U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 00–4160

Diurnal Variations in Metal Concentrations

in the Alamosa River and Wightman Fork,

Southwestern Colorado, 1995–97

By Roderick F. Ortiz and Robert W. Stogner, Sr.

Denver, Colorado

2001

Prepared in cooperation with the

U.S. ENVIRONMENTAL PROTECTION AGENCY
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

from:

District Chief

U.S. Geological Survey

Box 25046, Mail Stop 415

Denver Federal Center

Denver, CO 80225-0046
CONTENTS III

CONTENTS

Abstract.................................................................................................................................................................................. 1

Introduction............................................................................................................................................................................ 1

Purpose and Scope....................................................................................................................................................... 2

Description of Study Area ........................................................................................................................................... 2

Methods of Investigation ....................................................................................................................................................... 2

Data-Collection Methods............................................................................................................................................. 2

Data-Analysis Methods ............................................................................................................................................... 4

Diurnal Variations in Metal Concentrations .......................................................................................................................... 6

Aluminum .................................................................................................................................................................... 7

Copper.......................................................................................................................................................................... 7

Iron............................................................................................................................................................................... 7

Zinc .............................................................................................................................................................................. 9

Temporal Variations in Maximum and Minimum Concentrations .............................................................................. 9

Frequency of Exceeding Daily-Mean Concentration Threshold ................................................................................. 10

Implications for Sampling Strategies..................................................................................................................................... 10

Summary................................................................................................................................................................................ 13

References.............................................................................................................................................................................. 13

FIGURES

1. Map showing location of study area and water-quality sampling sites in the Alamosa River Basin ......................... 3

2. Hypothetical hydrograph showing sample points and graphical depiction of discrete and composite

sample sets .................................................................................................................................................................. 5

3. Example diagram of boxplot....................................................................................................................................... 7

4–7. Boxplots showing:

4. Variations in dissolved and total-recoverable aluminum concentrations in relation to dailymean concentration ............................................................................................................................................ 8

5. Variations in dissolved and total-recoverable copper concentrations in relation to daily-mean

concentration ...................................................................................................................................................... 8

6. Variations in dissolved and total-recoverable iron concentrations in relation to daily-mean

concentration ...................................................................................................................................................... 9

7. Variations in dissolved and total-recoverable zinc concentrations in relation to daily-mean

concentration ...................................................................................................................................................... 10

8. Graph showing percentage of days that daily-maximum and daily-minimum concentration occurred

during a given 6-hour time period............................................................................................................................... 11

9. Graph showing percentage of discrete sample concentrations that exceeded the daily-mean

concentration by a given threshold ............................................................................................................................. 12

TABLE

1. Percent change in metal concentrations over a 24-hour period at six sites in the Alamosa

River Basin, 1995–97 .............................................................................................................................................. 6
IV CONTENTS

CONVERSION FACTORS AND ACRONYMS

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic datum derived

from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

Acronyms used in this report:

CDPHE Colorado Department of Public Health and Environment

COC Constituents of Concern

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

EWI Equal-width increment

Multiply By To obtain

Length

inch 2.54 millimeter

foot 0.3048 meter

mile 1.609 kilometer

square mile 2.590 square kilometer

cubic foot per second 0.02832 cubic meter per second

gallon 3.785 liter
Abstract 1

Abstract

A comprehensive sampling network

was implemented in the Alamosa River Basin

from 1995 to 1997 to address data gaps identified

as part of the ecological risk assessment of

the Summitville Superfund site. Aluminum,

copper, iron, and zinc were identified as the

constituents of concern for the risk assessment.

Water-quality samples were collected at six sites

on the Alamosa River and Wightman Fork by

automatic samplers. Several discrete (instantaneous) samples were collected over 24 hours at

each site during periods of high diurnal variations

in streamflow (May through September). The

discrete samples were analyzed individually and

duplicate samples were composited to produce a

single sample that represented the daily-mean

concentration. The diurnal variations in concentration with respect to the theoretical daily-mean

concentration (maximum minus minimum

divided by daily mean) are presented.

