Assessing water quality impacts and cleanup effectiveness in streams dominated by episodic mercury discharges

Dc. Whyte et Jw. Kirchner, Assessing water quality impacts and cleanup effectiveness in streams dominated by episodic mercury discharges, SCI TOTAL E, 260(1-3), 2000, pp. 1-9
Citations number
Categorie Soggetti
Journal title
ISSN journal
0048-9697 → ACNP
Year of publication
1 - 9
SICI code
Accurate pollutant mass budgets are needed for identifying contaminant sour ces and establishing cleanup goals. We monitored mercury discharges from an abandoned mine site in northern California with the objectives of: (1) est imating the mass loading of mercury from the site; (2) evaluating the facto rs that control the mercury discharges; (3) assessing the significance of p eak flows in transporting contaminants; and (4) developing methods for meas uring the effectiveness of cleanup efforts. We sampled water downstream fro m the mine site over a wide range of streamflows. Mercury concentrations va ried over 2000-fold, from 485 to 1040000 ng/l, grossly exceeding the regula tory water quality objective of 12 ng/l at all times. Particulate mercury r epresented over 99.97% of the total mercury, and mercury concentrations wer e closely correlated to suspended sediment concentrations (r = 0.98). Thus, we can use suspended sediment concentrations as a proxy for mercury concen trations, and calculate a continuous record of mercury flux from continuous monitoring of streamflow (using a small flume) and turbidity (using an opt ical backscatter sensor). Mercury fluxes inferred in this way are consisten t with fluxes estimated from field samples. In January and February of 1998 , our small abandoned mine site released approximately 82 kg of mercury to downstream waters. Most of the mercury was released during brief intense ra instorms. For example, in one 200-min period we recorded 3.4 cm of rain, a 2.6-fold increase in streamflow (460-1120 l/s), and an 82-fold increase in mercury flux (1.2-99 g/min). Over 75% of the total mercury flux during this 2-month period occurred in less than 10% of the total time. In systems suc h as this one, where contaminant transport is highly episodic, sampling pro grams that miss the high-flow episodes may greatly underestimate the actual water quality threat. In addition, changes in pollutant fluxes or concentr ations in receiving waters may not reflect changes in pollutant sources (su ch as an environmental cleanup) if the stochastic forcing (e.g. intense rai nstorms) varies through time. We propose that water quality trends can be m ore accurately measured by changes in the relationship between contaminant flux and stochastic driving factors, as expressed by contaminant rating cur ves. (C) 2000 Elsevier Science B.V. All rights reserved.