RESUMO
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.
Assuntos
Monitoramento Ambiental/métodos , Recuperação e Remediação Ambiental , Mercúrio/análise , Poluentes Químicos da Água/análise , California , Sedimentos Geológicos/análise , Mineração , Nefelometria e Turbidimetria , ChuvaRESUMO
The Walker Creek intertidal delta of Tomales Bay, California is impacted by a former mercury mine within the watershed. Eleven short sediment cores (10 cm length) collected from the delta found monomethylmercury (MMHg) concentrations ranging from 0.3 to 11.4 ng/g (dry wt.), with lower concentrations occurring at the vegetated marsh and upstream channel locations. Algal mats common to the delta's sediment surface had MMHg concentrations ranging from 7.5 to 31.5 ng/g, and the top 1 cm of sediment directly under the mats had two times greater MMHg concentrations compared to adjacent locations without algal covering. Spatial trends in resident biota reflect enhanced MMHg uptake at the delta compared to other bay locations. Eighteen sediment cores, 1 to 2 m deep, collected from the 1.2 km2 delta provide an estimate of a total mercury (Hg) inventory of 2500+/-500 kg. Sediment Hg concentrations ranged from pre-mining background conditions of approximately 0.1 microg/g to a post-mining maximum of 5 microg/g. Sediment accumulation rates were determined from three sediment cores using measured differences of (137)Cs activity. We estimate a pre-mining Hg accumulation of less than 20 kg/yr, and a period of maximum Hg accumulation in the 1970s and 1980s with loading rates greater than 50 kg/yr, corresponding to the failure of a tailings dam at the mine site. At the time of sampling (2003) over 40 kg/yr of Hg was still accumulating at the delta, indicating limited recovery. We attribute observed spatial evolution of elevated Hg levels to ongoing inputs and sediment re-working, and estimate the inventory of the anthropogenic fraction of total Hg to be at least 1500+/-300 kg. We suggest ongoing sediment inputs and methylation at the deltaic surface support enhanced mercury levels for resident biota and transfer to higher trophic levels throughout the Bay.