Diurnal metal concentrations were highly

variable in the Alamosa River and Wightman

Fork. The concentration of a metal at a single site

could change by several hundred percent during

one diurnal cycle. The largest percent change in

metal concentrations was observed for aluminum

and iron. Zinc concentrations varied the least of

the four metals. No discernible or predictable

pattern was indicated in the timing of the daily

mean, maximum, or minimum concentrations.

The percentage of discrete sample concentrations

that varied from the daily-mean concentration

by thresholds of plus or minus 10, 25, and

50 percent was evaluated. Between 50 and

75 percent of discrete-sample concentrations

varied from the daily-mean concentration by more

than plus or minus 10 percent. The percentage of

samples exceeding given thresholds generally was

smaller during the summer period than the snowmelt period.

Sampling strategies are critical to accurately define variability in constituent concentration, and conversely, understanding constituent

variability is important in determining appropriate

sampling strategies. During nonsteady-state

periods, considerable errors in estimates of dailymean concentration are possible if based on one

discrete sample. Flow-weighting multiple discrete

samples collected over a diurnal cycle provides a

better estimate of daily-mean concentrations

during nonsteady-state periods.

INTRODUCTION

The upper Alamosa River Basin is a heavily

mineralized area in the San Juan Mountains of southwestern Colorado (fig. 1). Metal contamination of

streams in the basin from the Summitville Mine site,

other smaller mines, and from natural metal-enriched

acidic drainage in the basin has occurred for decades.

Mining operations have been active intermittently in

the Summitville area since the late 1800’s. Largescale open-pit mining began at the Summitville Mine

site in the mid-1980’s and continued until the mine

site was abandoned in late 1992. As a result, the

U.S. Environmental Protection Agency (USEPA)

Diurnal Variations in Metal Concentrations in the

Alamosa River and Wightman Fork, Southwestern

Colorado, 1995–97

By Roderick F. Ortiz and Robert W. Stogner, Sr.
2 Diurnal Variations in Metal Concentrations in the Alamosa River and Wightman Fork, Southwestern Colorado, 1995–97

assumed site-maintenance responsibilities for the site

under the emergency response provisions of Superfund. In 1998, the Colorado Department of Public

Health and Environment (CDPHE) assumed shared

responsibility of the Summitville site with USEPA.

In 1995, the Morrison-Knudsen Corporation

and ICF Kaiser Engineers (1995) identified multiple

data gaps needed for the ecological risk assessment

of the Summitville Superfund site. As a result, the

U.S. Geological Survey (USGS) developed a comprehensive data-collection plan for the basin to address

these data gaps. The sampling analysis plan included

the operation of several streamflow gages, waterquality monitors, and automatic samplers on the

Alamosa River and Wightman Fork (Edelmann and

Ortiz, U.S. Geological Survey, written commun., 1995

and 1997). Data collected from 1995 through 1997

were used to evaluate the diurnal variations in the

water quality of the Alamosa River and Wightman

Fork. The data also will be used in the current draft

Tier II Summitville ecological risk assessment (Camp

Dresser and McKee, Inc., 1999) to help address exposure of aquatic biota to metals in stream water.

Purpose and Scope

The purpose of this report is to characterize

the diurnal variations in metal concentrations during

periods of nonsteady-state streamflow conditions

at selected water-quality sites on the Alamosa River

and Wightman Fork. The report addresses the high

variability in metal concentrations in the basin and

quantifies the extent to which instantaneous sample

concentrations vary in comparison to the daily-mean

concentration. Additionally, the report addresses

the unpredictability of sample concentrations

throughout the diurnal discharge cycle and the implications to sampling strategies in the Alamosa River

Basin.

Description of Study Area

The upper Alamosa River Basin is located in

southwest Colorado (fig. 1). Elevations in the study

area range from 8,400 feet to nearly 13,000 feet above

sea level. Annual precipitation ranges from approximately 12 inches at the lower elevations to as much as

40 inches at the top of the highest peaks (Miller and

McHugh, 1994). Most of the precipitation is in the

form of snowfall.

