Prepared in cooperation with the Urban Drainage and Flood Control District

Summary and Evaluation of the Quality of Stormwater 

in Denver, Colorado, 2006–2010

By Michael R. Stevens and Cecil B. Slaughter

Open-File Report 2012–1052

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Stevens, M.R., and Slaughter, C.B., 2012, Summary and evaluation of the quality of stormwater 

in Denver, Colorado, 2006–2010: U.S. Geological Survey Open-File Report 2012–1052, 94 p.

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
Acknowledgments

The authors gratefully acknowledge the knowledge and insight provided by Ken 

MacKenzie, Manager, Master Planning Program, Urban Drainage and Flood Control District. 

Technical reviews were provided by Katherine Walton-Day and Michael Sweat, U.S. 

Geological Survey. Editorial reviews were provided by Michael Deacon and Betty Palcsak, U.S. 

Geological Survey. Field and technical assistance was provided by Dennis Smits, Clifford Bossong, 

and Andrea Fleming, U.S. Geological Survey. 

iii
Contents

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

Introduction......................................................................................................................................................... 2

Purpose and Scope......................................................................................................................................... 4

Description of Stations .................................................................................................................................... 4

Previous Investigations ................................................................................................................................... 8

Methods of Study................................................................................................................................................ 8

Streamflow Data Collection............................................................................................................................. 9

Sample Collection and Analysis ....................................................................................................................10

Stormwater Samples ................................................................................................................................. 16

Bacteriological Samples ............................................................................................................................18

Quality Assurance and Quality Control..........................................................................................................18

Summary of Stormwater Quality .......................................................................................................................23

Water Quality of Stormwater Samples ..........................................................................................................24

Comparison to Colorado Water-Quality Standards .......................................................................................52

Evaluation of Stormwater Quality between 2002–2005 and 2006–2010...........................................................55

Statistics........................................................................................................................................................55

Comparison to Previous Results ...................................................................................................................61

Summary ..........................................................................................................................................................61

References Cited..............................................................................................................................................64

Appendix...........................................................................................................................................................67

Figures

Figure 1. Map showing location of stations..................................................................................................... 3

Figure 2. Explanation for boxplots shown in this report ................................................................................ 17

Figure 3. Graphs showing daily values for streamflow and precipitation and dates of sample collection 

(2006–2010) at South Platte River below Union Avenue at Englewood, South Platte River at Denver,

Toll Gate Creek above 6th Avenue at Aurora, Sand Creek at mouth near Commerce City, and South

Platte River at Henderson................................................................................................................................ 23

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte 

River below Union Avenue at Englewood, South Platte River at Denver, Toll Gate Creek above 

6th Avenue at Aurora, Sand Creek at mouth near Commerce City, and South Platte River at 

Henderson, calendar years 1998–2001, 2002–2005, and 2006–2010 ............................................................ 32

iv
v

Tables

Table 1. Station characteristics....................................................................................................................... 5

Table 2. Land-cover characteristics. ............................................................................................................... 7

Table 3. Streamflow statistics for 2006–2010 water years and calendar years............................................... 9

Table 4. Statistics for concentrations and measurement values at stormwater sampling 

sites, 2006–2010 ............................................................................................................................................. 11

Table 5. Analytical results for quality-assurance blanks, 2006–2010............................................................ 19

Table 6. Relative percentage difference statistics for duplicate samples collected during 2006–2010 ......... 22

Table 7. Statistics for concentrations and measurement values at stormwater sampling 

sites, 2006–2010 ............................................................................................................................................. 27

Table 8. Summary of Colorado water-quality standards and properties and constituents that exceeded 

Colorado standards for this study, 2006–2010 ................................................................................................ 55

Table 9. Mann-Whitney test of differences between distributions of water-quality values and 

concentrations in stormwater samples collected in 2002–2005 and 2006–2010 at all sites. ........................... 58

Appendix

Table A1. Statistics for concentrations and measurement values at South Platte River below Union

Avenue at Englewood by year, 2006–2010 ..................................................................................................... 70

Table A2. Statistics for concentrations and measurement values at South Platte River at Denver by

year, 2006–2010.............................................................................................................................................. 75

Table A3. Statistics for concentrations and measurement values at Toll Gate Creek above 6th Avenue

at Aurora by year, 2006–2010 ......................................................................................................................... 80

Table A4. Statistics for concentrations and measurement values at Sand Creek at mouth near

Commerce City by year, 2006–2010................................................................................................................ 85 

Table A5. Statistics for concentrations and measurement values at South Platte River at Henderson

by year, 2006–2010 ......................................................................................................................................... 90
vi

Conversion Factors and Abbreviations

Multiply By To obtain

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

acre-foot (acre-ft) 1,233 cubic meter (m3)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3) 

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

ALERT Automated Local Evaluation in Real Time

CODWR Colorado Division of Water Resources

CDPHE Colorado Department of Public Health and Environment

QA/QC Quality assurance and quality control

MWRD Metropolitan Wastewater Reclamation District

NWIS National Water Information System

NWQL National Water Quality Laboratory

RPD Relative percent difference

UBL Universal blank water

UDFCD Urban Drainage and Flood Control District 

USGS U.S. Geological Survey

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD83)

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or micrograms 

per liter (μg/L).

Water year is defined in this report as the 12-month period October 1 through September 30, designated by 

the calendar year in which it ends.
1

Summary and Evaluation of the Quality of Stormwater 

in Denver, Colorado, 2006–2010

By Michael R. Stevens and Cecil B. Slaughter

Abstract

Stormwater in the Denver area was sampled by the U.S. Geological Survey, in cooperation 

with the Urban Drainage and Flood Control District, in a network of 5 monitoring stations—3 on 

the South Platte River and 2 on streams tributary to the South Platte River, Sand Creek, and Toll 

Gate Creek beginning in January 2006 and continuing through December 2010. Stormwater 

samples were analyzed at the U.S. Geological Survey National Water Quality Laboratory during 

2006–2010 for water-quality properties such as pH, specific conductance, hardness, and residue on 

evaporation at 105 degrees Celsius; for constituents such as major ions (calcium, magnesium), 

organic carbon and nutrients, including ammonia plus organic nitrogen, ammonia, dissolved nitrite 

plus nitrate, total phosphorus, and orthophosphate; and for metals, including total recoverable and 

dissolved phases of copper, lead, manganese, and zinc. Samples collected during selected storms 

were also analyzed for bacteriological indicators such as Escherichia coli and fecal coliform at the 

Metro Wastewater Reclamation Laboratory.

About 200 stormwater samples collected during storms characterize the quality of storm 

runoff during 2006–2010. In general, the quality of stormwater (2006–2010) has improved for 

many water-quality constituents, many of which had lower values and concentrations than those in 

stormwater collected in 2002–2005. However, the physical basis, processes, and the role of 

dilution that account for these changes are complex and beyond the scope of this report. 

The water-quality sampling results indicate few exceptions to standards except for 

dissolved manganese, dissolved zinc, and Escherichia coli. Stormwater collected at the South 

Platte River below Union Avenue station had about 10 percent acute or chronic dissolved 

manganese exceedances in samples; samples collected at the South Platte River at Denver station 

had less than 5 percent acute or chronic dissolved manganese exceedances. In contrast, samples 

collected at Toll Gate Creek above 6th Avenue at Aurora station, Sand Creek at mouth near 

Commerce City station, and the South Platte River at Henderson station, each had about 30 to 50 

percent exceedances of both acute and chronic dissolved manganese standards. Of the samples

collected at Sand Creek at mouth near Commerce City, 1 sample exceeded the acute standard and 4 

samples exceeded the chronic standard for dissolved zinc, but no samples collected from the other 

sites exceeded either standard for zinc. Almost all samples of stormwater analyzed for Escherichia 

coli exceeded Colorado numeric standards. A numerical standard for fecal coliform is no longer 

applicable as of 2004. 

Results from the 2002–2005 study indicated that the general quality of stormwater had 

improved during 2002–2005 compared to 1998–2001, having fewer exceedances of Colorado 

standards, and showing downward trends for many water-quality values and concentrations. These 

trends coincided with general downward or relatively similar mean streamflows for the 2002–2005 
2

compared to 1998–2001, which indicates that dilution may be a smaller influence on values and 

concentrations than other factors. For this report, downward trends were indicated for many

constituents at each station during 2006–2010 compared to 2002–2005. The trends for mean 

streamflow for 2006–2010 compared to 2002–2005 are upward at all sites except for the South 

Platte River at Henderson, indicating that dilution by larger flows could be a factor in the 

downward concentration trends. At the South Platte River below Union Avenue station, downward 

trends were indicated for hardness, dissolved ammonia, dissolved orthophosphate, and dissolved 

copper. Upward trends at South Platte River below Union Avenue were indicated for pH. At the 

South Platte River at Denver station, downward trends were indicated for total ammonia plus 

organic nitrogen, dissolved ammonia, dissolved nitrite plus nitrate, dissolved orthophosphate, total 

phosphorus, dissolved organic carbon, and dissolved lead, manganese, and zinc, and total 

recoverable zinc. An upward trend in properties and constituents at South Platte River at Denver

was indicated for pH. At Toll Gate Creek above 6th Avenue at Aurora, downward trends were 

indicated for residue on evaporation, total ammonia plus organic nitrogen, dissolved ammonia, 

dissolved orthophosphate, total phosphorus, and total recoverable copper, lead, manganese, and 

zinc. Upward trends in properties and constituents at Toll Gate Creek above 6th Avenue at Aurora 

were indicated for pH, specific conductance, and dissolved nitrite plus nitrate. At Sand Creek at 

mouth near Commerce City, downward trends were indicated for hardness, dissolved calcium, total 

ammonia plus organic nitrogen, and dissolved ammonia, orthophosphate, manganese, and zinc. An 

upward trend in properties and constituents at Sand Creek at mouth near Commerce City was 

indicated for pH. Downward trends at South Platte River at Henderson were indicated for specific 

conductance, hardness, dissolved magnesium, residue on evaporation, total ammonia plus organic 

nitrogen, dissolved ammonia, dissolved nitrite plus nitrate, dissolved orthophosphate, total 

phosphorus, dissolved lead and manganese, and total recoverable copper, lead, manganese, and 

zinc.

Introduction

The quality of stormwater runoff and trend of water quality in stormwater runoff is of 

interest to many management and regulatory agencies as well as scientists, recreational stream 

users, and the general public. The basic character of stormwater runoff is of general interest to all, 

and more detailed information concerning spatial and temporal variations in stormwater quality 

may be of interest to management and regulatory agencies. In response to these interests, the U.S. 

Geological Survey (USGS), in cooperation with the Urban Drainage and Flood Control District 

(UDFCD), collected stormwater samples during wet weather from a monitoring network of five 

stations in and around metropolitan Denver, Colorado. The monitoring network is the basis for 

evaluation of stormwater conditions in Aurora, Denver, and Lakewood, and consists of three 

stations on the South Platte River, the principal stream draining the area and two stations on 

streams that are tributary to the South Platte River, Sand Creek and Toll Gate Creek (fig. 1). 

Reaches of each of these streams have use classifications and associated numeric standards for 

some water-quality properties and constituents and bacteriological indicators included in this study, 

such as pH, dissolved nitrite plus nitrate, dissolved copper, lead, manganese, zinc, and Escherichia 

coli, (Colorado Department of Public Health and Environment, 2011a, 2011b).
3

Figure 1. Map showing location of stations.
4

Purpose and Scope

The purpose of this report is to present summaries and an evaluation of laboratory analyses 

for water-quality properties and constituents and bacteriological indicators in stormwater samples 

that characterize the quality of stormwater in and around metropolitan Denver, Colo., from January 

2006 through December 2010. Specifically, the summaries and evaluation in this report include 

• characterizations that summarize streamflow associated with stormwater samples,

• descriptive statistics that summarize stormwater quality,

• comparisons of stormwater quality with numeric standards for use classifications established by 

the CDPHE, and

• comparison of previous characterizations of stormwater quality, based on data collected during 

2002–2005 to the stormwater quality characterized in this report (2006–2010). 

The summary presented in this report (2006–2010) provides a basis for comparison to 

similar data (1998-2001 and 2002–2005); however, the physical basis and processes responsible 

for changes between the two periods are complex and are beyond the scope of this report. 

Description of Stations

The five stations in the monitoring network include three stations on the main stem of the 

South Platte River (fig. 1; table 1). These three stations, South Platte River below Union Avenue at 

Englewood, South Platte River at Denver, and South Platte River at Henderson, will be referred to 

in this report as “Union,” “Denver,” and “Henderson.” The stations on the South Platte River 

receive streamflow from several tributaries, and the South Platte River is sometimes referred to as 

a “receiving stream.” The two additional stations, Sand Creek at mouth near Commerce City and 

Toll Gate Creek above 6th Avenue at Aurora, represent tributaries and are referred to in this report 

as “Sand Creek” and “Toll Gate.” Sand Creek is tributary to the South Platte River, and Toll Gate

is tributary to Sand Creek. Collectively, these five stations define a drainage system that drains 

most of the area in and around metropolitan Denver.
5

Table 1. Station characteristics.

[Station number and name from the U.S. Geological Survey (USGS) National Water Inventory System. Latitude 

and Longitude (North American Datum of 1927), geographic coordinates in degrees minutes seconds; Operating 

agency indicates agency responsible for streamflow records: CODWR, Colorado Division of Water Resources; 

UDFCD, Urban Drainage and Flood Control District. RF location, location of precipitation gage associated with 

station, the source for all precipitation records is UDFCD] 

Station number Station name Station location Operating

agency

RF location

Latitude Longitude Latitude Longitude

06710247 South Platte River below Union Avenue at 

Englewood

39o37'57" 105o00'52" USGS 39o37'57" 105o00'52"

06714000 South Platte River at Denver 39o45'35" 105o00'10" CODWR 39o44'32" 104o59'58"

394329104490101 Toll Gate Creek above 6th Avenue at Aurora 39o43'29" 104o49'01" UDFCD 39o43'30" 104o49'06"

394839104570300 Sand Creek at mouth near Commerce City 39o48'36" 104o57'00" USGS/

UDFCD

39o48'39" 104o57'03"

06720500 South Platte River at Henderson 39o55'10" 104o52'04" CODWR 39o55'19" 104o52'00"
6

The South Platte River originates in central Colorado and has a contributing drainage area 

of 3,860 square miles (mi2) at the Denver station (U.S. Geological Survey, 2006–2010). Snowmelt 

from mountainous areas of central Colorado is a principal source of spring streamflow in the South 

Platte River; however, reservoirs and diversions along the South Platte River regulate streamflow. 

Chatfield Lake, which is just upstream from Denver, contains the vast majority of storm runoff 

generated upstream from the reservoir, essentially truncating the contributing drainage area for the 

stations used in this study (fig. 1). Consequently, most stormwater runoff in the South Platte River 

in the Denver area is derived from a local contributing drainage area that does not extend upstream 

from Chatfield Lake. 

Land-cover characteristics used in this report (table 2) represent percentages of the local 

contributing drainage areas for four aggregated categories developed on the basis of USGS 

National Land Cover data from 1992 (U.S. Geological Survey, 2003). In the aggregated 

classification, areas related to crops are classified as “agricultural”; areas identified as commercial, 

industrial, or transportation and areas related to mining, such as sand and gravel operations, are 

classified as “urban”; areas identified as residential or recreational are classified as “residential”; 

and all remaining areas are classified as “undeveloped.” The local contributing drainage areas are 

less than 10 percent urban and consist mostly of undeveloped land (ranging from 41.6 to 76.0 

percent in individual basins) and residential areas (17.8 to 50.4 percent). The agricultural landcover classification shows a tendency to increase with local contributing drainage area (from 0.7 to 

3.2 percent in the South Platte main-stem stations (Union, Denver and Henderson). In the Sand 

Creek drainage area, agricultural land cover increases from 5.0 to 8.7 percent from Toll Gate to 

Sand Creek at mouth (table 2). Undeveloped and urban land cover also increases from Toll Gate to 

Sand Creek.
7

Table 2. Land-cover characteristics.

[Station number and name from the U.S. Geological Survey (USGS) National Water Inventory System; Operating 

agency indicates agency responsible for streamflow records: USGS, U.S. Geological Survey; CODWR, Colorado 

Division of Water Resources; UDFCD, Urban Drainage and Flood Control District; CDA, contributing drainage 

area in square miles; LCDA, local contributing drainage area in square miles adjusted for regulation (excludes CDA 

upstream from Chatfield Lake for South Platte River stations). Land cover indicates percentage of LCDA on basis 

of National Land Cover data from 1992 (U.S. Geological Survey, 2003) for undeveloped areas (Und), agricultural 

areas (Ag), residential areas (Res), and urban areas (Urb); land-cover percentages may not sum to 100 percent due 

to rounding; comments provide important historical operational events] 

Station number CDA LCDA

Land cover, in percentage of 

local contributing drainage 

area Comments

Ag Urb Res Und

06710247 3,090 76 0.7 5.8 50.4 43.1 Gage operated by USGS as described in Sauer and Turnipseed 

(2010). 

06714000 3,860 841 1.2 3.4 19.4 76.0 Gage operated by CODWR as described in Sauer and 

Turnipseed (2010).

394329104490101 35 35 5.0 5.5 48.0 41.6 Gage operated as part of flood warning network (UDFCD). 

