U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5229

Comparison of Atmospheric Deposition among Three Sites 

In and Near the Flat Tops Wilderness Area,

Colorado, 2003–2005

Prepared in cooperation with Rio Blanco County, Colorado

Comparison of Atmospheric Deposition 

among Three Sites In and Near the

Flat Tops Wilderness Area,

Colorado, 2003–2005

By George P. Ingersoll, Donald H. Campbell, and M. Alisa Mast

Prepared in cooperation with Rio Blanco County, Colorado

Scientific Investigations Report 2008–5229

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

Ingersoll, G.P., Campbell, D.H., and Mast, M.A., 2008, Comparison of Atmospheric Deposition among Three Sites 

In and Near the Flat Tops Wilderness Area, Colorado, 2003–2005: U.S. Geological Survey Scientific Investigations 

Report 2008–5229, 17 p.
iii

Contents

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

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

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

Study Area..............................................................................................................................................2

Data Collection and Analysis.......................................................................................................................2

Data Collection......................................................................................................................................4

Seasonal Snowpack....................................................................................................................4

Bulk Deposition ............................................................................................................................4

Wet-Fall Deposition .....................................................................................................................4

Analytical Methods and Quality Assurance.....................................................................................6

Statistical Methods...............................................................................................................................6

Atmospheric Deposition Chemistry ............................................................................................................7

Chemical Concentrations ..................................................................................................................11

Precipitation Amounts .......................................................................................................................14

Summary and Conclusions.........................................................................................................................15

Acknowledgments.......................................................................................................................................15

References....................................................................................................................................................15

Appendix........................................................................................................................................................17

Figures

1. Map showing the study area in northwestern Colorado .......................................................3

2. Worker prepares snowpit for full-strata sampling at Ned Wilson Lake in Colorado ........5

3. Workers preparing bulk collector near Ripple Creek Pass National Atmospheric

Deposition Program site ..............................................................................................................5

4. Seasonal volume-weighted-mean concentrations of (A) ammonium, (B) nitrate,

and (B) sulfate in bulk deposition at the Ned Wilson Lake site (NWLB) compared

to the Ripple Creek Pass site (RCPB). Winter pair is on the left and summer pair

is on the right...............................................................................................................................12

5. Seasonal volume-weighted-mean concentrations of (A) ammonium, (B) nitrate,

and (B) sulfate at the Ripple Creek Pass bulk deposition site (RCPB) compared

to Ripple Creek Pass wet deposition site (RCBW). Winter pair is on the left and

summer pair is on the right .......................................................................................................13

Tables

1. Dissolved concentrations and volume-weighted-mean concentrations of

ammonium, nitrate, and sulfate and precipitation amounts in winter and

summer bulk-deposition samples collected at Ned Wilson Lake in Colorado

(NWLB) 2003–2005........................................................................................................................8
iv

2. Dissolved concentrations and volume-weighted-mean concentrations of

ammonium, nitrate, and sulfate, and precipitation amounts in winter and

summer bulk-deposition samples collected near Ripple Creek Pass in Colorado

(RCPB) 2003–2005 .........................................................................................................................9

3. Dissolved concentrations and volume-weighted-mean concentrations of

ammonium, nitrate, and sulfate and precipitation amounts at National

Atmospheric Deposition Program wet-fall site near Ripple Creek Pass in

Colorado (RCPW) 2003–2005.....................................................................................................10

4. Summary of seasonal volume-weighted mean concentrations for dissolved

ammonium, nitrate, and sulfate at study sites 2003–2005....................................................11

5. Wilcoxon rank-sum test results shown as p-values for three monitoring sites for 

seasonal volume-weighted-mean concentrations of ammonium, nitrate, and sulfate in 

atmospheric deposition samples .............................................................................................12

6. Location information and measured and modeled annual precipitation amounts

at sampling sites and nearby SNOTEL sites...........................................................................14

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square centimeter (cm2

) 0.001076 square foot (ft2)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 0.2642 gallon (gal)

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Concentrations of chemical constituents in water are given in microequivalents per liter (μeq/L).

Water year (WY) is defined in this report as the 12-month period October 1 through September 

30, designated by the calendar year in which it ends.
Abstract

Atmospheric deposition was monitored for ammonium, 

nitrate, and sulfate concentrations and precipitation amounts 

in the Flat Tops Wilderness Area of northwestern Colorado 

at Ned Wilson Lake beginning in 1984 to detect changes that 

might result from future emissions associated with development of oil-shale resources in northwestern Colorado. 

Renewed monitoring, by the U.S. Geological Survey, in 

cooperation with Rio Blanco County, to determine the current 

status of atmospheric deposition has been ongoing since 2003 

at Ned Wilson Lake. Two new monitoring sites were located 

near Ripple Creek Pass near the Flat Tops Wilderness area and 

about 12 kilometers north of Ned Wilson Lake because access 

to the area near Ripple Creek Pass is less difficult and less 

expensive, particularly in winter and spring. The intent of this 

study was to establish whether the new deposition data being 

collected near Ripple Creek Pass, near the northern boundary of the Flat Tops Wilderness Area, would be representative 

of deposition at sensitive sites within the wilderness such as 

Ned Wilson Lake and to compare more current (2003 through 

2005) deposition data with earlier data (1984 through 1991).

At Ned Wilson Lake, bulk ammonium and nitrate concentrations collected from 1984 through 1991 were similar to 

those from 2003 through 2005. However, in the same comparison significant differences in sulfate concentrations were 

observed, indicating a decrease consistent with other regional 

findings for similar periods. Comparison of concentrations of 

constituents at two bulk-deposition sites located at Ned Wilson 

Lake (NWLB) and near Ripple Creek Pass (RCPB) showed 

only one significant difference (p = 0.05) with the winter bulk 

nitrate concentrations for NWLB significantly lower than 

winter concentrations from RCPB. Another comparison of 

concentrations of constituents between the bulk deposition site 

RCPB and a wet deposition site 100 meters away (RCPW) 

showed no significant differences for concentrations of 

ammonium, nitrate, and sulfate for both the winter and summer comparisons. While results indicate many similarities in 

concentrations of constituents and in seasonal variability in 

those concentrations, they are based on a short period of study. 

Precipitation amounts from RCPB were less than the amounts 

collected at NWLB, and precipitation amounts from RCPW 

were less than the amounts collected at RCPB. Although 

RCPB may not be a perfect replacement site for NWLB, it 

may be similar enough to represent atmospheric deposition in 

areas of the Flat Tops Wilderness of northwestern Colorado.

Introduction

Energy development and the growth of urban areas in 

the Rocky Mountain region have increased concern about the 

environmental effects of atmospheric deposition on highelevation lakes and streams, particularly in protected areas 

such as national parks and wilderness areas. Atmospheric 

deposition monitoring began in the Flat Tops Wilderness 

Area of northwestern Colorado at Ned Wilson Lake in 1984 

(Campbell and others, 1991). Baseline chemistry of atmospheric deposition (including precipitation and dry deposition) and water quality was established to detect changes that 

might result from future emissions of nitrogen, sulfur, and 

acidity associated with development of oil-shale resources in 

northwestern Colorado. Monitoring of atmospheric deposition 

decreased in the 1990s due to different priorities. During the 

past two decades, atmospheric emissions and resulting deposition have changed both at regional and at national scales (Fenn 

and others, 2003a; Lehmann and others, 2005). Currently 

(2008), there is renewed interest in oil-shale development in 

northwestern Colorado, and natural gas and oil extraction is 

increasing in the area. Thus, there is a need for renewed monitoring to determine the current status of atmospheric deposition in the Flat Tops Wilderness Area. Such information is 

critical to enable resource managers to balance the protection 

of sensitive resources and economic development. 

Currently, nearly all sulfur dioxide (SO2

) emissions 

and more than 70 percent of nitrogen oxide (NOx

) emissions 

in the Rocky Mountain region are produced by fossil-fuel 

combustion in urban areas and by electric utilities (U.S. 

