Biosparging of Contaminated Groundwater at the T-Dock Site, South Prudence Bay Island Park, Rhode Island

Site Name:

South Prudence Bay Island Park


Portsmouth, Rhode Island

Period of

Pilot Test - October to January 1997
Full-scale Operation - February 1998 to February 2000


Full-scale Remedial Action


- A pilot test was performed at the site in 1997 and 1998. Results indicated that biosparging would be an effective technology to remediate the site, and that addition of SVE would not significantly enhance the performance of the system.
- The biosparging system was installed in 1997 and included 22 sparge wells. Wells were installed at depths ranging from 15 to 26 feet bgs, and well screens were installed in the bottom two feet of each well.
- Atmospheric air was delivered to the network of sparge wells via an air compressor, a blower and a distribution manifold. Air flow rates were adjusted between 0.2 to 9.5 cfm as necessary.
- Groundwater concentrations were monitored in thirteen wells installed at the site.

Cleanup Authority:
State (RIDEM)

Project Management
Christine Johnson-Battista
USACE, New England District
696 Virginia Road
Concord, MA 01742-2751
(978) 772-0148

Technology Expert:
Ian Ogersby
USACE, New England District
696 Virginia Road
Concord, MA 01742-2751
(978) 318-8631

Fred Symmes
Roy F. Weston
1 Wall Street
Manchester, NH 03101
(603) 656-5412
Regulatory Contacts
Matthew DeStefano
235 Promenade St.
Providence, RI 02908

Organic Compounds (BTEX)Maximum detected concentrations were:
- Benzene at 2,050 ug/L
- Toluene at 7,830 ug/L
- Ethylbenzene at 3,470 ug/L
- Xylene at 11,000 ug/L

Waste Source:
One gasoline UST and a fuel distribution system were operated at the site between 1940 and 1985. It is assumed that periodic leaks and spills occurred during this span.

Type/Quantity of Media Treated:
- The lateral extent of the groundwater plume at the site was estimated to be 35,000 SF
- The depth to groundwater at the site is typically 10 to 12 ft bgs, but decreases to 4 to 6 ft bgs near Narragansett Bay.

Purpose/Significance of Application:
The application treated BTEX-contaminated groundwater in-situ at a relatively remote site without collecting or discharging treated water.

Regulatory Requirements/Cleanup Goals:
Remedial goals for BTEX compounds in groundwater were:
Benzene below 5 ug/L, Toluene below 1,000 ug/L, Ethylbenzene below 700 ug/L and Xylene below 10,000 ug/L.
- The system was required to meet RIDEM air emission requirements for volatile organic compounds.

- Monitoring wells at the site were sampled quarterly from late 1997 through 2000. Results indicate that BTEX compounds in the groundwater have been treated to acceptable concentrations, with the exception of benzene in one well. The system has been shut down, and the site is currently in the closure process.ยท RIDEM air emission requirements were met throughout operation of the treatment system.

Cost Factors:
- The total cost for this project was $280,946.

South Prudence Bay Island Park is located in Portsmouth, Rhode Island and is currently used mainly for public recreation. The T-Dock site is located on a two-acre parcel at the southern edge of the park, and is directly adjacent to Narragansett Bay. From the 1940s until 1985, fuel storage and distribution operations were conducted at the T-Dock site. In 1985 and 1992, actions were taken to remove one 10,000-gallon gasoline UST and sections of the fuel distribution pipeline at the site. Investigations conducted at the site in 1996 and 1997 indicated that soil and groundwater contamination was present in the area of the former UST and pipeline. Contaminants of concern at the site included mainly BTEX compounds. Contaminated soil was excavated at the site under a separate project performed during 1997.

In 1996 and 1997 a pilot tests were conducted in a small section of the T-Dock site. Testing included installation of one sparge well, one SVE well and six groundwater monitoring wells. The first test was performed to evaluate the expected performance of a combination of SVE and sparging. After the first test indicated that the SVE well had a limited area of influence, a second test was conducted to determine if a low-flow sparging system without SVE could be operated at the site to treat BTEX in groundwater while not exceeding RIDEM air emission standards. It was determined that a system operated with pulsed air injection at a rate of 1 cfm for twenty minutes out of each hour, would meet the requirements of the project.

The biosparging system was constructed in 1997 and 1998, and included a total of 22 sparge wells and thirteen groundwater monitoring wells. Ten of the thirteen monitoring wells were five nests of two wells each, one screened at a shallow depth and one screened deeper. Atmospheric air was delivered to the wells via an air compressor, a blower and a distribution manifold. After it was determined that air emissions standards would not be violated by increasing flow to the sparging system, air flow rates were modified as necessary to address areas of the site with higher contaminant concentrations. Air flow rates to the system were varied from 0.2 to 9.5 cfm during the project. The full-scale system was operated from February 1998 through February 2000. Quarterly sampling results indicated that BTEX compounds in groundwater had been treated to the remediation goals, with the exception of one well, which still contained benzene at a concentration above the goal. The system was shut down in February 2000 and the site is currently in the closure process.Treatment was completed in April 1999.