title: Quantifying remediation effectiveness under variable external forcing using contaminant rating curves.
abstract: Remediation efforts are typically assessed through before-and-after comparisons of contaminant concentrations or loads. These comparisons can be misleading when external drivers, such as weather conditions, differ between the pre- and postremediation monitoring periods. Here, we show that remediation effectiveness may be better assessed by comparing pre- and postremediation contaminant rating curves, which permit "all else equal" comparisons of pre- and postremediation contaminant concentrations and loads under at any specified external forcing. We illustrate this approach with a remediation case study at an abandoned mercury mine in Northern California. Measured mercury loads in the stream draining the mine site were a factor of 1000 smaller after the remediation than before, superficially suggesting that the cleanup was 99.9% effective, but rainstorms were weaker and less frequent during the postremediation monitoring period. Our analysis shows that this difference in weather conditions alone reduced mercury loads at our site by a factor of 73-85, with a further factor of 12.6-14.5 being attributable to the remediation itself, implying that the cleanup was 92-93% (rather than 99.9%) effective. Our results illustrate the need to account for external confounding drivers when assessing remediation efforts, particularly in systems with highly episodic forcing.
authors.affiliation: Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA. james.kirchner@wsl.ch
authors.forename: James W
authors.initials: JW
authors.lastname: Kirchner
authors.forename: Carrie M
authors.initials: CM
authors.lastname: Austin
authors.forename: Alexandra
authors.initials: A
authors.lastname: Myers
authors.forename: Dyan C
authors.initials: DC
authors.lastname: Whyte
mesh_terms: D002140:California; D004784:Environmental Monitoring / Q000379:methods*; D052918:Environmental Restoration and Remediation; D019015:Geologic Sediments / Q000032:analysis; D008628:Mercury / Q000032:analysis*; D008906:Mining; D009391:Nephelometry and Turbidimetry; D011891:Rain; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D014874:Water Pollutants, Chemical; D008628:Mercury