Environmental Protection Agency, 2006a). SO2 is of environmental concern because it can be oxidized in the atmosphere 

to sulfate, which causes visibility impairment and is a 

major component of acidic deposition (U.S. Environmental 

Comparison of Atmospheric Deposition among Three Sites 

In and Near the Flat Tops Wilderness Area,

Colorado, 2003–2005

By George P. Ingersoll, Donald H. Campbell, and M. Alisa Mast
2 Comparison of Atmospheric Deposition among Three Sites In and Near the Flat Tops Wilderness Area, Colorado, 2003–2005

Protection Agency, 2006b). NOx also is a precursor of pollutants that cause acidic deposition and visibility impairment 

and produces ozone. Acidic deposition poses a threat to many 

high-elevation ecosystems in the Rocky Mountains because 

they are characterized by bedrock and soil types that have 

little capacity to buffer acidic inputs (Turk and Adams, 1983). 

Elevated nitrogen deposition to these sensitive ecosystems 

may provide an additional stress that can lead to changes in 

soil fertility, shifts in vegetation type, and eutrophication of 

surface waters (Baron and others, 2000).

Sulfur dioxide emissions have declined in the United 

States during the 1980s and 1990s, although most strikingly 

in the East, due to installation of pollution controls at coalfired generating plants, reductions in emissions from smelters, 

and reductions in the sulfur content of fuels (U.S. Environmental Protection Agency, 1998). This decline is reflected in 

decreases in sulfate concentrations in precipitation across the 

United States for the period 1980 through 1992, including 

several sites in the Rocky Mountain region (Lynch and others, 

1995). Although NOx

 emissions also have declined in the Eastern United States, they have increased slightly in the Rocky 

Mountain region from 1985 through 2002 (Lehmann and others, 2005). In addition, there is increasing concern about emissions of ammonia from agricultural and urban sources adjacent 

to pristine mountainous areas (Baron and others, 2000). As a 

result of increasing nitrogen emissions in the Western United 

States, the relative importance of nitrogen species to precipitation acidity is considerable, and ecological effects of increased 

deposition of nitrogen may be undesirable (Burns, 2002; Fenn 

and others, 2003b).

As concerns about potential changes in local and regional 

atmospheric deposition grow, year-round wet-fall monitoring 

is no longer possible at sites within the Flat Tops Wilderness 

Area because of legal and logistical restrictions. Therefore, it 

is important to establish whether the new deposition data being 

collected near Ripple Creek Pass, near the northern boundary 

of the Flat Tops Wilderness Area, are a representative substitute of deposition at sensitive sites within the wilderness such 

as Ned Wilson Lake. Snowpack and bulk deposition monitoring was restarted at Ned Wilson Lake and near Ripple Creek 

Pass in water year 2003.

To address this monitoring need, the U.S. Geological Survey (USGS), in cooperation with Rio Blanco County, Colorado, 

established a program to collect additional deposition samples 

from annual snowpacks and from bulk and wet-fall collectors 

at three sites in or near the Flat Tops Wilderness Area. The 

primary objectives of this program were the following:

1. Determine whether precipitation chemistry data from 

2003 through 2005 for Ned Wilson Lake compares to 

historical data collected during 1984 through 1991 and 

provide baseline data needed to determine future changes 

in atmospheric deposition.

2. Compare results of snowpack and bulk precipitation 

sampling at Ned Wilson Lake to results near Ripple Creek 

Pass from 2003 through 2005 to evaluate whether the two 

sites receive comparable atmospheric deposition.

3. Compare results of sampling from 2003 through 2005 

snowpack and bulk precipitation for the site near the Ripple Creek Pass National Atmospheric Deposition Program 

(NADP) site to wet-fall results to determine transferability 

of the different methods of collecting atmospheric deposition data.

Purpose and Scope

The purpose of this report is to present the statistical 

comparison of atmospheric deposition among three sites 

in and near the Flat Tops Wilderness Area, specifically of 

bulk-deposition and wet-fall chemistry during water years 

2003 through 2005, and to present a comparison of results for 

historical data from 1984 through 1991. This report contains 

a brief description of sample-collection, analysis, and qualitycontrol methods; volume-weighted mean concentrations of 

ammonium, nitrate, and sulfate; precipitation amounts; and 

suggestions for monitoring at three sites in and near the Flat 

Tops Wilderness Area of Colorado.

Study Area

The three sites compared in this study were located in 

similar mountainous terrain in alpine and subalpine areas 

of the White River National Forest in Rio Blanco County in 

northwestern Colorado (fig. 1). The bulk-deposition monitoring site at Ned Wilson Lake (NWLB) is in a high alpine 

setting near tree line at about 3,393-m elevation within the Flat 

Tops Wilderness Area. The bulk-deposition monitoring site 

located near Ripple Creek Pass (RCPB) is about 12 km north 

of NWLB and outside the Wilderness Area. The NADP wetfall site near Ripple Creek Pass (RCPW) is located below tree 

line in an open meadow adjacent to subalpine forest at about 

2,929-m elevation. The nearby bulk-deposition collection site, 

RCPB, is about 100 m north of RCPW in a similar setting at 

about 2,938-m elevation. Annual precipitation occurs mostly 

as snowfall at the three sites, and seasonal snowpacks generally persist from October through April (and as late as July at 

higher elevations) (Western Regional Climate Center, 2006). 

The sites generally are upwind from several large powerplants 

and numerous small emissions sources in the more densely 

populated areas of eastern Colorado (U.S. Environmental 

Protection Agency, 2006a).

Data Collection and Analysis

Two periods of data collection are discussed in this 

report. First, depositional data were collected from 1984 

through 1991 at Ned Wilson Lake (Ranalli and others, 1997). 

Second, depositional data from 2003 through 2005 were collected at the three sites (NWLB, RCPB, and RCPW) using 

different methods to represent most annual precipitation. 

Chemical analyses were done to determine concentrations of 
"$

!!"

#

#

RIO BLANCO 

COUNTY

Ripple Creek PassSNOTEL

Ripple Creek Pass snow, bulk, and NADP

Ned Wilson Lake,snow, and bulk

Bison LakeSNOTEL Glenwood Springs

EXPLANATION

! Ned Wilson Lake 

" Ned Wilson Lake snow

! Ned Wilson Lake bulk (NWLB) 

# Bison Lake SNOTEL

" Ripple Creek Pass snow

Ripple Creek Pass bulk (RCPB)

$ Ripple Creek Pass NADP (RCPW)

# Ripple Creek Pass SNOTEL

Grand Junction

Flat Tops Wilderness Area

White River National Forest

COLORADO

UTAH

Meeker

Base from U.S. Geological Survey,

North American Datum of 1983 (NAD83),Universal Transverse Mercator grid, 2005,and Street Map USA, 2004. 0 20 40 60 MILES

0 20 40 60 KILOMETERS

National Forest

40o

109o

Grand Junction

Flat Tops WilderneArea

COLORA

GlenwoodSprings

Study area

Figure 1. Map showing the study area in northwestern Colorado.
4 Comparison of Atmospheric Deposition among Three Sites In and Near the Flat Tops Wilderness Area, Colorado, 2003–2005

dissolved ammonium, nitrate, and sulfate, and precipitation 

was measured at each site. Concentrations were statistically 

compared among the three atmospheric-deposition sites to 

determine if deposition chemistry at two sites near Ripple 

Creek Pass (RCPB and RCPW) was representative of deposition chemistry at the Ned Wilson Lake site (NWLB) inside the 

Flat Tops Wilderness Area. Further comparisons were made 

of the chemistry of wet-fall samples to either bulk samples or 

snowpack samples collected near Ripple Creek Pass. Snowpack samples were collected instead of bulk samples during 

winter when bulk collection was less feasible.

Data Collection

For the data collected during 2003 through 2005, two 

different methods of sample collection were used to estimate annual atmospheric deposition: (1) seasonal snowpack 

plus spring and summer bulk precipitation, and (2) wet-fall 

precipitation. In this report, bulk atmospheric deposition refers 

to precipitation as snow or rain, and dry deposition (which 

is occurring constantly). Snowpack is a form of bulk deposition containing both wet and dry deposition from the snowaccumulation period. Wet-fall refers only to water containing 

dissolved and particulate materials captured during precipitation and does not include dry deposition. Dry deposition is an 

accumulation of materials deposited from the atmosphere in 

the absence of precipitation. NADP wet-fall samples include 

particulate matter deposited exclusively during precipitation 

events and at no other times. The period of comparison for 

the study began when the NADP site near Ripple Creek Pass 

began operation in May 2003 (although snowpack chemical data are included for the previous winter) and continued 

through October 2005. Volume-weighted-mean (VWM) 

concentrations were calculated separately as summertime and 

wintertime periods for the years 2003–2005. The VWM concentration is the product of the precipitation-volume-weighting 

factor (the sample precipitation volume divided by the sum 

of all sample precipitation volumes for the period) and the 

sample concentration.

