Chemical Processes Observed

The Eh of the injected waste dropped rapidly from +40 mV to -80 mV in the first 40 h after injection began and remained at about -80 mV thereafter. Denitrifying bacteria detoxified the acrylonitrile by mineralizing the compound, breaking it down into bicarbonate and ammonia. The nitrates were degraded to nitrogen gas. The backflow test produced data indicating that these transformations were about 90% complete within 25 m (82 ft) of the injection well and virtually 100% complete within 100 m (328 ft). These results are an example of a biodegradation-dispersion curve. Denitrifying-bacteria densities increased from traces (101 organisms/100 mL in the native groundwater) to large populations (10 to 108 organisms/100 mL) in injected wastes that had been in the aquifer for several days.

Sodium thiocyanate (NaSCN) was first detected in the closest monitoring well (300 m away) 260 days after injection began. Ammonium ions (a reaction product of biomineralization) did not appear as a contaminant until 580 days after initial injection. This delay was probably the result of ion exchange or other adsorption processes and may be an example of an adsorption-dispersion curve. Because sodium thiocyanate in the waste remained unchanged during its movement through the injection zone, it was used to detect the degree of mixing that took place between the waste liquid and native water in an observation well. Thus the appearance of sodium thiocyanate as well as an increase in chemical oxygen demand in the standby well 4 years after injection began signaled the arrival of wastes at that location.

This case study is interesting in that one hazardous waste (acrylonitrile) was quickly rendered nonhazardous after injection, whereas another (sodium thiocyanate) showed no evidence of decomposition during the duration of the study. The implication for geochemical fate assessment is that research should focus on the compounds likely to be most resistant to decomposition or immobilization, as they will be the ones most critical in demonstrating containment in a no-migration petition.

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