Formatted Title
Long-Term Performance of Microscale ZVI Delivered to Low-Permeability Formations via Hydraulic Fracturing
Background/Objectives
Zero-valent iron (ZVI) is a proven amendment for in situ chemical reduction of numerous compounds. Its effectiveness is closely linked to its longevity, which has been leveraged since the mid-1990s in the construction of trenched permeable reactive barriers (PRBs). Over the last 10 years ZVI has more commonly been delivered via injection and, in its granular form, must be delivered via fracturing. Hydraulic fracturing is an established technology for delivery of granular amendments into low-permeability and heterogeneous formations. The technique can readily deliver high mass loadings of microscale ZVI (mZVI) to precise locations in the subsurface and is commonly used for both source treatment and injected PRB applications. Here we present the remedial approach and performance monitoring results for three full-scale projects where ZVI-filled fractures facilitated treatment of target compounds for at least six years.
Approach/Activities
Three sites in different geologic settings utilized mZVI injected via hydraulic fracturing. At a site in New England, the source zone in glacial clay till was characterized by high chloropicrin concentrations in both soil and groundwater, requiring a high mZVI mass loading of more than 2% by weight of soil. The second site is in Denmark, where a source zone treatment was implemented to address TCE impacts in soil and groundwater. Both source treatments utilized a grid configuration of injection locations to meet the target mZVI mass loading and achieve the amendment distribution goals. At a third site, located in South Carolina, a 17-acre TCE plume extended onto neighboring properties with surface water receptors. Treatment at this site consisted of linear arrays of injection locations to create injected PRBs with fractures spanning three distinct lithologic units – saprolite, weathered bedrock, and fractured crystalline bedrock. Taken together, the three sites included injection of more than 1.6 million pounds of mZVI into nearly 600 fractures. Treatment performance at these sites was evaluated based on soil and/or groundwater sampling on a quarterly or semi-annual basis.
Results/Lessons Learned
At the Denmark site, ZVI treatment in glacial till has persisted for at least six years based on multiple lines of evidence. Dissolved oxygen, oxidation-reduction potential, and ferrous iron results showed persistent reducing geochemical conditions within and downgradient of the source zone throughout the 6-year monitoring period. The total contaminant mass in soil decreased by approximately 85%, and mass flux calculations indicate the discharge of TCE in groundwater downgradient of the treatment zone decreased by more than 98% at the 6-year mark, when most of the downgradient mass was composed of ethene and ethane. At the New England site, six years of performance monitoring show that concentrations of chloropicrin in groundwater have decreased by two to four orders of magnitude, and the total chloropicrin mass in the treatment area has decreased by 97%. At the South Carolina site seven years after remedy implementation, TCE concentrations in offsite wells have decreased by 97% from an average of 5,300 µg/L to 140 µg/L, and the extent of the 100 µg/L plume has decreased in area from 11 acres at baseline to 1.5 acres.