Seasonal Snowpack

Seasonal snowpack samples were collected to represent 

precipitation and total (wet and dry, or bulk) deposition that 

fell from October through April or May (fig. 2). As snowpacks accumulate during the winter and spring, atmospheric 

deposition occurs during dry periods as well as during periods 

of precipitation (wet-fall) providing a measurement of total 

deposition. Because the high-elevation snowpacks in the study 

area persisted through winter and spring with negligible melt, 

seasonal snowpack samples offered an efficient medium for 

collecting naturally composited atmospheric deposition in 

single samples. Snowpits were prepared and sampled according to USGS protocols before snowmelt began at Ripple Creek 

Pass (during March or April) and at Ned Wilson Lake (during 

April or May). Snowpack samples were collected in triplicate 

during 2005 as a test for precision and small-scale variability 

(see Appendix). Snowpack-sample collection was done earlier 

at the RCPB site because snowmelt begins earlier there than at 

the higher NWLB site. Detailed methods for seasonal snowpack sampling can be found in Ingersoll and others (2002).

Bulk Deposition

To continue collecting year-round wet and dry deposition after the seasonal snowpack began to melt, bulk collectors 

were deployed at the NWLB and RCPB sites during spring 

each year. Bulk collectors were installed the same day snowpack samples were collected and were operated until early 

fall when the next seasonal snowpack began to accumulate, 

providing a continuous sampling record — all four seasons for 

each of the 3 years of the study. Bulk collectors were constructed with lightweight materials that could be transported 

into remote areas efficiently by using skis, snowmobiles, or 

helicopters. Polyethylene buckets lined with Teflon bags were 

suspended on steel tripods at heights of 2 to 3 m above the 

snow or ground surface and were supported with wire-rope 

guys (fig. 3). Bulk collectors remained open for the duration 

of the individual sample-collection periods and collected both 

wet and dry deposition. Individual samples generally were 

retrieved from the collector at 2- to 3-week intervals, depending upon the amount of precipitation. If inadequate precipitation occurred (less than about 1 L) by the second week, collectors were allowed to remain a third week before samples were 

retrieved and new Teflon bags installed. During spring, when 

precipitation could occur as either snowfall or rainfall, 60-L 

containers were used. Large orifices (1,134 cm2

) and volumes 

were necessary to capture samples during heavy snowfall 

common to the area. During summer, when snowfall was less 

likely, smaller, 20-L buckets with smaller orifices (908 cm2) 

replaced the 60-L containers. The 20-L buckets were fitted 

with funnels to reduce evaporation caused by warmer summertime temperatures. The same collector configuration was used 

at both the NWLB and RCPB sites. Precipitation collected was 

calculated as the mass measurement divided by the area of the 

container orifices.

Wet-Fall Deposition

Wet-fall precipitation data presented in this study were 

collected by Air Sciences, Incorporated, as part of the National 

Atmospheric Deposition Program using NADP protocols 

(National Atmospheric Deposition Program, 2006) at the site 

near Ripple Creek Pass. Program information and data for 

the wet-fall site can be found at the NADP Web site at http://

nadp.sws.uiuc.edu/lib (accessed August 2, 2008). Monthly 

concentration and precipitation values for the study period 

were available beginning May 2003 and were exported from 

the NADP Web site for each month through October 2005. 

Monthly NADP data were selected for use in this comparison 

(instead of weekly or seasonal) to more closely resemble the 

bulk-deposition sampling intervals and to use a similar total 
Data Collection and Analysis 5

Figure 2. Worker prepares snowpit for full-strata sampling at Ned Wilson Lake in Colorado.

Figure 3. Workers prepare bulk collector near Ripple Creek Pass National Atmospheric Deposition 

Program site.
6 Comparison of Atmospheric Deposition among Three Sites In and Near the Flat Tops Wilderness Area, Colorado, 2003–2005

number of samples in statistical comparisons to data from the 

other methods. Monthly NADP data also were used because 

of the completeness relative to weekly data, which occasionally had missing samples. Monthly values computed by the 

NADP were thought to be the best available data when weekly 

data were missing. Although wet-fall- and bulk-samplecollection dates did not exactly coincide, the two sets of 

sample-collection dates generally matched within 1 to 2 weeks 

of each other.

A noteworthy distinction about the wet-fall data, when 

comparing it to bulk-deposition and snowpack data, is that 

wet-fall data exclude dry deposition, whereas bulk-deposition 

and snowpack samples contain both wet and dry deposition. The wet-fall collector at RCPW was designed to open 

the orifice and collect atmospheric deposition only during 

precipitation events. Once precipitation ceases, the orifice is 

automatically closed and remains closed until the next precipitation event begins. Thus, wet-fall collection protocol prevents 

collection of dry atmospheric deposition.

Analytical Methods and Quality Assurance

Concentrations of dissolved ammonium, nitrate, and 

sulfate in seasonal snowpack and bulk-deposition samples 

were determined in USGS laboratories in Lakewood, Colorado. Dissolved constituents are defined as those that have 

passed through a 0.45-micrometer membrane filter. Hereinafter, all concentrations of ammonium, nitrate, and sulfate 

refer to dissolved concentrations. Seasonal snowpack samples 

were melted in 8-L Teflon bags used for field collection; 

bulk-deposition samples were melted in 20- or 60-L Teflon 

bags used in field collection. Method detection limits were 

0.5 μeq/L for ammonium, 0.2 μeq/L for nitrate, and 0.3 μeq/L 

for sulfate. Ten quality-assurance samples were collected 

from 2003 through 2005 and included laboratory blanks, field 

blanks, and field replicates (see Appendix). Concentrations of 

ammonium, nitrate, and sulfate in field- and laboratory blanks 

were near or below detection limits in most cases. Small 

concentrations of nitrate (up to 1.5 μeq/L above the detection 

limit) were reflected in field blanks. Concentrations of nitrate 

detected in the field blanks were much lower than concentrations in environmental samples and were not believed to have 

had a discernible effect on the environmental samples. For 

comparison of concentrations of selected constituents between 

environmental samples and replicate samples, relative percent 

differences (RPD) were calculated. The RPD (expressed as a 

percentage in this report) is the absolute value of the difference of the environmental sample concentration (E) minus the 

replicate sample concentration (R), divided by the average of 

the environmental sample and the replicate sample concentrations, and multiplied by 100 (|E-R|/[(E+R)/2]) x 100). Relative 

percent differences between collected environmental samples 

and replicate samples ranged from 0.0 percent to 18.2 percent 

for ammonium, nitrate, and sulfate. Additional analytical laboratory methods and quality-assurance procedures for analyses 

of major-ion concentrations are described in Turk and others 

(2001) and Ingersoll and others (2005). USGS laboratories 

providing analytical results in this report participated with several other laboratories in continuing performance evaluations 

of analytical results. Further information about the interlaboratory comparison program for USGS can be found at http://bqs.

usgs.gov/srs#contacts (accessed December 2, 2008). 

Concentrations of dissolved ammonium, nitrate, and 

sulfate in wet-fall samples were determined using NADP protocols in the Central Analytical Laboratory at the Illinois State 

Water Survey in Champaign, Illinois. Detection limits reported 

for NADP wet-fall data for the constituents in this study were 

similar to those reported for snowpack and bulk deposition. For 

further information about analytical methods and quality assurance, see http://nadp.sws.uiuc.edu/ (accessed August 2, 2008).

Statistical Methods

For the historical data comparison, data from 1984 

through 1991 for NWLB for precipitation, and bulk ammonium, nitrate, and sulfate concentrations were compared to 

data from 2003 through 2005 for the same site. The Wilcoxon 

rank-sum test was used to test for differences among the variables (Wilcoxon, 1945; Iman and Conover, 1983).

