Water Quality of the Boulder Creek Watershed, Colorado

Located within the Rocky Mountain Front Range of Colorado, the 1,160-

km2 Boulder Creek Watershed encompasses a variety of climate zones and 

geologic units (figs. 1 and 2). Water quality of Boulder Creek is affected by 

discharge variations from snowmelt, agricultural diversions, and wastewater 

treatment plant (WWTP) effluent, by point and non-point solute sources, 

and by in-stream processes. As with many watersheds in the American West, 

dependable water quality and sufficient water supply are issues facing local 

water managers and users.

This fact sheet summarizes the results of a cooperative study by the U.S. 

Geological Survey (USGS) and the city of Boulder to characterize the water 

quality of the Boulder Creek Watershed. Complete results of the study are 

published in USGS Water-Resources Investigations Report 03-4045 (Murphy 

and others, 2003). This study was initiated by discussions at the Boulder 

Creek Watershed Forum, a monthly gathering of watershed residents, USGS 

scientists, city of Boulder personnel, and University of Colorado researchers. 

The primary goals of this study were to 1) characterize Boulder Creek 

water quality in the year 2000, 2) identify sources and sinks of chemical 

constituents, and 3) identify the processes controlling water chemistry. 

Results from this study provide 

community members with a better 

understanding of their watershed 

and a foundation for protecting and 

conserving water resources. 

STUDY AREA

Boulder Creek originates as 

headwater streams at the Continental 

Divide and flows through historical 

mining districts and mountain 

communities to the mouth of Boulder 

Canyon. Upon exiting Boulder 

Canyon, Boulder Creek flows through 

the city of Boulder and eastward 

through the plains to the confluence 

with Saint Vrain Creek, 75 km 

downstream from the headwaters. 

Similar to many Front Range areas, 

the Boulder Creek Watershed area 

experienced rapid population growth 

from 1990 to 2000. As a consequence 

of this growth, acres of farmland in 

Boulder County decreased by 19 

percent between 1992 and 1997 (U.S. 

Department of Agriculture, 1997).

APPROACH

Because of the large seasonal 

variation in flow (fig. 3), sampling was 

done during high-flow (mid-June) and 

low-flow (mid-October) conditions. 

Water-quality samples were collected 

from 18 sites along Boulder Creek 

and from 11 other sites (tributaries, 

WWTP effluent, pipelines, canals, 

and Saint Vrain Creek). Temperature, 

pH, dissolved oxygen, and specific 

conductance were measured on 

site. All samples were analyzed for 

54 inorganic constituents, carbon, 

nitrogen, phosphorous, and fecal 

coliform bacteria. At ten sites, samples 

were analyzed for 84 pesticides and 68 

wastewater compounds (Table 1).

RESULTS

Discharge

Low-flow conditions occur in 

Boulder Creek from October through 

March, and high-flow conditions occur 

from April to July, as snow melts, 

feeding the headwater streams. In 

addition to natural variations in 

discharge, water management, 

including impoundments, 

wastewater effluent, and 

agricultural diversions, causes 

temporal and spatial variations 

in discharge (fig. 3). The 

removal of water by diversions 

leaves less for dilution of 

constituents, such that WWTP 

Photo 1. Boulder Creek. Lower basin in foreground and foothills and Continental Divide in 

background.

U.S. Department of the Interior USGS Fact Sheet 043-03

U.S. Geological Survey August 2003

Photo 2. On-site processing, Boulder Creek.
effluent dominates the chemistry of the 

lower portion of Boulder Creek during 

low-flow periods. 

Inorganic Geochemistry

Specific conductance, an indicator 

of total dissolved ions, was relatively 

low above the mouth of Boulder 

Canyon, increased slightly in the 

reach between the mouth of Boulder 

Canyon and upstream of the Boulder 

75th Street WWTP (BC-aWWTP), and 

increased in a step-wise fashion below 

the WWTP (fig. 4a). Most dissolved 

inorganic constituents showed a 

similar downstream trend. Calcium, 

chloride, magnesium, silica, sodium, 

bicarbonate, and sulfate accounted 

for over 90 percent of the 

dissolved inorganic mass. 

