On October 22, 2018, at approximately 11:10 am, an incident regarding mine discharge and potential fish impacts to Left Hand Creek in Boulder County, CO was reported to Region 8 response. The incident location was reported near the intersection of Left Hand Canyon Drive and California Gulch Road, less than 1 mile downstream of the Captain Jack Mill Superfund site, Operable Unit 01 (Big Five adit). Notifications were made and EPA and the Colorado Department of Public Health and Environment (CDPHE) began investigating to assess if the fish kill was related to ongoing work at the Big Five adit.

An inspection of the site by CDPHE on the afternoon of October 22, showed the mine adit continuing to drain at a controlled discharge of approximately 20-30 gallons per minute into the ponds from a pipe connected to the flow through bulkhead.  Both upper and lower settling ponds were operating, with water overflowing into the upper pond stand pipe and discharging downstream to the second settling pond.

1. Coordinate with CDPHE  to provide timely updates to public and key stakeholders.
2. Implement a neutralization system to enhance removal of acidity and metals in the settling ponds at the Site.
3. Monitor discharge from the ponds.
4. Verify stream water quality is improving.
5. Assess in-tunnel treatment system performance, evaluate/implement actions as necessary to improve performance.
6. Transition back to (remedial authorities) CDPHE lead with EPA support.

On October 23, EPA response in conjunction with CDPHE, conducted water sampling and monitoring along Left Hand Creek starting just above the Left Hand Water District intake approximately 15 miles downstream of the Big Five adit.   Decreasing pH levels (below pH 6) were observed starting approximately 1.5 miles downstream from where the Big Five adit settling ponds discharge into Left Hand Creek.
On October 24, EPA and CDPHE temporarily closed the flow through valve on the bulkhead.  The settling ponds appear to be treating less effectively which may be a result of lower pH mine drainage discharge caused by rising elevations of the mine pool.

EPA response conducted the following.

Small scale field treatability tests on discharge water with NaOH.   

EPA response conducted the following operations.

1.  Partially opened the flow-through bulkhead valve and began assessing low flow treatment with a passive lime system.   
2.  Treated low flows with Sodium Hydroxide (at the ponds) as an interim measure pending passive  lime treatment assessment and potential implementation. Set up a temporary sodium hydroxide delivery system  to provide pH adjustment and mixing in the adit discharge pipe between the upper and lower ponds.  
3.  Began evaluating logistical requirements to initiate in-tunnel neutralization with caustic.  
4.  The bulkhead valve was opened to about 20-30 gpm and pH adjustment occurred for a few hours.  Before the crew left the site, the bulkhead valve was closed.

During this time:

EPA continued to operate the treatment plant.  The mine pool was lowered and returned to historic conditions.

The EPA and Colorado Department of Public Health and Environment (CDPHE) shut down the temporary active water treatment system at the Captain Jack Mill Superfund Site on December 23, 2019.  The treatment system was installed at the site in winter 2018 and was intended to be a temporary mitigation effort to stabilize the degraded water quality conditions in the Big Five Tunnel that occurred upon flooding the mine workings.  The goal of the temporary water treatment system was to lower the mine pool and return the metal concentrations back to historic conditions.  Those goals have been met.

Upon shutdown of the temporary treatment system, water from the Big Five Tunnel will again flow through the series of two settling ponds to allow for the removal of particulates, and where some metals removal is expected to occur.  Similar to historic conditions at the Site (as measured in 2015), the tunnel discharge will impact Left Hand Creek.  EPA and CDPHE will monitor water quality in Left Hand Creek downstream from the Site over the next several months and anticipate that the impact form the discharge on the fishery will not exceed historic impacts, as measured in a 2015 fish survey.