For the more recent data comparisons, two hypotheses 

were evaluated at three sites: (1) volume-weighted mean bulk 

concentrations at NWLB were different from VWM bulk 

concentrations at RCPB, and (2) VWM bulk concentrations at 

RCPB were different from NADP VWM wet-fall concentrations at RCPW. Distributions of concentrations mostly were 

non-normal during the study at the three monitoring sites 

(NWLB, RCPB and RCPW), so nonparametric statistical tests 

were used to evaluate differences between concentration data 

from site to site. The Wilcoxon rank-sum test was performed 

to compare volume-weighted mean concentrations of ammonium, nitrate, and sulfate among the three sites for the study 

period. For example, for a given constituent observed, such 

as ammonium concentrations, all concentrations for NWLB 

and RCPB first were divided into either the winter or summer 

group. At RCPB, the winter grouping generally included the 

period October through May, and the summer grouping generally included the period June through September (or October 

depending upon the onset of the snowfall season). At the 

higher elevation NWLB site, winterlike conditions often persist until July. So, the winter grouping there generally included 

the period October through June, and the summer grouping 

generally included the period July through September. Using 

these seasonal groupings, VWM bulk ammonium concentrations for NWLB were tested against VWM bulk ammonium 

concentrations for RCPB. Next, seasonally selected VWM 

concentrations for RCPB were tested against seasonal VWMwetfall-ammonium concentrations for RCPW. One-tailed 

tests were used to determine if concentration or precipitation 

were greater or lesser at one of two sites being compared. In 

this report, p-values of 0.05 indicate a significant difference, 

p-values of 0.20 indicate weak significant difference, and 

p-values above 0.20 indicate no significant difference.
Atmospheric Deposition Chemistry

Concentration and precipitation results for the NWLB, 

RCPB, and RCPW sites are presented in tables 1, 2, and 

3, respectively. Concentrations of dissolved ammonium, 

nitrate, and sulfate and precipitation amounts are shown for 

each sample collected for the comparison. Seasonal volumeweighted mean concentrations were calculated for two 

general groups: wintertime and summertime samples (table 

4). Precipitation samples including both snowpack and bulkdeposition samples were collected at NWLB and RCPB sites 

by using the same methods throughout the period May 2003 

through October 2005. The two complete water years (WY) 

of the period (2004 and 2005) were represented by a series of 

samples starting with seasonal snowpack samples collected 

in March or April (which had begun to accumulate during the 

previous fall) and ending with bulk-deposition samples collected through summer and into October. In all, 17 samples 

were collected at NWLB and 31 samples were collected at 

RCPB. Wet-fall samples at RCPW were collected differently 

but represent wet deposition during most of the same time 

period. Wet-fall collection began during May 2003 and continued through October 2005, for a total of 30 months. It is 

important to point out that these results are based on a limited 

sample set, and comparisons would be more robust if more 

data were available.

Atmospheric Deposition Chemistry 7
8 Comparison of Atmospheric Deposition among Three Sites In and Near the Flat Tops Wilderness Area, Colorado, 2003–2005

Table 1. Dissolved concentrations and volume-weighted-mean concentrations of ammonium, nitrate, and sulfate and precipitation 

amounts in winter and summer bulk-deposition samples collected at Ned Wilson Lake in Colorado (NWLB) 2003 through 2005.

[μeq/L, microequivalents per liter; VWM, volume-weighted mean; cm, centimeters; na, not applicable]

Sampling period Collection date Ammonium 

as NH4

+

Nitrate

as NO3

–

Sulfate

as SO4

2– Precipitation

(μeq/L) (μeq/L) (μeq/L) (cm)

Winter1,2 5/13/2003 5.4 9.3 6.1 97.7

Winter 6/25/2003 23.7 28.2 28.5 1.6

Seasonal VWM3 5.7 9.6 6.5 na

Winter 4/13/2004 3.7 7.2 3.7 75.6

Winter 7/11/2004 10.1 44.6 34.2 5.9

Seasonal VWM 4.2 9.9 5.9 na

Winter 4/22/2005 3.7 7.0 4.5 98.3

Winter 7/13/2005 18.8 24.6 24.4 12.3

Seasonal VWM 5.4 9.0 6.7 na

Summer4 8/27/2003 16.5 31.5 17.9 5.8

Summer 9/17/2003 8.2 15.0 8.7 2.9

Seasonal VWM 13.8 26.0 14.8 na

Summer 7/26/2004 7.5 17.6 8.4 4.2

Summer 8/9/2004 26.9 40.1 25.8 1.4

Summer 8/21/2004 14.0 21.1 14.9 1.9

Summer 9/29/2004 10.6 16.4 12.9 9.6

Seasonal VWM 11.6 19.2 13.1 na

Summer 8/13/2005 13.7 14.7 9.4 9.5

Summer 8/24/2005 17.6 19.7 16.1 3.5

Summer 9/9/2005 12.8 19.9 17.9 3.5

Summer 9/25/2005 4.4 12.6 11.0 1.1

Summer 10/12/2005 6.2 7.1 9.3 5.2

Seasonal VWM 12.0 14.4 11.8 na

1

Sample concentrations (ammonium, nitrate, and sulfate) and precipitation amounts collected by sample.

2

Winter season generally was grouped as October through June.

3

Seasonal VWM shown for ammonium, nitrate, and sulfate.

4

Summer season generally was grouped as July through September.
Atmospheric Deposition Chemistry 9

Table 2. Dissolved concentrations and volume-weighted-mean concentrations of ammonium, nitrate, and sulfate, and precipitation 

amounts in winter and summer bulk-deposition samples collected near Ripple Creek Pass in Colorado (RCPB) 2003 through 2005. 

[μeq/L, microequivalents per liter; VWM, volume-weighted mean; cm, centimeters; na, not applicable]

Sampling period Collection date Ammonium 

as NH4

+

Nitrate

as NO3

–

Sulfate

as SO4

2– Precipitation

(μeq/L) (μeq/L) (μeq/L) (cm)

Winter1,2 04/04/2003 4.6 11.0 5.7 66.1

Winter 05/14/2003 9.1 13.5 11.9 23.1

Winter 06/03/2003 25.9 28.5 21.1 1.5

Seasonal VWM3 6.1 12.0 7.5 na 

Winter 04/01/2004 4.3 10.9 4.7 82.9

Winter 05/25/2004 0.0 28.0 24.5 4.4

Seasonal VWM 4.1 11.8 5.7 na 

Winter 03/28/2005 4.5 9.4 6.3 48.2

Winter 05/09/2005 11.2 14.2 13.5 6.9

Winter 06/07/2005 11.5 15.8 15.2 9.7

Seasonal VWM 6.2 10.9 8.4 na 

Summer4 06/23/2003 72.6 26.2 32.2 1.8

Summer 07/29/2003 42.9 52.2 44.2 0.9

Summer 08/26/2003 13.9 15.6 7.7 3.1

Summer 09/02/2003 19.3 27.8 15.2 1.4

Summer 09/22/2003 5.9 16.4 15.8 5.6

Seasonal VWM 21.1 21.3 18.0 na 

Summer 06/26/2004 5.3 13.2 25.5 3.3

Summer 07/07/2004 39.9 16.1 15.1 2.8

Summer 07/17/2004 7.4 30.8 13.7 1.2

Summer 07/27/2004 13.9 25.9 11.7 1.1

Summer 08/10/2004 33.5 45.4 28.2 1.1

Summer 08/31/2004 13.1 18.7 15.0 6.8

Summer 09/13/2004 32.4 52.1 27.9 0.5

Summer 09/28/2004 8.3 10.7 8.5 6.3

Summer 09/28/2004 8.3 10.6 8.3 6.3

Summer 10/19/2004 8.2 15.8 11.7 4.3

Seasonal VWM 12.9 16.6 13.6 na 

Summer 06/21/2005 3.8 12.1 7.2 2.6

Summer 07/12/2005 16.4 31.0 34.6 2.1

Summer 07/26/2005 8.6 16.3 11.5 2.5

Summer 08/02/2005 36.5 32.8 23.3 0.5

Summer 09/06/2005 4.3 21.5 15.9 3.9

Summer 09/27/2005 0.9 14.3 12.6 3.1

Summer 10/18/2005 0.1 0.0 8.8 6.1

Summer 11/08/2005 2.1 12.9 10.5 3.2

Seasonal VWM 4.7 13.5 13.3 na

1

Sample concentrations (ammonium, nitrate, and sulfate) and precipitation amounts collected by sample.

