Upper Arkansas River Basin Toxics and Synoptic Studies

Our approach studied chemical processes within a hydrologic context, using a two-step approach.

• First, we employed instream experimentation to provide data about the processes affecting metals.
• Second, we used the resulting data sets to develop and apply solute transport models that help quantify rates and processes.

Tracer-injection studies in St. Kevin Gulch, near Leadville, Colorado, helped us design methods to characterize loadings from mining activities on a watershed scale. In 1995, we began to do tracer-injection studies in support of the planning needs of Federal Land Management Agencies, and as part of the U.S. Geological Survey's Abandonded Mine Land Initiative.

The objectives were to:

1. Characterize the instream chemical processes that control the transport and transformation of metals downstream from mine drainage.
2. Use tracer-injection methods to evaluate remediation efforts in selected basins, in support of district efforts to apply the watershed approach to the remediation of abandoned mine lands.
3. Quantify the time and length scales of chemical and hydrologic processes that affect the metals through development of solute-transport models.
4. Characterize the chemistry of colloids, sediment, and bed sedments that are active in controlling the dissolved concentrations of metals.

The primary study sites for the Upper Arkansas Toxics Project were located in the headwaters of the Arkansas River Basin near Leadville, Colorado at St. Kevin Gulch and the Upper Arkansas River. From 1986 to 1995, a series of studies were conducted to quantify the physical, chemical, and biological processes affecting trace metal fate and transport. Additional studies were conducted in other watersheds from 1996 to 2001 at the following locations.

Colorado

• Animas River
  • Eureka to Howardsville (1998)
  • A72 to Elk Park (1998)
• Cement Creek
  • Cement Creek (1996)
  • Ross Basin (1999)
  • Mayday Mine Dump(2000)
• Chalk Creek (1995)
• French Gulch (1996)
• Lake Fork of the Arkansas (2001)
• Mineral Creek (1999)
• Wightman Fork (Summitville CO) (1997)
• Willow Creek (Creede CO) (2000)

Montana

• Bullion Mine (1998)
• Lower Tenmile Creek (1998)
• Uncle Sam (1998)
• Soda Butte Creek (1999)

Utah

• Little Cottonwood (1998, 1999)

Many of the techniques employed by the Upper Arkansas Research Team at St. Kevin Gulch were used at the sites affected by acid mine drainage above. The streams that were studied were typically headwater systems that gain significant amounts of water as they flow down valley. The sources of additional water ranged from well-defined tributary inflows that appear on topographic maps, to diffused groundwater inflows that are not visible to the naked eye. The water quality associated with these sources of water also varied dramatically, ranging from dilute mountain springs to metal-rich waters emanating from mineralized areas. The challenge facing those interested in stream water quality is thus one of source determination: In a given watershed, what sources of water are most detrimental to the system under study? This question is of paramount importance to land managers who have limited resources with which to implement remedial actions.

The problem of source determination may be addressed by employing two well-established techniques: the tracer-dilution method and synoptic sampling. The tracer-dilution method provides estimates of stream discharge that are in turn used to quantify the amount of water entering the stream via tributary and groundwater inflow. Synoptic sampling of instream and inflow chemistry provides a spatially-detailed 'snapshot' of stream water quality. When used in unison, these techniques provide a description of the system that includes both discharge and concentration. Discharge and concentration data may be used to determine the mass loading associated with various sources of water. Sources representing the greatest contributions in terms of mass loading may then be the target of remedial actions.