A Deeper Look at Permeable Reactive Barriers
Mar 15, 2018 -
Cleanup action completed using zero-valent iron permeable reactive barriers (ZVI-PRB) technology at Defense National Stockpile Center Scotia Depot (Glenville, NY) Superfund site.
ZVI-PRBs – A Passive Remedy
In-situ permeable reactive barriers (PRBs), or treatment walls, are replacing pump and treat remedies for contaminated groundwater with life-cycle cost savings greater than 75%. Iron PRBs remediate chlorinated solvent contaminated groundwater by abiotic degradation of the halogenated volatile organic compounds to harmless daughter products. Zero-valent iron placed in the groundwater has been known to abiotically degrade a wide range of chlorinated compounds and energetics to non-toxic end products. Also, numerous heavy metals are immobilized and precipitated from the groundwater by the iron. The first iron reactive treatment wall was constructed in 1991 as a field trial. In almost three decades, over two hundred (200) full scale and pilot PRBs have been installed. The rapid increase in the number of PRBs installed reflects the maturity and continued acceptance of the zero valent iron technology and benefits of passive in-situ remediation. Iron PRB technology is now considered a well-established and proven long-term solution to groundwater remediation and is acknowledged by many experts and regulatory agencies to be a preferred remedy with a life of greater than thirty years.
The advantages and applicability of iron PRB technology are summarized below:
- Green and sustainable technology
- Accepted by the regulatory agencies as a preferred remedy and has replaced Pump & Treat in many Records of Decision (RODs)
- Environmentally benign - no pumping or energy needs - no toxic waste or emissions generated
- Destroys a wide variety of volatile organic compounds (VOCs) to non-toxic end products, with chemistry and kinetics well understood
- Immobilizes or precipitates numerous metals with a large effective capacity
- Demonstrated long-term performance, with expected life of >30 years, with no signs of clogging, no loss of permeability, or major changes in iron reactivity over time
- Significant cost advantage amounting to less than the life cycle cost of pump and treat
Azimuth orientated vertical hydraulic fracturing, also known as vertical inclusion propagation (VIP), is an alternate mode of constructing iron PRBs in-situ, resulting in significant cost savings and allowing reactive barriers to be installed to greater depths than conventional technologies. Controlled Vertical Inclusion Propagation (VIP) has been utilized extensively for construction of Trenchless Permeable Reactive Barriers (tPRBs) throughout the United States for the last twenty years. With over 10,000 tons of zero valent iron (ZVI) injected to construct miles of PRBs in variable lithologic environments nationwide, the single azimuth VIP technology has proven to be an effective method for constructing PRBs from 3-inch to 9-inches in thickness as deep as 150-feet below ground surface (bgs) with geologies ranging from silts to clays, glacial tills to highly permeable sands and gravels, with depths ranging from 15 feet to greater than 120 feet.
Some advantages of orientated VIP placed PRBs include:
- Passive treatment resulting in long-term cost savings
- Minimal site disturbance
- No excavation/disposal of contaminated material
- Deep application of treatment technology
- Minimal impact on groundwater flow regime
- Permanent remedy
Reactive barriers are installed to greater depths than conventional technologies with vertical inclusion propagation (VIP).
VIP placed PRBs are constructed from a series of wells installed along the barrier's azimuth or bearing. Each well is installed using conventional drilling methods and comprised of a specialized casing grouted into the boreholes. A controlled vertical inclusion is initiated in each well at the required azimuth orientation and depth using adjacent wells to control the direction of the inclusion. Iron filings are injected into the wells in a highly viscous cross-linked proprietary gel that forms the continuous permeable iron reactive barrier. The barrier's geometry can be monitored in real time to ensure the coalescence and construction of the barrier conforms to the design.
PRBs – A Solution as an Engineering Control
As part of the overall remediation goals in a ROD for a State Superfund Site in Glenville, NY, a ZVI-PRB was selected as a means for mid-plume reduction of an existing trichloroethene (TCE) plume to. The Site was constructed during World War II, serving as a storage and supply depot for Naval Forces. Two possible sources of TCE contamination were identified from numerous investigations between 1995 and 2007 after low levels of TCE were detected in a nearby public water supply.
