October 15, 2019 – Nearly three weeks after a major discharge event on the South Yuba River resulted in water quality impairment and a no-swim advisory issued by Nevada County Public Health, the exact cause has yet to be determined. This now infamous photo was taken Friday September 20, 2019 at the Highway 49 crossing. It shows people at a popular swimming location on a warm afternoon – the sole characteristic that makes the image striking is the color of the water.

For organizations like The Sierra Fund, who have been working to identify, assess, and address legacy mining impacts for over a decade, the telltale distinct orange brown color of the water points to an obvious source – 19th century hydraulic gold mines. Hydraulic mine sites are notorious for sedimentladen discharge after major storm events. The timing of the rain prior to the September 20 water quality event (on 9/17-9/19/19) was unique in that the precipitation fell early in the season and fell hard, while most of the watershed was just “wetting up.” Under such conditions, hydraulic mine pits become saturated and can release large amounts of silt and clay into surface flows, most of which eventually end up in our local watersheds.

Photo taken at Humbug Creek confluence 9/21/2019 by Nick Graham

This picture was taken on September 21, and it shows silts and clays stranded on rocks near the confluence of Humbug Creek and the South Yuba River. Humbug Creek receives surface flows from one of the largest hydraulic mines in our region – Malakoff Diggins. There was not evidence of this silt and clay material upstream of the Humbug Creek confluence on September 21, indicating that Humbug Creek was at least in part responsible for discharge of this type.

The Sierra Fund has been monitoring the discharge from Malakoff Diggins in Humbug Creek since 2011 and has a gage which monitors turbidity continuously in the reach just downstream of where Hiller Tunnel discharges to Humbug Creek. The gage data allows us to measure how much water and how much sediment is released from Malakoff Diggins every year. The relationship between turbidity and total suspended sediments and mercury have been well established for this site. Using this relationship, the amount of sediment and the amount of mercury that was released during the September storm can be calculated. Below is a picture of Hiller Tunnel, the sole outlet for the Malakoff Diggins hydraulic mine pit, that was taken on September 21. The silt and clay on the banks outside the tunnel is visible where it was left stranded after the rain ceased and the water level dropped.

Photo taken at Hiller Tunnel outlet 9/21/2019 by Nick Graham

Malakoff Diggins is by no means unique. There are literally hundreds of hydraulic mines on public and private lands across our forests that pose similar water quality threats. Furthermore, the issue of hydraulic mine discharge is not going anywhere. It is a relic of the Gold Rush. The Yuba and Bear Rivers were extensively hydraulic mined in the late 19th century and the large scars left on the landscape were never remediated. They are denuded areas that lack soil and have only minimal vegetative growth making them highly erosive and prone to water quality discharges with high suspended sediment loads.

The use of mercury at these sites to aid in the recovery of gold has resulted in the mine sites still discharging particulate-bound mercury, or mercury associated with fine silts and clays, which can travel long distances from the mine sites and can methylate in warm anoxic environments, like reservoirs, contaminating fish making them unsafe for consumption. Mercury is a neurotoxin, that can result in permanent developmental delays in children and fetuses and the primary exposure to humans is from fish consumption.

The legacy of mercury is a toxic legacy that needs to be addressed as part of a regional initiative to restore ecosystem and community resiliency in the Sierra, so that events like that on September 20 do not continue to pollute our watersheds. It is imperative to make it clear that this kind of discharge happens every time it rains. It just so happens that with the first flush, warm summer days following heavy early fall storms, the impact was visible for all to see. The rest of the time, sediment released from hydraulic mines is mixed in with the muddy waters that we see during winter rain events.

The Sierra Fund invites the interested public to learn more about this issue at our biennial conference, Reclaiming the Sierra: Headwater Mercury Source Reduction, taking place October 16-18, 2019 in Grass Valley. For more information and to register to attend please visit https://reclaimingthesierra.org/.