |
| E-mail: tarek.ladaa@shawgrp.com                                       |
|                                                                       |
| Knoxville, TN                                                         |
|                                                                       |
| Fax: (865) 690-3626                                                   |
+-----------------------------------------------------------------------+

+-----------------------------------------------------------------------+
| Charnjit Bhullar                                                      |
|                                                                       |
| US Environmental Protection Agency                                    |
|                                                                       |
| Region 9 (SFD 7-1)                                                    |
|                                                                       |
| Telephone: 415.972.3960                                               |
|                                                                       |
| 75 Hawthorne St.                                                      |
|                                                                       |
| E-mail: Bhullar.charnjit@epamail.epa.gov                              |
|                                                                       |
| San Francisco, CA 94105                                               |
|                                                                       |
| Fax: 415.947.3526                                                     |
+-----------------------------------------------------------------------+

+-----------------------------------------------------------------------+
| Waleed Shaheen P.E.                                                   |
|                                                                       |
| U.S. Army Corps of Engineers                                          |
|                                                                       |
| Telephone: (402)293-2517                                              |
|                                                                       |
| P.O.Box 13287                                                         |
|                                                                       |
| E-mail: waleed.w.shaheen@nwo02.usace.army.mil                         |
|                                                                       |
| Offutt AFB, NE 68113                                                  |
|                                                                       |
| Fax: (402)291-8177                                                    |
+-----------------------------------------------------------------------+