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Assessment of environmental impacts of mercury treatability test using suction dredging, South Yuba River

Image of an underwater diverThe Bureau of Land Management (BLM) is planning a treatability study using suction dredge equipment on the South Yuba River in the vicinity of the Humbug Creek confluence. This area receives drainage from the Malakoff Diggings State Park and is a known “hot spot” for elemental mercury. Due to the large quantities of Hg recovered previously by hobby dredge operations, the location has been off-limits to future dredging. It is of great interest of BLM to systematically remove the Hg-rich sediments from this area of the South Yuba River where previous non-systematic dredge operations have recovered large amounts of Hg. The goal of the systematic removal is to mitigate potential ecological threats to the South Yuba ecosystem from the Hg-laden sediments.

Picture of hydrologists and equipmentTreatability studies are designed to test the effectiveness of a technology and to determine its potential environmental effects. Because dredge operators have collected and recovered large amounts of Hg from the South Yuba River near Humbug Creek, the BLM wishes to test the effectiveness of the suction dredge to remove and recover these Hg-contaminated sediments, thus removing a potential environmental hazard from the ecosystem. However, it is unknown how effective the method is in recovering Hg from the disturbed sediments and what potential effect any Hg losses from the suction dredge system might have in the immediate vicinity of the dredge operation or downstream. Qualitative evidence suggests that the dredge may actually enhance Hg transport downstream and increase the threat of Hg to the ecosystem. Knowing the effectiveness of Hg recovery using the suction dredge is crucial to the utilization of this technology for recovery and restoration efforts in all Hg-contaminated river sediments.

The goals of the BLM suction dredging treatability study are to (1) assess the effectiveness of suction dredging in removing mercury from the environment and (2) assess potential impacts of suction dredging with regard to discharging mercury-contaminated suspended sediment to the aquatic environment.

Research related to these overarching goals will address the following topical questions:

  1. What is the effectiveness of the suction dredge in recovering Hg from the contaminated sediments?
    1. How much Hg is recovered using a standard 8” suction dredge set-up?
    2. How much Hg is displaced downstream by the suction dredge operation?
    3. How far are the different Hg fractions transported downstream?
  2. What effect does the suction dredge operation have on the character of any Hg not recovered in the dredge operation?
    1. In what size fraction is the Hg not captured by the dredge found?
    2. How reactive is the sediment pre- and post- dredge on-site?
    3. How reactive is the sediment transported and deposited downstream?
  3. What is the ultimate ecological effect of any Hg not recovered by the operation?
    1. Are methylation rates increased on-site or downstream following the dredge operation?
    2. Are methylmercury concentrations in water or sediment affected on-site or downstream following the dredge operation?
    3. Are Hg levels in biota affected on-site or downstream following the dredge operation?

In addition to the practical evaluation of the efficiency of a standard 8” suction dredge as a method for recovering Hg contaminated streambeds and its potential ecological impacts, this research directly relates to local, state, and federal goals related to the assessment and cleanup potential of mine contaminated watersheds.

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Project Chief: Jacob Flec
Phone: 916-278-3000
Email: jafleck@usgs.gov

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Page Last Modified: Monday, 03-Oct-2011 13:41:58 EDT