The study area extends from the headwaters of

the Alamosa River to just above Terrace Reservoir and

has a drainage area of approximately 110 square miles

(Stogner, 1996). Several areas in the basin are hydrothermally altered and contain sulfide minerals and

precious metals. Runoff from mined areas and

unmined areas that are hydrothermally altered can

adversely affect the quality of the water in the basin.

Low-pH water with high concentrations of trace

metals from the Summitville Mine site affects

Wightman Fork and the Alamosa River downstream

from its confluence with Wightman Fork (fig. 1).

Upstream from the confluence, the Alamosa River

receives drainage from hydrothermally altered areas.

The Alamosa River flows east from the confluence

with Wightman Fork through the Alamosa Canyon for

14 miles before reaching Terrace Reservoir. Several

small tributaries enter the Alamosa River along this

reach.

METHODS OF INVESTIGATION

Six sites were selected for streamflow gaging

and water-quality sampling in the Alamosa River

Basin (fig. 1). Four sites were located on the Alamosa

River: Alamosa River above Terrace Reservoir

(AR34.5), Alamosa River below Castleman Gulch

(AR41.2), Alamosa River above Jasper (AR43.6),

and Alamosa River above Wightman Fork (AR45.5).

The remaining two sites were located on Wightman

Fork: Wightman Fork at the mouth (WF0.0) and

Wightman Fork below Cropsy Creek (WF5.5).

Site WF5.5 is located immediately downstream from

the Summitville Mine site. Each of the six sites was

equipped with instantaneous streamflow-gaging

equipment and automatic pumping samplers. With the

exception of AR43.6, all the sites were equipped with

instantaneous water-quality monitors.

Data-Collection Methods

Streamflow-gaging procedures were conducted

in accordance with USGS methods (Rantz and others,

1982). Generally, streamflow-gaging equipment

was installed before snowmelt conditions began
METHODS OF INVESTIGATION 3

AR34

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

EXPLANATIO

Hydrology modified by Denver Water, 1998From U.S. Geological Survey Digital Line Graphs, 1:100,000, 1982Universal Transverse Mercator projection, Zone 13, Feet

SummitvilleAltered Area

AlteredArea

Stunner Altered Area

WF0.0

AR45.5

Creek Creek

WF5.5

AR43.6

Iron Bitter

Alum Creek

Summitville Mine site

Water-quality sampling site 

WF0.0

Approximate extent of hydr

Jasper CO

AR41.2

Alamosa River

Jasper

Burnt Creek

STUDY AREA

106 22' 30" o 106 37' 30" o

37 22'30" o

Lieutenant C

r

S

i

vl er

Creek Fern Cre ek

Castleman Gulch

Jasper

C

r

e

e

k

Spring Creek

Cropsy Cr

Wightman Fork

Ranger Cree

k

Figure 1. Location of study area and water-quality sampling sites in the Alamosa River Basin.
4 Diurnal Variations in Metal Concentrations in the Alamosa River and Wightman Fork, Southwestern Colorado, 1995–97

(mid-April) and was operated through the summer

period (mid-September to early October). Instantaneous streamflow data were recorded every

15 minutes.

Automatic pumping samplers, hereinafter

referred to as “samplers,” were installed at all six sites.

The automatic samplers were used from mid-April to

September when large variations in diurnal flow and,

potentially, metal concentrations would be expected. A

diurnal cycle began at the low-flow trough of the day

and ended on the trough of the following day. Generally, the sampling events occurred once in May, twice

in June, and once each in July and August. A sampler

may have been used in September if nonsteady-state

streamflow conditions existed at that time. The

samplers were operated from 1995 through 1997.

Samplers were programmed to collect multiple

water-quality samples at each of the sampling

sites at predetermined intervals over a 24-hour period.

