The Horizontal Reactive Media Treatment Well (HRX Well®) for Passive In‐Situ Remediation

Abstract

A new in‐situ remediation concept termed a Horizontal Reactive Media Treatment Well (HRX Well®) is presented that utilizes horizontal wells filled with reactive media to passively treat contaminated groundwater in‐situ. The approach involves the use of large‐diameter directionally drilled horizontal wells filled with granular reactive media generally installed parallel to the direction of groundwater flow. The design leverages natural “flow‐focusing” behavior induced by the high in‐well hydraulic conductivity of the reactive media relative to the aquifer hydraulic conductivity to passively capture and treat proportionally large volumes of groundwater within the well. Clean groundwater then exits the horizontal well along its downgradient sections. Many different types of solid granular reactive media are already available (e.g., zero valent iron, activated carbon, ion exchange resins, zeolite, apatite, chitin); therefore, this concept could be used to address a wide range of contaminants. Three‐dimensional flow and transport simulations were completed to assess the general hydraulic performance, capture zones, residence times, effects of aquifer heterogeneity, and treatment effectiveness of the concept. The results demonstrate that capture and treatment widths of up to tens of feet can be achieved for many aquifer settings, and that reductions in downgradient concentrations and contaminant mass flux are nearly immediate. For a representative example, the predicted treatment zone width for the HRX Well is approximately 27 to 44 feet, and contaminant concentrations immediately downgradient of the HRX Well decreased an order of magnitude within 10 days. A series of laboratory‐scale physical tests (i.e., tank tests) were completed that further demonstrate the concept and confirm model prediction performance. For example, the breakthrough time, peak concentration and total mass recovery of methylene blue (reactive tracer) was about 2, 35, and 20 times (respectively) less than chloride (conservative tracer) at the outlet of the tank‐scale HRX Well.

The horizontal reactive media treatment well (HRX Well®) for passive in situ remediation: Design, implementation, and sustainability considerations

“…Test data clearly highlight the hydraulic effects of the carbon‐packed well on flow through the pit (Exhibit ) and demonstrate that the HRX Well captured approximately 39% of the flow while accounting for only 0.5% of the test pit volume. Both previous laboratory tank experiments (Divine et al., ) and field pilot scale tests demonstrate that the HRX Well captures and treats contaminated water consistent with design model predictions. The measured capture width was approximately 40 inches (based on observed head measurements).…”

“…Additional tracer tests and further data evaluation will be completed during future phases of the project. Previous laboratory work using methyl orange (as a surrogate contaminant) demonstrated that ZVI packed into an HRX Well could treat groundwater effectively after more than 100 pore volumes of throughput (Divine et al., ). Overall, the physical model tests further validate the remediation concept and confirm the numerical modeling as a reliable tool for field‐scale design.…”

“…Also, the HRX Well configuration presented above and in Divine et al. () is fully passive. However, Divine et al.…”

“…Conceptual diagram of the HRX Well. Impacted groundwater (indicated by red colorflood) is passively focused and flows into the HRX Well (gray cylinder) where it is treated as it flows through solid‐phase reactive media before exiting back into the aquifer (indicated by blue colorflood) (from Divine et al., )…”

“…However, actual treatment widths and treatment zone geometries are determined by the specific well design and aquifer conditions; consequently, numerical modeling will generally be necessary to understand the effects of complex aquifer geometry, anisotropy, and permeability distribution more accurately on treatment zone geometry. The passive flow rate through an HRX Well ( Q HRX ) can be described as follows (Divine et al., ): where K HRX is the hydraulic conductivity of the media in the HRX Well, r HRX is the radius of the well, and i HRX is the hydraulic gradient along the well (which is assumed to be approximately equal to the ambient aquifer hydraulic gradient, i A for wells that are mostly horizontal and oriented parallel to groundwater flow). The approximate average flow velocity and minimum average residence time of groundwater within the well can be estimated by where is the mobile or effective porosity of the treatment media within the HRX Well and L HRX is the average distance water travels within the HRX Well.…”