The Susie Mine is located within the Community of Rimini, in
Lewis and Clark County near Helena, Montana. The mine is located within the 53-square mile Upper Tenmile Creek Mining
Area Superfund Site, which includes approximately 150 active and abandoned mines. Mining for gold, lead, copper, and zinc began
in the area in the 1870’s and continued through the 1930’s. Waste rock containing heavy metals was used
as fill for roads, yards, and local waterways, resulting in soil and
groundwater contamination with heavy metals.
The Susie Adit discharges acidic mine-impacted
water containing heavy metals and arsenic at a rate of 4 to 10 gallons per minute
with a pH between 3.5 to 4.3. The portal
of the mine tunnel has partially collapsed causing mine-impacted water to accumulate
behind the blockage. This blockage
creates the potential for a sudden, uncontrolled release of mine-impacted water
(i.e. “blowout”) and sediment into Tenmile Creek. EPA Region 8 is working with the Montana Department of Environmental Quality to mitigate
the risk by reducing the volume of mine-impacted water pooled behind the
blockage and removing the blockage. Additional drainage structures will also be installed. This work is scheduled for July – August 2021.
On July 12, 2021, a small amount of sludge, approximately
20-40 gallons, was released during an EPA dewatering operation at the Susie Adit
site. EPA estimates that a 100-foot stretch of Tenmile
Creek was impacted by this release, which occurred when a hose removing water from the adit
site collected excess sediment and sludge material as the water level dropped
in the area being dewatered. This event was not a “blowout.” The City of Helena drinking water will not be impacted by this event
as there are no City Water intakes downstream of this area.
EPA will resume the dewatering effort in the next few days and
will implement supplemental control measures to ensure that additional sludge
releases do not occur. Water quality sampling following
the July 12 discharge was conducted analytical results will be available soon and crews will also be removing sludge from the creek at the point of
discharge.