Concentrations were lower 

during high flow because 

of dilution, primarily from 

snowmelt.

Bedrock geology is 

an important control of 

water chemistry in the 

watershed. When dissolved 

major ion concentrations 

in headwater sampling 

sites are compared to those 

in precipitation collected 

along the Continental 

Divide at Niwot Ridge, 

most constituents increased 

by factors of 10 to 20, which is 

consistent with minor weathering of 

the local bedrock. A greater amount 

of mineral dissolution occurs, due 

to longer ground-water flow paths 

and greater residence time, at the 

most downstream site in crystalline 

bedrock, BC-CAN. Although Boulder 

Creek passes through historical metalmining districts, most dissolved metal 

concentrations, including arsenic, 

cadmium, chromium, copper, lead, 

nickel, and zinc, were 

extremely low, generally less 

than 1 microgram per liter. 

As Boulder Creek 

flows from the foothills to 

the plains, not only does 

bedrock composition change, 

but potential anthropogenic 

sources increase. In the 

reach between Boulder 

Canyon and the Boulder 

WWTP, concentrations of 

most dissolved inorganic 

constituents increased. 

Differentiation between 

natural and anthropogenic 

sources of solutes is difficult because 

both likely contribute to the chemistry 

in this reach. However, the increase 

in bicarbonate, calcium, magnesium, 

sodium, and sulfate is consistent 

with weathering of the underlying 

sedimentary bedrock. In the reach 

through the city of Boulder, dissolved 

metal concentrations remained low. 

The largest increase in dissolved 

constituents occurs downstream of the 

Boulder WWTP at site BC-75, with 

the greatest increases being chloride, 

nitrogen, sulfate, and sodium. The 

WWTP discharge accounted for 

about 75 percent of the discharge 

during the low-flow sampling and 

about 35 percent during high flow. 

The WWTP effluent had a positive 

gadolinium spike (Fig. 5) that can 

Table 1. Groups of constituents analyzed in this investigation. 

Number in parentheses is the number of constituents determined in 

each group.[C, carbon; N, nitrogen; P, phosphorus]

Basic water-quality mea- Organic constituents

surements (9) Pesticides (84)

 Herbicides

Inorganic constituents Insecticides

Major anions (4) Wastewater compounds (68)

Major cations (5) Surfactants

Trace elements (45) Hormones

 Steroids

Nutrients (C, N, and P) Prescription drugs

Bacteria Nonprescription drugs

Fecal coliform

Figure 1. Boulder Creek Watershed: sampling sites and streamgaging stations.

Figure 2. Generalized elevation profile with 

climate zones and bedrock type.
be used as a geochemical signature 

of the effluent. Gadolinium (Gd), 

a rare earth element (REE), is not 

naturally enriched relative to other 

REEs, but it has industrial sources, 

primarily as gadopentetic acid, which 

is used as a contrasting agent in 

magnetic resonance imaging (MRI) 

that is then discharged to the urban 

wastewater system (Bau and Dulski, 

1996). In contrast, the REE pattern of 

Nederland WWTP effluent does not 

have a Gd spike, which is consistent 

with the absence of MRI facilities in 

Nederland. 

Another increase in inorganic 

dissolved constituents occurs from the 

inflow of Coal Creek, which receives 

WWTP effluent from Erie, Lafayette, 

Louisville, and Superior. The decrease 

in specific conductance during low 

flow in the reach between Coal Creek 

and the confluence with Saint Vrain 

Creek is likely due to dilution by 

ground-water inflow.

Nutrients 

Concentrations of nitrogen (N) 

species and phosphorous (P) were 

near or below detection limits for 

main-stem sites upstream from the 

Boulder WWTP during both highand low-flow sampling (Fig 4b). 

