Water Board Studies Show No Threat From Suction Dredging

Continued studes by the California Water Board fail to find link between suction dredging and mercury levels in rivers

Latest Water Board Study Shows No Link Between Ongoing Mining and Mercury Levels


Above. Studies show river flooding results in elevated levels of mercury in wildlife. Photo credit Ian O'Halloran

The latest in a series of studies by the Water Board was released last last year. This is the most comprehensive study to date of what causes elevated mercury levels in fish in some lakes and reservoirs in California.

The study finds mines and mining aren't related to the levels of mercury in fish in the lakes. The study looked at a variety of factors which could be related to elevated mercury levels in some lakes, and in some species of fish.

The more studies the Water Board prepares the more evident it becomes the Mother Lode is free from significant levels of mercury. The new study finds no linkage between active, productive mines in an area and elevated mercury levels. The study refutes the environmental group's position there is a "toxic legacy" from mining in the gold country. The vast majority of the mercury released during the gold rush has largely been removed over the past 150 years by winter storms and also by suction dredge miners. Based on statistics collected from the California Department of Fish and Wildlife (CDFW) it's likely suction dredge miners have removed over 14 tons of mercury.

The California Water Board has prepared a series of studies over the past ten years evaluating mercury levels in the Sierra Nevada area. These studies evaluated mercury levels in fish in reservoirs; levels of mercury of fish in rivers; linkages between environmental factors and mercury; ability of a suction dredge to recover mercury and a separate study evaluated the effects of winter storms on mercury distribution. A summary of each study is provided:

Mercury Contamination in Fish in Northern California Lakes and Reservoirs, 2007

Between 2000-2001 twenty lakes and reservoirs in historic gold mining areas were tested for elevated levels of mercury in fish tissue. Researchers had theorized fish in historic mining areas would contain higher levels of mercury than fish outside of these areas, but the results of the study failed to show this linkage.

The report concluded non-native fish such as bass, rainbow trout and catfish tended to have significantly higher levels of mercury than native trout. The report confirms "trout species display the lowest bioaccumulation of mercury..."

The results of this sampling is shown in the table below which provides the measured mercury levels in the fish against the US EPA standard for safe levels of mercury.


Above. Evaluation of mercury levels in fish in lakes and reservoirs in dredging areas show fish are consistently below US EPA threshold levels

Contaminants in Sport Fish, 2013

This study was a follow on to the study of lakes and reservoirs by looking primarily at the rivers and streams in historic gold mining areas to evaluate the theory fish in these areas should be higher in mercury levels.This was the most comprehensive study in the history of California. Over the course of two years (2011-2013) they sampled 568 fish from 63 rivers and streams in Central California.

The researchers collected multiple species and multiple types of each species from each site and evaluated these results against their expectations.They found, once again, virtually all trout were safe to eat, and of those species which typically had higher levels of mercury, such as largemouth bass, these species only had moderate levels of mercury but not sufficient to warrant fish advisories.

The study found every river and stream in the historic gold mining areas was not just below the threshold for issuing fish advisories, but well below the threshold. "Most locations (51%) of the 63 locations sampled had low concentrations of mercury (<0.07ppm)...Only 13% of the locations sampled had levels which exceeded the threshold advisory value of 0.3ppm and these fish had only slightly elevated levels measured at 0.44ppm."

The locations which exceeded the threshold criteria were all located in the Sacramento delta. Low lying elevations where mercury accumulates.

The results were reflected the fundamental argument the Western Mining Alliance made in the 2012 suction dredging environmental impact report, that mercury capture by suction dredges should be encouraged to prevent the mercury from moving into the low elevation areas where it has the potential to transform into a harmful type of mercury which can be ingested by the food chain. The evaluation of rivers in gold mining country by the Water Board reflects a much higher mercury leve from fish in low-lying areas than in the gold mining regions.

The standard dredge sluice box, as shown below, is designed to capture all material with a specific gravity higher than six. Mercury, being nearly as dense as gold is captured by suction dredges at nearly 100%.

Capture Graph

Above. A modern sluice box found on a suction gold dredge is nearly 100% efficient at capturing mercury.