Sampling times were selected to cover the observed

diurnal streamflow cycle at each site (fig. 2). Typically,

sample sets were collected every 4 hours, which

resulted in six sample sets for the day. Each sample set

included four instantaneous samples collected in separate 1-liter bottles. Water from two of the instantaneous samples was processed for analysis of totalrecoverable and dissolved constituents, one sample

was composited using flow-weighted techniques

(U.S. Environmental Protection Agency, 1991),

and one sample was used for rinsing equipment

prior to processing samples (fig. 2). At the end of the

24-hour sample-collection period, the samples were

retrieved and transported to a nearby staging area for

processing. Discrete samples were processed and

submitted for analysis of selected anions and trace

metals. The results of chemical analyses on these

discrete samples were used to evaluate the variation

in concentration at each site over the 24-hour period.

Samples designated for compositing were flowweighted to produce a single composite sample that

represented the daily-mean concentration at a particular site (U.S. Environmental Protection Agency,

1991).

The USGS recommends that samples be

retrieved from automatic samplers at the earliest

possible time to reduce the chance of chemical and

biological alteration of the sample (Shelton, 1994).

For the majority of the discrete samples collected from

the samplers, the holding times prior to processing was

less than 24 hours. However, it was assumed that the

longest a discrete sample set might remain unprocessed was about 36 hours. As such, quality-control

measures were taken to address the concern that

sample degradation could occur during these extended

holding times (Edelmann and Ortiz, U.S. Geological

Survey, written commun., 1995 and 1997). During

each sampling trip, two of the sites were selected for

additional sample collection using conventional USGS

equal-width increment (EWI) techniques (Edwards

and Glysson, 1988); the sites were rotated throughout

the study period to represent all six sites. The EWI

samples were processed immediately according to

USGS techniques for major anions, total-recoverable

metals, and dissolved metals (Ward and Harr, 1990).

The remaining sample water was set aside for approximately 36 hours before being processed in the same

manner for the same analytes. Overall, the percent

difference in metal concentrations between the

samples processed immediately and the samples

held for 36 hours indicated that there was good

replication of data. Nearly 90 percent of the totalrecoverable metals data from the sample proceeded

immediately varied by less than 12 percent from the

sample held for 36 hours. Likewise, 80 percent of the

dissolved metal data, excluding iron, varied by less

than 15 percent.

Data-Analysis Methods

For the purpose of this report, the constituents of

concern (COC) were dissolved and total-recoverable

aluminum, copper, iron, and zinc. The four metals

were identified as COCs in the ecological risk

characterization prepared by Morrison-Knudsen

Corporation and ICF Kaiser Engineers (1995) and the

draft Summitville ecological risk assessment prepared

by Camp Dresser and McKee, Inc. (1999). Discrete

and flow-weighted composite data were used for the

analysis of diurnal variations in metal concentrations.

Water-quality data were tabulated and plotted to evaluate variations in water quality associated with diurnal

variations in streamflow.

The raw data used in the analyses were not

included in the text of this report; however, the data

are available from the Colorado Department of Public

Health and Environment, Hazardous Materials and

Waste Management Division, 4300 Cherry Creek

Drive South, Denver, CO 80222–1530. The data
METHODS OF INVESTIGATION 5

Χ

Χ

Χ

Χ

Χ

Χ

Sample #1 Sample #2 Sample #3 Sample #4 Sample #5 Sample #6

Diurnal (24-hour) cycle and sample sets

Stream hydrograph

Sample point

Sample set

A - Total recoverable (discrete)

A

B C

D

B - Dissolved (discrete)

C - Composite

D - Equipment rinse

bottle designations

Composite

sample

1C

2C

3C

4C

5C

6C

Increasing streamflow

Time

Figure 2. Hypothetical hydrograph showing sample points and graphical depiction of discrete and composite sample

sets.
6 Diurnal Variations in Metal Concentrations in the Alamosa River and Wightman Fork, Southwestern Colorado, 1995–97

include all dissolved and total-recoverable concentrations for discrete and composite samples collected

from 1995 through 1997. Discrete- and compositesample groups are found in the data base under the

general heading of “sample type.”

