Picture a relatively homogeneous silty formation, with a modest hydraulic gradient, contaminated by chlorinated hydrocarbons. Now, imagine a single sand stringer a few inches in width suddenly appearing through the heart of this contaminated zone. Groundwater will flow towards this higher permeability zone, first slowly as it moves through the silt to the sand stringer, and then at higher flows within the sand stringer. You have created the proverbial “underground river” within the silt. This isn’t an imaginary situation; groundwater remediation experts understand that higher concentrations of contaminants move through preferential pathways or higher permeability regions of
heterogeneous formations. Groundwater flow is preferentially “focused” through the higher permeability zones.
Returning to the thought experiment, instead of the sand stringer, place a horizontal
well, stretching out parallel with the direction of groundwater flow. Now, place a treatment media, say Zero-Valent Iron, contained in removable and replaceable cartridges within that
well. The result is the same, except your horizontal well becomes the engineered preferential pathway, groundwater flows passively from a capture zone much larger than the well diameter, enters the well contaminated, and emerges clean at the other end.
What you’ve just imagined, in simplistic form, is the Horizontal Reactive Media Treatment Well (HRX Well®), developed and patented by Arcadis. As we noted in our website blog post of August 2018, Arcadis and Ellingson-DTD installed the first full scale prototype of an HRX Well at Vandenberg Air Force Base in California. Initial testing indicated near complete treatment of captured groundwater. However, some design limitations were identified with the in-situ point velocity probes (PVPs) used to measure flow through the HRX Well. To address these limitations, several months after the initial installation, a small Arcadis and Ellingson-DTD team returned to Vandenberg to remove, retrofit and replace the instrumentation cartridges with improved seals to prevent water from bypassing measurement. While on site, they also conducted a pump test to force captured groundwater through the treatment well at a higher than natural rate – up to twice the flow under passive operation – to evaluate how capture width could be further increased through “active” operation. Testing included placement of transducers in adjacent monitoring wells to capture changes in water level as well as testing with the in situ PVPs in the well itself.
Results from more than a year of performance monitoring are now in – and are impressive. With the monitoring improvements in place, the prototype HRX Well® is clearly capturing contaminated groundwater from a zone approximately 50 feet wide in passive mode (with no pumping) and exceeding 70 feet when flow is augmented by low-flow pumping. Captured water
is treated nearly 100% by the ZVI and enhanced biodegradation induced by the residual carbon from the biodegradable drilling mud before exiting the downgradient end of the HRX Well. The
clean water exiting the well then begins to flush and clean the aquifer. Four performance monitoring wells located downgradient in the aquifer have already shown dramatic reductions in contaminant concentrations, from 50 to 74%, providing additional confirmation of the validity of the design.