The concentrations of N and P were 

highest immediately downstream 

of the Boulder WWTP (BC-75), 

and then decreased with distance as 

these nutrients are utilized by plants 

and microorganisms or attached to 

particulates in the water column. 

In the upper watershed, total 

organic carbon (TOC) concentrations 

(fig. 4c) were greater during high 

flow than low flow due to flushing of 

natural organic matter from soils by 

spring runoff. The greatest increase 

in TOC occurred below the Boulder 

WWTP, and concentrations 

decreased downstream due 

to biodegradation, sorption, 

and photolysis.

Fecal coliform

Throughout most 

of Boulder Creek, fecal 

coliform concentrations 

were well below the 

Colorado Department 

of Public Health 

and Environment 

recreation class 1a 

standard of 200 colonies per 

100 milliliters (Colorado 

Department of Health and the 

Environment, 2002). However, 

four low-flow, main-stem 

Boulder Creek samples (BC-30, 

BC-75, BC-aDC, and BC-aCC) 

exceeded this standard.

Wastewater compounds

During high flow, 40 

wastewater compounds were 

detected in the main stem of 

Boulder Creek at concentrations 

ranging from less than 1 to 

100,000 nanograms per liter 

(ng/L). During low flow, 41 of 

these compounds were detected 

at concentrations up to 270,000 

ng/L. 

The most abundant 

wastewater compounds detected 

were ethylenediaminetetraa

cetic acid (EDTA), a widely used 

metal complexing agent, and nonylphenol ethoxycarboxlyates (NPEC), 

nonionic surfactant degradation 

products. Maximum concentrations 

of both EDTA and NPEC occurred 

downstream from the Boulder WWTP 

during low-flow conditions. The next 

most abundant compounds detected 

were the steroids (cholesterol and 

coprostanol). Steroid concentrations 

were an order of magnitude less 

than those of EDTA and NPEC, 

except in the upper basin, where 

natural sources produce steroids 

and there are limited sources of 

EDTA and NPEC. Other wastewater 

compounds detected in at least one 

Boulder Creek sample include: 1) 

the nonprescription drugs cotinine (a 

nicotine metabolite), acetaminophen, 

ibuprofen, and caffeine; 2) the 

prescription drugs cimetidine, 

diltiazem, diphenhydramine, 

sulfamethoxazole, and trimethoprim; 

3) the hormones cis-androsterone, 

17-alpha-estradiol, and equilinen; 

and 4) other organic wastewater 

compounds used as antioxidants, 

disinfectants, deodorizers, plasticizers, 

and surfactants. The concentrations 

of these compounds were much 

lower than the steroids, and they 

contributed only a small fraction of the 

total organic load. The results for the 

wastewater compounds measured in 

this study are consistent with results 

from a national reconnaissance for 

Figure 3. Graph showing estimated discharge along 

Boulder Creek, June 13 and October 10, 2000.

Figure 4. Graphs showing downstream variation of specific conductance (A), nitrogen (B), 

and carbon (C).
CONCLUSIONS

Rivers receive water that has interacted with upstream regions, such that 

water chemistry reflects both natural and human activity in a watershed. The 

detailed sampling and analysis in this report provide a baseline for future 

reference, as well as information on the relation between land use, geology, 

and downstream changes in water chemistry. The chemical analyses presented 

in this report include traditional water-quality constituents as well as lowlevel determinations of trace elements, pesticides, and wastewater compounds 

determined by relatively new analytical techniques.

 Results include:

• High-flow concentrations were lower than low-flow concentrations due to 

greater dilution by snowmelt. 

• Water quality of the headwater region reflects minor mineral dissolution 

of the crystalline bedrock; historical hardrock mining did not have a 

major effect on stream chemistry during these flow regimes.