2

Winter season generally was grouped as October through May.

3

Seasonal VWM shown for ammonium, nitrate, and sulfate.

4

Summer season generally was grouped as June through October.
10 Comparison of Atmospheric Deposition among Three Sites In and Near the Flat Tops Wilderness Area, Colorado, 2003–2005

Table 3. Dissolved concentrations and volume-weighted-mean concentrations of ammonium, nitrate, and sulfate and precipitation 

amounts at NADP wet-fall site near Ripple Creek Pass in Colorado (RCPW), 2003 through 2005. 

[μeq/L, microequivalents per liter; VWM, volume-weighted mean; cm, centimeters; na, not applicable]

Sampling period Month Year Ammonium 

as NH4

+

Nitrate

as NO3

–

Sulfate

as SO4

2– Precipitation

(μeq/L) (μeq/L) (μeq/L) (cm)

Winter1,2 May 2003 18.9 16.3 16.0 3.0

Seasonal VWM3 18.9 16.3 16.0 na

Winter Oct4 2003 15.0 23.4 16.3 0.4

Winter Nov 2003 12.4 12.4 9.7 11.8

Winter Dec 2003 2.7 7.9 4.5 5.0

Winter Jan 2004 2.2 7.5 2.0 5.2

Winter Feb 2004 2.1 8.7 4.7 5.1

Winter Mar 2004 13.0 22.4 16.2 3.9

Winter Apr 2004 8.9 8.9 7.0 7.6

Winter May 2004 8.5 14.9 11.7 3.9

Seasonal VWM 7.9 11.5 7.9 na

Winter Nov 2004 2.0 6.1 4.2 5.2

Winter Dec 2004 8.8 10.5 3.4 2.0

Winter Jan 2005 0.9 6.9 2.3 6.4

Winter Feb 2005 4.1 8.6 5.3 4.1

Winter Mar 2005 5.7 7.1 6.8 4.5

Winter Apr 2005 4.6 6.7 5.8 7.9

Winter May 2005 7.3 10.7 11.4 5.2

Seasonal VWM 4.3 7.7 5.7 na

Summer5 June 2003 29.9 26.5 27.3 3.3

Summer July 2003 48.1 34.3 25.0 1.0

Summer Aug 2003 18.0 18.2 10.5 9.0

Summer Sept 2003 7.8 9.5 6.3 1.9

Seasonal VWM 21.3 20.0 14.6 na

Summer June 2004 21.7 27.6 21.3 2.7

Summer July 2004 9.9 12.7 8.3 5.1

Summer Aug 2004 10.7 16.8 13.2 4.4

Summer Sept 2004 6.2 8.5 8.1 8.8

Summer Oct 2004 3.3 8.2 6.2 8.6

Seasonal VWM 8.1 12.1 9.5 na

Summer June 2005 10.6 12.6 12.4 11.6

Summer July 2005 15.7 17.3 13.7 4.2

Summer Aug 2005 9.7 14.9 10.9 5.6

Summer Sept 2005 7.0 10.1 9.6 3.7

Summer Oct 2005 5.4 6.4 8.0 9.2

Seasonal VWM 9.3 11.6 10.8 na

1

Sample concentrations (ammonium, nitrate, and sulfate) and precipitation amounts collected by sample.

2

Winter season generally was grouped as October through May.

3

Seasonal VWM shown for ammonium, nitrate, and sulfate.

4

The October sample in 2003 was included in the winter grouping so that the end of the Summer 2003 sampling period in this table would correspond to the end 

of the summer sampling period in 2003 at RCPB (09/22/2003, table 2).

5

Summer season generally was grouped as June through October.
Atmospheric Deposition Chemistry 11

Table 4. Summary of seasonal volume-weighted mean concentrations for dissolved ammonium, nitrate, and sulfate at study sites, 

2003 through 2005.

[Units in microequivalents per liter; NWLB, bulk deposition at Ned Wilson Lake in Colorado; RCPB, bulk deposition near Ripple Creek Pass in Colorado; 

RCPW, wet fall at NADP site near Ripple Creek Pass in Colorado]

Constituent 

and site Winter 2003 Winter 2004 Winter 2005 Winter average

Summer 2003 Summer 2004 Summer 2005 Summer 

average

Ammonium as NH4

+

NWLB 5.7 4.2 5.4 5.1 13.8 11.6 12.0 12.5

RCPB 6.1 4.1 6.2 5.5 21.1 12.9 4.7 12.9

RCPW 18.9 7.9 4.3 10.3 21.3 8.1 9.3 12.9

Nitrate as NO3

–

NWLB 9.6 9.9 9.0 9.5 26.0 19.2 14.4 19.9

RCPB 12.0 11.8 10.9 11.5 21.3 16.6 13.5 17.1

RCPW 16.3 11.5 7.7 11.8 20.0 12.1 11.6 14.6

Sulfate as SO4

2–

NWLB 6.5 5.9 6.7 6.4 14.8 13.1 11.8 13.2

RCPB 7.5 5.7 8.4 7.2 18.0 13.6 13.3 15.0

RCPW 16.0 7.9 5.7 9.9 14.6 9.5 10.8 11.7

Chemical Concentrations

Three comparisons of results of atmospheric-deposition 

monitoring were performed in this study. First, historical data 

for precipitation and bulk deposition at NWLB were compared 

to more recent data collected at the same site. Second, comparisons between the two bulk-deposition sites, NWLB and RCPB, 

were done with data collected during similar time periods. 

These comparisons were done to evaluate whether bulk-deposition data collected near the more accessible RCPB site were 

different from bulk-deposition data from the remote site within 

the Flat Tops Wilderness Area at NWLB. Third, bulk-deposition data from RCPB were compared to wet-fall data at nearby 

RCPW as a test of comparability of different methods.

Data for precipitation and bulk ammonium, nitrate, and 

sulfate concentrations for the Ned Wilson site from 2003 

through 2005 were compared to data from the same site from 

1984 through 1991, which were collected using the same 

sampling and analytical methods. Nearly all of the samples 

from the earlier period were collected during the early summer through early fall and did not contain winter samples that 

could be compared to the winter samples collected during the 

2003 through 2005 period. Therefore, the comparison between 

periods was made only for samples collected during the early 

summer through early fall timeframe (June through October).

Volume-weighted mean bulk sulfate concentration was 

the only variable that differed significantly between the two 

periods. The Wilcoxon rank-sum test (Wilcoxon, 1945; Iman 

and Conover, 1983), which was used to test for differences, 

showed the sulfate concentration for the recent period was 

significantly less than that for the earlier period (p = 0.02). 

The p-value indicates a 2-percent random chance of finding the same difference in concentrations from the same 

population. The VWM sulfate bulk concentration for the 

recent period (n = 3) was 13.1 μeq/L compared to 16.9 μeq/L 

(n = 8) for the earlier period. This difference is consistent 

with other regional analyses showing a general decline in 

atmospheric sulfate deposition resulting from implementation of increasingly more stringent emission standards on 

coal-fired electric plants (Nilles and Conley, 2001; Lehmann 

and others, 2005).

The second set of comparisons was made between 

the two bulk-deposition sites, NWLB and RCPB. Samples 

were grouped by wintertime or summertime collection, and 

volume-weighted mean concentrations were calculated for 

each group (figs. 4 A–C). Results indicated little significant 

difference in concentrations of ammonium, nitrate, or sulfate 

(table 5). Wintertime nitrate concentrations at NWLB were 

significantly lower (p = 0.05) than concentrations from RCPB, 

and summertime sulfate concentrations at NWLB were lower 

than concentrations at RCPB but were weakly significant 

(p = 0.20). However, seasonal fluctuations resulting in higher 

concentrations during summer were reflected in seasonal 

averages for all three constituents for both NWLB and RCPB 

(table 4). Consistent with the seasonal fluctuations noted, the 

significantly lower winter nitrate concentration at NWLB 

matches the pattern of lower wintertime concentration at both 

sites. Although these results suggest that seasonal concentrations of ammonium, nitrate, and sulfate at NWLB generally 

were similar to those at RCPB, the small sample size from 3 

years of data limits confidence because it is uncertain if these 

sample values are representative of the larger population.
12 Comparison of Atmospheric Deposition among Three Sites In and Near the Flat Tops Wilderness Area, Colorado, 2003–2005

Table 5. Wilcoxon rank-sum test results shown as p-values for three monitoring sites for seasonal volume-weighted-mean 

concentrations of ammonium, nitrate, and sulfate in atmospheric deposition samples. 

