Preferential flow path development and its influence on long-term PRB performance: column study

Area of Science:

• Environmental Engineering
• Geochemistry
• Hydrogeology

Background:

• Permeable reactive barriers (PRBs) using iron (Fe(0)) are effective for groundwater remediation.
• The operational lifespan of Fe(0) PRBs is constrained by various degradation pathways.
• Uncertainty exists regarding the dominant mechanisms limiting PRB performance.

Purpose of the Study:

• To investigate the primary mechanisms of performance degradation in Fe(0)-based PRBs.
• To quantify the impact of mineral precipitation, gas production, and microbial activity on PRB function.
• To establish early indicators for monitoring PRB hydraulic changes.

Main Methods:

• Controlled field study using large-volume, flow-through columns.
• Treatment of nitrate-rich groundwater with Fe(0) media.
• Monitoring of chemical and hydraulic parameters, including tracer tests and ionic profiles.

Main Results:

• Mineral precipitation and accumulation identified as the main cause of pore clogging at the column inlet.
• Nitrogen gas (N(2)) production from biotic processes contributed secondarily to hydraulic effects.
• Significant porosity reduction (up to 45.3%) observed near the inlet within 72 days under accelerated flow.
• Preferential flow is predicted within approximately 1 year for similar site conditions.

Conclusions:

• Mineral precipitation is the dominant factor limiting Fe(0) PRB longevity.
• pH serves as a critical early indicator of hydraulic residence time changes and heterogeneity development.
• Groundwater bypass can occur if PRB media becomes clogged and less conductive than surrounding soil, compromising treatment efficacy.