394839104570300 184 184 8.7 6.3 22.4 62.6 Gage operated by USGS as described in Sauer and Turnipseed 

(2010) from 2006 to 2009. Gage operated as part of flood 

warning network in 2010 (UDFCD). 

06720500 4,770 1,740 3.2 3.7 17.8 75.3 Gage operated by CODWR as described in Sauer and 

Turnipseed (2010).
8

Previous Investigations

Several previous investigations of stormwater quality have been completed in the Denver 

area. From 1979 through 1983, annual data reports were issued summarizing stormwatermonitoring activities by the USGS for various stations in the Denver metropolitan area (Ellis, 1978; 

Ellis and Alley, 1979; Hall and Duncan, 1980, 1981; Gibbs, 1981; Gibbs and Doerfer, 1982; Gibbs 

and others, 1983). Although these activities included some sampling in the South Platte River, most 

of the activities focused on tributaries to the South Platte River. Blakely and others (1983) reported 

on washoff characteristics from two experimental plots, and Ellis and others (1984), Ellis and 

Mustard (1985), and Mustard and others (1985) provided additional stormwater information on the 

basis of the USGS stormwater-monitoring activities. The Denver Regional Council of 

Governments (1983) also summarized their stormwater-monitoring activities in the Denver 

metropolitan area.

More recently, Bossong and others (2005) summarized stormwater quality in the Denver 

area on the basis of samples collected during October 1997 through September 2001. The 

conclusions from the 2005 report indicated that concentrations in stormwater tended to increase in 

a downstream direction along the South Platte River, that concentrations for many water-quality 

properties and constituents in stormwater were relatively smaller compared to concentrations in 

base flow, that exceptions to CDPHE numeric standards (measured concentrations that did not 

meet standards) for some metals occurred in as many as 10 to 25 percent of samples, and that there 

were consistent exceedances of numeric standards for bacteriological indicators such as 

Escherichia coli and fecal coliform. In previously published USGS reports, data were grouped by 

water years (October 1 through September 30 of the following year). In this report, data are 

grouped and analyzed by calendar year. 

A second report (Bossong and Fleming, 2008) summarized stormwater quality for water 

years 2002–2005 using methods similar to those in Bossong and others (2005). The 2002–2005 

report indicated that statistically significant downward trends in concentrations were identified for 

the combined means of all stations for dissolved copper, manganese, and zinc, and total 

recoverable zinc. Downward trends between 1998–2001 means and 2002–2005 means were also 

relatively widespread for many constituents including hardness, ammonia plus organic nitrogen, 

total phosphorus, most dissolved metals (lead, manganese, and zinc), and all total recoverable 

metals (copper, lead, manganese, and zinc). Some trends were indicated only at individual stations. 

Downward trends were identified for ammonia as nitrogen at Union, Denver, and Henderson, for 

orthophosphate at Sand Creek, and for dissolved nitrite plus nitrate at Denver and Henderson. Only 

dissolved nitrite plus nitrate at Union and total recoverable copper and lead at Sand Creek showed

consistent upward trends. There were many more exceedances to water-quality standards in the 

1998–2001 study than in the 2002–2005 study. Escherichia coli and fecal coliform consistently 

exceeded bacteria standards in the 1998–2001 and the 2002–2005 reports. 

Methods of Study

The methods used to collect and process streamflow data, collect and process stormwater 

samples, and perform laboratory analyses are described in this section. The section also describes 

quality-assurance and quality-control (QA/QC) methods and results. 
9

Streamflow Data Collection

Streamflow records are maintained for each station by various operating agencies (table 3). 

Union is operated by the USGS in accordance with methods described by (Turnipseed and Sauer, 

2010; Sauer and Turnipseed, 2010). The streamflow data for Union is maintained in the USGS 

National Water Information System (NWIS; http://waterdata.usgs.gov/nwis/). Daily data, which 

represent various statistics for a 24-hour period, and subdaily data that are instantaneous values 

used to develop daily values, are available from NWIS. In addition, the records were published in 

USGS annual data reports for 2006–2010 (U.S. Geological Survey, 2006–2010).

Table 3. Streamflow statistics for 2006–2010 water years and calendar years.

[Station number and name from the U.S. Geological Survey National Water Inventory 

System; ft3/s, cubic foot per second; Toll Gate Creek is not included in streamflow statistics 

because the gage is only operated seasonally; water year, the 12-month period from October 

1 through September 30, and designated by the calendar year in which it ends and which

includes 9 of the 12 months; calendar year, 12-month period from January 1 through 

December 31]

Descriptor

Station name (number)

Union

(06710247)

Denver

(06714000)

Sand Creek 

(394839104570300)

Henderson

(06720500)

Water-year statistics1

Period, in water years 2006–2010 2006–2010 2006–2010 2006–2010

Annual mean, in ft3/s 209 382 33 541

Highest annual mean, in ft3/s 411 684 42 813

 Year it occurred 2007 2007 2009 2007

Lowest annual mean, in ft3/s 105 219 21 333

 Year it occurred 2006 2006 2006 2006

Highest daily mean, in ft3/s 2,388 3,610 1,035 4,570

 Date it occurred May 2, 2007 April 24, 2007 April 24, 2007 June 2, 2009

Lowest daily mean, in ft3/s 5.84 80 0.74 44.6

 Date it occurred June 23, 2006 June 14, 2006 March 6, 2009 January 18, 2010

Calendar-year statistics

Period, in calendar years 2006–2010 2006–2010 2006–2010 2006–2010

Annual mean, in ft3/s 209 386 33 531

Highest annual mean, in ft3/s 412 685 45 813

 Year it occurred 2007 2007 2009 2007

Lowest annual mean, in ft3/s 112 228 21 333

 Year it occurred 2006 2006 2006 2006

Highest daily mean, in ft3/s 2,388 3,610 1,035 4,570

 Date it occurred May 2, 2007 April 24, 2007 April 24, 2007 June 2, 2009

Lowest daily mean, in ft3/s 5.84 80 0.74 44.6

 Date it occurred June 23, 2006 June 14, 2006 March 6, 2009 January 18, 2010 1

Previous reports (Bossong and others, 2005; Bossong and Fleming, 2008) grouped and analyzed data by water years,

but this report groups and analyzes data by calendar year
10

Denver and Henderson are operated by the Colorado Division of Water Resources 

(CODWR). The CODWR streamflow records at Denver and Henderson were reviewed and 

published by USGS for 1998–2008, but have not been reviewed and published by USGS since 

2009. Daily values for 2006–2010 and instantaneous values for the most recent year are maintained

on a publicly available CODWR Web-based system

http://www.dwr.state.co.us/SurfaceWater/Default.aspx. All instantaneous values from Denver and 

Henderson are available on a UDFCD Web-based system (http://alert.udfcd.org).

Toll Gate and Sand Creek currently are operated by UDFCD using systematic but 

undocumented methods that involve theoretical stage and streamflow relations developed on the 

basis of channel geometry (Sand Creek was operated by the USGS 1998–2009 until the USGS 

gage location was moved). The Toll Gate and Sand Creek data are used as part of an Automated 

Local Evaluation in Real Time (ALERT) system operated by UDFCD to assess field conditions in 

a real-time mode in order to facilitate flood forecasting. Because the UDFCD data are focused on 

real-time information, data may be available only for periods of storm runoff. The UDFCD records 

consist only of instantaneous values that are archived but do not receive additional processing or

review. 

Sample Collection and Analysis

During 2006–2010, composite stormwater samples were collected for water-quality 

analysis. Composite samples integrate several aliquots of water collected over a period of time as 

described in the following “Stormwater Samples” section. All composite samples were collected 

using refrigerated automatic pumping samplers that withdraw water from a fixed point in the 

channel cross section. Bacteriological samples were manually collected during selected storms. 

The composite and bacteriological samples are collectively referred to as “stormwater samples.” 

Stormwater samples collected during 2006–2010 were analyzed by the USGS National 

Water Quality Laboratory (NWQL) in Denver, and bacteriological samples were analyzed at

Metropolitan Wastewater Reclamation District (MWRD) in Denver. The results of all laboratory 

analyses obtained as part of this study are available in the NWIS database and can be accessed 

using the USGS Web interface to water data at http://waterdata.usgs.gov/co/nwis/qw/. Table 4 

describes the laboratory analytical methods used in this study. Results referred to as “dissolved” 

are from water that has been passed through a filter that removes all particles greater than 0.45 

micrometer (μm) in their smallest diameter. Results referred to as “total recoverable” are from 

unfiltered water that has undergone an in-bottle digestion procedure (Hoffman and others, 1996). 

Results referred to as “whole water” are from water that has not been filtered or digested. Table 4 

also lists the reporting levels. The general term “reporting level” specifies the level at which 

laboratory results are censored, or reported as “less than” (Helsel, 2005). The reporting level is 

generally twice the long-term method detection level (Childress, 1999). The long-term method 

detection level is determined by the NWQL based on long-term quality control performance 

(Childress, 1999). Laboratory quantifications of values below the reporting level are sometimes 

made by the laboratory on a particular analysis and are qualified as estimated. 
11

Table 4. Statistics for concentrations and measurement values at stormwater sampling sites, 2006–2010.

[ft3

/s, cubic foot per second; mL, milliliter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; 

mg/L, milligrams per liter; μg/L, micrograms per liter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of 

samples; nlt, number of “less thans” or censored data; --, not calculated; %, percentile; MPN, most probable number] 

Property or 

constituent

Summary statistics

n nlt

Range of uncensored values Censoring level Mean SD 25% Median 

50% 75% Minimum Maximum

06710247 South Platte River below Union Avenue at Englewood, Colorado

Streamflow, mean, ft3/s 410 312 131 346 632. 38 0 33 1,490 --

Streamflow, volume, acre-feet 378 282 136 322 574 38 0 30 1,360 --

pH, standard units 7.7 0.2 7.6 7.7 7.8 38 0 6.9 8.4 --

Specific conductance, μS/cm 490 210 345 409 585 38 0 226 993 --

Hardness, total, mg/L as CaCO3 145 47.4 113 133 164 38 0 81.5 290 --

Calcium, mg/L 43 13 33 39 47 38 0 24 84 --

Magnesium, mg/L 9.3 3.6 6.8 7.9 11 38 0 5.1 20 --

Suspended solids, residue on evaporation 

at 105 °C, mg/L 226 183 77 196 326 37 1 16 670 --

Nitrogen, total, ammonia plus organic as 

N, mg/L 1.4 0.72 0.83 1.4 1.7 8 0 0.46 4.00 --

Nitrogen, dissolved, ammonia as N, mg/L 0.06 0.06 0.02 0.05 0.08 38 6 0.01 6.00 0.01

Nitrogen, dissolved, nitrite plus nitrate as 

N, mg/L 0.82 0.56 0.42 0.59 1.03 38 0 0.17 2.71 --

Orthophosphate, dissolved as P, mg/L 0.059 0.051 0.024 0.043 0.068 38 0 0.004 0.237 --

Phosphorus, total as P, mg/L 0.38 0.24 0.22 0.35 0.49 38 0 0.07 0.98 --

Carbon, dissolved, organic as C, mg/L 6.7 3.0 4.8 6.1 7.5 38 0 2.9 18.6 --

Escherichia coli, MPN/100 mL 5,380 5,620 -- 5,380 -- 4 0 20 11,000 --

Fecal coliform, MPN/100 mL 6,180 6,200 -- 6,180 -- 4 0 20 12,000 --

Copper, dissolved, μg/L 2.1 0.8 1.6 1.9 2.6 38 0 1.1 4.6 --

Copper, total recoverable, μg/L 11 7.9 5.4 11 14 36 0 2.9 34 --

Lead, dissolved, μg/L 0.08 0.06 0.04 0.06 0.07 38 6 0.03 0.27 0.04, 0.06

Lead, total recoverable, μg/L 8.3 7.2 2.6 7.4 10.3 36 0 0.62 26.9 --

Manganese, dissolved, μg/L 19.5 26.0 6.4 11.6 20.6 38 0 0.2 119 --

Manganese, total recoverable, μg/L 304 242 143 263 379 36 0 42.2 884 --

Zinc, dissolved, μg/L 5.3 3.0 2.8 4.8 6.9 38 0 1.8 13.6 --

Zinc, total recoverable, μg/L 40 29 19 34 53 36 0 7 134 --
12

Property or

constituent

Summary statistics

n nlt

Range of uncensored values Censoring level

Mean SD 25% Median 

50% 75% Minimum Maximum

06714000 South Platte River at Denver, Colorado

Streamflow, mean, ft3/s 1,480 1,370 656. 1,090 1,790 49 0 184 8,083 --

Streamflow, volume, acre-feet 1,410 1,280 667 1,010 1,630 49 0 168 7,350 --

pH, standard units 7.5 0.3 7.5 7.5 7.7 48 0 6.6 8.0 --

Specific conductance, μS/cm 522 225 384 491 597 46 0 260 1,390 --

Hardness, total, mg/L as CaCO3 141 43 108 138 160 48 0 74.4 251 -- Calcium, mg/L 42 12 32 42 49 48 0 22 76 -- Magnesium, mg/L 8.6 3.1 6.2 8 9.8 48 0 4.3 18 -- Suspended solids, residue on evaporation 

at 105 °C, mg/L 202 125 122 180 258 47 0 20.0 660 --

Nitrogen, total, ammonia plus organic as 

N, mg/L 1.8 0.82 1.2 1.5 2.2 7 0 0.51 4.2 --

Nitrogen, dissolved, ammonia as N, mg/L 0.12 0.13 0.03 0.07 0.13 47 4 0.01 0.60 0.01

Nitrogen, dissolved, nitrite plus nitrate as 

N, mg/L 1.21 0.65 0.80 0.98 1.38 46 0 0.55 3.69

--

Orthophosphate, dissolved as P, mg/L 0.150 0.087 0.098 0.122 0.167 47 0 0.046 0.487 -- Phosphorus, total as P, mg/L 0.54 0.24 0.39 0.47 0.65 47 0 0.14 1.33 -- Carbon, dissolved, organic as C, mg/L 6.3 2.0 5.0 5.8 7.2 48 0 4.1 12.2 -- Escherichia coli, MPN/100 mL 8,900 9,926 -- 6,250 -- 4 0 110 23,000 -- Fecal coliform, MPN/100 mL 10,520 9,486 -- 9,450 -- 4 0 170 23,000 --

Copper, dissolved, μg/L 2.9 1.2 2.2 2.5 3.3 48 1 1.5 7.0 1 

Copper, total recoverable, μg/L 17 11 9.1 15 21 45 0 3.8 67 --

Lead, dissolved, μg/L 0.118 0.062 0.075 0.1 0.155 48 0 0.04 0.31 --

Lead, total recoverable, μg/L 15.2 13.7 6.72 13.1 18 45 0 2.13 79.5 --

Manganese, dissolved, μg/L 11 25 1.1 3.3 9.7 48 0 0.1 156 --

Manganese, total recoverable, μg/L 333 182 215 288 412 45 0 77.9 873 --

Zinc, dissolved, μg/L 8.3 4.5 5.0 6.8 10.6 48 0 3.3 21.3 --

Zinc, total recoverable, μg/L 71 51 33 60 91 45 0 13 287 --

Table 4. Statistics for concentrations and measurement values at stormwater sampling sites, 2006–2010.—Continued

[ft3

/s, cubic foot per second; mL, milliliter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; 

mg/L, milligrams per liter; μg/L, micrograms per liter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of 

samples; nlt, number of “less thans” or censored data; --, not calculated; %, percentile; MPN, most probable number] 
13

Property or

constituent

Summary statistics

n nlt

Range of uncensored values Censoring level Mean SD 25% Median 

50% 75% Minimum Maximum

394329104490101 Toll Gate Creek above 6th Avenue near Aurora, Colorado

Streamflow, mean, ft3/s 261 289 73 182 345 30 0 4 1,400 --

Streamflow, volume, acre-feet 242 241 67 156 313 30 0 4 921 --

pH, standard units 7.7 0.2 7.5 7.6 7.8 33 0 7.4 8.0 --

Specific conductance, μS/cm 1,112 506 720 957 1,500 31 0 363 2,230 --

Hardness, total, mg/L as CaCO3 316 167 193 251 418 33 0 84.6 746 --

Calcium, mg/L 95 50 60 77 129 33 0 26 226 --

Magnesium, mg/L 19 10 11 15 24 33 0 4.6 44 --

Suspended solids, residue on evaporation 

at 105 °C, mg/L 180 186 46 97 254 31 0 10 695 --

Nitrogen, total, ammonia plus organic as 

N, mg/L 1.6 0.78 1.1 1.4 2.0 32 0 0.63 3.5 --

Nitrogen, dissolved, ammonia as N, mg/L 0.14 0.15 0.04 0.09 0.20 32 3 0.01 0.66 0.01

Nitrogen, dissolved, nitrite plus nitrate as 

N, mg/L 0.64 0.21 0.49 0.59 0.77 32 0 0.36 1.28 --

Orthophosphate, dissolved as P, mg/L 0.022 0.020 0.007 0.013 0.035 32 1 0.003 0.077 0.003