[Values shown in bold are significant at p=0.05 level; p-values shown in italics indicate weak significance (p=0.20); NWLB, bulk deposition at Ned Wilson 

Lake in Colorado; RCPB, bulk deposition near Ripple Creek Pass in Colorado; RCPW, wet-fall at NADP site near Ripple Creek Pass, Colorado]

Ha: x>y1 NWLB compared to RCPB RCPB compared to RCPW

Winter Summer Winter Summer

Ammonium 0.80 0.65 0.65 0.65

Nitrate 1.00 0.35 0.35 0.20

Sulfate 0.80 0.90 0.35 0.20

Ha

: x<y NWLB compared to RCPB RCPB compared to RCPW

Winter Summer Winter Summer

Ammonium 0.35 0.50 0.50 0.50

Nitrate 0.05 0.80 0.80 0.90

Sulfate 0.35 0.20 0.80 0.90

1

 P-values for alternate hypotheses of NWLB > RCPB or RCPB > RCPW (Ha: x>y), and NWLB < RCPB or RCPB < RCPW (Ha: x<y) are results of 1-tailed 

Wilcoxon rank-sum tests.

A, Ammonium

Explanation

0.0

5.0

10.0

15.0

20.0

25.0

NWLB RCPB NWLB RCPB

MICROEQUIVALENTS PER LITER

winter

summer

B, Nitrate

Explanation

0.0

5.0

10.0

15.0

20.0

25.0

30.0

NWLB RCPB NWLB RCPB

MICROEQUIVALENTS PER LITER

winter

summer

C, Sulfate

Explanation

0.0

5.0

10.0

15.0

20.0

NWLB RCPB NWLB RCPB

MICROEQUIVALENTS PER LITER

winter

summer

Figure 4. Seasonal volumeweighted-mean concentrations 

of (A) ammonium, (B) nitrate, and 

(C) sulfate in bulk deposition at 

the Ned Wilson Lake site (NWLB) 

compared to the Ripple Creek Pass 

site (RCPB). Winter pair is on the 

left and summer pair is on the right.
Atmospheric Deposition Chemistry 13

The third set of comparisons was made between RCPB 

and RCPW (figs. 5A–C). Bulk samples at RCPB for the period 

before the wet-fall collector became operational in mid-May 

2003 at RCPW (collection dates of 04/04/03 and 05/14/03 

[table 2]) were excluded from figures 5A–C in order to match 

the initial date of collection at RCPW. Thus, different seasonal, volume-weighted means for RCPB were used in the 

comparison between RCPB and RCPW (25.9 μeq/L of ammonium, 28.5 μeq/L of nitrate, and 21.1 μeq/L of sulfate) (table 

2; figs. 5A–C) than were used for the comparison between 

NWLB and RCPB (6.1 μeq/L of ammonium, 12.0 μeq/L of 

nitrate, and 7.5 μeq/L of sulfate) (table 2; figs. 4A–C).

Results for both the winter and summer seasons showed 

no difference between ammonium concentrations but showed 

weakly significant differences between nitrate (p = 0.20) and 

sulfate (p = 0.20) concentrations during the summer seasons 

(table 5). Wet-fall collection may have been affected by the 

exclusion of dry deposition during periods of no precipitation, particularly with nitrate and sulfate. Generally, somewhat 

higher concentrations of nitrate and sulfate at RCPB compared 

to RCPW were consistent with the inclusion of dry deposition 

in bulk samples. Previous work at the NWLB site indicated 

consistently higher concentrations in bulk compared to wetfall samples collected side-by-side (Ranalli and others, 1997). 

Other analysis of total deposition data for ammonium, nitrate, 

and sulfate also indicated the importance of dry deposition 

in the region (Clow and others, 2002). Further, dry deposition monitored in the Rocky Mountain region from 2002 to 

2004 by the Clean Air Status and Trends Network (CASTNet) 

program constituted a substantial fraction of total atmospheric 

deposition in Colorado, particularly during summertime 

(National Park Service, 2007). However, dry deposition of 

ammonium at six CASTNet sites in the Rocky Mountain 

region was small relative to dry deposition of nitrate or sulfate 

(U.S. Environmental Protection Agency, 2007). This is one 

explanation why there were generally higher concentrations 

in bulk samples for nitrate and sulfate but not for ammonium. 

This comparison suggests that wet-fall deposition chemistry 

monitored near Ripple Creek Pass may not fully represent 

total deposition (for nitrate and sulfate, in particular).

A, Ammonium 

Explanation

0.0

5.0

10.0

15.0

20.0

25.0

30.0

RCPB RCPW RCPB RCPW

MICROEQUIVALENTS PER LITER

winter

summer

B, Nitrate 

Explanation

0.0

5.0

10.0

15.0

20.0

25.0

30.0

RCPB RCPW RCPB RCPW

MICROEQUIVALENTS PER LITER

winter

summer

C, Sulfate

Explanation

0.0

5.0

10.0

15.0

20.0

25.0

RCPB RCPW RCPB RCPW

MICROEQUIVALENTS PER LITER

winter

summer

Figure 5. Seasonal volumeweighted-mean concentrations of 

(A) ammonium, (B) nitrate, and (C) 

sulfate at the Ripple Creek Pass bulk 

deposition site (RCPB) compared to 

Ripple Creek Pass wet deposition site 

(RCPW). Winter pair is on the left and 

summer pair is on the right.
14 Comparison of Atmospheric Deposition among Three Sites In and Near the Flat Tops Wilderness Area, Colorado, 2003–2005

Precipitation Amounts

Total annual precipitation amounts at sampling sites 

and nearby snow telemetry (SNOTEL) sites, operated by the 

U.S. Department of Agriculture Natural Resources Conservation Service (Western Regional Climate Center, 2006), were 

measured or modeled during the study for the complete water 

years 2004 and 2005 (table 6). Two SNOTEL sites in the 

general area (within 45 km) recorded annual precipitation during the study at comparable elevations in mountainous terrain. 

SNOTEL sites at Ripple Creek Pass and Bison Lake (fig. 1) 

reported similar ranges of total annual precipitation as compared to those of NWLB and RCPB. However, considerably 

less precipitation was recorded for RCPW. In common with 

the bulk-deposition site at NWLB, the Bison Lake SNOTEL 

site is at a higher elevation (3,316 m) than the Ripple Creek 

Pass SNOTEL site (3,151 m). For additional comparison, 

modeled estimates of annual precipitation for WY 2004 and 

WY 2005 for NWLB and RCPB are available from the PRISM 

(Parameter-elevation Regressions on Independent Slopes 

Model) Group (2007). PRISM estimates also are in the same 

general range of measured precipitation amounts but were not 

always in close agreement. In all cases, the 2005 precipitation 

was greater than the 2004 precipitation except for RCPB. At 

the Ripple Creek Pass SNOTEL site, total annual precipitation 

was below the 30-year average of 118 cm in 2004 (94 cm) and 

in 2005 (98 cm). At the Bison Lake SNOTEL site, total annual 

precipitation was below the 30-year average of 117 cm in 2004 

(106 cm) and above average in 2005 (128 cm). According to 

30-year averages for 1971 through 2000 at the SNOTEL sites, 

the study period was characterized both by wet and by dry 

periods, but few extreme deviations from average were noted. 

However, dry periods occurred during summer, especially 

during 2003, and limited the number of precipitation samples. 

This explains the relative shortage of bulk-deposition samples 

both at RCPB and NWLB and also is reflected in reduced precipitation amounts and wet-fall samples that year at RCPW. 

For the comparison between NWLB and RCPB, total 

precipitation for water year 2004 and water year 2005 was 11 

percent greater at NWLB (as would be expected of the higher 

elevation site). However, in the comparison between RCPB 

and RCPW, total precipitation for the same period was substantially higher (58 percent) at RCPB, particularly in winter. 

