mL total gas g~ cellulose with excess NO3 . Carbon dioxide production under anaerobic
conditions was as follows: 27.4   5.8 pmol CO2 g1 cellulose in the absence of nutrients,
66.9   1.1 pmol CO2 g-1 cellulose with them, and 251   5 pmol CO2 g-1 cellulose with excess
NO (after 6 years of incubation 2.24   0.24 x 108 bacterial cells mL1 were detected in these
samples). The overall rate of total gas production in anaerobic samples was 6.7 x 10-4, 6.7 x
10-4, and 2.5 x 10-3 mL g1 cellulose day~', respectively, and for CO2 production it was 6.4 x
10~3, 1.4 x 10-2, and 5.6 x 102 [mol CO2 g 1 cellulose day , respectively.
" Cellulose degradadation products deterred in solution include fumaric, lactic, oxalic,
oxalacetic, propionic, and succinic acids indicating fermentative microbial activity.
" Methane was first detected after 7.4 years incubation in brine inundated samples. The amount
of methane detected at 9.5 years under anaerobic conditions were without nutrients 5.89
1.30 nmol g1 cellulose; 2.74   0.90 nmol g1 cellulose with nutrients, and 2.57   0.79 nmol g-
1 cellulose with excess NO3 . In samples when the initial conditions were aerobic the amount
of CH4 detected was very low: 1.34   0.03 nmol g1 cellulose without nutrients, 0.84   0.05
nmol g1 cellulose with nutrients, and 1.27   0.37 nmol g-i cellulose with excess NO3 .
" The presence of bentonite, once proposed as a backfill additive, enhanced the total gas
production, concentration of gaseous and aqueous metabolites by several fold.
" Under humid conditions, total gas and CO2 production was much lower than smples
incubated under iundated conditions. Adding nutrients lowered gas production; under
initially aerobic conditions: 6.09   2.41 pmol CO2 was produced g1 cellulose after -9 years
without nutrients while 0.48   0.29 pmol CO2 was produced g1 cellulose with a nutrient
amendment. The same held true under anaerobic conditions: unamended samples produced
115   20 pmol CO2 g-1 cellulose while nutrient-amended ones generated 21.9   3.3 pmol
CO2 g-1 cellulose after -9 years incubation. Bentonite enhanced gas production: anaerobic
unamended samples produced 591   135 pmol CO2 g-1 cellulose, and amended samples
produced 673   49 moles CO2 g-1 cellulose. Methane (32.6   9.3 nmoles g1 cellulose) was
detected only in bentonite containing samples.
* Plastic and rubber-materials subjected to an absorbed radiation dose of up to 4,000 Mrad to
determine if radiation damage could affect biodegradability of the polymers and gas
generation. Microbial gas production was not observed after -7 years incubation in
polyethylene or polyvinylchloride samples, whereas irradiated rubber materials neoprene and
hypalon showed enhanced CO2 production.
* After 6 years of incubation, the microbial population in anaerobic brine-inundated samples
were enumerated by direct microscopy: unamended uninoculated samples contained 5.12
3.41 x 105 bacterial cells mL~1, unamended inoculated samples contained 1.59   0.15 x 107
cells ml~1, amended inoculated samples contained 1.62   0.07 x 108 cells mL~1, and amended
inoculated samples with excess NO3 contained 2.24   0.24 x 108 cells mL~1. DNA analyses
showed a diverse assemblage of bacteria and archae in unamended and nutrient-amended
inundated cellulose samples.

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