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Natural Attenuation of Perchlorate in Denitrified Groundwater

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Abstract

Monitoring of a well-defined septic system groundwater plume and groundwater discharging to two urban streams located in southern Ontario, Canada, provided evidence of natural attenuation of background low level (ng/L) perchlorate (ClO4) under denitrifying conditions in the field. The septic system site at Long Point contains ClO4 from a mix of waste water, atmospheric deposition, and periodic use of fireworks, while the nitrate plume indicates active denitrification. Plume nitrate (NO3-N) concentrations of up to 103 mg/L declined with depth and downgradient of the tile bed due to denitrification and anammox activity, and the plume was almost completely denitrified beyond 35 m from the tile bed. The ClO4 natural attenuation occurs at the site only when NO3-N concentrations are <0.3 mg/L, after which ClO4 concentrations decline abruptly from 187 ± 202 to 11 ± 15 ng/L. A similar pattern between NO3-N and ClO4 was found in groundwater discharging to the two urban streams. These findings suggest that natural attenuation (i.e., biodegradation) of ClO4 may be commonplace in denitrified aquifers with appropriate electron donors present, and thus, should be considered as a remediation option for ClO4 contaminated groundwater.

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