This document summarizes a study of the hydraulic performance of a zero-valent iron permeable reactive barrier (PRB) installed in 2002 at a former military base in Texas to treat groundwater contamination. Monitoring data showed increasing contaminant levels downstream, indicating the PRB was not functioning as intended. A detailed assessment was conducted in 2011-2012 involving analyzing existing data, surveying water levels, installing new monitoring wells, and using groundwater modeling. Results showed the PRB formed a partial barrier to groundwater flow that has intensified over 10 years, indicating a steady deterioration of hydraulic conductivity. Preferential flow paths through the PRB impact its effectiveness in controlling the plume's movement and contaminants' residence time for degradation. Model