Around 1990 – the Colorado Water Quality Control Division (WQCD) starts doing extensive water quality testing in the Upper Animas River in preparation for a water quality standards hearing before the Colorado Water Quality Control Commission.
1991- The Sunnyside Mine closes. It was the last major mine in the area and was easily the biggest. Sunnyside Gold Corp. begins reclamation activities.
1994- Animas River Stakeholders Group (ARSG) forms at the urging of WQCD which helps retain a facilitator.
1995- WQCC adopts strict goal based (numeric) standards but delays the effective date to 1998 to allow ARSG to investigate metal sources and strategies to reduce them. ARSG hires a coordinator.
1996- Sunnyside Gold and WQCD sign a consent decree which allows Sunnyside to bulkhead its mine workings and turn off its treatment plant in Gladstone in return for remediating a number of historic mine sites. The goal is to ensure that water quality will be no worse in the Animas River below Silverton than it was before the treatment plant was turned off. Zinc concentrations were used as the surrogate metal by which this goal was determined. The treatment plant is used to treat Cement Creek while bulkheads are completed and the mine pool behind the bulkheads reaches equilibrium.
1997- With ARSG’s urging, WQCC grants an extension of the effective date of the water quality standards to 2001. The Dept of Interior begins the Abandoned Mined Lands Initiative to study the effects of abandoned mines on water quality and designates the Animas as one of the two initial, national pilot project sites.
1999- Sunnyside Gold completes approximately 17 remediation projects that it began in 1991. Several other mining companies, US, BLM and ARSG complete several more projects.
2001- ARSG presents a Use Attainability Study (UAA) to WQCC with recommendation for use classifications and water quality standards. WQCC adopts the recommendations. TMDL (Total Maximum Daily Load) are also determined based on the UAA.
2002- Gold King #7 level begins to discharge significant amounts of acid mine drainage.
2003- Sunnyside and Gold King complete another group of remediation sites as part of an agreement to terminate the consent degree including bulkheading the Mogul and Koehler mines. Operation of the treatment plant in Gladstone stops after the mine pool behind the bulkheads reaches equilibrium and water quality below Silverton indicates no statistical degradation has occurred as a result of the consent degree.
2003- ARSG has pilor project Good Samaritan legislation introduced into Congress.
2007- EPA supports reports on treatment plant options at Gladstone.
2007- Two-volume USGS report from Department of Interior Abandoned Mined Land Initiative released.
2008- EPA begins to collect data for Superfund assessment.
2011- EPA announces that Animas probably could meet listing criteria for Superfund, although no official determination is made.
2011- Sunnyside Gold Corporation offers $6.5 million (later $10 million) towards remediation conditioned on a release from future liability.
2013- EPA opens the Red & Bonita mine.
2015- EPA installs a bulkhead in the Red & Bonita but the valve is not closed. On August 5th, EPA causes Gold King spill.