Judson Mead Geologic Field Station

Watershed interactions and water quality assessment of previously mined mineralized areas. Willow Creek Demonstration Watershed, Madison County MT, 2006-2011

Download the presentation

The drainage divide of the Tobacco Root Mountains bounds the upper watershed and is underlain by granite and granite-gneiss. Tertiary age deposits composed of weathered sediments fill in the lower watershed and create a dual aquifer system in the valley. The region has a long history of mining – gold, copper, iron, and zinc associated with the Tobacco Root Batholith. Most mines in the watershed are now abandoned. The ore minerals are the result of magma and hydrothermal intrusion into the gneiss – water in the fractures became supersaturated and then precipitated the ore minerals.

Glacial activity in the upper reaches of the watershed and freeze–thaw weathering produced silica–rich sediments from the igneous and metamorphic rock. The Tertiary fill in the lower reaches is composed of the weathered sedimentary materials carried down from the upper watershed and deposited by braided stream systems NE of the Elk Creek fault in the basin. Tertiary fill gravel, sand, silt and clay beds are well to poorly–sorted and variable in thickness.

• Did previous mining have a long-term impact on the watershed?
• How does the ground water interact with the local geology?
• What flow paths does the water take to the reservoir?
• Is the drinking water safe for agriculture and human use, given current EPA guidelines?

• Soil samples collected by the G329 class (2006)
• Water samples collected by G329 classes (2006 and 2011)

Soil samples: Bulk XRF and sequential extractions (after Sposito et al. 1982) exchangeable, sorbed, organic, and carbonate fractions analyzed Water samples: Flame/Furnace atomic absorption and ion chromatography Soil sample collection: top 10cm of A horizon via trowel; dried, and returned to the lab.

Water sample collection: Temp, pH, alkalinity, specific conductance, and redox potential measured in field; collected in polyethylene bottles and returned to the lab.

Soil analysis: bulk XRF (X-Ray fluorescence) – gave breakdown of everything, regardless of what soil fraction it's in.

Sequential extractions break down the molecular bonds; each sequentially; stronger acid breaks even more bonds.