Phosphorus, total as P, mg/L 0.33 0.32 0.12 0.22 0.44 32 0 0.04 1.38 --

Carbon, dissolved, organic as C, mg/L 10.4 3.7 8.5 10.4 11.7 33 0 5.3 20.9 --

Escherichia coli, MPN/100 mL 2,310 1,715 -- 3,300 -- 3 0 330 3,300 --

Fecal coliform, MPN/100 mL 2,310 1,715 -- 3,300 -- 3 0 330 3,300 --

Copper, dissolved, μg/L 2.5 1.1 1.8 2.2 2.8 33 0 1.0 4.9 --

Copper, total recoverable, μg/L 12 8.8 5.8 8.3 15 33 0 2.9 33 --

Lead, dissolved, μg/L 0.097 0.07 0.053 0.08 0.12 33 4 0.04 0.34 0.04, 0.06

Lead, total recoverable, μg/L 7.18 7.67 2.35 3.45 9.18 33 0 0.71 28.3 --

Manganese, dissolved, μg/L 75.29 78.83 20.3 50.9 105 33 0 2.8 330 --

Manganese, total recoverable, μg/L 381 269 192 244 480 33 0 103 1,120 --

Zinc, dissolved, μg/L 8.8 5.4 4.7 8.0 11 33 0 1.5 26.1 --

Zinc, total recoverable, μg/L 47 35 24 33 59 33 0 9 132 --

Table 4. Statistics for concentrations and measurement values at stormwater sampling sites, 2006–2010.—Continued

[ft3

/s, cubic foot per second; mL, milliliter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; 

mg/L, milligrams per liter; μg/L, micrograms per liter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of 

samples; nlt, number of “less thans” or censored data; --, not calculated; %, percentile; MPN, most probable number] 
14

Property or

constituent

Summary statistics

n nlt

Range of uncensored values Censoring level

Mean SD 25% Median 

50% 75% Minimum Maximum

394839104570300 Sand Creek at mouth near Commerce City, Colorado

Streamflow, mean, ft3/s 285 275 100 229 374 47 0 21 1,620 --

Streamflow, volume, acre-feet 255 249 95 205 348 47 0 19 1,473 --

pH, standard units 7.6 0.2 7.4 7.6 7.8 47 0 7.3 8.0 --

Specific conductance, μS/cm 811 392 575 684 908 45 0 330 2,020 --

Hardness, total, mg/L as CaCO3 195 79 153 171 210 47 0 74.8 455 --

Calcium, mg/L 59 22 47 52 64 47 0 23 131 -- Magnesium, mg/L 12 5.6 8.4 10 13 47 0 4.1 31 --

Suspended solids, residue on evaporation 

at 105 °C, mg/L 351 306 128 236 470 47 0 56.0 1,360 -- Nitrogen, total, ammonia plus organic as 

N, mg/L 2.6 1.6 1.6 2.0 3.2 45 0 1.1 9.5 --

Nitrogen, dissolved, ammonia as N, mg/L 0.11 0.11 0.02 0.09 0.18 45 8 0.01 0.47

0.005, 0.01, 

0.02

Nitrogen, dissolved, nitrite plus nitrate as 

N, mg/L 0.88 0.43 0.59 0.78 0.95 45 0 0.41 2.55 -- Orthophosphate, dissolved as P, mg/L 0.121 0.113 0.059 0.089 0.141 45 0 0.017 0.627 --

Phosphorus, total as P, mg/L 0.79 0.47 0.49 0.63 0.90 45 0 0.27 2.36 -- Carbon, dissolved, organic as C, mg/L 10.8 4.2 7.8 9.3 13.2 46 0 5.8 23.6 --

Escherichia coli, MPN/100 mL 3,530 2,370 -- 2,950 -- 3 0 790 4,900 -- Fecal coliform, MPN/100 mL 2,630 2,090 -- 2,140 -- 3 0 790 4,900 --

Copper, dissolved, μg/L 3.0 2.0 1.6 2.4 3.6 47 0 1.2 12.5 --

Copper, total recoverable, μg/L 22 14 11 17 28 47 0 6.5 55 --

Lead, dissolved, μg/L 0.19 0.14 0.10 0.14 0.25 47 0 0.04 0.74 --

Lead, total recoverable, μg/L 21.0 17.3 6.16 14.5 31.6 47 0 3.67 60.5 --

Manganese, dissolved, μg/L 45 78 2.5 11 59 47 0 0.5 391 --

Manganese, total recoverable, μg/L 678 661 283 466 640 47 0 144 3,210 --

Zinc, dissolved, μg/L 67.2 191 7.8 10.9 21.0 47 0 2.8 1,170 --

Zinc, total recoverable, μg/L 524 1205.4 44 75 241 47 0 22 5,230 --

Table 4. Statistics for concentrations and measurement values at stormwater sampling sites, 2006–2010.—Continued

[ft3

/s, cubic foot per second; mL, milliliter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; 

mg/L, milligrams per liter; μg/L, micrograms per liter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of 

samples; nlt, number of “less thans” or censored data; --, not calculated; %, percentile; MPN, most probable number] 
15

Property or

constituent

Summary statistics

n nlt

Range of uncensored values Censoring level Mean SD 25% Median 

50% 75% Minimum Maximum

06720500 South Platte River at Henderson, Colorado

Streamflow, mean, ft3/s 2,260 3,290 754 1,470 2,390 33 0 218 19,000 --

Streamflow, volume, acre-feet 1,720 1,570 663 1,340 2,040 33 0 198 6,920 --

pH, standard units 7.3 0.2 7.2 7.4 7.5 34 0 6.6 7.7 --

Specific conductance, μS/cm 557 196 454 528 601 34 0 205 1,260 --

Hardness, total, mg/L as CaCO3 141 31 119 133 154 34 0 95 226 --

Calcium, mg/L 42 9 36 41 47 34 0 28 64 --

Magnesium, mg/L 8.9 2.5 7.3 8.3 10 34 0 6.0 17 --

Suspended solids, residue on evaporation 

at 105 °C, mg/L 174 112 94 133 242 32 0 44 545 --

Nitrogen, total, ammonia plus organic as 

N, mg/L 2.9 1.7 2.0 2.6 3.4 3 0 0.96 9.4 --

Nitrogen, dissolved, ammonia as N, mg/L 0.80 0.88 0.24 0.58 0.73 33 1 0.11 3.38 0.01

Nitrogen, dissolved, nitrite plus nitrate as 

N, mg/L 2.08 0.81 1.53 1.87 2.24 33 0 0.96 4.64 --

Orthophosphate, dissolved as P, mg/L 0.461 0.279 0.262 0.367 0.621 33 0 0.152 1.15 --

Phosphorus, total as P, mg/L 1.02 0.46 0.72 0.96 1.18 33 0 0.37 2.85 --

Carbon, dissolved, organic as C, mg/L 9.1 5.3 6.2 7.5 10.6 34 0 4.8 34.4 --

Escherichia coli, MPN/100 mL 10,400 11,550 -- 7,900 -- 3 0 310 23,000 --

Fecal coliform, MPN/100 mL 12,100 11,370 -- 13,000 -- 3 0 310 23,000 --

Copper, dissolved, μg/L 3.04 0.92 2.4 2.75 3.4 34 0 1.7 5.5 --

Copper, total recoverable, μg/L 24 13 13 22 28 32 0 9.1 63 --

Lead, dissolved, μg/L 0.22 0.08 0.17 0.21 0.29 34 0 0.07 0.41 --

Lead, total recoverable, μg/L 22.6 15.9 10.2 17.3 32.4 32 0 5.09 70.9 --

Manganese, dissolved, μg/L 62 127 18 33 62 34 0 0.6 757 --

Manganese, total recoverable, μg/L 408 305 243 359 472 32 0 138 1,840 --

Zinc, dissolved, μg/L 18.3 7.5 13.1 17.3 22.1 34 0 6.7 45.8 --

Zinc, total recoverable, μg/L 114 68 68 91 148 32 0 32 308 --

Table 4. Statistics for concentrations and measurement values at stormwater sampling sites, 2006–2010.—Continued

[ft3

/s, cubic foot per second; mL, milliliter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; 

mg/L, milligrams per liter; μg/L, micrograms per liter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of 

samples; nlt, number of “less thans” or censored data; --, not calculated; %, percentile; MPN, most probable number] 
16

For statistical summaries and boxplots, the number of censored values in the data are 

considered in the computations, but the values of statistics (in the tables and boxplots) that are 

affected by the presence of censored data (such as mean, standard deviation, and the lower 

percentiles) are predicted based on log-probability regression on the uncensored values (Helsel and 

Hirsch, 1992). If more than 25 percent of the data are censored, statistics were not computed for the 

types affected by censored values. In this report, generally only the 25th percentile and the mean 

statistics on a few water-quality constituents required the log-probability regression methods. For 

Mann-Whitney rank sum testing in this report, no groups of water-quality constituents for a period 

had more than 50-percent censored values, which is the limit for successful use of the test to 

determine differences (Helsel and Hirsch, 1992).

Stormwater Samples

About 200 stormwater samples were collected by using refrigerated, automatic pumping 

samplers that were manually activated when stormwater runoff began. Once activated, the samplers 

obtained aliquots, or sample portions, by withdrawing fixed volumes of water from the stream 

every 60 minutes. The samplers were equipped with eight 2-liter (L) bottles and were programmed 

to add three aliquots to each bottle. Time-weighted composite stormwater samples were collected 

by compositing individual aliquots of stormwater that were obtained at fixed-time increments 

typically during 12-hour intervals; bacteriological samples were collected manually from streams 

during some storms (see Bacteriological Samples section). Each station was visited by USGS 

project staff every 12 to 24 hours during storm events. As the stations were visited, the contents of 

four sequential bottles were composited into a single time-weighted composite sample covering 

about 12 hours of the storm hydrograph, and individual sample bottles were filled by using a churn 

splitter for subsequent laboratory analysis. The churn splitter is a plastic container for compositing 

samples that allows mixing of raw sample water while bottles are filled using a spigot on the churn 

(U.S. Geological Survey, variously dated). The compositing, filtration, bottling, and preservation of 

samples submitted to NWQL took place in the field using procedures described in the USGS 

National Field Manual (U.S. Geological Survey, variously dated). If individual sample bottles were 

found to contain different volumes of water, it was assumed that individual aliquot volumes varied, 

and the aliquot sample volumes were adjusted, if possible, or the samples were discarded. The 

samples were analyzed by the NWQL for physical properties such as pH, specific conductance, 

hardness, and residue on evaporation at 105°C; for major ions such as calcium and magnesium, and 

organic carbon, and nutrients including ammonia, total ammonia plus organic nitrogen, dissolved 

nitrite plus nitrate, orthophosphate, and total phosphorus; and for metals, including total

recoverable and dissolved phases of copper, lead, manganese, and zinc (table 4).

The composite samples integrate concentrations over time and are appropriate for 

evaluating water-quality criteria developed for aquatic life (Colorado Department of Public Health 

and Environment, 2011a, 2011b). Generally, the most stringent criteria in place are the acute 

standards, which specify a maximum allowable concentration for a period of time, usually 24 

hours. The acute criteria, like the composite samples, describe concentration over a period of time. 

Decisions to activate samplers in this study were made by USGS project staff on the basis 

of local weather and streamflow conditions, which were monitored continuously through a variety 

of sources, including data available in near real-time from the World Wide Web 

(http://alert.udfcd.org/ ). The ALERT system, operated by UDFCD, provides data in near real-time 

from an extensive network of precipitation gages and information from many UDFCD streamflowgaging stations (Urban Drainage and Flood Control District, 2012). It also integrates streamflow 
17

data from USGS and CODWR streamflow-gaging stations into Web-based reports in near realtime. The ALERT data typically are available within a few minutes of measurement.

Boxplots were constructed for values and concentrations at all sites. The data from each site 

are displayed side-by-side for selected water-quality measurements and constituents. The 

explanation for the boxplots is shown in figure 2. The boxplot shows the quartiles (25th, 50th or 

median, and 75th percentiles). The whiskers on the boxplot extend to the maximum and minimum

values. An open circle is plotted for the mean value. The number of values used in the boxplot, the 

number of values that were censored, and the highest censoring level that was applied to the data 

for a particular boxplot are shown at the top of the boxplot. A line representing one-half of the 

censoring level, essentially the long-term method detection level, is drawn on the boxplot 

corresponding to the y-axis value of that long-term method detection level. Any boxplot that has a 

censoring level greater than the 25th percentile is drawn with a dashed line for the box below that 

level. The dashed box indicates that statistical methods were used to estimate the 25th percentile 

(same as methods used to compute the statistics for this report). Boxplots provide a visual 

representation of basic site statistics and are useful for interpreting differences among the 1998–

2001, 2002–2005, and 2006–2010 periods.

Figure 2. Explanation for boxplots shown in this report. 

The Mann-Whitney rank-sum test (Iman and Conover, 1983) was used to test for 

statistically significant temporal differences in streamflow and water-quality data collected from

the 2002–2005 and the 2006–2010 periods at the same monitoring station. The Mann-Whitney 

rank-sum test is a nonparametric procedure in which data for the two populations (data for both 

monitoring periods) are ranked; a two-sample t-test is then performed on the ranks of the data. The 

advantages of the Mann-Whitney rank-sum test over a simple two-sample t-test (Iman and 

Conover, 1983) are that (1) the data need not be normally distributed (and hydrologic data often are 

not normally distributed), and (2) the Mann-Whitney rank-sum test is less sensitive to the 
18

assumption of equal variances compared to other tests. The null hypothesis states that samples from 

both stations are from the same population; for example, the samples are not statistically different 

(Helsel and Hirsch, 1992). The Mann-Whitney rank-sum tests used in this report are two-tailed 

tests, and the probability of error (p-value) was used in this report to determine the significance of 

statistical tests for all statistical methods. In this report, p-values less than 0.10 are considered to be 

statistically significant. The use of the word “significant” herein only applies to the p-value of the 

statistical test to reduce any confusion of descriptive terms. The median values for both periods 

tested are presented in a table with the p-values to provide the reader a frame of reference for the 

data periods. The test assesses the probability of the two datasets belonging to the same population, 

not necessarily just the difference in the medians. 

Bacteriological Samples

A total of 17 bacteriological samples were collected during selected storms. The samples 

were collected manually as grab samples by filling bottles at the streambank and transporting them 

directly to MWRD for analysis, generally within an hour of sample collection. The bacteriological 

samples were analyzed for Escherichia coli and fecal coliform (table 4).

Quality Assurance and Quality Control

Quality-control (QC) samples (blanks and duplicates) were collected to characterize 

contamination and precision resulting from sampling equipment or ambient field conditions. 

Approximately 10 percent of all samples collected were QC samples. No blanks or duplicates were 

collected during 2006-2010 for bacteriological samples. 

Blank samples were processed in the field using universal blank water (UBL), obtained 

from the NWQL and certified to have very low concentrations of inorganic and organic 

constituents. The UBL water was poured directly into autosampler bottles in the field, processed 

through churn splitters and filters, and transported to the NWQL for analysis. The results for blanks 

in table 5 indicate no substantial contamination to analyzed concentrations from ambient field 

conditions or from the use of sampling and processing equipment in the field or in the laboratory. 
19

Table 5. Analytical results for quality-assurance blanks, 2006–2010.

