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Thousands of abandoned and inactive mines are located in environmentally
sensitive mountain watersheds. Cost-effective remediation of the effects
of metals from mining in these watersheds requires knowledge of the most
significant sources of metals. The significance of a given source depends
on the toxicity of a particular metal, how much of the metal enters the
stream, and whether or not the metal remains in the stream in a toxic form.
This discussion deals with accounting for how much metal enters the stream
and whether it stays in the stream. The amount of metal entering the stream
is called the mass loading and is calculated as the product of metal concentration
and stream discharge. The overall effect of high metal concentrations on
streams and aquatic organisms is unclear without discharge measurements.
A traditional discharge measurement is obtained by dividing a stream
into small sections and measuring the cross-sectional area and the average
water velocity in each section. Summing the measurements of all the sections
gives the discharge of the entire stream. This method works well where the
channel bottom and banks are smooth. In mountain streams, however, the stream
bottom typically is covered with cobbles, allowing much of the water to
flow through the cobbles of the streambed where it cannot be measured by
a flow meter (fig. 1). Thus, accurate discharge measurements are difficult
to obtain in mountain streams, even under the best of conditions.
Figure 1. Block diagram illustrating the flow of mountain streams through cobbles below the streambed. Flow through the cobbles cannot be measured by a flow meter.
A recent study by the U.S. Geological Survey (USGS) as part of the Toxic
Substances Hydrology Program illustrates a practical approach to obtaining
and using discharge measurements in mountain streams. The study area was
in Chalk Creek, a tributary of the Arkansas River in Colorado that receives
mine drainage from the Golf Tunnel adit (figs. 2 and 3). Metal-rich mine
drainage from the Golf Tunnel is routed around waste rock and a capped tailings
pile into a constructed wetland. From the wetland, the mine drainage enters
Chalk Creek from small springs and seeps along the stream. Regulatory and
land-management agencies have asked two basic questions about Chalk Creek.
First, is there more than one source of mine drainage that affects the stream
and, if so, does a remediation plan need to account for drainage from more
than one source? Second, have past remediation efforts been successful?
To address these questions, we employed a tracer-dilution method and synoptic
sampling. Synoptic sampling is the collection of samples from many locations
during a short period of time, typically a few hours. Thus, it is like a
"snapshot" of the changes along a stream at a given point in time.
Figure 2. Looking downstream at old mine workings and the capped tailings pile in Chalk Creek, Colorado.
Figure 3. Chalk Creek study section.
Discharge in mountain streams can be measured precisely by adding a
dye or salt tracer to a stream, measuring the dilution of the tracer as
it moves downstream, and calculating discharge from the amount of dilution.
Because we know the concentration of the injected tracer and the rate at
which it is added to the stream, we know the mass that is added to the stream.
By measuring the concentration of the tracer upstream and downstream from
the injection point, we can calculate the discharge in the stream downstream
from the injection point. Mathematically, this is written
The tracer also helps define hydrologic properties such as the velocity
of the streamflow, travel time in the stream, the mixing of solutes, the
quantity of inflow from tributaries and ground water, and the transient
storage in streambed cobbles. To define these properties in Chalk Creek,
a sodium chloride tracer was added at a constant rate for 24 hours at a
point upstream from the mine drainage and chloride concentration was monitored
at several sites downstream from the injection point. There are three main
segments on a plot of time versus discharge for two sites (fig. 4). First,
there is the arrival of the tracer at the two sites (fig. 4, segment A).
The difference between these times of arrival indicates the "time of
travel" between sites. Second, there is a "plateau" segment
(fig. 4, segment B). The difference between concentrations at the two sites
during the plateau is an indication of the difference in discharge at the
two sites. The tracer is diluted by the additional water that enters the
stream. Finally, there is the decrease of the tracer concentration at two
sites after we stopped adding the tracer (fig. 4, segment C). This segment
gives the same information as the arrival of the tracer.
Figure 4. Chloride concentration versus time for two sites in Chalk Creek, Colorado. The difference in discharge is represented by the lower concentration at the downstream site. The travel time between the sites is represented by the difference in time between the arrival and the departure of the tracer at both sites.
Tracer-dilution and flow-meter discharge measurements at various distances
from the tracer-injection point are compared in figure 5. Discharge calculated
from the tracer dilution is greater because it includes water flowing through
the cobbles. The "smoother" pattern in the tracer-dilution discharge
helps in determining where ground-water inflow is occurring and in the calculation
of more accurate mass loading of metals.
