Delivering Sulfate to <scp>LNAPL</scp> Body for Enhanced Source Zone Depletion ( <scp>ESZD</scp> )

Abstract This field pilot study evaluates the feasibility of applying sulfate to a light non‐aqueous phase liquid (LNAPL) body for achieving enhanced source zone depletion (ESZD) at a refinery site. Two rounds of gypsum land application (GLA) delivered sulfate to an 8000 ft 2 area encompassing LNAPL‐containing monitoring wells MW‐2 and MW‐3, while MW‐1 served as a natural source zone depletion (NSZD) control. Over the 1044‐day study period, LNAPL and groundwater samples were collected to assess changes in LNAPL composition and key groundwater geochemical indicators, respectively. Principal component analysis revealed distinct and consistent compositional shifts in LNAPL under sulfate‐enhanced biodegradation conditions. LNAPL depletion rate for ESZD was 3‐fold faster than for NSZD. Notably, BTEX compounds also exhibited three‐ to sixfold enhanced depletion, while normal‐, iso‐, and cyclo‐alkanes also showed significant increases in depletion rates. Groundwater from GLA‐influenced wells demonstrated elevated sulfide, Fe(II), methane, and enriched δ 34 S‐sulfate, supporting active sulfate‐enhanced biodegradation. These results demonstrate that sulfate delivery can accelerate depletion of LNAPL and of its individual constituents in source zones, offering a practical and cost‐effective strategy for long‐term management at sites with persistent, unrecoverable LNAPL. Continued monitoring and adaptive sulfate delivery are recommended to further optimize BTEX attenuation in groundwater.