[SPRU, South Platte River below Union Avenue; SPRD, South Platte River at Denver; TGC6, Toll Gate Creek above 6th Avenue; 

SCRM, Sand Creek at mouth near Commerce City; SPRH, South Platte River at Henderson; Samples consist of universal blank water 

(UBL) passed through automated sampler bottles and all processing equipment; μS/cm at 25°C, microsiemens per centimeter at 25 

degrees Celsius; °C, Celsius; mg/L, milligrams per liter; N, nitrogen; P, phosphorus; CaCO3, calcium carbonate; μg/L, micrograms per 

liter; E, estimated value less than reporting level; na, not applicable; <, less than]

Station name SPRU TGC6 TGC6 SPRH SPRU SCRM SPRH SCRM SPRH SPRD SPRD TGC6 SPRD

Date (yyyymmdd) 20060709 20070329 20070425 20070516 20070530 20080806 20080808 20080810 20080913 20090418 20090527 20090720 20100704

Time (hhmm) 1432 0700 1340 1415 1220 1030 1100 1111 1245 1630 159 2256 1900

pH, in standard units 7.6 6.0 8.2 7.8 8 8.5 7.9 8 8.1 6.2 8.1 7.2 7.9

Specific conductance, 

in μS/cm at 25°C na na na na na na na na na na na 1 1

Hardness, total as 

CaCO3, in mg/L 1.09 na na na na na na na na na na na < .18

Calcium, in mg/L 0.41 0.04 0.05 <.02 0.02 E.03 0.09 0.04 E.04 0.05 0.04 < .02 < .04

Magnesium, in mg/L 0.014 <.014 <.014 E.008 <.014 <.020 <.020 <.020 <.020 < .012 < .012 < .012 < .016

Residue on evaporation at 105°C, in 

mg/L

<10 na na <10 <10 na na na na na < 15 < 15 < 15

Nitrogen, Ammonia + 

organic, total as N, 

in mg/L

<.10 na <.10 <.10 <.10 <.14 <.14 <.14 E.10 < .10 E .06 < .10 < .10

Nitrogen, ammonia, 

dissolved as N, in 

mg/L

E.008 na <.020 <.020 <.020 <.020 <.020 <.020 <.020 < .020 < .020 < .020 < .020

Nitrogen, nitrite + 

nitrate, dissolved as 

N, in mg/L

<.06 na <.06 <.06 <.06 <.04 E.02 <.04 <.04 < .04 < .04 < .04 < .04

Orthophosphate, 

dissolved as P, in 

mg/L

E.003 na <.006 E.003 <.006 E.003 E.004 E.003 <.006 < .008 0.017 < .008 < .008

Phosphorus, total as 

P, in mg/L <.04 na <.04 <.04 <.04 E.02 <.04 <.04 <.04 < .04 < .04 < .04 <.04
20

Carbon, organic, 

dissolved as C, in 

mg/L

0.8 <.4 0.4 E.3 <.4 0.5 E.3 E.3 na E .2 < .4 0.5 < .7

Copper, dissolved, in 

μg/L <.4 <.40 <.40 <.40 <.40 <1.0 <1.0 <1.0 <1.0 < 1.0 < 1.0 < 1.0 < 1.0

Copper, total 

recoverable, in μg/L <.6 <1.2 <1.2 <1.2 <1.2 <1.2 <1.2 <1.2 <1.2 < 4.0 < 4.0 < 4.0 < 1.4

Lead, dissolved, in 

μg/L <.08 <.12 <.12 <.12 <.12 <.08 <.08 <.08 <.08 < .06 E .05 < .06 < .03

Lead, total 

recoverable, in μg/L E.05 <.06 <.06 0.07 <.06 0.09 E.04 E.04 <.06 < .10 0.13 < .10 < .06

Manganese, 

dissolved, in μg/L <.2 <.2 E.1 <.2 <.2 <.2 0.9 <.2 <.2 < .2 0.9 < .2 < .3

Manganese, total 

recoverable, in μg/L 1.5 <.6 <.6 E.4 <.6 <.8 E.7 E.6 E.6 < .4 2.6 < .4 < .8

Zinc, dissolved, in 

μg/L <.6 2.4 <.60 <.60 1.1 <1.8 <1.8 <1.8 <1.8 E 1.8 < 2.0 < 2.0 < 2.8

Zinc, total 

recoverable, in μg/L <2 <2 <2.0 <2.0 E1.0 <2.0 <2.0 <2.0 <2.0 < 2.0 E 1.1 < 2.0 2.1

Table 5. Analytical results for quality-assurance blanks, 2006–2010.—Continued

[SPRU, South Platte River below Union Avenue; SPRD, South Platte River at Denver; TGC6, Toll Gate Creek above 6th Avenue; 

SCRM, Sand Creek at mouth near Commerce City; SPRH, South Platte River at Henderson; Samples consist of universal blank water 

(UBL) passed through automated sampler bottles and all processing equipment; μS/cm at 25°C, microsiemens per centimeter at 25 

degrees Celsius; °C, Celsius; mg/L, milligrams per liter; N, nitrogen; P, phosphorus; CaCO3, calcium carbonate; μg/L, micrograms per 

liter; E, estimated value less than reporting level; na, not applicable; <, less than]

Station name SPRU TGC6 TGC6 SPRH SPRU SCRM SPRH SCRM SPRH SPRD SPRD TGC6 SPRD

Date (yyyymmdd) 20060709 20070329 20070425 20070516 20070530 20080806 20080808 20080810 20080913 20090418 20090527 20090720 20100704

Time (hhmm) 1432 0700 1340 1415 1220 1030 1100 1111 1245 1630 159 2256 1900
21

Duplicate samples were used to evaluate the reproducibility (precision) of analytical results. 

The duplicate samples consisted of stormwater split into two separate samples and sent to the 

NWQL for analysis. The relative percent difference (RPD) for two values (a and b) is calculated 

according to the following equation: 

 RPD = |{(a – b) ÷ [(a + b) ÷ 2]} × 100| (1)

For example, if the environmental sample (a) result is 18, and the duplicate (b) is 20, the 

RPD (absolute value) is 10.52 percent. If censored values were reported for both results in a 

comparison, they were counted as a “zero” RPD. If one result was censored and the other result 

was not, the RPD was not computed. For small concentrations, small differences in analytical 

results near the reporting level can result in large RPDs. Some constituents showed this tendency, 

but analysis of the distribution of the RPDs and the associated concentration or measured value 

seemed to indicate mostly random distribution of RPD values. For the 151 duplicate analyses, the 

median RPD was 1.9 percent, and RPDs ranged from 0 to 115 percent (table 6). For the 151 

duplicate analyses, the 90th-percentile RPDs ranged from 0.4 to 45 percent. Among those results 

dissolved copper, lead, and manganese, accounted for the largest RPDs (90th-percentile RPDs 

ranged from 34 to 45 percent). Excluding these dissolved metals, the range of 90th-percentile 

values in table 6 was 0.4 to 20 percent (table 6). The RPD results indicate generally satisfactory 

reproducibility for the majority water-quality analysis types. For the dissolved metals—copper, 

lead, and manganese—reproducibility was only fair.
22

Table 6. Relative percentage difference statistics for duplicate samples collected during 2006–2010.

[ft3

/s, cubic foot per second; μS/cm at 25°C, microsiemens per centimeter at 25 degrees Celsius; °C, Celsius; CaCO3, calcium 

carbonate; mg/L, milligrams per liter; μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; n, number 

of samples]

Property or constituent n Minimum Maximum 10th 

percentile

25th 

percentile Median 75th percentile

90th 

percentile

Specific conductance, field, μS/cm at 25°C 8 0 0.5 0 0 0 0.2 0.4

Hardness, total as CaCO3, in mg/L 8 0 5.4 0 0.5 0.8 3.2 4.5

Calcium, dissolved, mg/L 8 0.2 4.4 0.2 0.4 0.9 3.0 3.6

Magnesium, dissolved, mg/L 8 0 2.7 0 0.2 1.0 1.5 2.3

Residue on evaporation at 105°C, mg/L 8 1.3 24 2.3 3.8 6.6 13 18

Nitrogen, ammonia plus organic, total as N, mg/L 7 0 10 0 0.6 6.9 8.9 9.7

Nitrogen, ammonia, dissolved as N, mg/L 8 0 22 0 0.0 0.9 3.2 12

Nitrogen, nitrite plus nitrate, dissolved as N, mg/L 8 0 1.9 0 1.0 1.5 1.7 1.9

Orthophosphate, dissolved as P, mg/L 8 0 20 0 0 1.2 7.6 20

Phosphorus, total as P, mg/L 8 0 10 0 1.4 3.5 5.8 9.4

Carbon, organic, dissolved as C, mg/L 8 0 5.7 0.9 1.3 2.4 3.8 4.6

Copper, dissolved, μg/L 8 0 115 2.9 6.4 12 15 45

Copper, total recoverable, μg/L 8 1.0 17 1.4 2.1 2.7 3.1 7.7

Lead, dissolved, μg/L 8 0 55 0 0 26 37 43

Lead, total recoverable, μg/L 8 0.2 7.3 0.2 1.9 3.1 4.2 6.9

Manganese, dissolved, μg/L 8 0 108 0.1 0.6 0.9 1.7 34

Manganese, total recoverable, μg/L 8 0.2 5.1 0.3 0.3 1.3 2.3 4.7

Zinc, dissolved, μg/L 8 0.4 19 0.4 5.4 8.1 9.4 13

Zinc, total recoverable, μg/L 8 0 16 0.6 1.6 2.0 5.9 9.1
23

Summary of Stormwater Quality

The results of laboratory analyses of stormwater samples for water-quality properties and 

constituents and biological indicators are summarized in this section. Summary statistics of 

laboratory results are presented for each of the five stations. In addition, analytical results are 

compared to numeric standards established by the CDPHE (Colorado Department of Public Health 

and Environment, 2011a, 2011b). Hydrographs of daily mean streamflow for each streamflowgaging station and the dates of sampling events are presented in figure 3. The streamflow plots, in 

addition to the streamflow characteristics of the stations listed in table 3, provide a context for the 

hydrologic setting of stormwater collection. Precipitation data on the hydrographs were collected at 

the same station (Union, Toll Gate, Sand Creek, and Henderson) or at a nearby location (Denver), 

and the exact locations of the precipitation gages are listed in table 1.

Figure 3. Graphs showing daily values for streamflow and precipitation and dates of sample collection 

(2006-2010) at (A) South Platte River below Union Avenue at Englewood, (B) South Platte River at 

Denver, (C) Toll Gate Creek above 6th Avenue at Aurora, (D) Sand Creek at mouth near Commerce 

City, and (E) South Platte River at Henderson. 
24

Figure 3. Graphs showing daily values for streamflow and precipitation and dates of sample collection 

(2006-2010) at (A) South Platte River below Union Avenue at Englewood, (B) South Platte River at 

Denver, (C) Toll Gate Creek above 6th Avenue at Aurora, (D) Sand Creek at mouth near Commerce 

City, and (E) South Platte River at Henderson.—Continued
25

Figure 3. Graphs showing daily values for streamflow and precipitation and dates of sample collection 

(2006-2010) at (A) South Platte River below Union Avenue at Englewood, (B) South Platte River at 

Denver, (C) Toll Gate Creek above 6th Avenue at Aurora, (D) Sand Creek at mouth near Commerce City, 

and (E) South Platte River at Henderson.—Continued
26

Water Quality of Stormwater Samples

A total of 197 composite samples were collected from 2006 through 2010; about 30 to 50 

samples were collected at each station. Samples were collected during periods of rising, peak, or 

falling surface-water levels. Sampling events (2006–2010) are shown on hydrographs for each of 

the five stations (fig. 3).

Summary statistics of streamflow, water-quality properties and constituents, including 

bacteriological results, are presented in table 7 and illustrated with boxplots in figure 4 for each of 

the five stations. Table 7 also reports the number of samples that had censored values—

unquantifiable concentrations that were less than the reporting level. Some reporting levels varied 

during 2006–2010 (table 7). Water-quality constituents having censored values included residue on 

evaporation, dissolved ammonia, dissolved orthophosphate, and dissolved copper and lead.
27

Table 7. Statistics for concentrations and measurement values at stormwater sampling sites, 2006–2010.

[ft3/s, cubic foot per second; mL, milliliter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; mg/L, 

milligrams per liter; μg/L, micrograms per liter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; 

nlt, number of “less thans” or censored data; --, not calculated; %, percentile; MPN, most probable number] 

Property or 

constituent

Summary statistics n nlt

Range of uncensored values Censoring level Mean SD 25% Median 

50% 75% Minimum Maximum

06710247 South Platte River below Union Avenue at Englewood, Colorado

Streamflow, mean, ft3/s 410 312 131 346 632. 38 0 33 1,490 --

Streamflow, volume, acre-feet 378 282 136 322 574 38 0 30 1,360 --

pH, standard units 7.7 0.2 7.6 7.7 7.8 38 0 6.9 8.4 --

Specific conductance, μS/cm 490 210 345 409 585 38 0 226 993 --

Hardness, total, mg/L as CaCO3 145 47.4 113 133 164 38 0 81.5 290 --

Calcium, mg/L 43 13 33 39 47 38 0 24 84 --

Magnesium, mg/L 9.3 3.6 6.8 7.9 11 38 0 5.1 20 --

Suspended solids, residue on evaporation 

at 105 °C, mg/L 226 183 77 196 326 37 1 16 670 --

Nitrogen, total, ammonia plus organic as 

N, mg/L 1.4 0.72 0.83 1.4 1.7 8 0 0.46 4.00 --

Nitrogen, dissolved, ammonia as N, mg/L 0.06 0.06 0.02 0.05 0.08 38 6 0.01 6.00 0.01

Nitrogen, dissolved, nitrite plus nitrate as 

N, mg/L 0.82 0.56 0.42 0.59 1.03 38 0 0.17 2.71 --

Orthophosphate, dissolved as P, mg/L 0.059 0.051 0.024 0.043 0.068 38 0 0.004 0.237 --

Phosphorus, total as P, mg/L 0.38 0.24 0.22 0.35 0.49 38 0 0.07 0.98 --

Carbon, dissolved, organic as C, mg/L 6.7 3.0 4.8 6.1 7.5 38 0 2.9 18.6 --

Escherichia coli, MPN/100 mL 5,380 5,620 -- 5,380 -- 4 0 20 11,000 --

Fecal coliform, MPN/100 mL 6,180 6,200 -- 6,180 -- 4 0 20 12,000 --

Copper, dissolved, μg/L 2.1 0.8 1.6 1.9 2.6 38 0 1.1 4.6 --

Copper, total recoverable, μg/L 11 7.9 5.4 11 14 36 0 2.9 34 --

Lead, dissolved, μg/L 0.08 0.06 0.04 0.06 0.07 38 6 0.03 0.27 0.04, 0.06

Lead, total recoverable, μg/L 8.3 7.2 2.6 7.4 10.3 36 0 0.62 26.9 --

Manganese, dissolved, μg/L 19.5 26.0 6.4 11.6 20.6 38 0 0.2 119 --

Manganese, total recoverable, μg/L 304 242 143 263 379 36 0 42.2 884 --

Zinc, dissolved, μg/L 5.3 3.0 2.8 4.8 6.9 38 0 1.8 13.6 --

Zinc, total recoverable, μg/L 40 29 19 34 53 36 0 7 134 --
28

Property or

constituent

Summary statistics

 n nlt

Range of uncensored values Censoring level

Mean SD 25% Median 

50% 75% Minimum Maximum

06714000 South Platte River at Denver, Colorado

Streamflow, mean, ft3/s 1,480 1,370 656 1,090 1,790 49 0 184 8,083 --

Streamflow, volume, acre-feet 1,410 1,280 667 1,010 1,630 49 0 168 7,350 --

pH, standard units 7.5 0.3 7.5 7.5 7.7 48 0 6.6 8.0 --

Specific conductance, μS/cm 522 225 384 491 597 46 0 260 1,390 --

Hardness, total, mg/L as CaCO3 141 43 108 138 160 48 0 74.4 251 -- Calcium, mg/L 42 12 32 42 49 48 0 22 76 -- Magnesium, mg/L 8.6 3.1 6.2 8 9.8 48 0 4.3 18 -- Suspended solids, residue on evaporation 

at 105 °C, mg/L 202 125 122 180 258 47 0 20.0 660 --

Nitrogen, total, ammonia plus organic as 

N, mg/L 1.8 0.82 1.2 1.5 2.2 7 0 0.51 4.2 --

Nitrogen, dissolved, ammonia as N, mg/L 0.12 0.13 0.03 0.07 0.13 47 4 0.01 0.60 0.01

Nitrogen, dissolved, nitrite plus nitrate as 

N, mg/L 1.21 0.65 0.80 0.98 1.38 46 0 0.55 3.69

--

Orthophosphate, dissolved as P, mg/L 0.150 0.087 0.098 0.122 0.167 47 0 0.046 0.487 -- Phosphorus, total as P, mg/L 0.54 0.24 0.39 0.47 0.65 47 0 0.14 1.33 -- Carbon, dissolved, organic as C, mg/L 6.3 2.0 5.0 5.8 7.2 48 0 4.1 12.2 -- Escherichia coli, MPN/100 mL 8,900 9,926 -- 6,250 -- 4 0 110 23,000 -- Fecal coliform, MPN/100 mL 10,520 9,486 -- 9,450 -- 4 0 170 23,000 --

Copper, dissolved, μg/L 2.9 1.2 2.2 2.5 3.3 48 1 1.5 7.0 1 

Copper, total recoverable, μg/L 17 11 9.1 15 21 45 0 3.8 67 --

Lead, dissolved, μg/L 0.118 0.062 0.075 0.1 0.155 48 0 0.04 0.31 --

Lead, total recoverable, μg/L 15.2 13.7 6.72 13.1 18 45 0 2.13 79.5 --

Manganese, dissolved, μg/L 11 25 1.1 3.3 9.7 48 0 0.1 156 --

Manganese, total recoverable, μg/L 333 182 215 288 412 45 0 77.9 873 --

Zinc, dissolved, μg/L 8.3 4.5 5.0 6.8 10.6 48 0 3.3 21.3 --

Zinc, total recoverable, μg/L 71 51 33 60 91 45 0 13 287 --

Table 7. Statistics for concentrations and measurement values at stormwater sampling sites, 2006–2010.—Continued

[ft3

/s, cubic foot per second; mL, milliliter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; mg/L, 

milligrams per liter; μg/L, micrograms per liter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; 

nlt, number of “less thans” or censored data; --, not calculated; %, percentile; MPN, most probable number] 
29

Property or

constituent

Summary statistics

 n nlt

Range of uncensored values Censoring level Mean SD 25% Median 

50% 75% Minimum Maximum

394329104490101 Toll Gate Creek above 6th Avenue near Aurora, Colorado

Streamflow, mean, ft3/s 261 289 73 182 345 30 0 4 1,400 --

Streamflow, volume, acre-feet 242 241 67 156 313 30 0 4 921 --

pH, standard units 7.7 0.2 7.5 7.6 7.8 33 0 7.4 8.0 --

Specific conductance, μS/cm 1,112 506 720 957 1,500 31 0 363 2,230 --

Hardness, total, mg/L as CaCO3 316 167 193 251 418 33 0 84.6 746 --

Calcium, mg/L 95 50 60 77 129 33 0 26 226 --

Magnesium, mg/L 19 10 11 15 24 33 0 4.6 44 --

Suspended solids, residue on evaporation 

at 105 °C, mg/L 180 186 46 97 254 31 0 10 695 --

Nitrogen, total, ammonia plus organic as 

N, mg/L 1.6 0.78 1.1 1.4 2.0 32 0 0.63 3.5 --

Nitrogen, dissolved, ammonia as N, mg/L 0.14 0.15 0.04 0.09 0.20 32 3 0.01 0.66 0.01