There are three likely explanations for this. First, annual precipitation measured in 2004 at RCPB (120.9 cm) was greater 

than would be expected because the snowpack water content 

was much higher at the nearby alternate location chosen. The 

2004 snowpack site was relocated about 200 m east of the 

RCPB site, at a similar elevation, due to early snowmelt at the 

original site. Relocation of the snowpack-sampling site to a 

steeper site with less exposure to the sun was necessary to collect the annual snowpack sample intact. A second explanation 

for this difference likely is the undercatch at the gage at the 

NADP site, a common problem in precipitation measurement 

(Goodison and others, 1998; Yang and others, 2000). Last, the 

larger amount of precipitation also was due to the large fraction of winter precipitation at RCPB that was represented by 

the annual snowpack and the tendency for undercatch in gages 

to be highest in winter due to blowing snow. For these reasons, 

precipitation measurements from SNOTEL sites, or modeled 

precipitation estimates for the study area, may be more useful 

for calculation of wet deposition.

NWLB is about 455 m higher in elevation than RCPB, 

which may account for the larger amounts of precipitation 

collected at NWLB. Given the general similarities in constituent concentrations, it is reasonable to assume that data collected from the two sites are from similar, but not identical, 

populations.

Table 6. Location information and measured and modeled annual precipitation amounts at sampling sites and nearby SNOTEL sites. 

[Latitude (north) and Longitude (west) in decimal degrees; m, meters; cm, centimeters; NWLB, bulk deposition at Ned Wilson Lake in 

Colorado; RCPB, bulk deposition near Ripple Creek Pass in Colorado; RCPW, wet fall at NADP site near Ripple Creek Pass in Colorado; 

SNOTEL, snow-telemetry site]

Site Latitude Longitude Elevation Annual precipitation (cm)

(m) Water year 2004 Water year 2005

NWLB 39.9608 107.3214 3,393 98.6 133.4

NWLB1 39.9608 107.3214 3,393 95.7 102.8

RCPB 40.0861 107.3119 2,938 120.9 88.8

RCPB1 40.0861 107.3119 2,938 93.8 96.1

RCPW 40.0851 107.3118 2,929 63.9 69.0

Bison Lake SNOTEL 39.7600 107.3600 3,316 106.4 127.5

Ripple Creek Pass 

SNOTEL 40.1100 107.2900 3,151 94.5 98.0

1

Estimated from modeled information (PRISM Group, 2007).
References 15

Summary and Conclusions

The U.S. Geological Survey has been monitoring bulk 

atmospheric deposition of ammonium, nitrate, and sulfate 

intermittently at a site at Ned Wilson Lake (NWLB) in the Flat 

Tops Wilderness Area since 1984. This site is key to monitoring the impacts of oil-shale development and the continuing 

expansion of gas and oil development on atmospheric deposition in northwestern Colorado. For many reasons, it is no 

longer feasible to continue operating the Ned Wilson Lake 

site. In 2003 a potential substitute bulk atmospheric deposition 

monitoring site (RCPB) and a wet-deposition monitoring site 

(RCPW) were established near Ripple Creek Pass just outside 

of the Flat Tops Wilderness Area. The U.S. Geological Survey, 

in cooperation with Rio Blanco County, Colorado, established 

a program to determine if atmospheric deposition of ammonium, nitrate, and sulfate from 2003 through 2005 at NWLB 

compares to historical data collected during 1984 through 

1991; to determine if bulk atmospheric deposition data at the 

NWLB site are similar to that collected at the RCPB site; 

and to compare bulk atmospheric deposition chemistry and 

wet-deposition chemistry collected at the RCPB and RCPW, 

respectively.

For the comparison of VWM bulk ammonium, nitrate, 

and sulfate concentrations collected from 1984 through 1991 

to those from 2003 through 2005, the Wilcoxon rank-sum test 

showed only a significant difference in sulfate concentrations. 

The sulfate concentration for the 2003 through 2005 period 

was significantly lower than that for the 1984 through 1991 

period, which is consistent with other regional findings for 

similar periods.

For comparison of the VWM concentrations of 

constituents at the bulk sites, NWLB and RCPB, seasonal 

variations in VWM concentrations of ammonium and sulfate 

were similar for the two sites. The Wilcoxon rank-sum test 

showed only one significant difference (p = 0.05): the winter 

bulk nitrate concentrations for NWLB were significantly lower 

than winter concentrations from RCPB. Summer bulk VWM 

concentrations of sulfate also tended to be lower at NWLB, 

but at a weak level of significance (p = 0.20). Because only 

three VWM concentrations were used for each of the winter 

and for each of the summer statistical comparisons of bulk 

concentrations, these results may not be representative of the 

larger, unsampled population of concentrations.

The statistical comparison of VWM concentrations 

of constituents between the bulk deposition site (RCPB) 

and the wet deposition site (RCPW), showed no significant 

differences for concentrations of ammonium, nitrate, and 

sulfate for both the winter and summer comparisons. Summer VWM concentrations of nitrate and sulfate from RCPB 

tended to be higher than concentrations from RCPW but at a 

weak level of significance (p = 0.20). Further, the bulk VWM 

concentrations for nitrate and sulfate showed generally 

higher ranges of concentrations than those associated with 

the wet deposition, which indicates that the RCPW does not 

represent the full spectrum of atmospheric deposition. Note 

that slightly different VWM concentrations were used for 

these comparisons for 2003 because the RCPW site did not 

begin collecting data until mid-May of that year, and the bulk 

samples collected in April and early May 2003 were not used 

in the comparison.

Precipitation amounts from NWLB were greater than 

the amounts collected at RCPB, probably because of the 

difference in elevations. Precipitation amounts from RCPB 

were greater than the amounts collected at RCPW. The 

comparison of RCPB and RCPW precipitation amounts was 

confounded by the need to temporarily relocate the snowpack 

site in 2004. Also, wet-fall collectors tend to undercollect 

precipitation. For these reasons, precipitation measurements 

from SNOTEL sites, or modeled precipitation estimates for 

the study area, may be more useful for calculation of wet 

deposition.

Although data were not available to generate truly 

rigorous statistical comparisons of the NWLB and RCPB 

data, the statistical results indicate many similarities in 

VWM concentrations of constituents and in seasonal variability in those concentrations. Also, NWLB is about 455 m 

higher in elevation than RCPB, which may account for some 

of the larger amounts of precipitation collected at NWLB 

relative to RCPB. Given the general similarities, it is reasonable to assume that data collected from the two sites are 

from similar, but not identical, populations. Further, although 

RCPB is not a perfect replacement site for NWLB, it may be 

similar enough to represent atmospheric deposition in northwestern Colorado.

Acknowledgments

The authors greatly appreciate the cooperation from Rio 

Blanco County, Colorado, and especially the assistance provided by Jan Henderson. Expert field work and logistical support from Mark Scritchfield and Gene Scritchfield contributed 

substantially to the success of this study. Thanks also are due 

to Chester Crabb, Christine Dolliver, Heather Handran, and 

Lisa Smith for their field and laboratory assistance. Technical 

reviews from Rodger Steen and Clifford Bossong substantially 

improved this report.

References

Baron, Jill, Rueth, H.M., Wolfe, A.M., Nydick, K.R., Allstott, E.J., Minear, J.T., and Moraska, B., 2000, Ecosystem 

responses to nitrogen deposition in the Colorado Front 

Range: Ecosystems, v. 3, p. 352–368.

Burns, D.A., 2002, The effects of atmospheric nitrogen deposition in the Rocky Mountains of Colorado and southern 

Wyoming—A synthesis and critical assessment of published 

results: U.S. Geological Survey Water-Resources Investigations Report 2002–4066, 36 p.
16 Comparison of Atmospheric Deposition among Three Sites In and Near the Flat Tops Wilderness Area, Colorado, 2003–2005

Campbell, D.H., Turk, J.T., and Spahr, N.E., 1991, Response 

of Ned Wilson Lake Watershed, Colorado, to changes 

in atmospheric deposition of sulfate: Water Resources 

Research v. 27, no. 8, p. 2047–2060.

Clow, D.W., Ingersoll G.P., Mast M.A., Turk J.T., and Campbell D.H., 2002, Comparison of snowpack and winter wetfall chemistry in the Rocky Mountains, USA—Implications 

for winter dry deposition: Atmospheric Environment, v. 36, 

p. 2337–2348.

