Tracers as Tools for Design and Evaluation of Injection-Based In Situ Groundwater Remediation Systems DENICE K. NELSON ARCADIS G&M, 430 First Avenue North, Suite 720, Minneapolis, MN 55401 CRAIG E. DIVINE ARCADIS G&M, 630 Plaza Drive, Suite 200, Highlands Ranch, CO 80129 Key Terms: Groundwater Tracers, Hazardous Waste, Remediation System Design, Hydrogeology, Environ- mental Geology ABSTRACT In situ treatment of contaminants by the direct injection of a reagent solution into the subsurface is an emerging remedial strategy. Three case studies are presented where tracers were used to evaluate various design and operational components at sites treating contaminants in situ. Tracers were used to determine site-specific design parameters including groundwater velocity, effective radius of injected fluids, and re- quired frequency of injections. Additionally, tracers were used to verify hydraulic connectivity between the injection field and down-gradient observation wells, provide information on hydrogeology and solute- transport behavior (i.e., permeability differences, preferential channeling, and fractured flow) within the targeted treatment areas, and in two instances, used in conjunction with reagent delivery for deter- mination of decay half-life coefficients. These case studies demonstrate that tracers can be used as practical tools to aid in designing full-scale systems and evaluating operation and long-term performance of in situ treatment strategies. INTRODUCTION Historically, plumes of contaminated groundwater have been typically managed by conventional pump-and-treat programs. Although pump-and-treat strategies can be suc- cessfully used to control plume migration, they generally require long-term system operation, are expensive to operate and maintain, and are rarely effective in reducing contaminant concentrations enough to meet stringent groundwater clean-up standards throughout the entire plume. Mass-transfer remediation technologies, such as conventional air-sparging, have been developed during the past 2 decades to overcome some limitations of pump and treat systems by treating contaminants in situ. These technologies have had varying degrees of success because of their inability to effectively address non-aqueous phase liquid (NAPL) and sorbed-phase contamination. However, a significant advance in in situ treatment has emerged within the past 10 years in which specific re- agents or amendment solutions are directly injected into the aquifer. This approach is designed to facilitate the creation (and in some cases, the propagation) of zones where contaminants are treated in situ through biotic or abiotic processes. These zones can be referred to as in situ reactive zones (IRZs). A reactive zone can either be sus- tained over a long period of time thereby acting as a contaminant migration barrier, or it can be a short-term treatment strategy for elimination of contaminant source zones. This general technique may offer significant ad- vantages over other conventional in situ techniques be- cause (1) specific injected reagents may enable complete in situ contaminant destruction, (2) injected reagents may have better access to sorbed-phase contaminants and con- taminants present in less permeable zones, and (3) reme- dial design of reactive zones is highly flexible and can be easily adapted to address changes in plume character- istics. In situ reactive zones have been used to treat a wide variety of contaminants, including volatile and semi- volatile organic compounds (VOCs and SVOCs), pesti- cides, and metals (for an in-depth discussion on the broad application of IRZ strategies for treating groundwater, the reader is referred to Suthersan and Payne [2005]). The inherent design flexibility is one of the primary advantages of injected reagent-based remedial strategies; however, there are important site characteristics that can dramatically affect remediation system performance. Some examples of important site-specific characteristics and related design and operation components that are important for successful implementation are listed in Table 1. The relative importance of these various site characteristics depends on the specific remedial approach; however, in most cases, designing and operating effec- tive reagent injection-based remedial strategies primarily Environmental & Engineering Geoscience, Vol. XI, No. 4, November 2005, pp. 383–393 383