Nitrogen, dissolved, nitrite plus nitrate as 

N, mg/L 0.64 0.21 0.49 0.59 0.77 32 0 0.36 1.28 --

Orthophosphate, dissolved as P, mg/L 0.022 0.020 0.007 0.013 0.035 32 1 0.003 0.077 0.003

Phosphorus, total as P, mg/L 0.33 0.32 0.12 0.22 0.44 32 0 0.04 1.38 --

Carbon, dissolved, organic as C, mg/L 10.4 3.7 8.5 10.4 11.7 33 0 5.3 20.9 --

Escherichia coli, MPN/100 mL 2,310 1,715 -- 3,300 -- 3 0 330 3,300 --

Fecal coliform, MPN/100 mL 2,310 1,715 -- 3,300 -- 3 0 330 3,300 --

Copper, dissolved, μg/L 2.5 1.1 1.8 2.2 2.8 33 0 1.0 4.9 --

Copper, total recoverable, μg/L 12 8.8 5.8 8.3 15 33 0 2.9 33 --

Lead, dissolved, μg/L 0.097 0.07 0.053 0.08 0.12 33 4 0.04 0.34 0.04, 0.06

Lead, total recoverable, μg/L 7.18 7.67 2.35 3.45 9.18 33 0 0.71 28.3 --

Manganese, dissolved, μg/L 75.29 78.83 20.3 50.9 105 33 0 2.8 330 --

Manganese, total recoverable, μg/L 381 269 192 244 480 33 0 103 1,120 --

Zinc, dissolved, μg/L 8.8 5.4 4.7 8.0 11 33 0 1.5 26.1 --

Zinc, total recoverable, μg/L 47 35 24 33 59 33 0 9 132 --

Table 7. Statistics for concentrations and measurement values at stormwater sampling sites, 2006–2010.—Continued

[ft3

/s, cubic foot per second; mL, milliliter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; mg/L, 

milligrams per liter; μg/L, micrograms per liter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; 

nlt, number of “less thans” or censored data; --, not calculated; %, percentile; MPN, most probable number] 
30

Property or

constituent

Summary statistics

 n nlt

Range of uncensored values Censoring level Mean SD 25% Median 

50% 75% Minimum Maximum

394839104570300 Sand Creek at mouth near Commerce City, Colorado

Streamflow, mean, ft3/s 285 275 100 229 374 47 0 21 1,620 --

Streamflow, volume, acre-feet 255 249 95 205 348 47 0 19 1,473 --

pH, standard units 7.6 0.2 7.4 7.6 7.8 47 0 7.3 8.0 --

Specific conductance, μS/cm 811 392 575 684 908 45 0 330 2,020 --

Hardness, total, mg/L as CaCO3 195 79 153 171 210 47 0 74.8 455 --

Calcium, mg/L 59 22 47 52 64 47 0 23 131 -- Magnesium, mg/L 12 5.6 8.4 10 13 47 0 4.1 31 --

Suspended solids, residue on evaporation 

at 105 °C, mg/L 351 306 128 236 470 47 0 56.0 1,360 -- Nitrogen, total, ammonia plus organic as 

N, mg/L 2.6 1.6 1.6 2.0 3.2 45 0 1.1 9.5 --

Nitrogen, dissolved, ammonia as N, mg/L 0.11 0.11 0.02 0.09 0.18 45 8 0.01 0.47

0.005, 0.01, 

0.02

Nitrogen, dissolved, nitrite plus nitrate as 

N, mg/L 0.88 0.43 0.59 0.78 0.95 45 0 0.41 2.55 -- Orthophosphate, dissolved as P, mg/L 0.121 0.113 0.059 0.089 0.141 45 0 0.017 0.627 --

Phosphorus, total as P, mg/L 0.79 0.47 0.49 0.63 0.90 45 0 0.27 2.36 -- Carbon, dissolved, organic as C, mg/L 10.8 4.2 7.8 9.3 13.2 46 0 5.8 23.6 --

Escherichia coli, MPN/100 mL 3,530 2,370 -- 2,950 -- 3 0 790 4,900 -- Fecal coliform, MPN/100 mL 2,630 2,090 -- 2,140 -- 3 0 790 4,900 --

Copper, dissolved, μg/L 3.0 2.0 1.6 2.4 3.6 47 0 1.2 12.5 --

Copper, total recoverable, μg/L 22 14 11 17 28 47 0 6.5 55 --

Lead, dissolved, μg/L 0.19 0.14 0.10 0.14 0.25 47 0 0.04 0.74 --

Lead, total recoverable, μg/L 21.0 17.3 6.16 14.5 31.6 47 0 3.67 60.5 --

Manganese, dissolved, μg/L 45 78 2.5 11 59 47 0 0.5 391 --

Manganese, total recoverable, μg/L 678 661 283 466 640 47 0 144 3,210 --

Zinc, dissolved, μg/L 67.2 191 7.8 10.9 21.0 47 0 2.8 1,170 --

Zinc, total recoverable, μg/L 524 1205.4 44 75 241 47 0 22 5,230 --

Table 7. Statistics for concentrations and measurement values at stormwater sampling sites, 2006–2010.—Continued

[ft3

/s, cubic foot per second; mL, milliliter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; mg/L, 

milligrams per liter; μg/L, micrograms per liter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; 

nlt, number of “less thans” or censored data; --, not calculated; %, percentile; MPN, most probable number] 
31

Property or

constituent

Summary statistics

 n nlt

Range of uncensored 

values Censoring level Mean SD 25% Median 

50% 75% Minimum Maximum

06720500 South Platte River at Henderson, Colorado

Streamflow, mean, ft3/s 2,260 3,290 754 1,470 2,390 33 0 218 19,000 --

Streamflow, volume, acre-feet 1,720 1,570 663 1,340 2,040 33 0 198 6,920 --

pH, standard units 7.3 0.2 7.2 7.4 7.5 34 0 6.6 7.7 --

Specific conductance, μS/cm 557 196 454 528 601 34 0 205 1,260 --

Hardness, total, mg/L as CaCO3 141 31 119 133 154 34 0 95 226 --

Calcium, mg/L 42 9 36 41 47 34 0 28 64 --

Magnesium, mg/L 8.9 2.5 7.3 8.3 10 34 0 6.0 17 --

Suspended solids, residue on evaporation 

at 105 °C, mg/L 174 112 94 133 242 32 0 44 545 --

Nitrogen, total, ammonia plus organic as 

N, mg/L 2.9 1.7 2.0 2.6 3.4 3 0 0.96 9.4 --

Nitrogen, dissolved, ammonia as N, mg/L 0.80 0.88 0.24 0.58 0.73 33 1 0.11 3.38 0.01

Nitrogen, dissolved, nitrite plus nitrate as 

N, mg/L 2.08 0.81 1.53 1.87 2.24 33 0 0.96 4.64 --

Orthophosphate, dissolved as P, mg/L 0.461 0.279 0.262 0.367 0.621 33 0 0.152 1.15 --

Phosphorus, total as P, mg/L 1.02 0.46 0.72 0.96 1.18 33 0 0.37 2.85 --

Carbon, dissolved, organic as C, mg/L 9.1 5.3 6.2 7.5 10.6 34 0 4.8 34.4 --

Escherichia coli, MPN/100 mL 10,400 11,550 -- 7,900 -- 3 0 310 23,000 --

Fecal coliform, MPN/100 mL 12,100 11,370 -- 13,000 -- 3 0 310 23,000 --

Copper, dissolved, μg/L 3.04 0.92 2.4 2.75 3.4 34 0 1.7 5.5 --

Copper, total recoverable, μg/L 24 13 13 22 28 32 0 9.1 63 --

Lead, dissolved, μg/L 0.22 0.08 0.17 0.21 0.29 34 0 0.07 0.41 --

Lead, total recoverable, μg/L 22.6 15.9 10.2 17.3 32.4 32 0 5.09 70.9 --

Manganese, dissolved, μg/L 62 127 18 33 62 34 0 0.6 757 --

Manganese, total recoverable, μg/L 408 305 243 359 472 32 0 138 1,840 --

Zinc, dissolved, μg/L 18.3 7.5 13.1 17.3 22.1 34 0 6.7 45.8 --

Zinc, total recoverable, μg/L 114 68 68 91 148 32 0 32 308 --

Table 7. Statistics for concentrations and measurement values at stormwater sampling sites, 2006–2010.—Continued

[ft3

/s, cubic foot per second; mL, milliliter; μS/cm, microsiemens per centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; mg/L, 

milligrams per liter; μg/L, micrograms per liter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; 

nlt, number of “less thans” or censored data; --, not calculated; %, percentile; MPN, most probable number] 
32

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.
33

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
34

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
35

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
36

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
37

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
38

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
39

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
40

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
41

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
42

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
43

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
44

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
45

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
46

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
47

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
48

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
49

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
50

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
51

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
52

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce 

City (SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.—Continued
53

Figure 4. Boxplots showing distribution of water-quality properties and constituents at South Platte River below Union Avenue at Englewood

(SPRU), South Platte River at Denver (SPRD), Toll Gate Creek above 6th Avenue at Aurora (TGC6), Sand Creek at mouth near Commerce City

(SCRM), and South Platte River at Henderson (SPRH), calendar years 1998–2001, 2002–2005, and 2006–2010. Plotted values and 

concentrations. Boxplot explanation provided in figure 2.
54

The results from laboratory analyses indicate, in general, and particularly for nutrients, that 

some values and concentrations for water-quality constituents increase with contributing drainage 

area in the South Platte River, but no well-defined relation was identified between concentration 

and the amount of urban land cover identified using the U.S. Geological Survey National Land 

Cover data (table 2; fig. 4; table 7; appendix tables A1-A5). For example, the mean concentration 

for ammonia plus organic nitrogen is 1.4 milligrams per liter (mg/L) at Union, 1.8 mg/L at Denver, 

and 2.9 mg/L at Henderson. This pattern corresponds with the size of the local contributing 

drainage area, which also increases downstream (table 2). Sand Creek, a tributary that flows into 

the South Platte River between the Denver and Henderson sites, has Toll Gate Creek as tributary. 

Some values and concentrations of results also increase in a downstream direction from Toll Gate

to Sand Creek (fig. 4; table 7; appendix tables A1-A5). For some water-quality constituents, 

concentrations measured in the tributary streams (Toll Gate Creek and Sand Creek) are higher than 

concentrations measured in the upstream reach of the South Platte River (Union) (table 7, fig. 4). 

Comparison to Colorado Water-Quality Standards

Results from laboratory analyses of stormwater samples were compared to numeric waterquality standards for pH, Escherichia coli, dissolved nitrite plus nitrate, dissolved copper, 

lead, manganese, and zinc (Colorado Department of Public Health and Environment, 

2011a, 2011b) (table 8). All numeric standards do not apply to all stations. The numeric standards 

are fixed values for pH, Escherichia coli, dissolved nitrite plus nitrate, and a water-supply-related 

standard for dissolved manganese. The numeric values for acute and chronic aquatic-life toxicity 

standards for dissolved copper, lead, manganese (in addition to the fixed standard just mentioned), 

and zinc are each calculated according to equations published by the CDPHE (Colorado 

Department of Public Health and Environment, 2011a, 2011b). The calculations for these values, 

often referred to as “table-value standards,” are based on hardness; in this study, the hardness value 

associated with the composite sample was used in the calculation.
55

Table 8. Summary of Colorado water-quality standards and properties and constituents that exceeded Colorado standards for this study, 2006–2010.

[--, no data or no standard; ml, milliliter; mg/L, milligrams per liter; μg/L, micrograms per liter; <, less than; TVS, table value standards 

equation; standards from Colorado Department of Public Health and Environment (2011a), 2011b); only data greater than or equal to 

the reporting level were compared to standards; nitrite plus nitrate concentration is used instead of nitrate for comparison to standard as 

nitrite concentrations are generally negligible; samples without hardness for trace element standards computation were not included in 

the exceedance evaluation; water temperature was not available for the computation of ammonia standards for composite samples] 

Property or constituent 

Total number 

of

samples

Total number 

of

uncensored

samples

Type of

standard

Range of concentration Standards Number of 

uncensored 

sample exceedances Minimum Maximum Chronic Acute

South Platte River below Union at Englewood

pH, lab, standard units 38 38 Fixed 6.9 8.4 -- 6.5 to 9.0 0

Escherichia coli, colonies/100ml 4 0 Fixed 20 11,000 -- 120 3

Nitrate, dissolved, mg/L 38 38 Fixed 0.17 2.71 -- 10 0

Copper, dissolved, μg/L 38 38 TVS acute 1.1 4.6 -- 33 to 109 0

38 38 TVS chronic 1.1 4.6 26 to 77 -- 0

Lead, dissolved, μg/L 38 32 TVS acute 0.03 0.27 -- 35 to 135 0

38 32 TVS chronic 0.03 0.27 0.6 to 2.2 -- 0

Manganese, dissolved, μg/L 38 38 TVS acute 0.2 119 -- 37 to 57 3

38 38 TVS chronic 0.2 119 31 to 47 -- 4

38 38 Fixed chronic 0.2 119 190 -- 0

Zinc, dissolved, μg/L 38 38 TVS acute 1.8 1.8 -- 103 to 304 0 

38 38 TVS chronic 13.6 13.6 91 to 269 -- 0

South Platte River at Denver

pH, lab, standard units 48 48 Fixed 6.6 8.0 -- 6.5 to 9.0 0

Escherichia coli, colonies/100ml 4 0 Fixed 110 23,000 120 3

Nitrate, dissolved, mg/L 46 46 Fixed 0.60 3.7 -- 10 0

Copper, dissolved, μg/L 48 47 TVS acute 1.5 7.0 -- 11 to 35 0

48 47 TVS chronic 1.5 7.0 30 to 96 -- 0

Lead, dissolved, μg/L 48 48 TVS acute 0.04 0.31 -- 31 to 116 0

48 48 TVS chronic 0.04 0.31 0.5 to 

1.9 -- 0 

Manganese, dissolved, μg/L 48 48 TVS acute 0.1 156 -- 36 to 54 2 

48 48 TVS chronic 0.1 156 30 to 45 -- 2

48 48 Fixed chronic 0.1 156 190 -- 0

Zinc, dissolved, μg/L 48 48 TVS acute 3.3 21.3 -- 363 to 1,390 0

48 48 TVS chronic 3.3 21.3 84 to 238 -- 0
56

Toll Gate Creek above 6th Avenue at Aurora

pH, lab, standard units 33 33 Fixed 7.4 8.0 -- 6.5 to 9.0 0

Escherichia coli, colonies/100ml 3 0 Fixed 330 3,300 -- 120 3

Nitrate, dissolved, mg/L 32 32 Fixed 0.009 0.66 -- 10 0

Copper, dissolved, μg/L 33 33 TVS acute 0.97 4.9 -- 13 to 99 0

33 33 TVS chronic 0.97 4.9 10 to 64 -- 0

Lead, dissolved, μg/L 33 29 TVS acute 0.04 0.34 -- 58 to 352 0

33 29 TVS chronic 0.04 0.34 0.93 to 5.7 -- 0

Manganese, dissolved, μg/L 33 33 TVS acute 3 330 -- 38 to 78 15

33 33 TVS chronic 3 330 31 to 64 -- 17

Zinc, dissolved, μg/L 33 33 TVS acute 1.5 26.1 -- 419 to 4,620 0

33 33 TVS chronic 1.5 26.1 94 to 603 -- 0

Sand Creek at mouth near Commerce City

pH, lab, standard units 47 47 Fixed 7.3 8.0 -- 6.5 to 9.0 0

Escherichia coli, colonies/100ml 3 0 Fixed 790 4,900 -- 120 3

Nitrate, dissolved, mg/L 45 45 Fixed 0.41 2.6 -- 10 0

Copper, dissolved, μg/L 47 47 TVS acute 1.2 12.5 -- 29 to 161 0

47 47 TVS chronic 1.2 12.5 23 to 110 -- 0

Lead, dissolved, μg/L 47 47 TVS acute 0.04 0.74 -- 32 to 215 0

47 47 TVS chronic 0.04 0.74 0.51 to 3.46 -- 0

Manganese, dissolved, μg/L 47 47 TVS acute 0.5 391 -- 36 to 66 13

47 47 TVS chronic 0.5 391 30 to 54 -- 15

Zinc, dissolved, μg/L 47 47 TVS acute 2.8 1,170 -- 366 to 2,680 1 

47 47 TVS chronic 2.8 1,170 85 to 395 -- 4

South Platte River at Henderson

pH, lab, standard units 34 34 Fixed 6.6 7.7 -- 6.5 to 9.0 0

Escherichia coli, colonies/100ml 3 0 Fixed 310 23,000 -- 120 3

Nitrate, dissolved, mg/L 33 33 Fixed 0.96 4.6 -- 10 0

Copper, dissolved, μg/L 34 34 TVS acute 1.7 5.5 -- 33 to 74 0

34 34 TVS chronic 1.7 5.5 25 to 53 -- 0

Lead, dissolved, μg/L 34 34 TVS acute 0.07 0.41 -- 41 to 104 0

34 34 TVS chronic 0.07 0.41 0.66 to 1.67 -- 0

Manganese, dissolved, μg/L 34 34 TVS acute 0.6 757 -- 39 to 52 13

34 34 TVS chronic 0.6 757 32 to 43 -- 16

34 34 Fixed chronic 0.6 757 400 -- 1

Zinc, dissolved, μg/L 34 34 TVS acute 6.7 46 -- 476 to 1,240 0

34 34 TVS chronic 6.7 46 104 to 218 -- 0

Table 8. Summary of Colorado water-quality standards and properties and constituents that exceeded Colorado standards for this study, 2006–2010.—Continued

[--, no data or no standard; ml, milliliter; mg/L, milligrams per liter; μg/L, micrograms per liter; <, less than; TVS, table value standards 

equation; standards from Colorado Department of Public Health and Environment (2011a), 2011b); only data greater than or equal to 

the reporting level were compared to standards; nitrite plus nitrate concentration is used instead of nitrate for comparison to standard as 

nitrite concentrations are generally negligible; samples without hardness for trace element standards computation were not included in 

the exceedance evaluation; water temperature was not available for the computation of ammonia standards for composite samples] 
A summary of results from 2006–2010 stormwater samples that did not meet (exceeded) 

CDPHE water-quality standards (Colorado Department of Public Health and Environment, 2011a, 

2011b) is presented in table 8 for individual stations. The results indicate few exceptions to 

standards except for dissolved manganese, dissolved zinc, and Escherichia coli. Stormwater 

collected at Union had about 10 percent acute or chronic dissolved manganese exceedances in 

samples; samples collected at Denver had less than 5 percent acute or chronic dissolved manganese 

exceedances (table 8). In contrast, samples collected at Toll Gate, Sand Creek, and Henderson, each 

had about 30 to 50 percent exceedances of both acute and chronic dissolved manganese standards. 

