CHAPTER 5 MANAGING ACID ROCK DRAINAGE AND THE CHALLENGES OF THE CLEAN WATER ACT

• Jurisdiction: United States

Idle Mining Properties
(Sep 1998)
CHAPTER 5
MANAGING ACID ROCK DRAINAGE AND THE CHALLENGES OF THE CLEAN WATER ACT
Mark J. Logsdon ¹
Geochimica, Inc.
Ojai, California

PROBLEM DEFINITION

The discussion session will evaluate a "composite" case study of an idle mine property. The "site" is a relatively small, open-pit gold mine, located in the Western U.S., in the foothills to mountainous terrain; geology is complex. [Figures 1 and 2] The area has been mined for over 100 years, and historic mine waste and workings are present at the site. A limited baseline study was performed before the new mining and indicated that some of the local surface and ground water had very high TDS and sulfate; arsenic was detected at low concentrations in a few ground-water wells. These water quality data indicate that the local waters were not universally suitable for drinking water prior to the new mining operation.

Average annual precipitation is moderate, but has very high seasonal variability, with 80% of the annual precipitation falling in a four-month period. The site is in the headwaters of a major regional river, but the local drainages are ephemeral. The mine permit area is about 20% of the local drainage basins, and the disturbed ground of the mine is only about 1/2 of that. The main-stem river water is classified for potable, recreational, agricultural and industrial uses. The local streams are not classified, but under the local rules, unclassified tributaries are assumed to have the same beneficial uses as the main stem. There is a water-supply reservoir located less than 10 miles downstream from the site. This reservoir receives seasonal surface-water flow, including flow from the ephemeral drainages of the mine, and releases water during the dry season, primarily for agricultural use. The storage capacity of the reservoir is part of a regional flood-control program, and there also is some recreational use of the reservoir. [Figures 3 — 4]

The...