Case Study: NARD

2022-07-21 | CGS Admin

Are pristine mountain waters always clean and pure? Can streams unaffected by human activities and livestock influences be unfit for human consumption, or for aquatic life? The existence of natural acid rock drainage (NARD) suggests a “no” to the former, and a “yes” to the latter question.

But what exactly is NARD?  Natural groundwater movement can eventually cause significant surface water contamination, depending on the local geology. As meteoric water infiltrates the ground—becoming groundwater—it slowly percolates through soils and rock where it can pick up substantial contaminants that are naturally present.

An award-winning 2011 study by the CGS—examining specific areas in Colorado that have naturally poor surface-water quality due to local geology—contains some surprising answers. B-54 Natural Acid Rock Drainage Associated with Hydrothermally Altered Terrane in Colorado reveals the geology behind poor water quality in a number of locations across the state. It identifies and examines streams in eleven different headwater areas of Colorado where surface water is acidic and has high concentrations of metals upstream of any significant human impacts.

Rocks in these areas were altered by intensely hot water circulating in the Earth’s crust, often associated with volcanic activity during the geologic past. The hydrothermal alteration of the rocks changed their composition by dissolving some minerals and depositing others. In the affected areas, this alteration process deposited metal-sulfide minerals, commonly pyrite (fool’s gold), in the rocks.

When these rocks are exposed at the surface of the earth, they interact with oxygen in the atmosphere and the iron sulfide “rusts” to form iron oxide minerals, creating striking yellow, orange, and red colors—similar to the oxidation of metal on an old rusty car. Acid rock drainage occurs when the sulfur that is displaced by the oxygen combines with water to form weak sulfuric acid. The acidic water then dissolves minerals from the bedrock, often adding significant amounts of dissolved metals to these headwater streams. These acidic metal-enriched waters may cause severe degradation of water resources and impact existing ecosystems. Natural acid rock drainage has been active in Colorado for thousands, possibly millions of years.

The CGS collected 101 water samples from headwater areas and identified specific streams as being affected by NARD. Examined in the report are the Silverton, Lake City, Platoro-Summitville, Kite Lake areas and East Trout Creek in the San Juan Mountains, the La Plata and Rico mountain areas, the headwaters of Lake Creek south of Independence Pass and of the Snake River in eastern Summit County, the Ruby Range near Crested Butte, the Red Amphitheater near Alma, and the Rabbit Ears Range.

Through detailed geologic mapping, the study characterizes the type and intensity of hydrothermal alteration and correlates the geology with surface-water chemistry. Many of the areas exhibiting intense hydrothermal alteration also contain historic mine sites. Frequently, acid rock drainage from natural sources and mine sites combine to cause severe downstream surface water quality problems. In these situations, it is important to distinguish a natural baseline, or background, water quality so that realistic clean-up goals for overall downstream water quality can be set.

Funding for this study came from the CGS portion of the Department of Natural Resources Severance Tax Operational Account. Colorado Severance Taxes (STAX) are derived from the production of gas, oil, coal, and metallic minerals.

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