Of the samples collected at Sand Creek, 1 sample exceeded the acute standard and 4 samples 

exceeded the chronic standard for dissolved zinc (table 8), but no samples collected from the other 

sites exceeded either standard for zinc. Almost all samples of stormwater analyzed for Escherichia 

coli exceeded the Colorado numeric standards (table 8). A numerical standard for fecal coliform is 

no longer applicable as of 2004 (Colorado Department of Health and Environment, 2006). 

Evaluation of Stormwater Quality between 2006–2010 and 2002–2005

The water quality of stormwater samples collected in this study is evaluated in this section 

by using the Mann-Whitney rank-sum test, which is a nonparametric statistical test (Helsel and 

Hirsch, 1992). The evaluation compares 2006–2010 results to 2002–2005 results. Previous reports 

(Bossong and others, 2005; Bossong and Fleming, 2008) grouped data by water years, but this 

report groups and analyzes data by calendar year. 

Statistics

The distribution of data from stormwater samples collected during 2006–2010 was

compared to the corresponding distribution of 2002–2005 data to determine if water quality 

changed between periods. In this report, an upward trend indicates increasing values or 

concentrations, and a downward trend indicates decreasing values or concentrations (table 9). 

Trends in stormwater quality also are illustrated in boxplots for each of the five sites (fig. 4) and 

presented in the statistics of table 7 and appendix tables A1-A5. Although boxplots also are 

presented for 1998–2001 data (Bossong and others, 2005), the statistical comparisons and trend 

analysis with the 2002–2006 data were made in Bossong and Fleming (2008) and are not repeated 

herein.

57
58

Table 9. Mann-Whitney test of differences between distributions of water-quality values and concentrations in stormwater samples collected in 2002–

2005 and 2006–2010 at all sites.

[n, number of samples, each time period separated by a comma; shaded values indicate statistical significance for this report with a

p-value less than or equal to 0.1000; mg/L, milligrams per liter; μg/L, micrograms per liter; °C, degrees Celsius; μS/cm, microsiemens 

per centimeter at 25°C; CaCO3, calcium carbonate; cm, centimeter; N, nitrogen; P, phosphorus; C, carbon; MPN, most probable 

number] 

Property or constituent

South Platte River below Union Avenue

n 

Median value or 

concentration 

2002-2005

Median value or 

concentration 

2006-2010 p-value Trend

Streamflow, mean, in ft3/s 37,38 154 346 0.0059 Upward

pH, lab 37,38 7.5 7.7 0.0018 Upward

Specific conductance, field, μS/cm at 25°C 31,38 478 410 0.4509 None

Hardness, as CaCO3, in mg/L 37,38 155 133 0.0388 Downward

Calcium, dissolved, mg/L 28,38 42.6 39.6 0.3811 None

Magnesium, dissolved, mg/L 28,38 8.6 7.9 0.7753 None

Residue on evaporation at 105°C 37,38 199 196 0.6601 None

Nitrogen, ammonia plus organic, total as N, mg/L 37,38 1.40 1.35 0.2068 None

Nitrogen, ammonia, dissolved as N, mg/L 28,38 0.080 0.046 0.0422 Downward

Nitrogen, nitrite plus nitrate, dissolved as N, mg/L 28,38 0.775 0.585 0.1650 None

Orthophosphate, dissolved as P, mg/L 37,38 0.082 0.0435 0.0044 Downward

Phosphorus, total as P, mg/L 37,38 0.36 0.35 0.4908 None

Carbon, organic, dissolved as C, mg/L 25,38 6.7 6.1 0.3357 None

Copper, dissolved, μg/L 37,38 2.1 1.8 0.0431 Downward

Copper, total recoverable, μg/L 28,35 12.9 10.6 0.3125 None

Lead, dissolved, μg/L 28,38 0.05 0.06 0.9426 None

Lead, total recoverable, μg/L 28,36 9.6 7.4 0.2585 None

Manganese, dissolved, μg/L 28,38 12.8 11.6 0.4556 None

Manganese, total recoverable, μg/L 28,36 285 263 0.4690 None

Zinc, dissolved, μg/L 28,38 4.6 4.8 0.8609 None

Zinc, total recoverable, μg/L 28,36 45 34 0.2819 None
59

Property or constituent

South Platte River at Denver

 n 

Median value or 

concentration 

2002-2005

Median value or 

concentration 

2006-2010

Mann-Whitney rank 

sum p-value Trend

Streamflow, mean, in ft3/s 46,49 636 1,080 0.0010 Upward

pH, lab 46,48 7.5 7.5 0.0284 Upward

Specific conductance, field, μS/cm at 25°C 41,46 509 491 0.2956 None

Hardness, as CaCO3, in mg/L 45,48 154 138 0.1222 None

Calcium, dissolved, mg/L 35,48 42.9 41.6 0.8537 None

Magnesium, dissolved, mg/L 35,48 8.22 8.03 0.7329 None

Residue on evaporation at 105°C 45,47 186 186 0.7281 None

Nitrogen, ammonia plus organic, total as N, mg/L 45,47 2.1 1.5 0.0059 Downward

Nitrogen, ammonia, dissolved as N, mg/L 35,47 0.220 0.074 <0.0001 Downward

Nitrogen, nitrite plus nitrate, dissolved as N, mg/L 35,46 2.00 0.98 <0.0001 Downward

Orthophosphate, dissolved as P, mg/L 45,47 0.238 0.122 <0.0001 Downward

Phosphorus, total as P, mg/L 45,47 0.67 0.47 0.0068 Downward

Carbon, organic, dissolved as C, mg/L 31,48 7.8 5.8 0.0006 Downward

Copper, dissolved, μg/L 45,48 3.0 2.5 0.3597 None

Copper, total recoverable, μg/L 35,45 18.0 15.4 0.1063 None

Lead, dissolved, μg/L 35,48 0.14 0.10 0.0036 Downward

Lead, total recoverable, μg/L 35,45 15.4 13.1 0.1888 None

Manganese, dissolved, μg/L 35,48 16.4 3.25 <0.0001 Downward

Manganese, total recoverable, μg/L 35,45 285 288 0.9961 None

Zinc, dissolved, μg/L 35,48 10.1 6.75 0.0147 Downward

Zinc, total recoverable, μg/L 35,45 84 60 0.0896 Downward

Table 9. Mann-Whitney test of differences between distributions of water-quality values and concentrations in stormwater samples collected in 2002–

2005 and 2006–2010 at all sites.—Continued

[n, number of samples, each time period separated by a comma; shaded values indicate statistical significance for this report with a

p-value less than or equal to 0.1000; mg/L, milligrams per liter; μg/L, micrograms per liter; °C, degrees Celsius; μS/cm, microsiemens 

per centimeter at 25°C; CaCO3, calcium carbonate; cm, centimeter; N, nitrogen; P, phosphorus; C, carbon; MPN, most probable 

number] 
60

Property or constituent

Toll Gate Creek above 6th Avenue

 n 

Median value or 

concentration 

2002-2005

Median value or 

concentration 

2006-2010

Mann-Whitney rank 

sum p-value Trend

Streamflow, mean, in ft3/s 20,30 47 182 0.0020 Upward

pH, lab 24,33 7.3 7.6 <0.0001 Upward

Specific conductance, field, μS/cm at 25°C 21,31 690 957 0.0188 Upward

Hardness, as CaCO3, in mg/L 24,33 250 251 0.2934 None

Calcium, dissolved, mg/L 21,33 67.4 76.7 0.2142 None

Magnesium, dissolved, mg/L 21,33 12.4 14.6 0.1315 None

Residue on evaporation at 105°C 22,31 246 97 0.0109 Downward

Nitrogen, ammonia plus organic, total as N, mg/L 24,32 2.25 1.40 0.0142 Downward

Nitrogen, ammonia, dissolved as N, mg/L 21,32 0.190 0.088 0.0194 Downward

Nitrogen, nitrite plus nitrate, dissolved as N, mg/L 21,32 0.46 0.58 0.0373 Upward

Orthophosphate, dissolved as P, mg/L 32,32 0.032 0.013 0.0405 Downward

Phosphorus, total as P, mg/L 24,32 0.54 0.22 0.0016 Downward

Carbon, organic, dissolved as C, mg/L 21,33 8.4 10.4 0.4887 None

Copper, dissolved, μg/L 24,33 2.8 2.2 0.1038 None

Copper, total recoverable, μg/L 21,33 16.6 8.3 0.0051 Downward

Lead, dissolved, μg/L 21,33 0.08 0.08 0.5930 None

Lead, total recoverable, μg/L 21,33 9.17 3.45 0.0101 Downward

Manganese, dissolved, μg/L 21,33 41.8 50.9 0.6573 None

Manganese, total recoverable, μg/L 21,33 683 244 0.0042 Downward

Zinc, dissolved, μg/L 21,33 7.4 8.0 0.7494 None

Zinc, total recoverable, μg/L 21,33 94 32 0.0010 Downward

Table 9. Mann-Whitney test of differences between distributions of water-quality values and concentrations in stormwater samples collected in 2002–

2005 and 2006–2010 at all sites.—Continued

[n, number of samples, each time period separated by a comma; shaded values indicate statistical significance for this report with a

p-value less than or equal to 0.1000; mg/L, milligrams per liter; μg/L, micrograms per liter; °C, degrees Celsius; μS/cm, microsiemens 

per centimeter at 25°C; CaCO3, calcium carbonate; cm, centimeter; N, nitrogen; P, phosphorus; C, carbon; MPN, most probable 

number] 
61

Property or constituent

Sand Creek at Mouth near Commerce City

n 

Median value or 

concentration 

2002-2005

Median value or 

concentration 

2006-2010

Mann-Whitney rank 

sum p-value Trend

Streamflow, mean, in ft3/s 34,47 102 229 0.0145 Upward

pH, lab 34,47 7.4 7.6 0.0018 Upward

Specific conductance, field, μS/cm at 25°C 30,45 765 684 0.1781 None

Hardness, as CaCO3, in mg/L 33,47 234 171 0.0106 Downward

Calcium, dissolved, mg/L 25,47 68.6 52.5 0.0790 Downward

Magnesium, dissolved, mg/L 25,47 14.0 10.1 0.1051 None

Residue on evaporation at 105°C 34,47 347 236 0.4999 None

Nitrogen, ammonia plus organic, total as N, mg/L 33,45 2.90 2.00 0.0579 Downward

Nitrogen, ammonia, dissolved as N, mg/L 25,45 0.28 0.09 0.0003 Downward

Nitrogen, nitrite plus nitrate, dissolved as N, mg/L 25,45 0.87 0.78 0.1096 None

Orthophosphate, dissolved as P, mg/L 33,45 0.152 0.089 0.0023 Downward

Phosphorus, total as P, mg/L 33,45 0.83 0.63 0.5141 None

Carbon, organic, dissolved as C, mg/L 24,46 12.9 9.3 0.1132 None

Copper, dissolved, μg/L 33,47 3.0 2.4 0.1222 None

Copper, total recoverable, μg/L 25,47 19.2 17.0 0.4246 None

Lead, dissolved, μg/L 25,47 0.13 0.14 0.3970 None

Lead, total recoverable, μg/L 25,47 19.1 14.5 0.5385 None

Manganese, dissolved, μg/L 25,47 35.1 10.6 0.0482 Downward

Manganese, total recoverable, μg/L 25,47 602 466 0.5191 None

Zinc, dissolved, μg/L 32,47 34.4 10.9 0.0322 Downward

Zinc, total recoverable, μg/L 24,47 148 75 0.2288 None

Table 9. Mann-Whitney test of differences between distributions of water-quality values and concentrations in stormwater samples collected in 2002–

2005 and 2006–2010 at all sites.—Continued

[n, number of samples, each time period separated by a comma; shaded values indicate statistical significance for this report with a

p-value less than or equal to 0.1000; mg/L, milligrams per liter; μg/L, micrograms per liter; °C, degrees Celsius; μS/cm, microsiemens 

per centimeter at 25°C; CaCO3, calcium carbonate; cm, centimeter; N, nitrogen; P, phosphorus; C, carbon; MPN, most probable 

number] 
62

Property or constituent

South Platte River at Henderson

n 

Median value or 

concentration 

2002-2005

Median value or 

concentration 

2006-2010

Mann-Whitney rank 

sum p-value Trend

Streamflow, mean, in ft3/s 35,33 1,060 1,470 0.2538 None

pH, lab 35,34 7.4 7.4 0.6197 None

Specific conductance, field, μS/cm at 25°C 30,34 655 528 0.0102 Downward

Hardness, as CaCO3, in mg/L 34,34 172 134 0.0009 Downward

Calcium, dissolved, mg/L 23,34 44 41 0.1796 None

Magnesium, dissolved, mg/L 23,34 10.1 8.28 0.0735 Downward

Residue on evaporation at 105°C 34,32 264 142 0.0033 Downward

Nitrogen, ammonia plus organic, total as N, mg/L 34,33 4.95 2.60 <0.0001 Downward

Nitrogen, ammonia, dissolved as N, mg/L 23,33 1.48 0.578 0.0003 Downward

Nitrogen, nitrite plus nitrate, dissolved as N, mg/L 23,33 2.57 1.87 0.0066 Downward

Orthophosphate, dissolved as P, mg/L 34,33 0.598 0.367 0.0094 Downward

Phosphorus, total as P, mg/L 34,33 1.585 0.96 <0.0001 Downward

Carbon, organic, dissolved as C, mg/L 19,34 10.6 7.5 0.1669 None

Copper, dissolved, μg/L 34,34 3.0 2.8 0.6673 None

Copper, total recoverable, μg/L 23,32 31.9 22.4 0.0238 Downward

Lead, dissolved, μg/L 23,34 0.26 0.21 0.0478 Downward

Lead, total recoverable, μg/L 23,32 33.2 17.3 0.0212 Downward

Manganese, dissolved, μg/L 23,34 85.8 33.1 0.0060 Downward

Manganese, total recoverable, μg/L 23,32 542 360 0.0217 Downward

Zinc, dissolved, μg/L 23,34 17.0 17.3 0.7634 None

Zinc, total recoverable, μg/L 23,32 146 91 0.0732 Downward

Table 9. Mann-Whitney test of differences between distributions of water-quality values and concentrations in stormwater samples collected in 2002–

2005 and 2006–2010 at all sites.—Continued

[n, number of samples, each time period separated by a comma; shaded values indicate statistical significance for this report with a

p-value less than or equal to 0.1000; mg/L, milligrams per liter; μg/L, micrograms per liter; °C, degrees Celsius; μS/cm, microsiemens 

per centimeter at 25°C; CaCO3, calcium carbonate; cm, centimeter; N, nitrogen; P, phosphorus; C, carbon; MPN, most probable 

number] 
63

Comparison to Previous Results

For this report, the Mann-Whitney nonparametric rank sum statistical test was used to 

determine if the distribution of values and concentrations for 2002–2005 samples was different 

from the distributions of values and concentrations in stormwater determined for 2006–2010 

(table 9). Additional corroboration of many of the statistical tests trends are illustrated in the 

boxplots of figure 4. A probability value (p-value) less than or equal to 0.10 was used as the 

criteria to determine if an apparent trend was statistically significant. These results are presented 

for each of the five stations in table 9. 