Figure 5. Comparison of discharge calculated by flow-meter and tracer-dilution methods.
Site-to-site differences in the tracer concentration during the plateau
segment are a result of dilution. These differences are used to calculate
stream discharge. Synoptic samples are collected during this period, giving
a "snapshot" of the stream profile that includes both discharge
and metal concentrations and providing a detailed profile of zinc concentrations
in both the stream and the inflows along the stream reach (fig. 6a). To
interpret these concentrations, the concentrations and the discharge were
multiplied to develop a mass-loading profile (fig. 6b). These profiles help
answer the basic questions about the effectiveness of remediation and the
sources of metals.
Figure 6. (a) Zinc concentration and (b) zinc mass loading with distance for Chalk Creek, Colorado. The zinc concentration in the Golf Tunnel discharge is shown for reference to the inflow concentrations along the stream.
About 6 percent of the total zinc load enters the stream reach from
upstream sources. There is a large range in zinc concentration among the
inflows, from about 0.1 to 82 milligrams per liter (mg/L). The increase
in zinc concentration of the stream from about 0.04 to 0.4 mg/L corresponds
to the highest inflow concentrations. Inflows between 80 and 300 meters
(m) downstream have about the same zinc concentration as water from the
Golf Tunnel, about 10 mg/L. These high concentrations indicate that zinc
is not removed as the water flows from the Golf Tunnel through the constructed
wetland area to the stream. Altogether, the seeps and springs with Golf
Tunnel water represent about 72 percent of the total zinc load at 615 meters,
the end of the study reach.
A few of the inflows near 250 m downstream have higher zinc concentrations
than those from the Golf Tunnel (fig. 6a). These inflows are located where
a fracture, which has been identified in several ground-water wells, crosses
the stream (fig. 3). The higher zinc concentrations represent a second source
of metal-rich water. Water from the fracture contributes only about 8 percent
of the total zinc load (fig. 6b); thus, the high concentrations do not always
indicate the most substantial sources to the stream. Downstream from 350
m, inflow concentrations of zinc generally are about 2 mg/L. These inflows
are in the section of the stream where tailings piles have been removed.
These concentrations represent loading of zinc, most likely from a ground-water
plume that originated from the old tailings. If this source is from a plume,
the contribution is expected to diminish over time, but currently the area
of the old tailings contributes about 14 percent of the zinc load to the
stream.
Measuring stream discharge using tracer-dilution methods allows the
calculation of mass loading for Chalk Creek and provides answers for the
two basic questions. First, there appears to be more than one source of
mine drainage, and each source contributes water with different zinc concentrations.
Water from two sources, the Golf Tunnel and a fracture that crosses the
stream, contributes zinc, but most of the load comes from the Golf Tunnel.
Second, there are still effects of metals on the stream in the area where
old tailings were removed; those effects likely will decrease with time.
The data obtained from many samples collected along just 600 m of stream
provides the information necessary to calculate a loading curve and guide
future cleanup efforts. The detailed mass-loading curve indicates the sections
of the stream where metals enter the stream. The example of Chalk Creek
shows that the highest inflow concentrations do not always indicate the
most significant sources of metal loading. This would not be apparent, however,
without using tracer-dilution discharge measurements to help focus on those
sections of the stream with the greatest loading. On the basis of this and
other work, the regulatory agencies are now directing remediation efforts
toward both sources of mine drainage.
- Briant A. Kimball
The following publications contain additional information on tracer-dilution methods and other applications related to mine-drainage problems.
Briant Kimball
U.S. Geological Survey
Administration Building, Room 1016
1745 West 1700 South
Salt Lake City, UT 84104
http://toxics.usgs.gov/toxics/
Information on mine-drainage issues can be obtained by accessing the home page on the World Wide Web at:
Information about the U.S. Geological Survey Abandoned Mine Lands Initiative
can be obtained by accessing the
home page on the World Wide Web at:
U.S. Department of the Interior
U.S. Geological Survey
2329 Orton Circle, West Valley City, Ut, 84119
Maintainer: GS-W-UTslc_Webmaster@usgs.gov
Last Modified:
Thursday, 24-May-2001 10:21:21 EDT
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