Environmental Engineering Reference
In-Depth Information
During pump testing, the radii of influence were found to be 0.3-0.5 m.
Upstream and downstream groundwater monitoring wells were used for an
integrated mass flow evaluation using a 3D groundwater flow model. More
wells in the vicinity and their long-term monitoring data were taken into
account to determine the influence of the gas PRB on ongoing natural attenu-
ation processes.
Neon and helium trace gases were used to determine an initial ROI of
the coherent gas flow and gas escape to the vadose zone. These tests were
repeated during a subsequent stationary period. An average ROI of gas flow
of 5-8 m was detected in the groundwater zone; oxygen gas storage and dis-
solution efficiency of oxygen at approximately 60% was found using mass
balance modeling. The slight aeration of the vadose zone was anticipated
during the short-term testing of the gas PRB, and the effects of stripping and
the safety implications were monitored in the vadose zone.
The operation of the Profen oxygen gas PRB consisted of three stages. Gas
injection rates at lances were in the range of 0.25-0.5 m³/h STP.
Period I (116 days):
Pre-oxidation by continuous injection of 26 kg O
2
/day
Period II (77 days):
Forced sediment conditioning and initiation of biodegradation by
continuous injection of 37 kg O
2
/day
Period III (128 days):
Stabilization of biodegradation by pulsed injection of 21 kg O
2
/day
In periods I and II, almost the entire dissolved oxygen mass was needed
for pyrite oxidation. Initial high sulfate production rates led to a temporary
decrease in downstream pH and increased iron and manganese dissolution.
During period III, the conditions for an optimized biodegradation of pol-
lutants were established and pH >6.5 were found. Approximately, 20% of
the dissolved oxygen was consumed in the transformation of contaminants.
Additional details are reported in Zittwitz et al. (2012).
Aerobic biodegradation rates were estimated from mass balances, and
were proven by laboratory testing and field-scale transport modeling. First-
order rate coefficients of 0.07/day
−1
for benzene and 0.04/day for naphtha-
lene were found. Degradation ratios for the total mass flows of benzene and
naphthalene were approximately 96% and 80% respectively.
In summary, the implementation of the oxygen gas PRB at the Profen site
was performed during an ongoing MNA application. The ability to enhance
natural attenuation potentials in a plume of dissolved aliphatic and aro-
matic hydrocarbons was demonstrated. No meaningful interference to the
accorded MNA prognoses outside the PRB zone was found, due to miminal
impacts on the groundwater flow. However, the efficiency of oxygen gas stor-
age and dissolution should be increased significantly in order to optimize
the cost effectiveness of a full-scale application.
The gas PRB technology was found to be suitable for the technical risk cov-
erage of MNA. The full-scale application was based on groundwater flow and
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