In an earlier study, Bossong and others (2008) indicated that the general quality of 

stormwater had improved during 2002–2005 compared to 1998–2001, having fewer exceedances 

of Colorado standards, and showing downward trends for many water-quality values and 

concentrations. These trends coincided with general downward or relatively similar mean 

streamflows for the 2002–2005 compared to 1998–2001, which indicates that dilution may be a 

smaller influence on values and concentrations than other factors. For this report, downward trends 

were indicated for many constituents at each station during 2006–2010 compared to 2002–2005 

(fig. 4; table 9). The trends for mean streamflow for 2006–2010 compared to 2002–2005 are 

upward at all sites except for the South Platte River at Henderson, indicating that dilution by larger 

streamflows could be a factor in the downward concentration trends (fig. 4). At Union, downward 

trends were indicated for hardness, dissolved ammonia, dissolved orthophosphate, and dissolved 

copper. Upward trends in properties and constituents at Union were indicated for pH. At Denver, 

downward trends were indicated for total ammonia plus organic nitrogen, dissolved ammonia, 

dissolved nitrite plus nitrate, dissolved orthophosphate, total phosphorus, dissolved organic carbon, 

and dissolved lead, manganese, and zinc, and total recoverable zinc. An upward trend in properties 

and constituents at Denver was indicated for pH. At Toll Gate, downward trends were indicated for 

residue on evaporation, total ammonia plus organic nitrogen, dissolved ammonia, dissolved 

orthophosphate, total phosphorus, and total recoverable copper, lead, manganese, and zinc. Upward 

trends in properties and constituents at Toll Gate were indicated for pH, specific conductance, and 

dissolved nitrite plus nitrate. At Sand Creek, downward trends were indicated for hardness, 

dissolved calcium, total ammonia plus organic nitrogen, dissolved ammonia, orthophosphate, 

manganese, and zinc. An upward trend in properties and constituents at Sand Creek was indicated 

for pH. Downward trends at Henderson were indicated for specific conductance, hardness, 

dissolved magnesium, dissolved residue on evaporation, total ammonia plus organic nitrogen, 

dissolved ammonia, dissolved nitrite plus nitrate, dissolved orthophosphate, total phosphorus, 

dissolved lead and manganese, and total recoverable copper, lead, manganese, and zinc. 

In general, the quality of stormwater (2006–2010) has improved for many water-quality 

constituents, which had lower values and concentrations than those in stormwater collected in 

2002–2005. However, the physical basis, processes, and the role of dilution that account for these 

changes are complex and are beyond the scope of this report. 

Summary 

The quality of stormwater runoff is of interest to many management and regulatory 

agencies as well as scientists, recreational stream users, and the general public. In response to this

interest, the U.S. Geological Survey, in cooperation with the Urban Drainage and Flood Control 

District, systematically collected stormwater samples from a network of five monitoring stations in 
64

and around metropolitan Denver, Colorado. This report describes the quality of stormwater 

sampled from October 1, 2006, to December 31, 2010. The network includes three stations: South 

Platte River below Union Avenue at Englewood, South Platte River at Denver, and South Platte 

River at Henderson, known as Union, Denver, and Henderson. The stations on the South Platte 

River receive streamflow from several tributaries, and the South Platte River is sometimes referred 

to as a “receiving stream.” The network also includes stations on two tributary streams: Sand 

Creek, which is tributary to the South Platte River, and Toll Gate Creek, which is tributary to Sand 

Creek. Sampling was accomplished by using refrigerated automatic pumping samplers that were 

used to collect composite stormwater samples. About 200 time-weighted composite stormwater 

samples were collected by compositing individual aliquots of stormwater that were obtained at 

fixed-time increments typically during 12-hour intervals; bacteriological samples were collected 

manually from streams during some storms. 

Stormwater samples were analyzed at the U.S. Geological Survey National Water Quality 

Laboratory during 2006–2010 for water-quality properties pH, specific conductance, hardness, and 

residue on evaporation at 105 degrees Celsius; for major ions (calcium, magnesium) and organic 

carbon; for nutrients such as ammonia plus organic nitrogen, ammonia, dissolved nitrite plus 

nitrate, orthophosphate, and total phosphorus; and for metals, including total recoverable and 

dissolved phases of copper, lead, manganese, and zinc. Samples analyzed for bacteriological 

indicators including Escherichia coli and fecal coliform were collected during selected storms and 

were analyzed at the Metro Wastewater Reclamation Laboratory.

Quality-control samples were collected and the analyses of blank sample results indicated 

no substantial contribution to concentrations from ambient field conditions or sampling equipment. 

For the 151 duplicate analyses, the median RPD was 1.9 percent, and RPDs ranged from 0 to 115 

percent. For the 151 duplicate analyses, the 90th-percentile RPDs ranged from 0.4 to 45 percent. 

Among those results dissolved copper, lead, and manganese, accounted for the largest RPDs (90th 

percentile RPDs ranged from 34 to 45 percent). Excluding these dissolved metals, the range of 90th 

percentile values in table was 0.4 to 20 percent. The RPD results indicate generally satisfactory 

reproducibility for the majority water-quality analysis types. For the dissolved phase of the metals 

copper, lead, and manganese, reproducibility was only fair.

The results from laboratory analyses indicate, in general, and particularly for nutrients, that 

some values and concentrations for water-quality constituents increase with contributing drainage 

area in the South Platte River. No well-defined relation was identified between concentration and 

the amount of urban land cover identified using the U.S. Geological Survey National Land Cover 

data. For example, the mean concentration for ammonia plus organic nitrogen is 1.4 mg/L at the 

South Platte River below Union Avenue station, 1.8 mg/L at South Platte River at Denver station, 

and 2.9 mg/L at South Platte River at Henderson station. This pattern correlates with the size of the 

local contributing drainage area, which also increases downstream. Sand Creek, a tributary that 

flows into the South Platte River between the South Platte River at Denver and South Platte River 

at Henderson stations, has Toll Gate Creek above 6th Avenue at Aurora station as tributary. Some 

values and concentrations of results also increase in a downstream direction from the Toll Gate

above 6th Avenue at Aurora station to the Sand Creek at mouth near Commerce City station. For 

some water-quality constituents, concentrations measured in the tributary streams (Toll Gate above 

6th Avenue at Aurora and Sand Creek at mouth near Commerce City) are higher than 

concentrations measured in the upstream reach of the South Platte River (South Platte River below 

Union Avenue).
65

A summary of results from 2006–2010 stormwater samples that did not meet (exceeded) 

CDPHE water-quality standards is presented in this report for individual stations. The results 

indicate few exceptions to standards except for dissolved manganese and zinc and Eschericia coli. 

Stormwater collected at South Platte River below Union Avenue had about 10 percent acute or 

chronic dissolved manganese exceedances in samples; samples collected at South Platte River at 

Denver had less than 5 percent acute or chronic dissolved manganese exceedances. In contrast, 

samples collected at Toll Gate above 6th Avenue at Aurora, Sand Creek at mouth near Commerce 

City, and South Platte River at Henderson, each had about 30 to 50 percent exceedances of both 

acute and chronic dissolved manganese standards. Of the samples collected at Sand Creek at mouth 

near Commerce City, 1 sample exceeded the acute standard and 4 samples exceeded the chronic 

standard for dissolved zinc, but no samples collected from the other sites exceeded either standard 

for zinc. Almost all samples of stormwater analyzed for Escherichia coli exceeded the Colorado 

numeric standards. A numerical standard for fecal coliform is no longer applicable as of 2004. 

The Mann-Whitney nonparametric rank sum statistical test was used to determine if the 

distribution of concentrations and values from 2006–2010 was different from the distributions 

determined for 2002–2005. A probability value (p-value) less than or equal to 0.10 was the criteria 

used to determine if an apparent trend was statistically significant. 

Results from the 2002–2005 study indicated that the general quality of stormwater had 

improved during 2002–2005 compared to 1998–2001, having fewer exceedances of Colorado 

standards, and showing downward trends for many water-quality values and concentrations. These 

trends coincided with general downward or relatively similar mean streamflows for the 2002–2005 

compared to 1998–2001, which indicates that dilution may be a smaller influence on values and 

concentrations than other factors. For this report, downward trends were indicated for many

constituents at each station during 2006–2010 compared to 2002–2005. The trends for mean 

streamflow for 2006–2010 compared to 2002–2005 are upward at all stations except for South 

Platte River at Henderson, indicating that dilution by larger flows could be a factor in the 

downward concentration trends. At South Platte River below Union Avenue, downward trends 

were indicated for hardness, dissolved ammonia, dissolved orthophosphate, and dissolved copper. 

Upward trends in properties and constituents at South Platte River below Union Avenue were 

indicated for pH. At South Platte River at Denver, downward trends were indicated for total 

ammonia plus organic nitrogen, dissolved ammonia, dissolved nitrite plus nitrate, dissolved 

orthophosphate, total phosphorus, dissolved organic carbon, and dissolved lead, manganese, and 

zinc, and total recoverable zinc. An upward trend in properties and constituents at South Platte 

River at Denver was indicated for pH. At Toll Gate above 6th Avenue at Aurora, downward trends 

were indicated for residue on evaporation, total ammonia plus organic nitrogen, dissolved 

ammonia, dissolved orthophosphate, total phosphorus, and total recoverable copper, lead, 

manganese, and zinc. Upward trends properties and constituents at Toll Gate above 6th Avenue at 

Aurora were indicated for pH, specific conductance, and dissolved nitrite plus nitrate. At Sand 

Creek at mouth near Commerce City, downward trends were indicated for hardness, dissolved 

calcium, total ammonia plus organic nitrogen, and dissolved ammonia, orthophosphate, 

manganese, and zinc. Upward trends for properties and constituents at Sand Creek at mouth near 

Commerce City was indicated for pH. Downward trends at South Platte River at Henderson were 

indicated for specific conductance, hardness, dissolved magnesium, residue on evaporation, total 

ammonia plus organic nitrogen, dissolved ammonia, dissolved nitrite plus nitrate, dissolved 

orthophosphate, total phosphorus, dissolved lead and manganese, and total recoverable copper, 

lead, manganese, and zinc.
66

In general, the quality of stormwater (2006–2010) has improved for many water-quality 

constituents, which had lower values and concentrations than those in stormwater collected in 

2002–2005. However, the physical basis, processes, and the role of dilution that account for these 

changes are complex beyond the scope of this report.

References Cited

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and storm runoff to the South Platte River in the southern Denver metropolitan area, Colorado: 

U.S. Geological Survey Water-Resources Investigations Report 83–4138, 35 p. 

Bossong, C.R., Stevens, M.R., Doerfer, J.T., and Glass, B.R., 2005, Summary and evaluation of 

the quality of stormwater in Denver, Colorado, water years 1998–2001: U.S. Geological Survey 

Scientific Investigations Report 2005–5150, 90 p. 

Bossong, C.R., and Fleming, A.C., 2008, Summary and evaluation of the quality and stormwater in 

Denver, Colorado, October 2001 to October 2005: U.S. Geological Survey Scientific 

Investigations Report 2008–5134, 102 p. 

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persulfate oxidation and infrared spectrometry: U.S. Geological Survey Open-File Report 

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procedures based on long-term method detection levels and some considerations for 

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standards and methodologies for surface water (5 CCR 1002–31): Colorado Department of 

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Denver Regional Council of Governments, 1983, Urban runoff quality in the Denver region: 

Denver, Colo., Denver Regional Council of Governments, 156 p. 

Ellis, S.R., 1978, Hydrologic data for urban storm runoff from three localities in the Denver 

metropolitan area, Colorado: U.S. Geological Survey Open-File Report 78–410, 135 p. 

Ellis, S.R., and Alley, W.B., 1979, Hydrologic data for urban storm runoff from three localities in 

the Denver metropolitan area, Colorado: U.S. Geological Survey Water-Resources Investigations 

Report 79–64, 60 p. 

Ellis, S.R., Doerfer, J.T., Mustard, M.H., Blakely, S.R., and Gibbs, J.W., 1984, Analysis of urban 

storm-runoff data and the effects on the South Platte River, Denver metropolitan area, Colorado: 

U.S. Geological Survey Water-Resources Investigations Report 84–4159, 66 p. 

Ellis, S.R., and Mustard, M.H., 1985, A summary of urban runoff studies in the Denver 

metropolitan area, Colorado: U.S. Geological Water-Resources Investigations Report 84–4072, 

31 p. 

Faires, L.M., 1993, Methods of analysis by the U.S. Geological Survey National Water Quality 

Laboratory—Determination of metals in water by inductively coupled plasma-mass 

spectrometry: U.S. Geological Survey Open-File Report 92–634, 28 p. 
67

Fishman, M.J., ed., 1993, Methods of analysis by the U.S. Geological Survey National Water 

Quality Laboratory—Determination of inorganic and organic constituents in water and fluvial 

sediments: U.S. Geological Survey Open-File Report 93–125, 217 p. 

Fishman, M.J., and Friedman, L.C., 1989, Methods for determination of inorganic substances in 

water and fluvial sediments: U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 5, chap. A1, 545 p. 

Garbarino, J.R., and Struzeski, T.M., 1998, Methods of analysis by the U.S. Geological Survey 

National Water Quality Laboratory—Determination of elements in whole-water digests using 

inductively coupled plasma-optical emission spectrometry and inductively coupled plasma-mass 

spectrometry: U.S. Geological Survey Open-File Report 98–165, 101 p. 

Gibbs, J.W., 1981, Hydrologic data for urban storm runoff from nine sites in the Denver 

metropolitan area, Colorado: U.S. Geological Survey Open-File Report 81–682, 142 p. 

Gibbs, J.W., Arnold, L.M., and Reed, R.L., 1983, Hydrologic data for the drainage basins of 

Chatfield and Cherry Creek Lakes, Denver metropolitan area: U.S. Geological Survey Open-File 

Report 83–857, 244 p. 

Gibbs, J.W., and Doerfer, J.T., 1982, Hydrologic data for urban storm runoff in the Denver 

metropolitan area, Colorado: U.S. Geological Survey Open-File Report 82–872, 553 p. 

Hall, D.C., and Duncan, A.C., 1980, Hydrologic data from Upper Grange Hall Creek Basin, 

Northglenn, Adams County, Colorado: U.S. Geological Survey Open-File Report 80–578, 132 p. 

Hall, D.C., and Duncan, A.C., 1981, Characterization of urban runoff from Grange Hall Creek at 

Northglenn, Adams County, Colorado: U.S. Geological Survey Water-Resources Investigations 

Report 81–28, 50 p. 

Helsel, D.R., 2005, Nondetects and data analysis—statistics for censored environmental data: New 

York, John Wiley and Sons, 250 p. 

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in water resources—Studies in 

environmental science 49: New York, Elsevier, 529 p. 

Hem, J.D., 1985, Study and interpretation of the chemical characteristics of natural water (3d ed.): 

U.S. Geological Survey Water-Supply Paper 2254, 264 p. 

Hoffman, G.L., Fishman, M.J., and Garbarino, J.R., 1996, Methods of analysis by the U.S. 

Geological Survey National Water Quality Laboratory—In-bottle acid digestion of whole-water 

samples: U.S. Geological Survey Open-File Report 96–225, 28 p. 

Iman, R.L., and Conover, W.J., 1983, A modern approach to statistics: New York, John Wiley and 

Sons, 497 p. 

Mustard, M.H., Ellis, S.R., and Gibbs, J.W., 1985, Runoff and washoff loads from rainfallsimulation experiments on a street surface and a native pasture in the Denver metropolitan area, 

Colorado: U.S. Geological Survey Open-File Report 84–820, 44 p. 

Patton, C.J., and Truitt, E.P., 1992, Methods of analysis by the U.S. Geological Survey National 

Water Quality Laboratory—Determination of total phosphorus by a Kjeldahl digestion method 

and an automated colorimetric finish that includes dialysis: U.S. Geological Survey Open-File 

Report 92–146, 39 p. 

Patton, C.J., and Truitt, E.P., 2000, Methods of analysis by the U.S. Geological Survey National 

Water Quality Laboratory—Determination of ammonium plus organic nitrogen by a Kjeldahl 

digestion method and an automated photometric finish that includes digest cleanup by gas 

diffusion: U.S. Geological Survey Open-File Report 2000–170, 31 p. 
68

Rantz, S.E., and others, 1982, Measurement and computa-tion of streamflow—Volume 1, 

Measurement of stage and discharge, and Volume 2, Computation of discharge: U.S. Geological 

Survey Water-Supply Paper 2175, 630 p. 

Turnipseed, D. P., and Sauer, V.B., 2010, Discharge measurements at gaging stations: U.S. 

Geological Survey Techniques and Methods 3-A8, xiv, 87 p. 

Sauer, V.B., and Turnipseed, D. P., 2010, Stage measurement at gaging stations: U.S. Geological 

Survey Techniques and Methods 3-A7, x, 45 p. 

Urban Drainage and Flood Control District, 2012, ALERT system real-time flood detection and 

current weather conditions: Urban Drainage and Flood Control District Web site accessed March 

2012 at http://alert.udfcd.org/ . 

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data: U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chaps. 

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http://eros.usgs.gov/#/Find_Data/Products_and_Data_Available/Land_Cover_Products
69

Appendix
70

Table A1. Statistics for concentrations and measurement values at South Platte River below Union Avenue at Englewood, 2006–2010.

