Iron Mountain


Pipe Scale Studies At Iron Mountain Mines

As part of the remediation at Iron Mountain Mine being overseen by the U.S. Environmental Protection Agency, a water treatment plant has been in operation since 1994 to neutralize the acid mine drainage (AMD) coming from the old mines. AMD is a major environmental concern because it can degrade water quality with elevated concentrations of acidity, sulfate, iron, and other associated trace metals and metalloids including copper, zinc, cadmium, and arsenic. To manage AMD at Iron Mountain, contaminated water is transported from the portals of the old mines to the treatment plant through two underground pipelines. The first pipeline transports water pumped from "Old Mine #8" (also known as pump station PW3) which has pH of about 2.5, while the second pipeline drains water from the Richmond and Hornet mines, which has a pH less than 1.5. The PW3 pipeline has developed substantial scaling over several kilometers, resulting in occasional clogging and spilling of AMD. The pipe scaling problem requires costly pipe clean-out, which has been done approximately every two to four years.

Map of Iron Mountain pipelines with scale forming due to the transport of acid mine drainage

(Click map to enlarge.) Aerial photograph showing pipelines carrying acid mine drainage to the water treatment plant at Iron Mountain. Iron-rich scale is forming in the pipeline (shown in red), which carries water from station PW3, where water is pumped from collapsed underground workings in Old mine and Number 8 mine, and water from the Slickrock Creek Retention Reservoir. The pH of the water in the PW3/SCRR pipeline (red) is approximately 2.5, whereas the pH in the pipeline that carries AMD from the Richmond and Hornet mines (green) is approximately 1.0, too low to form scale.

What is Pipe Scale?

Pipe scale is simply a mineral precipitate that forms within a pipe as water flows through it. A common example is the calcium-rich precipitate that forms when hard water is heated in a household water heater. In the PW3 pipeline at Iron Mountain, as AMD is transported away from its source, microbial iron oxidation causes formation of iron oxide minerals which accumulate on the inside of the pipe. The encrusted pipes cause costly management problems when excessive buildup interferes with treatment efforts by clogging pipelines or other treatment structures.

photo series of iron-rich pipe scale on site at Iron Mountain

The formation of iron-rich pipe scale at Iron Mountain is caused by oxidation in the pipeline of ferrous iron (Fe(II)), a highly soluble form, to ferric iron (Fe(III)), a less soluble form. The iron-oxidation reaction is mediated by microbes. The ferrous iron in filtered acid mine water, from which nearly all microbes have been removed, oxidizes at a much slower rate than in unfiltered acid mine water.

The Study

In cooperation with the U.S. Environmental Protection Agency, the USGS began investigating the biogeochemistry and mineralogy of the pipe scale in the summer of 2012. The pipe scale investigation has five objectives:

  1. Determine the mineralogy and chemistry of the pipe scale;
  2. Determine seasonal variability of water in pipelines;
  3. Determine rates of oxidation (biotic and abiotic) of Fe(II) in water from Slickrock Creek pipeline using laboratory experiment and detailed field observations;
  4. Construct a geochemical model of pipe scale formation; and
  5. Determine proportion of Richmond Mine water (pH ~1) that would need to be added to Slickrock Creek pipeline water (pH ~2.5) to prevent scale formation

Sampling of water and pipe scale occurred in August and December of 2012, and in April of 2013. A fourth sampling event is planned for October, 2013.