Percentage of variation in minimum and

maximum discrete concentrations relative to

the theoretical daily-mean concentration (the flowweighted composite concentration) was computed

as follows:

where

Dvar is percent diurnal variation,

Cmax is maximum concentration observed during

the diurnal cycle,

Cmin is minimum concentration observed during

the diurnal cycle, and

Cmean is daily-mean concentration.

DIURNAL VARIATIONS IN METAL

CONCENTRATIONS

Metal concentrations were highly variable in the

Alamosa River and Wightman Fork during a 24-hour

period when nonsteady-state streamflow conditions

existed. Samples collected at the same site but at

different times during a diurnal streamflow cycle could

vary several hundred percent. The percent change

between the maximum and minimum concentration

in 15 percent of the samples collected at an individual

site over 24 hours ranged from about 83 percent

to about 1,940 percent (table 1). Percent change

between maximum and minimum concentrations

varied the least for copper, 94 to 344 percent, and

zinc, 83 to 226 percent. Iron was the most variable,

with variations between the daily maximum and

minimum concentration ranging from 185 to

1,940 percent.

In addition, samples collected over a diurnal

cycle in the Alamosa River and Wightman Fork

were highly variable when compared to the dailymean concentration as defined by the flow-weighted

composite sample (daily mean). The variability of the

maximum and minimum concentrations with respect

to the theoretical daily-mean concentration provides

an estimate of the daily variability relative to the

mean concentration at a site. This estimate is particularly important when addressing the question of variability in instantaneous concentration relative to the

mean concentration. In other words, how much variation from the mean concentration might be expected

from any one sample collected during the day during

nonsteady-state streamflow conditions in the basin?

The following sections describe the general

tendencies in variation and present boxplots showing

the variation around the daily mean for each COC.

An example of a boxplot is in figure 3. The descriptor

Dvar

Cmax Cmin –

Cmean

= --------------------------- × 100

Table 1. Percent change in metal concentrations over a 24-hour period at six sites in the Alamosa River Basin, 1995–97

Site

(n = number of

sample events)

Percentile

Aluminum Copper Iron Zinc

Dissolved Total

recoverable Dissolved Totalrecoverable Dissolved Totalrecoverable Dissolved Totalrecoverable

AR34.5

(n = 15)

85th 605 400 139 344 757 305 117 226

AR41.2

(n = 14)

85th 190 394 161 125 263 259 98 83

AR43.6

(n = 14)

85th 275 228 121 94 185 282 100 83

AR45.5

(n = 15)

85th 596 756 173 204 283 284 132 160

WF0.0

(n = 15)

85th 311 215 166 146 1,940 355 111 100

WF5.5

(n = 14)

85th 154 130 123 142 224 236 108 119
DIURNAL VARIATIONS IN METAL CONCENTRATIONS 7

is defined as the difference between the maximum and

minimum concentration divided by the daily-mean

concentration (a composite sample) expressed as a

percentage (see eq. 1). Hereinafter, this descriptor is

referred to as the “percent diurnal variation” of the

metal concentration.

Aluminum

Large diurnal variations in aluminum concentration occurred in the Alamosa River Basin from 1995

through 1997. The percent diurnal variation was

typically less than 160 percent (fig. 4) but exceeded

2,000 percent at various sites. With the exception of

AR45.5, more than one-half the reported dissolved

aluminum concentrations varied from the daily-mean

concentration by 50 percent or more; as much as onefourth of the concentrations at AR34.5, AR41.2,

AR43.6, and WF0.0 varied from the daily-mean

concentration by more than 100 percent (fig. 4). The

percent diurnal variation for total-recoverable

aluminum was less than that for dissolved aluminum;

about one-half of the concentrations varied from the

composited concentration by more than 35 percent.

Copper

The percent diurnal variation for copper

was less extreme than the diurnal variation for

aluminum. Typically, the diurnal variation was

less than 80 percent (fig. 5), but variations as much

as 600 percent were observed. With the exception

of dissolved copper at AR45.5 and total-recoverable

copper at WF0.0, more than one-half the reported

dissolved and total-recoverable copper concentrations

varied from the daily-mean concentration by more

than 40 percent (fig. 5). As many as one-fourth

of the dissolved copper concentrations at AR34.5,

AR41.2, AR43.6, WF0.0, and WF5.5 varied

from the daily-mean concentration by more than

80 percent.

