Open-File Report 1998–0297
Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998
Runoff and drainage from mines, mine dumps, tailings, and springs contain elevated dissolved-zinc concentrations in the upper Animas River watershed. Discharge in cubic feet per second (ft3/s) and concentration in micrograms per liter (µg/L) from these sources vary seasonally. As a result, dissolved-zinc loads in pounds per day (lb/d) in the mainstem streams of the upper Animas River watershed vary seasonally. Better documentation of this seasonal variation is the first step in determining the correlation between source inputs and mainstem loads. The correlation is established by first observing fluctuations in mainstem loading and then relating these mainstem fluctuations to fluctuations measured in source loads. Sampling of sources can be tailored to coincide or precede periods of increased loading after the first year of data collection. This correlation can aid remediation prioritization in that remediation efforts can be focused on sources that have the highest loading impact to the system. Until recently, sampling in the basin was limited to spring runoff, summer, and early autumn. As a result, seasonal variations were not adequately defined. Although some variations in flow and loading were observed, a better characterization of low flow in winter and early spring, of spring runoff, and during periods of rainfall runoff is needed to improve the definition of dissolved-zinc concentrations and loads throughout the annual hydrograph. Sampling began at a network of four gages in November 1996 and is expected to continue through 1999. Two gages are located on the major tributaries of the Animas River (Cement Creek and Mineral Creek), and two are located on the Animas River—one upstream from the confluence of Cement Creek and one downstream from the confluence of Mineral Creek. During low flow in winter, one sample per month was taken. During spring runoff, samples were taken biweekly as the peak flow approached and then weekly during the periods of highest discharge. As the spring runoff receded, sampling decreased to biweekly and then monthly during low flow. Grab samples of several rainfall-runoff events from Cement and Mineral Creeks were obtained on the rising and falling limbs of the hydrograph. During water year 1997, dissolved-zinc loads (and corresponding concentrations) at the four gages ranged from 45.4 lb/d (679 µg/L) to 2,140 lb/d (197 µg/L), and streamflow ranged from 12.4 to 2,010 ft3/s. During low flow in winter (November-March), loads ranged from 42.9 lb/d (938 µg/L) to 562 lb/d (868 µg/L). During spring runoff (April-July), loads ranged from 92.1 lb/d (71 µg/L) to 2,140 lb/d (197 µg/L). And, in autumn (August-October), loads ranged from 84.4 lb/d (174 µg/L) to 626 lb/d (248 µg/L). Rainfall runoff sampled at the Mineral Creek gage in August indicated dissolved-zinc loads of 84.4 lb/d (174 µg/L) for the rising limb of the hydrograph (89.9 ft3/s) and 90.2 lb/d (177 µg/L) for the falling limb of the hydrograph (94.4 ft3/s). Maximum dissolved-zinc loads occurred between the first and second weeks of June at all four gages. Maximum concentration values occurred between February and March at all the gages except Cement Creek, where the concentration peak occurred in the second week of May. The reason for this late peak in concentration may be due to a flushing effect from spring snowmelt.
1U.S. Geological Survey, P.O. Box 3367, Durango, CO 81302 (kjleib@usgs.gov)
2U.S. Geological Survey, MS 415, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (mamast@usgs.gov)
3U.S. Geological Survey, P.O. Box 3367, Durango, CO 81302 (wgwright@usgs.gov)
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