Formatted Title
Time-Critical Gravity-Powered PFAS Remediation of Groundwater Seeps
Background/Objectives
At an active chemical manufacturing facility (the Site) in North Carolina, Geosyntec’s investigation of the fate and transport of per- and polyfluoroalkyl substances (PFAS) identified groundwater seeps as one of the major sources of PFAS loading to the Cape Fear River. The groundwater seeps coalesce into four distinct surface water discharges to the river in a remote, forested area of the Site. A time-critical interim remedy was needed that would treat the impacted groundwater seeps at their point of confluence with the river and be operational in only six months. The interim remediation objective was to capture total dry weather baseflow and reduce the concentration of PFAS by at least 80% assessed on a monthly average basis. The total PFAS concentrations ranged from 108 to 228 micrograms per liter (µg/L). The 95th percentile value of flume data were used as the design basis, so that the upper range of possible baseflow would be captured; these design basis flow rates for the four seeps ranged from 76 to 226 gallons per minute (gpm), for a total flow capture basis of 690 gpm.
Approach/Activities
Conventional PFAS water treatment systems pump influent water into engineered adsorption systems. This requires electricity for the mechanical equipment and permitted disharge of the treated effluent. The compressed six-month schedule was insufficient time to obtain the necessary permits and to deliver power to the remote areas of the site. Geosyntec resolved these constraints by developing a sustainable, passive, in situ gravity-powered remedy (flow-through cells [FTCs]). Isotherm studies had demonstrated the viability of granular activated carbon (GAC) to remove PFAS from the seeps. To overcome head loss through the media, the baseflow was impounded, and the FTCs were designed with the base of concrete chambers set approximately five feet below the pond elevation. An inlet weir controls the elevation of the flow into the FTC, such that the influent elevation is sufficiently higher than the effluent elevation to provide enough hydraulic head to power the remedy and drive the baseflow through the GAC. During construction, baseflow was temporarily impounded and pumped around to allow excavation to the design elevation. After concrete work was complete, GAC and a gravel drainage layer were added to the filter beds, and the baseflow was allowed to flow down into the chambers and percolate vertically through the GAC.
To avoid potential upset conditions, Geosyntec’s design included treatment in series. Manifold piping and valving allow operators significant flexibility in modifying the duty cycle: by oscillating which filter bed is lead or lag; isolating filter beds for maintenance or changeouts; running both beds in parallel mode to temporarily (and significantly) increase process flow rates when necessary; and finally, to backwash filter beds and maintain the effectiveness of the GAC media.
Results/Lessons Learned
The FTCs have consistently over-performed both in terms of flow capture and PFAS removal. PFAS has been removed from both dry weather and wet weather flow at an average removal efficiency of 99.5%. In total, to date, the four FTCs have treated over 400 million gallons of seep water and removed approximately 600 lbs of PFAS. This removal of PFAS mass from the four groundwater seeps has resulted in a significant reduction on the downstream river PFAS concentrations and mass loads.
Design constraints, including the time-critical schedule, the characteristics of the contaminant, and the environmental conditions at the remedy location, required a creative approach to meet project demands and an adaptive management strategy to maintain effectiveness and reliability. This project has highlighted the need for modular, passive remedies that can be scaled to fit, installed quickly, and require a limited infrastructure and permitting burden, but still nonetheless actively remove contaminants from the environment. The FTCs have demonstrated that in situ, gravity-powered remedies are a viable option for persistent, mobile compounds and should be a component of feasibility studies for impacted groundwater seeps. This remedy won the National Groundwater Association (NGWA) 2023 award for Outstanding Groundwater Remediation Project.