Natural Attenuation of Perchlorate in Denitrified Groundwater

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 ( ClO 4 − ) under denitrifying conditions in the field. The septic system site at Long Point contains ClO 4 − from a mix of waste water, atmospheric deposition, and periodic use of fireworks, while the nitrate plume indicates active denitrification. Plume nitrate ( NO 3 − ‐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 ClO 4 − natural attenuation occurs at the site only when NO 3 − ‐N concentrations are <0.3 mg/L, after which ClO 4 − concentrations decline abruptly from 187 ± 202 to 11 ± 15 ng/L. A similar pattern between NO 3 − ‐N and ClO 4 − was found in groundwater discharging to the two urban streams. These findings suggest that natural attenuation (i.e., biodegradation) of ClO 4 − may be commonplace in denitrified aquifers with appropriate electron donors present, and thus, should be considered as a remediation option for ClO 4 − contaminated groundwater.