• Concentrations of many constituents increased as Boulder Creek flows 

through the city of Boulder, in part reflecting the more easily weathered 

sedimentary bedrock.

• Treated effluent from the Boulder WWTP, which meets Colorado state 

water quality standards, dominates the chemistry of lower Boulder Creek, 

in part because upstream flow is diverted for municipal and agricultural 

uses and cannot provide instream dilution.

• Wastewater compounds were identified during both high- and low-flow 

conditions, indicating they are not removed during secondary treatment.

• Few pesticides were detected and were likely derived from urban and 

agricultural land uses.

• Boulder Creek contains a complex mixture of inorganic and organic 

constituents that are useful for determining geochemical and 

anthropogenic sources.

REFERENCES

Bau, Michael, and Dulski, Peter, 1996, 

Anthropogenic origin of positive gadolinium 

anomalies in river waters: Applied 

Geochemistry, v. 143, 245-255.

Colorado Department of Public Health 

and Environment, Water Quality 

Control Division, 2002, Surface water 

quality classifications and standards, 

Regulation 31: Basic standards and 

methodologies for surface water (5 CCR 

1002-31), accessed August 23, 2002, at 

http://www.cdphe.state.co.us/op/regs/

waterqualityregs.asp

Kolpin, D.W., Furlong, E.T., Meyer, M.T., 

Thurman, E.M., Zaugg, S.D., Barber, L.B., 

and Buxton, H.T., 2002, Pharmaceuticals, 

hormones, and other organic wastewater 

contaminants in U.S. Streams, 1999-2000, 

A national reconnaissance: Environmental 

Science and Technology, v. 36, p. 1202-

1211.

Murphy, S.F., Verplanck, P.L., and Barber, 

L.B., eds., 2003, Comprehensive water 

quality of the Boulder Creek Watershed, 

Colorado, during high-flow and low-flow 

conditions, 2000: U.S. Geological Survey 

Water-Resources Investigations Report 03-

4045.

U.S. Department of Agriculture, National 

Agricultural Statistics Service, 1997, 

accessed January 15, 2003 at http://

www.nass.usda.gov/census/

Additional information about the Boulder 

Creek Watershed can be obtained at the 

Boulder Area Sustainability Information 

Network (BASIN) website at www.basin.org.

many of the same compounds (Kolpin 

and others, 2002).

Pesticides

About 7,890 kilograms of 

pesticides (active ingredient) are 

applied annually to agricultural land 

(primarily alfalfa, barley, corn, and 

wheat) in Boulder County, mostly in 

the lower basin. The most commonly 

applied pesticides are the herbicides 

atrazine, 2,4-D, dicamba, metolachlor, 

and glyphosate, and the insecticides 

terbufos, carbofuran, and chlorpyrifos.

Eleven pesticides were found 

at one or more sites in Boulder 

Creek or its inflows. This study used 

improved analytical methods that 

provided a broader range of pesticides 

analyzed and lower detection levels 

than previous studies. Pesticides 

were detected during both the highand low-flow sampling, with more 

pesticides detected during low flow. 

Pesticides were mainly detected 

in samples from the lower basin, 

and the most frequently detected 

pesticide was diazinon (found at three 

Boulder Creek sites, Boulder WWTP 

effluent, and Coal Creek). Dichlobenil 

was the pesticide found at highest 

concentration (8 μg/L). Atrazine, 

metolachlor and parathion-methyl, 

used mainly in corn production, were 

Figure 5. Graph showing the 

rare earth element patterns of 

selected low-flow samples. 

Concentrations normalized 

to concentrations in North 

American Shale Composite 

(NASC).

found in Boulder Creek, but none 

of the other pesticides that are most 

commonly applied to lands in Boulder 

County were detected.

Authors: Verplanck, P.L., Murphy, 

S.F., and Barber, L.B.

For reprints contact U.S. Geological Survey, 

Branch of Information Services, Box 25286, 

Denver, CO 80226.