The long-term capability of soil pore air sampling to detect graves

We examined the concentrations of methane (CH₄), carbon dioxide (CO₂) and nitrous oxide (N₂O) in the pore air of a well-drained soil at which pig carcasses were buried over 12-13 years at depths between 50 and 100 cm. Sites included disturbed pits without carcasses, an undisturbed site, a mass grave (20 pigs) and single carcass graves buried at two depths with and without burial in a plastic bag. Twelve years after burial, the concentrations of CH₄ were mostly below atmospheric values (∼ 2.0 ppm), showing that this gas was of little value in detecting graves under these conditions. Concentrations of CO₂ were above values found under undisturbed conditions. Much of the increase is related to the decomposition of the carcasses, though disturbance during pit refilling and root respiration may account for part of the increase. The best indicator of burial over 13 years was increases in the concentration of N₂O associated with inclusion of carcasses and with evidence of lateral flow of N₂O in the soil. The N₂O concentration is well above that of undisturbed or disturbed soils providing a detectible effect in sites with carcasses.