Fenn, M.E., Haeuber, R., Tonnesen, G.S., Baron, J.S., Grossman-Clark, S., Hope, D., Jaffe, D.A., Copeland, S., Geiser, L., Rueth, H.M., and Sickman, J.O., 2003a, Nitrogen 

emissions, deposition, and monitoring in the western United 

States: BioScience, v. 53, no. 4, p. 391–403. 

Fenn, M.E., Baron, J.S., Allen, E.B., Rueth, H.M., Nydick, 

K., Geiser, L., Bowman, W.D., Sickman, J.O., Meixtner, T., 

Johnson, D.W., and Neitlich, P., 2003b, Ecological effects 

of nitrogen deposition in the Western United States: BioScience, v. 53, no. 4, p. 404–420.

Goodison, B.E., Louie, B.Y.T., and Yang, D., 1998, WMO 

solid precipitation intercomparison, final report: World 

Meteorological Organization, Geneva, Switzerland, 1998, 

WMO/TD 872, 212 p.

Iman, R.L., and Conover, W.J., 1983, A modern approach to 

statistics: New York, John Wiley and Sons, Inc., 498 p.

Ingersoll, G.P, Turk, J.T., Mast, M.A., Clow, D.W., Campbell, 

D.H., and Bailey, Z.C., 2002, Rocky Mountain Snowpack 

Chemistry Network—History, methods, and the importance 

of monitoring mountain ecosystems: U.S. Geological Survey Open-File Report 2001–466, 14 p., accessed 08/02/08 

at http://pubs.usgs.gov/of/2001/ofr01-466/.

Ingersoll, G.P., Mast, M.A., Nanus, L., Manthorne, D.,J., 

Campbell, D.H., Handran, H., Hultstrand, D.,H., and Winterringer, J., 2005, Rocky Mountain snowpack chemistry 

at selected sites, 2003: U.S. Geological Survey Open-File 

Report 2005–1332, 15 p., accessed 08/02/08 at http://pubs.

usgs.gov/of/2005/1332/.

Lehmann, C.M.B., Bowersox, V.C., and Larson, S.M., 2005, 

Spatial and temporal trends of precipitation chemistry in the 

United States, 1985–2002: Environmental Pollution, v. 135, 

p. 347–361.

Lynch, J.A., Grimm, J.W., and Bowersox, V.C., 1995, Trends 

in precipitation chemistry in the United States—A national 

prespective, 1980-1992: Atmospheric Environment, v. 29, 

p. 1231–1246.

National Atmospheric Deposition Program, 2006, National 

Atmospheric Deposition Program 2005 Annual Summary: 

NADP Data Report 2006–01, NADP Program Office, Illinois State Water Survey, 2204 Griffith Drive, Champaign, 

IL 61820, accessed 08/02/08 at http://nadp.sws.uiuc.edu/lib.

National Park Service, 2007, Dry deposition monitoring: 

http://www2.nature.nps.gov/air/Monitoring/drymon.cfm, 

accessed 08/02/08.

Nilles, M.A. and Conley, B.E., 2001, Changes in the chemistry 

of precipitation in the United States, 1981–1998: Water, Air, 

and Soil Pollution, v. 130, p. 409–414.

PRISM Group, 2007, Parameter-elevation Regressions on Independent Slopes Model, Oregon State 

University, Corvallis, Oregon, accessed 08/02/08 at 

http://www.prismclimate.org/.

Ranalli, A.J., Turk, J.T., and Campbell, D.H., 1997, The use 

of bulk collectors in monitoring wet deposition at highaltitude sites in winter: Water, Air, and Soil Pollution, v. 95, 

p. 237–255.

Turk, J.T., and Adams, D.B., 1983, Sensitivity to acidification 

of lakes in the Flat Tops Wilderness Area, Colorado: Water 

Resources Research, v. 19, no. 2, p. 346–350.

Turk, J.T., Taylor, H.E., Ingersoll, G.P., Tonnessen, K.A., 

Clow, D.W., Mast, M.A., Campbell, D.H., and Melack, 

J.M., 2001, Major-ion chemistry of the Rocky Mountain snowpack, USA: Atmospheric Environment, v. 35, 

p. 3957–3966.

U.S. Environmental Protection Agency, 1998, National Air 

Quality and Emissions Trends Report, 1997: 

EPA 454/R–9B–016.

U.S. Environmental Protection Agency, 2006a, National Emissions Inventory: Emissions Inventory & Analysis Group, 

Air Quality Assessment Division, Office of Air Quality 

Planning and Standards, U.S. EPA, Research Triangle Park, 

NC 27711, accessed 8/02/08 at http://www.epa.gov/ttn/

chief/eiinformation.html/.

U.S. Environmental Protection Agency, 2006b, Office 

of Air and Radiation, Six common air pollutants, 

Chief causes for concern, accessed 9/01/08 at 

http://www.epa.gov/air/urbanair/so2/chf1.html/.

U.S. Environmental Protection Agency, 2007, Clean Air Status 

and Trends Network (CASTNET), accessed 08/02/08 at 

http://www.epa.gov/castnet/.

Western Regional Climate Center, 2006, accessed 08/02/08 at 

http://www.wcc.nrcs.usda.gov/snow/.

Wilcoxon, F., 1945, Individual comparisons by ranking 

methods: Biometrics, v. 1, p. 80–83.

 Yang, D., Cane, D.L., Hinzman, L.D., Goodison, B.E., Metcalfe, J.R., Louie, P.Y.T, Leavesley, G.H., Emerson, D.G., 

and Hanson, C.L., 2000, An evaluation of the Wyoming 

gauge system for snowfall measurement: Water Resources 

Research, v. 36, p. 2665–2677.
Appendix 17

Appendix

Quality-assurance data for laboratory blanks, field blanks, and field replicates.

[μeq/L, microequivalents per liter; RPD, relative percent difference; DFC, Denver Federal Center, Lakewood, Colo.; NWLB, bulk-deposition at Ned Wilson 

Lake in Colorado; RCPB, bulk-deposition near Ripple Creek Pass in Colorado; bulk, summertime bulk collector; snow, wintertime snowpack; <, less than; 

na, not applicable]

Ammonium, dissolved, 

as NH4

+

Nitrate, dissolved, 

as NO3

–

Sulfate, dissolved, 

as SO4

2–

Sample type Site name Collect date (μeq/L) RPD (μeq/L) RPD (μeq/L) RPD

Laboratory blank DFC 09/18/2003 <0.5 na <0.2 na <0.3 na

Laboratory blank DFC 06/04/2004 <0.5 na <0.2 na <0.3 na

Field blank NWLB (bulk) 08/21/2004 <0.5 na 1.6 na <0.3 na

Field blank NWLB (bulk) 07/23/2005 <0.5 na 1.7 na <0.3 na

Environmental NWLB (snow) 04/22/2005 3.7 na 7.0 na 4.5 na

Field replicate NWLB (snow) 04/22/2005 3.3 11.3 7.8 11.0 4.9 7.6

Field replicate NWLB (snow) 04/22/2005 3.1 17.2 6.3 10.1 4.6 1.1

Field blank RCPB (bulk) 09/22/2003 <0.5 na 0.3 na <0.3 na

Environmental RCPB (bulk) 09/28/2004 8.3 na 10.7 na 8.5 na

Field replicate RCPB (bulk) 09/28/2004 8.3 0.0 10.6 0.9 8.3 2.4

Environmental RCPB (snow) 03/28/2005 4.5 na 9.4 na 6.3 na

Field replicate RCPB (snow) 03/28/2005 4.9 9.0 9.2 3.1 6.5 2.5

Field replicate RCPB (snow) 03/28/2005 4.0 10.6 8.2 13.9 5.2 18.2

Publishing support provided by: 

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For more information concerning this publication, contact:

Director, USGS Colorado Water Science Center

Box 25046, Mail Stop 415

Denver, CO 80225

(303)236-4882

Or visit the Colorado Water Science Center Web site at:

http://co.water.usgs.gov/
Ingersoll and others—Atmospheric Deposition, Flat Tops Wilderness Area, Colorado—U.S. Geological Survey Scientific Investigations Report 2008–5229