A Pre-Design Investigation (PDI) conducted in 2013 provided an updated site conceptual model with the following parameters:
- Groundwater exists at depth of 65 to 70 ft below ground surface (bgs) beneath the site and flows west-southwest at velocities ranging from 0.0086 to 5.2 ft/day;
- Four separate, commingled plumes from different sources exist at the site, the predominant one consisting mostly of TCE with concentrations in the center of the plume ranging from 200 to 700 μg/L;
- The TCE plume extends down to a depth of 110 ft bgs with TCE levels below criteria at by a depth of 120 ft bgs;
- The plume core is at a depth of 100 to 110 ft bgs and appears correlated with a low permeability layer.
The site conceptual model recommended a continuous ZVI-PRB of two designs - one extending from approximately 65-feet below ground surface (ft bgs) to 110-ft bgs over a 700-foot length and the second extending from approximately 65 to 75-ft bgs over a 250-foot length. Based on reaction kinetics and hydrogeologic data provided in the PDI Report, the estimated thickness required for destruction of chlorinated VOCs from approximately 800-ug/l to less than maximum contaminant levels (MCLs) of 5 ug/l was approximately 3-inches, providing a safety factor of ten.
Design and Modeling Activities
While the performance of ZVI-PRBs is well documented in literature, each site is custom designed. As part of the VIP design, GeoSierra conducts bench column studies and probabilistic design scenarios to determine a confidence level of the reduction of CVOCs. An additional critical component of the design is to ensure adequate residence time within the PRB based on intrinsic groundwater flow velocities. To validate site hydrogeologic data, four compliance monitoring well pairs were installed along the azimuth of the ZVI-PRB and a baseline evaluation of hydrogeologic characteristics of the aquifer was conducted using hydraulic pulse interference test (HPIT). HPIT provides a highly sensitive evaluation of the aquifer characteristics compared to industry standard testing methods, resulting in a calculated hydraulic conductivity and storativity. HPIT results confirmed a groundwater flow velocity in the upper 15 feet of the target depth, which was comprised of a gravel and cobble zone, to be an order of magnitude faster than the lower 30 feet, which was predominantly sand. This resulted in a change in design thickness (3” to 6”) for the upper portion of the ZVI-PRB to provide the necessary residence time, following consultation with our design partner, AECOM, based on their groundwater expertise and knowledge of the area. The spacing of the injection points was also revised to 12 feet versus the conventional 15 feet to ensure minimal loss of ZVI to the highly conductive cobble zone.
Site Implementation and Challenges
In February 2016, GeoSierra mobilized to begin drilling activities at 77 locations – 21 for the “shallow” 250-foot long PRB and 56 for the 650-foot long “deep” PRB, totaling 189 specialized casings, and 31 customized active resistivity strings for monitoring the PRB injections followed by initiation of injection of 1,050 tons of ZVI beginning in May 2016. The PRB was aligned within the constraints of the property boundaries, in close proximity to the compliance monitoring wells, and parallel to an existing easement owned by a utility company. The easement consisted of 115 kilovolt (kV) and 34 kV high voltage transmission lines with a second lower voltage line running perpendicular near the south end of the PRB. The combination of the upper cobble zone that comprised the full length of the 900-foot long PRB and the presence of the high voltage lines presented its challenges both in drilling and resistivity monitoring. A typical installation of the specialized casings is conducted by conventional mud rotary drilling. However, the presence of large cobbles required an additional drilling step, installation of a temporary conductor casing to 10 to 15 ft bgs to minimize fluid mud loss to the formation. Additionally, this highly transmissive cobble zone was obviously, more conductive than other sites. The presence of the tower grounds from the power lines located within 100 feet and approximately midpoint of the PRB required some innovative methods of baseline recording of existing conditions, installation of electrical grounds for the resistivity monitoring system, and the collection of over 16,000 images to capture the injections due to interference from peak electrical usage and weather.
The presence of utility lines and subsurface cobble zone created unique site challenges for the PBR placement and monitoring.
Learn more at Battelle’s 2018 conference on remediation of chlorinated and recalcitrant compounds. Visit GeoSierra at our booth #803 or speak with the author at Poster Session 1 – C3 to find out how these PRBs are designed, more about HPIT, and how these site challenges were addressed and overcome.
To stay apprised with everything Cascade is doing at this year’s Battelle conference, sign up here.
About the Author
Deborah Schnell, P.E. is a Senior Project Manager with GeoSierra Environmental, A Cascade Company. She has over twenty years of experience in site investigation and environmental remediation. Deborah is a nationally-recognized expert in the innovative remediation technologies of trenchless permeable reactive barriers and pneumatic fracturing.