The environmental remediation industry has increasingly considered the sustainability impacts of remediation systems. The ideal remediation system has a greater environmental benefit than detriment. While sustainable systems are increasingly available for many contaminants, per-and polyfluoroalkyl substances have presented new challenges for remediation practitioners seeking effective and sustainable systems. Here the sustainability and cost impacts of an ultrasonic reactor installed in a horizontal well (referred to as the In situ reactor technology [inSRT] system) at a hypothetical site are quantified and compared to hypothetical pump-and-treat (PT) system impacts. Direct costs of emissions from remediation were compared using levelized cost of energy data. The indirect impacts from pollution were quantified based on literature values. The results showed that the InSRT system had lower lifecycle impacts and costs compared to the PT system when each was considered under individually optimal conditions. InSRT was found to meet sustainability goals when used in a low hydraulic conductivity source zone and the PT system lifecycle impacts were reduced when the system was used in a highhydraulic conductivity area.

This column reviews the general features of PHT3D Version 2, a reactive multicomponent transport model that couples the geochemical modeling software PHREEQC-2 (Parkhurst and Appelo 1999) with three-dimensional groundwater flow and transport simulators MODFLOW-2000 and MT3DMS (Zheng and Wang 1999). The original version of PHT3D was developed by Henning Prommer and Version 2 by Henning Prommer and Vincent Post (Prommer and Post 2010). More detailed information about PHT3D is available at the website http://www.pht3d.org. The review was conducted separately by two reviewers. This column is presented in two parts.

Per-and polyfluoroalkyl substances (PFAS) have been found in air, water, and soil globally. Groundwater is used as a drinking water supply in many locations and PFAS detections in groundwater occur frequently. Cavitation occurs when ultrasound is applied to liquids and has been shown to be effective for PFAS degradation. Sonolysis is often used to describe the cavity collapse that occurs after cavity expansion. An ideal scenario implements sonolysis in a horizontal well, which allows passive capture of contaminated water while reducing the costs and hazards associated with ex situ remediation. Four PFAS-contaminated site groundwaters were treated in the sonolytic reactor in the laboratory. Data from those experiments were used in a series of hypothetical case studies to evaluate the influence of chemical and physical properties on feasibility of installation of a novel in situ sonolytic reactor. When PFAS precursor species and perfluorobutanoic acid were included in the remediation goals they were found to drive the hydraulic retention times necessary for effective treatment compared to longer-chain PFAS. Improved implementation efficiency occurred when the target treatment area was restricted to a concentrated source zone, rather than less concentrated areas of the downgradient plume.