Iron

Large diurnal variations in iron concentration

occurred in the Alamosa River Basin (fig. 6). The

percent diurnal variation was typically less than

140 percent but exceeded 1,000 percent for various

sites. More than one-half of the reported dissolved iron

Figure 3. Example diagram of boxplot.

Outlier data value less than or equal

to 3 and more than 1.5 times the

interquartile range outside the quartile

Data value less than or equal to 1.5 times

the interquartile range outside the quartile

75th percentile (three-fourths of data are

50th percentile (median)

25th percentile (one-fourth of data are less

Interquartile range

(one-half of data

are within this range){ less than this value)

than this value)
8 Diurnal Variations in Metal Concentrations in the Alamosa River and Wightman Fork, Southwestern Colorado, 1995–97

Figure 4. Variations in dissolved and total-recoverable aluminum concentrations in relation

to daily-mean concentration.

Figure 5. Variations in dissolved and total-recoverable copper concentrations in relation to

daily-mean concentration.

AR34.5 AR41.2 AR43.6 AR45.5 WF0.0 WF5.5

0

200

20

40

60

80

100

120

140

160

180

PERCENT DIURNAL VARIATION (MAXIMUM-MINIMUM)/COMPOSITE

Dissolved aluminum Total-recoverable aluminum

Note: Boxplots are truncated for better viewing of interquartile range

AR34.5 AR41.2 AR43.6 AR45.5 WF0.0 WF5.5

0

160

20

40

60

80

100

120

140

PERCENT DIURNAL VARIATION (MAXIMUM-MINIMUM)/COMPOSITE

Dissolved copper Total-recoverable copper

Note: Boxplots are truncated for better viewing of interquartile range
DIURNAL VARIATIONS IN METAL CONCENTRATIONS 9

concentrations varied more than 60 percent when

compared to the daily-mean concentration. At least

one-fourth of the dissolved concentrations at all the

sites varied from the daily-mean concentration by

about 100 percent. The percent diurnal variation for

total-recoverable iron was generally less than that for

dissolved iron; about one-half of the concentrations

varied from the daily-mean concentration by more

than 25 percent.

Zinc

The percent diurnal variations for zinc concentrations were the least variable of the four constituents

of concern (fig. 7). Typically, the diurnal variation was

less than 80 percent, but variations as much as

290 percent were observed. At least one-fourth

of the dissolved and total-recoverable zinc concentrations varied more than 50 percent when compared to

the daily-mean concentration. The percent diurnal

variation for total-recoverable zinc was similar to that

for dissolved zinc.

Temporal Variations in Maximum and

Minimum Concentrations

The timing of maximum and minimum concentrations was evaluated. The 24-hour sampling periods

were subdivided into 6-hour periods. The occurrence

of maximum and minimum COC concentrations were

then tabulated. Analysis indicated no discernible

pattern in the timing of maximum or minimum

concentrations during a diurnal cycle (fig. 8). In

general, maximum and minimum dissolved and totalrecoverable COC concentrations were fairly evenly

distributed throughout the 6-hour periods, with the

exception of dissolved iron. Maximum dissolved iron

concentrations were more likely to occur (about

75 percent) between 0600 hours and 1800 hours and

least likely to occur between midnight and 0600 hours.

Minimum dissolved iron concentrations were least

likely to occur between 0600 hours and 1200 hours

and more like to occur between 1800 hours and

midnight. Therefore, determining the likely time of

day when COC concentrations might represent a daily

mean, minimum, or maximum concentration is generally unpredictable.

Figure 6. Variations in dissolved and total-recoverable iron concentrations in relation to

daily-mean concentration.

