Improved field methods to quantify methane oxidation in landfill cover materials using stable carbon isotopes

Stable carbon isotopes provide a robust approach toward quantification of methanotrophic activity in landfill covers. The field method often applied to date has compared the delta C-13 of emitted to anaerobic zone CH4. Recent laboratory mass balance studies have indicated that this approach tends to underestimate CH4 oxidation. Therefore, we examined the CH4-delta C-13 at various soil depths in field settings and compared these values to emitted CH4. At 5-10 cm depth, we observed the most enrichment in CH4-delta C-13 (-46.0 to -32.1 parts per thousand). Emitted CH4-delta C-13 was more negative, ranging from -56.5 to -43.0 parts per thousand. The decrease in CH4-delta C-13 values from the shallow subsurface to the surface is the result of processes that result in selective emission of (CH4)-C-12 and selective retention of (CH4)-C-13 within the soil. Seasonal percent oxidation was calculated at seven sites representing four cover materials. Probe samples averaged greater (21 +/- 2%, p < 0.001, n = 7) oxidation than emitted CH4 data. We argue that calculations of fraction oxidized based on soil derived CH4 should yield upper limit values. When considered with emitted CH4 values, this combined approach will more realistically bracket the actual oxidation value. Following this guideline, we found the percent oxidation to be 23 +/- 3% and 38 +/- 16% for four soil and three compost covers, respectively.