Metals and acid mine drainage

Overview

Heavy metal contamination in the Colorado River (including dissolved and particulate zinc, cadmium, lead, arsenic, copper, and iron) is primarily concentrated in the headwater tributaries of the Upper Basin. Contamination is relatively widespread throughout these headwaters tributaries, and is found in tributaries to the Eagle, Gunnison, Animas, East, and Dolores Rivers; however, contamination rarely travels far downstream. The metal-rich geology in the Rocky Mountains creates natural background levels of metal contamination, which have then been exacerbated by 150 years of mining activities. Thousands of abandoned mines in Colorado (~5,100), Utah (~10,600), Arizona (~24,000) and New Mexico (~3,900) continue to contribute waters with elevated metals concentrations. Dissolved heavy metals require specific chemical conditions to persist as dissolved contaminants in water (e.g., low pH) and thus do not often propagate into the mainstem of the Colorado River.

Figure 1: The mixing zone of the Animas River (left) and Cement Creek (right). Water in Cement Creek is metal-rich, low-pH while water in the Animas River has low metal concentrations and near-neutral pH. The dilution of Cement Creek in the Animas River increases the pH of Cement Creek water, leading many of the dissolved metals to precipitate back into solid form, from solution. (Photo credit: U.S. Geological Survey).

Sulfide-rich minerals, such as pyrite, react with oxygen and water to form sulfuric acid; this sulfuric acid can then leach metals from surrounding host rock and these dissolved metals can travel downstream. When this process occurs naturally, it is referred to as acid rock drainage; enhanced metal concentrations in waters due to mining activities is often referred to as acid mine drainage. Mining activities can increase the concentrations of heavy metals in water by crushing rock to increase surface area, digging tunnels which act as conduits of oxygen and water deep into the subsurface, and exposing new horizons of sulfide-rich minerals to oxidizing agents and water. Most metals are soluble only at acidic pH and precipitate back into solid form at near-neutral pH; most metal-rich acidic waters are attenuated at confluences with cleaner streams that have higher pH. Much of the contamination from acid mine drainage in the headwaters of the Colorado River has persisted for decades and is likely to continue given the complexity of the situation.

Acid mine drainage in the Colorado River Basin gained notoriety following the Gold King Mine Spill in 2015 in Cement Creek, a tributary to the Animas River, which flows into the San Juan River and the Colorado River. During the Gold King Mine Spill, 3 million gallons of acid mine drainage were accidentally released following remediation efforts on a collapsed mine tunnel. Since the spill happened abruptly, the acid mine drainage plume had severely concentrated metals and traveled downstream through the Animas River where the precipitating iron turned the stream a vibrant and alarming orange color. The spill resulted in extensive litigation between the states of Colorado, New Mexico, Utah, and Navajo Nation; following the Gold King Mine spill, the region was designated as an EPA superfund site.

Metal contamination in headwater tributaries typically impacts local communities immediately downstream; the Gold King Mine Spill was a notable exception. While metal contamination is insidious and poses many challenges for remediation, extensive cleanup work carried out by the EPA, nonprofits, and state and local agencies has improved metal concentrations in many reaches of headwaters streams in the Upper Colorado River. However, these improvements are complicated by continued sulfide mineral oxidation at abandoned mine sites and modern mines.

Data and tools

USGS abandoned mine water quality database

The USGS abandoned mine and water quality database is hosted by the USGS and houses USGS publications, data releases, and relevant reports pertaining to the introduction of harmful substances into the environment from mines and tailings.

EPA water quality database

The EPA reports and releases relevant data from acid mine drainage sites at which the EPA is involved. Specific pages host data specifically relevant to the Gold King Mine spill (2015).

Additional resources

Colorado Abandoned Mines Water Quality Study

The Colorado Department of Natural Resources surveyed and sampled 145 abandoned mine sites with actively discharging water. These data are presented in a 2017 report.