Stream-discharge

measurements are needed to

understand the effects of metal

loading from mine drainage

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.

An approach for mountain

streams

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

Use of tracer injections and synoptic

sampling to measure metal loading from

acid mine drainage U.S. Department of the Interior—U.S. Geological Survey

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.

Flow below

streambed

Riffle

Mine-drainage

inflows

Figure 2. Looking downstream at old mine workings and the capped tailings pile in Chalk

Creek, Colorado.

Capped tailings

Stream
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 landmanagement 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 tracerdilution 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.

Adding a tracer: Discharge by

dilution

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

 Qs = (Ci*Qi)/(CB-CA)

 where

 is the discharge of the stream,

 is the tracer concentration in the

 injection solution,

 is the rate of the injection into the

 stream,

 is the tracer concentration downstream

 from the injection point, and

 is the tracer concentration upstream

 from the injection point.

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

Qs

Ci

Qi

CB

CA

Golf Tunnel

Waste rock

Chalk Creek

Mary Murphy Tailings

 (consolidated and capped)

Flow from

Golf Tunnel

Injection of sodium chloride

upstream from

mine drainage,

0 meters distance

downstream

Area of discharge

from fracture,

about 252 meters

distance downstream

Discharge from

former tailings

pile, 350 to 615 meters

distance downstream

Figure 3. Chalk Creek study section.

Area of greatest

mass loading to

Chalk Creek,

between 80 and

300 meters distance

downstream

Denver

Chalk Creek area

COLORADO

Constructed

wetland

0 400 800 1,200 1,600 FEET

0 100 200 300 400 METERS

Leadville

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.

Segment C

Segment B

Segment A

Measured chloride concentration

 28 meters downstream

 28 meters smoothed

 615 meters downstream

 615 meters smoothed

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

0

1

2

3

4

5

October 12, 1995

0700 1300 1900

October 13, 1995

0100 0700 1300

Difference in concentration (CB-CA)

Travel time between sites
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.

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.

Synoptic sampling: A “snapshot” in

time to compare metal loading of

sources

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.

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.

0 100 200 300 400 500 600 700

8

10

12

14

16

Figure 5. Comparison of discharge calculated by flow-meter and tracer-dilution

methods.

Discharge measured by flow meter

Discharge measured by tracer dilution

DISCHARGE, IN CUBIC FEET PER SECOND

DISTANCE DOWNSTREAM, IN METERS

-100 0 100 200 300 400 500 600 700

-100 0 100 200 300 400 500 600 700

0.05

0.10

0.15

0.20

0.25

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. ZINC LOAD, IN MILLIGRAMS PER SECOND

DISTANCE DOWNSTREAM, IN METERS

0.01

0.1

1

10

100

Golf Tunnel

 Stream

 Inflows affected by Golf Tunnel

 Inflows from fracture

 Inflows from area of old tailings

ZINC CONCENTRATION, IN MILLIGRAMS PER LITER

a

b

6 percent of total load

from upstream watershed

Old tailings, 14 percent

Fracture, 8 percent

Greatest

mass loading

0
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 groundwater 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.

Conclusions

Measuring stream discharge using tracerdilution 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

Sources of Additional

Information

 The following publications contain additional

information on tracer-dilution methods and

other applications related to mine-drainage

problems.

Bencala, K.E., McKnight, D.M., and

 Zellweger, G.W., 1990, Characterization of

 transport in an acidic and metal-rich mountain

 stream based on a lithium tracer injection

 and simulations of transient storage: Water

 Resources Research, v. 26, no. 5,

 p. 989-1000.

Broshears, R.E., Bencala, K.E.,

 Kimball, B.A., and McKnight, D.M., 1993,

 Tracer-dilution experiments and solute-

 transport simulations for a mountain stream,

 Saint Kevin Gulch, Colorado: U.S.

 Geological Survey Water-Resources

 Investigations Report 92-4081, U.S.

 Geological Survey, Denver, Colorado, 18 p.

Stream Solute Workshop, 1990, Concepts

 and methods for assessing solute dynamics

 in stream ecosystems: Journal of North

 American Benthological Society,

 v. 9, p. 95-119.

Kimball, B.A., Broshears, R.E., Bencala, K.E.,

 and McKnight, D.M., 1994, Coupling of

 hydrologic transport and chemical reactions

 in a stream affected by acid mine drainage:

 Environmental Science & Technology,

 v. 28, p. 2065-2073.

Information on technical reports and data

related to mine-drainage studies can be

obtained from:

 Briant Kimball

 U.S. Geological Survey

 Administration Building, Room 1016

 1745 West 1700 South

 Salt Lake City, UT 84104

 email: bkimball@usgs.gov

Information on the Toxic Substances

Hydrology Program can be obtained by

accessing the home page on the

World Wide Web at:

 http://toxics.usgs.gov/toxics/

Information on mine-drainage issues can

be obtained by accessing the home page

on the World Wide Web at:

 http://water.wr.usgs.gov/mine

January 1997 FS–245–96