AR34.5 AR41.2 AR43.6 AR45.5 WF0.0 WF5.5

0

300

20

40

60

80

100

120

140

160

180

200

220

240

260

280

PERCENT DIURNAL VARIATION (MAXIMUM-MINIMUM)/COMPOSITE

Dissolved iron Total-recoverable iron

Note: Boxplots are truncated for better viewing of interquartile range
10 Diurnal Variations in Metal Concentrations in the Alamosa River and Wightman Fork, Southwestern Colorado, 1995–97

Frequency of Exceeding Daily-Mean

Concentration Threshold

The probability that a discrete (instantaneous)

sample concentration will vary from the daily-mean

(composite) concentration by selected thresholds

was evaluated for each COC. Two seasonal periods,

snowmelt and summer flow, also were evaluated.

The percentage of discrete sample concentrations

within thresholds of plus or minus 10, 25, and

50 percent of the daily-mean sample concentration

was determined (fig. 9). Analysis indicated that,

depending on the COC and flow period, about 50 to

75 percent of discrete sample concentrations varied

from the daily-mean concentration by more than

plus or minus 10 percent. About 20 to 50 percent

of discrete sample concentrations varied from dailymean sample concentrations by more than plus

or minus 25 percent. Finally, between about 5

and 25 percent of discrete sample concentrations

varied from the daily-mean sample concentration by more than plus or minus 50 percent. The

percentage of variations between discrete and dailymean sample concentrations during the summer

period generally were smaller than percentage of

variations between discrete and daily-mean sample

concentrations during the snowmelt period for most

COCs.

IMPLICATIONS FOR SAMPLING

STRATEGIES

Sampling strategies need to accurately represent

variations and trends in metal concentrations and loads

in the Alamosa River Basin in order to evaluate the

effectiveness of remediation efforts on the water

quality of the basin. Given the variability and lack of

predictable temporal patterns of COC concentrations

during a diurnal cycle in the Alamosa River Basin, one

discrete sample collected during a diurnal cycle may

not accurately represent the daily-mean concentration.

On the basis of data collected from 1995 through

1997, during nonsteady-state periods in the Alamosa

River Basin, flow-weighting multiple discrete samples

collected over a diurnal cycle provides a better estimate of a representative daily-mean concentration as

compared to a single discrete sample collected at any

one time during the day.

Figure 7. Variations in dissolved and total-recoverable zinc concentrations in relation to

daily-mean concentration.

AR34.5 AR41.2 AR43.6 AR45.5 WF0.0 WF5.5

0

140

20

40

60

80

100

120

PERCENT DIURNAL VARIATION (MAXIMUM-MINIMUM)/COMPOSITE

Dissolved zinc

Note: Boxplots are truncated for better viewing of interquartile range

Total-recoverable zinc
IMPLICATIONS FOR SAMPLING STRATEGIES 11

Figure 8. Percentage of days that daily-maximum and daily-minimum concentration occurred during a given 6-hour

time period.

0

100

20

40

60

80

PERCENT

maximum

minimum

0

100

20

40

60

80

0

100

20

40

60

80

PERCENT

0

100

20

40

60

80

0

100

20

40

60

80

PERCENT

0

100

20

40

60

80

0000–0600 0601–1200 1201–1800 1801–2359

6-HOUR TIME PERIOD

0

100

20

40

60

80

PERCENT

0000–0600 0601–1200 1201–1800 1801–2359

6-HOUR TIME PERIOD

0

100

20

40

60

80

Dissolved aluminum Total-recoverable aluminum

Dissolved copper Total-recoverable copper

Dissolved iron Total-recoverable iron

Dissolved zinc Total-recoverable zinc
12 Diurnal Variations in Metal Concentrations in the Alamosa River and Wightman Fork, Southwestern Colorado, 1995–97

Figure 9. Percentage of discrete sample concentrations that exceeded the daily-mean concentration by a given threshold.