Factors Contributing to Mercury Levels in Rivers, 2014

This study was the third in a three-part study extending over seven years that sampled 63 rivers and 568 fish to determine the health of California sport fish. Without exception the fish in the mountains and gold bearing areas of California were either well below or significantly below established criteria for consumption. 87% of locations sampled tested below mercury threshold levels and 100% of rivers in the gold country tested below US Environmental Protection Agency (EPA) thresholds. The only areas found with levels of mercury exceeding advisory criteria were in the Sacramento Delta, which is consistent with earlier testing.

The Water Board report concluded "This could indicate that safe fish consumption at a frequency of more than one serving per week is possible at the vast majority of these locations if the cleaner species are selected. Comparing the data to a high standard of safety, 28 of the 63 locations (44%) had at least one species that can be safely consumed at a higher consumption rate of 3 servings per week."

Confirms Prior Studies

The results are consistent with earlier studies finding California trout are some of the safest for consumption in the country. Measured amounts of toxins, including mercury, show levels well below established health advisories. This new information throws into question the continued push by environmental groups to place additional restrictions on fishing and outdoor activities.

The Water Board studies reflect a continuing trend in California for decreasing mercury levels. A 2011 U.S. Geological Survey (USGS) report on mercury levels across the United States found mercury levels across the West had been dropping, including over a 7% drop in mercury levels in the Sacramento River and a 3% drop in the Klamath River. The USGS study evaluated measurements from a span of twenty years.

Of interest to scientists the Water Board study also measured levels of the naturally occurring element selenium. According to newly released guidance from the US National Oceanic and Atmospheric Agency (NOAA) the presence of selenium acts to completely offset the effects of mercury. The new NOAA guidance states if the balance of selenium to mercury in fish is achieved then fish are safe to eat. The Water Board study found levels of selenium in the fish exceeded the levels of mercury.

Annual Floods Cause Mercury to Move to Lower Elevations, 2013

The Singer Study was a short, six page report, which evaluated the ability of rivers to move gravel, sediment and accompanying historic mercury deposits from the higher elevation watersheds to low lying areas such as the valley and the delta. This study found major flooding events were primarily responsible for the mercury loading of the low elevation areas and estimated it would take another 10,000 years for the historic residual mercury to completely flush to the ocean.

The Humphries Report, 2005

This was the first look by the Water Board at the supposed linkage between suction dredges and mercury levels. Prepared in 2005 by a Water Board staff member this study used an unmodified suction dredge to evaluate the ability of a dredge to recover mercury. The report found a suction dredge was 98% efficient at capturing mercury. However, the Water Board staff member recommended not using a suciton dredge to recover mercury because the 2% not recovered was in his opinion too high. Ironically, this meant 100% of the mercury would be left in the river which apparently isn't too high.

USGS, The Fleck Report, 2011

The Fleck Report was the primary source for the findings in the 2012 Suction Dredging EIR that suction dredges had the potential to re-suspend mercury. Although this report was referenced, only part of it was referenced. The original purpose of this study was to evaluate suction dredges as a tool for removing mercury from the watersheds, specifically to remove mercury from a supposed mercury hotspot at the confluence of Humbug Creek and the South Yuba River.

This two year study conducted by the USGS was designed to evaluate an actual suction dredge by measuring the effluent from the dredge for trace amounts of mercury or other toxins. The first year of the study they used a 3" suction dredge to dredge some test holes in the mid-river gravels. The results from this portion of the study were never mentioned in the EIR, but they confirmed the earlier Humphries findings that a suction dredge was highly efficient at capturing mercury. The source data from the study indicated at least 98% of the mercury was captured, but the actual efficiency appears to be closer to 100%.

The results from the suction dredge test showed very minor amounts of mercury in the water which were within normal variations of mercury readings present in the river. Mercury levels in some cases actually dropped from prior to dredging to ongoing dredging but all data indicated very miniscule amounts of mercury in the water. The below table provides the results from the dredge test:


Above. Data from the USGS indicates suction dredges emit miniscule amounts of mercury compared to what they capture.