ELEVATED LEAD IN THE BIGHORN RIVER MAY BE NATURALLY DISTRIBUTED THROUGH REACTIVATED PRECAMBRIAN FAULT SYSTEMS

The Bighorn River in Montana was a declared 303d impaired waterway once it passes through the Crow Reservation due to elevated Pb and Hg. The river flows northeast through the reservation and bisects two major structural lineaments, the Lake Basin fault zone on the north and the Nye-Bowler Lineament on the south. These two parallel structural domains are characterized by a core of northwest-southeast trending faults and domes that are overprinted by a set of smaller en echelon north-northeast striking normal faults across south central Montana and are interpreted as surface expressions of left-lateral movement along basement wrench faults. The Pryor Mountains are situated between these two structural lineaments as well and consist of five uplifted Precambrian cored blocks that lie west of and proximal to the Bighorn River. The Pryor Mountain uplift is divided into northern and southern segments by the Sage Creek fault zone. The Sage Creek fault zone is the eastern extension of the Nye-Bowler Lineament.

A GIS (geographic information study) on available United States Geological Survey water data shows elevated lead values concentrated along these lineaments. Lead values range from several hundred to over 23,000 ppb in the data studied. Lead may be being mobilized and distributed through Precambrian faults reactivated during the Laramide orogeny. These faults may be serving as conduits and provide the source of natural contamination of lead for the Bighorn River.