Intercomparison of in-well, depth profile and online soil gas measurements for estimating petroleum natural source zone depletion rates: Will in-well gas samples suffice?

Natural Source Zone Deletion (NSZD) is a viable long-term management option for sites impacted by petroleum hydrocarbon fuels. NSZD rate estimation methods for petroleum mass losses often use soil gas gradients of oxygen, carbon dioxide, methane or vapour concentrations through the vadose zone. Seeking greater efficiencies, we investigated if existing short-screened wells are reliable for representative sampling of soil gases in a vadose zone undergoing NSZD. At gasoline, diesel, aviation-gasoline, crude oil and background locations, we compared soil gas compositions from three methods: (i) multilevel samplers (VZ) on three occasions over 18 months (three per location); (ii) oxygen sensors (GP) at multiple depths and tracked over 3 years (one per location); and (iii) short-screened monitoring wells (MW) across the water table (five per location) and sampled as per VZ. GP sensors showed seasonal and other variations, yielding variable NSZD estimates. Oxygen depth profiles from GP and VZ measurements showed good correspondence despite fluctuations. Oxygen, carbon dioxide and methane concentrations from the deepest sampling ports of VZ samplers correlated strongly with concentrations from adjacent MW installations, indicating in-well sampling in the field can provide reliable data. The advantages and limitations of using MW data in accelerating NSZD field assessments are outlined.