Effect of in-situ gas accumulation on the hydraulic conductivity of peat

The in situ hydraulic conductivity of moderately decomposed, catotelm (subsurface) peat is often anomalously low (10 to 10 cm s) considering that the porosity (n) of this peat is often as high as 80–97%, and the dry bulk density (pb) as low as 0.05–0.25 g cm. It has recently been hypothesized that this is the result of a progressive occlusion of pores in the peat matrix by bubbles of gas (especially methane) which are generated in-situ by ongoing anaerobic respiration. To test this hypothesis, the change with time of hydraulic conductivity (K), volumetric water content (Θ), volumetric gas content (γ), and gaseous methane concentration (M) were measured in three, repacked laboratory columns of catotelm peat through which de-aired, temperature equilibrated water flowed continuously under positive pressure. Two of the columns contained microbially active peat which was packed to average n and pb values of 96.2% and 0.06 g cm, respectively. The third column was similarly packed (n = 95.5%, Pb = 0.07 g cm) and then sterilized using 5.5 Mrad Co gamma radiation to render it microbially inactive. In the two unsterilized columns, K decreased on average from 7.0 × 10 (±6.1 × 10) cms to 1.8 × 10 (±1.7 × 10) cms; Θ decreased from 83.6% (±3.6) to 64.6% (±6.4); γ increased from 12.5% (±3.2) to 31.6% (±6.4); and M increased from 3.9 (±1.5) μmol ml to 50.2 (±14.9) μmol ml during four separate measurement periods (two perods per column) which varied in length from 44 to 78 days. During a 41-day measurement period, K and Θ increased in the sterilized comumn to stable, relatively high values of 5.7 × 10 cms and 91.0%, respectively; γ decreased to a stable low value of 4.5%; and only trace levels of M were detected. The chages and eventual stabilization of K, Θ and γ in the sterilized column were due to the gradual removal by the flowing water of artifact carbon dioxide gas produced by the sterilization process. The lack of M and the low final γ value in the sterilized column indicate that no significant anaerobic respiration occurred during the measurement period. It was concluded that it is indeed possible for gas generated in situ by anaerobic respiration to cause a progressive and significant decrease in the K of moderately decomposed catotelm peat.