0

100

20

40

60

80

PERCENTAGE OF SAMPLES THAT

EXCEED THRESHOLD

all data

snowmelt period

summer period

Dissolved aluminum

0

100

20

40

60

80

Total-recoverable aluminum

0

100

20

40

60

80

PERCENTAGE OF SAMPLES THAT

EXCEED THRESHOLD

Dissolved copper

0

100

20

40

60

80

Total-recoverable copper

0

100

20

40

60

80

PERCENTAGE OF SAMPLES THAT

EXCEED THRESHOLD

Dissolved iron

0

100

20

40

60

80

Total-recoverable iron

THRESHOLD

0

100

20

40

60

80

PERCENTAGE OF SAMPLES THAT

EXCEED THRESHOLD

Dissolved zinc

THRESHOLD

0

100

20

40

60

80

Total-recoverable zinc

Plus or minus

10 percent

Plus or minus

25 percent

Plus or minus

50 percent Plus or minus

10 percent

Plus or minus

25 percent

Plus or minus

50 percent
SUMMARY 13

SUMMARY

From 1995 to 1997, a comprehensive sampling

network was implemented in the Alamosa River

Basin to address data gaps identified as part of the

Summitville Superfund site ecological risk assessment. Aluminum, copper, iron, and zinc were

identified as the constituents of concern for the risk

assessment. Water-quality samples were collected

at six locations where automatic samplers were

installed on the Alamosa River and Wightman

Fork. Several discrete samples were collected over

24 hours at each site during periods of high diurnal

variations in streamflow (May through September).

The diurnal variations in concentration (maximum

minus minimum divided by composite) with

respect to the theoretical daily-mean concentration

are presented.

Diurnal metal concentrations were highly variable in the Alamosa River and Wightman Fork. The

concentration of a metal at a single site could vary by

several hundred percent during one diurnal cycle.

Diurnal variations with respect to the daily-mean

concentration (composited sample concentration)

also were large. The percentage of diurnal variation

for aluminum was typically less than 160 percent but

exceeded 2,000 percent at various sites. More than

one-half the dissolved aluminum concentrations varied

by at least 50 percent from the daily-mean concentration, and about one-fourth of the concentrations varied

by as much as 100 percent. The variation for totalrecoverable aluminum was not as large. The percent

diurnal variation for copper was generally less than

80 percent, but variations of as much as 600 percent

were determined. Generally, about one-half the

dissolved and total-recoverable copper concentrations

varied from the daily-mean concentration by more

than 40 percent. Large diurnal variations in iron

concentration occurred in the basin. Typically, the

percentage of diurnal variation was less than

140 percent but could exceed 1,000 percent. More

than one-half the dissolved iron concentrations varied

by more than 60 percent when compared to the

daily-mean concentration; one-fourth of the concentrations varied by as much as 100 percent. The variation for total-recoverable iron was not as large.

Percent diurnal variation for zinc was the least

variable of the constituents of concern. Typically,

the variations were less than 80 percent but did exceed

290 percent. At least one-fourth of the dissolved and

total-recoverable zinc concentrations varied from the

daily-mean concentration by more than 50 percent.

Except for dissolved iron, maximum and

minimum dissolved and total-recoverable COC

concentrations were fairly evenly distributed

throughout a 24-hour period. Therefore, predicting

the timing of the daily mean, minimum, or maximum

concentration is not feasible.

Percentage of sample concentrations that

varied from the daily-mean concentration by more

than plus or minus 10 percent ranged from 50 to

75 percent. About 20 to 50 percent of samples

exceeded the daily-mean concentration by plus or

minus 25 percent, and about 5 to 25 percent of

samples exceeded the mean daily concentration by

plus or minus 50 percent. The percentage of samples

exceeding given thresholds generally was smaller

during the summer period than the snowmelt

period.

Sampling strategies are critical to accurately

define variability in constituent concentration; and

conversely, understanding constituent variability is

important in determining appropriate sampling

strategies. During nonsteady-state periods, considerable errors in estimates of daily-mean concentration

are possible if based on one discrete sample. Flowweighting multiple discrete samples collected

over a diurnal cycle provides a better estimate of

daily-mean concentrations during nonsteady-state

periods.

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