[μS/cm at 25°C, microsiemens per centimeter at 25 degrees Celsius; mL, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per 

liter; μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; 

nlt, number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed; multiple censoring level

statistics uses log-probability regression on values greater than the censoring level to predict statistics that are affected by the censoring 

level (mean, standard deviation, and 25th percentile; conventional univariate statistics are used when no censored values are used; 

multiple detection limit techniques are used when censored values are part of the data for a particular value or concentration] 

06710247 South Platte River below Union Avenue near Englewood, Colorado, 2006

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.8 0.2 7.6 7.8 7.8 10 0 7.6 8.4

Specific conductance, μS/cm at 25°C 531 214 373 429 645 10 0 356 993

Hardness, total, mg/L as CaCO3 159 42 124 141 186 10 0 121 247

Calcium, mg/L 47.4 11.3 38.1 43.2 55.9 10 0 37.1 72.2

Magnesium, mg/L 9.71 3.32 7.11 7.94 11.6 10 0 6.84 16.1

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 199 190 60 178 296 10 0 16 645

Nitrogen, total, ammonia plus organic as N, mg/L 1.38 1.02 0.73 1.10 1.40 10 0 0.50 4.00

Nitrogen, dissolved, ammonia as N, mg/L 0.073 0.042 0.042 0.056 0.12 10 0 0.013 0.138

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.758 0.507 0.39 0.6 0.94 10 0 0.17 1.71

Orthophosphate, dissolved as P, mg/L 0.054 0.029 0.031 0.053 0.065 10 0 0.024 0.123

Phosphorus, total as P, mg/L 0.326 0.246 0.140 0.300 0.400 10 0 0.080 0.930

Carbon, dissolved, organic as C, mg/L 6.8 3.0 4.3 7.0 8.7 10 0 2.9 13.2

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 9,400 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 12,000 --

Copper, dissolved, μg/L 2.06 0.57 1.70 2.30 2.50 0 0 1.10 2.80

Copper, total recoverable, μg/L 10.8 8.9 4.6 9.7 12.9 10 0 3.3 33.5

Lead, dissolved, μg/L 0.06 0.02 0.04 0.06 0.07 10 2 0.04 0.10

Lead, total recoverable, μg/L 7.0 7.2 2.2 6.2 8.1 10 0 1.0 25.7

Manganese, dissolved, μg/L 21.7 25.0 6.4 11.0 29.8 10 0 2.6 81.2

Manganese, total recoverable, μg/L 286 204 183 257 307 10 0 42.2 806

Zinc, dissolved, μg/L 5.6 2.0 4.3 5.9 7.3 10 0 1.9 8.2

Zinc, total recoverable, μg/L 37 37 11 30 42 10 0 7 134
71

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.8 0.1 7.8 7.8 7.9 6 0 7.6 7.9

Specific conductance, μS/cm at 25°C 335 103 288 301 311 6 0 268 545

Hardness, total, mg/L as CaCO3 103 27 89 93 106 6 0 82 156

Calcium, mg/L 30.2 7.3 25.7 27.6 32.2 6 0 24.3 44.3

Magnesium, mg/L 6.65 2.21 5.60 6.00 6.18 6 0 5.06 11.1

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 196 190 58 118 330 6 0 34 520

Nitrogen, total, ammonia plus organic as N, mg/L 1.3 0.7 0.67 1.25 2 6 0 0.46 2.20

Nitrogen, dissolved, ammonia as N, mg/L 0.065 0.027 0.039 0.067 0.076 6 1 0.039 0.108

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.521 0.215 0.38 0.46 0.58 6 0 0.33 0.92

Orthophosphate, dissolved as P, mg/L 0.059 0.045 0.017 0.050 0.113 6 0 0.010 0.114

Phosphorus, total as P, mg/L 0.428 0.318 0.130 0.395 0.740 6 0 0.070 0.840

Carbon, dissolved, organic as C, mg/L 9.0 5.3 5.7 6.5 11.7 6 0 4.9 18.6

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 1,100 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 1,700 --

Copper, dissolved, μg/L 2.15 1.21 1.60 1.75 1.80 6 0 1.40 4.60

Copper, total recoverable, μg/L 13.1 9.7 5.1 10.5 23.7 6 0 2.9 25.8

Lead, dissolved, μg/L 0.072 0.095 0.014 0.043 0.07 6 3 0.06 0.26

Lead, total recoverable, μg/L 11.2 9.3 4.0 8.3 22.4 6 0 1.6 22.7

Manganese, dissolved, μg/L 25.9 35.6 11.0 11.7 18.1 6 0 4.8 98.0

Manganese, total recoverable, μg/L 301 199 89 298 522 6 0 72 529

Zinc, dissolved, μg/L 4.0 1.9 2.8 3.5 3.8 6 0 2.8 7.8

Zinc, total recoverable, μg/L 44 31 21 34 79 6 0 13 87

Table A1. Statistics for concentrations and measurement values at South Platte River below Union Avenue at Englewood, 2006–2010.—Continued

[μS/cm at 25°C, microsiemens per centimeter at 25 degrees Celsius; mL, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per 

liter; μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; 

nlt, number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed; multiple censoring level

statistics uses log-probability regression on values greater than the censoring level to predict statistics that are affected by the censoring 

level (mean, standard deviation, and 25th percentile; conventional univariate statistics are used when no censored values are used; 

multiple detection limit techniques are used when censored values are part of the data for a particular value or concentration] 
72

06710247 South Platte River below Union Avenue near Englewood, Colorado, 2008

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.8 0.1 7.8 7.8 7.9 3 0 7.8 7.9

Specific conductance, μS/cm at 25°C 424 77 360 403 510 3 0 360 510

Hardness, total, mg/L as CaCO3 129 26 107 121 158 3 0 107 158

Calcium, mg/L 38.0 6.8 31.9 36.8 45.4 3 0 31.9 45.4

Magnesium, mg/L 8.13 2.31 6.56 7.05 10.8 3 0 6.56 10.8

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 201 108 77 248 277 3 0 77 277

Nitrogen, total, ammonia plus organic as N, mg/L 1.41 0.54 0.83 1.5 1.9 3 0 0.83 1.90

Nitrogen, dissolved, ammonia as N, mg/L 0.095 0.096 0.024 0.057 0.205 3 0 0.024 0.205

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.9 0.213 0.66 0.97 1.07 3 0 0.66 1.07

Orthophosphate, dissolved as P, mg/L 0.049 0.016 0.037 0.044 0.068 3 0 0.037 0.068

Phosphorus, total as P, mg/L 0.326 0.153 0.150 0.410 0.420 3 0 0.150 0.420

Carbon, dissolved, organic as C, mg/L 7.4 2.7 4.6 7.8 9.9 3 0 4.6 9.9

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 11,000 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 11,000 --

Copper, dissolved, μg/L 1.83 0.40 1.40 1.90 2.20 3 0 1.40 2.20

Copper, total recoverable, μg/L 10.6 5.1 4.8 13.0 14.1 3 0 4.8 14.1

Lead, dissolved, μg/L Fewer than 3 uncensored values 3 1 0.05 0.07

Lead, total recoverable, μg/L 6.9 3.9 2.4 8.6 9.7 3 0 2.4 9.7

Manganese, dissolved, μg/L 4.6 3.7 1.5 3.7 8.7 3 0 1.5 8.7

Manganese, total recoverable, μg/L 312 141 156 351 429 3 0 156 429

Zinc, dissolved, μg/L 3.4 1.3 2.6 2.7 4.9 3 0 2.6 4.9

Zinc, total recoverable, μg/L 36.3 21.2 12.7 42.5 53.8 3 0 12.7 53.8

Table A1. Statistics for concentrations and measurement values at South Platte River below Union Avenue at Englewood, 2006–2010.—Continued

[μS/cm at 25°C, microsiemens per centimeter at 25 degrees Celsius; mL, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per 

liter; μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; 

nlt, number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed; multiple censoring level

statistics uses log-probability regression on values greater than the censoring level to predict statistics that are affected by the censoring 

level (mean, standard deviation, and 25th percentile; conventional univariate statistics are used when no censored values are used; 

multiple detection limit techniques are used when censored values are part of the data for a particular value or concentration] 
73

06710247 South Platte River below Union Avenue near Englewood, Colorado, 2009

Multiple censoring level statistics Range of uncensored 

values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.6 0.2 7.5 7.7 7.8 8 0 7.4 7.9

Specific conductance, μS/cm at 25°C 484 142 350 462 620 8 0 331 680

Hardness, total, mg/L as CaCO3 139 24 113 141 164 8 0 111 167

Calcium, mg/L 40.3 6.3 33.8 40.4 46.2 8 0 33.3 48.6

Magnesium, mg/L 9.23 2.32 7.14 9.66 10.45 8 0 6.22 13.2

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 260 168 101 299 408 8 0 27 440

Nitrogen, total, ammonia plus organic as N, mg/L 1.35 0.65 0.72 1.5 1.75 8 0 0.5 2.40

Nitrogen, dissolved, ammonia as N, mg/L 0.086 0.088 0.023 0.048 0.144 8 1 0.018 0.252

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.742 0.367 0.44 0.595 1.105 8 0 0.38 1.28

Orthophosphate, dissolved as P, mg/L 0.068 0.044 0.029 0.054 0.116 8 0 0.022 0.127

Phosphorus, total as P, mg/L 0.371 0.182 0.195 0.440 0.509 8 0 0.100 0.580

Carbon, dissolved, organic as C, mg/L 5.8 1.9 4.2 5.3 7.4 8 0 3.7 8.8

Escherichia Coli, MPN/100 mL -- -- -- -- -- -- -- -- --

Fecal coliform, MPN/100 mL -- -- -- -- -- -- -- -- --

Copper, dissolved, μg/L 1.90 0.50 1.60 1.75 2.35 8 0 1.20 2.60

Copper, total recoverable, μg/L 8.97 4.7 5.75 7.45 13.15 8 0 3.2 16

Lead, dissolved, μg/L 0.06 0.06 0.04 0.05 0.06 8 0 0.03 0.20

Lead, total recoverable, μg/L 6.3 4.7 3.1 4.1 10.7 8 0 1.1 13.2

Manganese, dissolved, μg/L 29.9 37.0 12.5 21.3 26.3 8 0 0.2 119

Manganese, total recoverable, μg/L 238 180 101 142 379 8 0 90 567

Zinc, dissolved, μg/L 4.7 2.0 2.8 5.0 6.6 8 0 2.2 6.9

Zinc, total recoverable, μg/L 32 17 22 28 45 8 0 7 56

Table A1. Statistics for concentrations and measurement values at South Platte River below Union Avenue at Englewood, 2006–2010.—Continued

[μS/cm at 25°C, microsiemens per centimeter at 25 degrees Celsius; mL, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per 

liter; μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; 

nlt, number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed; multiple censoring level

statistics uses log-probability regression on values greater than the censoring level to predict statistics that are affected by the censoring 

level (mean, standard deviation, and 25th percentile; conventional univariate statistics are used when no censored values are used; 

multiple detection limit techniques are used when censored values are part of the data for a particular value or concentration] 
74

06710247 South Platte River below Union Avenue near Englewood, Colorado, 2010

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.6 0.2 7.6 7.6 7.7 11 0 6.9 7.8

Specific conductance, μS/cm at 25°C 558 282 330 419 923 11 0 226 955

Hardness, total, mg/L as CaCO3 166 63 120 135 223 11 0 97 290

Calcium, mg/L 48.6 17.4 36.2 41.0 63.2 11 0 29.5 83.8

Magnesium, mg/L 10.81 4.72 7.15 8.68 15.8 11 0 5.71 19.6

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 250 219 110 220 284 11 1 46 670

Nitrogen, total, ammonia plus organic as N, mg/L 1.52 0.6 1.1 1.4 1.7 11 0 0.83 2.80

Nitrogen, dissolved, ammonia as N, mg/L 0.02 0.018 0.006 0.012 0.031 11 4 0.012 0.058

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 1.063 0.829 0.5 0.59 1.37 11 0 0.41 2.71

Orthophosphate, dissolved as P, mg/L 0.058 0.080 0.010 0.022 0.053 11 0 0.004 0.237

Phosphorus, total as P, mg/L 0.426 0.262 0.240 0.350 0.430 11 0 0.210 0.980

Carbon, dissolved, organic as C, mg/L 5.8 1.0 4.8 5.5 6.5 11 0 4.4 7.7

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 20 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 20 --

Copper, dissolved, μg/L 2.44 0.85 1.70 2.60 2.90 11 0 1.40 4.00

Copper, total recoverable, μg/L 13.7 9.3 7.4 12.2 14.4 9 0 3.1 30.4

Lead, dissolved, μg/L 0.10 0.07 0.06 0.06 0.14 11 0 0.03 0.27

Lead, total recoverable, μg/L 9.9 8.5 4.6 8.2 10.8 9 0 0.6 26.9

Manganese, dissolved, μg/L 10.5 8.7 2.5 7.6 16.9 11 0 0.5 28.4

Manganese, total recoverable, μg/L 364 273 176 267 430 9 0 91 884

Zinc, dissolved, μg/L 6.86 4.37 2.6 5.3 9.7 11 0 1.8 13.6

Zinc, total recoverable, μg/L 49.1 32.0 29.3 39.1 59.8 9 0 10.6 108

Table A1. Statistics for concentrations and measurement values at South Platte River below Union Avenue at Englewood, 2006–2010.—Continued

[μS/cm at 25°C, microsiemens per centimeter at 25 degrees Celsius; mL, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per 

liter; μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; 

nlt, number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed; multiple censoring level

statistics uses log-probability regression on values greater than the censoring level to predict statistics that are affected by the censoring 

level (mean, standard deviation, and 25th percentile; conventional univariate statistics are used when no censored values are used; 

multiple detection limit techniques are used when censored values are part of the data for a particular value or concentration] 
75

Table A2. Statistics for concentrations and measurement values at South Platte River at Denver by year, 2006–2010.

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; μg/L, 

micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, number of 

“less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

06714000 South Platte River at Denver, Colorado, 2006

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.6 0.2 7.5 7.7 7.7 6 0 7.3 7.9

Specific conductance, μS/cm at 25°C 408 42 375 416 439 4 0 349 447

Hardness, total, mg/L as CaCO3 141 38 121 129 159 6 0 99.3 209

Calcium, mg/L 42.4 9.9 37.2 39.6 48.5 6 0 30.6 59

Magnesium, mg/L 8.51 3.34 6.89 7.25 9.24 6 0 5.57 14.9

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 189 50 166 186 220 6 0 115 260

Nitrogen, total, ammonia plus organic as N, mg/L 1.9 1.0 1.3 1.5 2.0 6 0 1.1 3.9

Nitrogen, dissolved, ammonia as N, mg/L 0.192 0.200 0.095 0.119 0.131 6 0 0.087 0.600

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 1.56 1.07 0.91 1.21 1.49 6 0 0.83 3.69

Orthophosphate, dissolved as P, mg/L 0.179 0.089 0.121 0.147 0.197 6 0 0.110 0.349

Phosphorus, total as P, mg/L 0.57 0.26 0.39 0.51 0.62 6 0 0.35 1.07

Carbon, dissolved, organic as C, mg/L 6.38 2.74 5 5.7 6 6 0 4.1 11.8

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 7,900 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 7,900 --

Copper, dissolved, μg/L 2.2 0.5 1.8 2.1 2.5 6 0 1.5 3.2

Copper, total recoverable, μg/L 16.6 6.3 12.0 16.4 18.2 6 0 9.0 27.3

Lead, dissolved, μg/L 0.10 0.07 0.06 0.08 0.13 6 0 0.04 0.24

Lead, total recoverable, μg/L 12.2 4.3 8.2 13.4 15.4 6 0 6.0 17.0

Manganese, dissolved, μg/L 27.7 62.9 1.4 2.1 3.5 6 0 1.1 156.0

Manganese, total recoverable, μg/L 337 93 272 297 412 6 0 254 487

Zinc, dissolved, μg/L 7.9 6.6 4.9 5.6 6.3 6 0 3.7 21.1

Zinc, total recoverable, μg/L 64 32 40 63 68 6 0 30 121
76

Table A2. Statistics for concentrations and measurement values at South Platte River at Denver by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; μg/L, 

micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, number of 

“less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

06714000 South Platte River at Denver, Colorado, 2007

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median

50% 75% n nlt Minimum Maximum

pH, standard units 7.7 0.2 7.5 7.7 7.8 6 0 7.5 8.0

Specific conductance, μS/cm at 25°C 410 99 367 384 429 5 0 303 568

Hardness, total, mg/L as CaCO3 108.9 24.9 89.2 110 126 6 0 74.4 144

Calcium, mg/L 32.6 7.2 26.9 33.4 37.6 6 0 22.2 42.2

Magnesium, mg/L 6.71 1.76 5.35 6.54 7.81 6 0 4.59 9.49

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 231 140 156 205 355 6 0 39 428

Nitrogen, total, ammonia plus organic as N, mg/L 1.45 0.61 1.3 1.4 1.7 6 0 0.51 2.4

Nitrogen, dissolved, ammonia as N, mg/L 0.106 0.091 0.024 0.089 0.186 6 0 0.014 0.234

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.94 0.29 0.73 0.80 1.21 6 0 0.69 1.39

Orthophosphate, dissolved as P, mg/L 0.106 0.028 0.074 0.106 0.133 6 0 0.073 0.142

Phosphorus, total as P, mg/L 0.472 0.216 0.39 0.445 0.65 6 0 0.14 0.76

Carbon, dissolved, organic as C, mg/L 7.08 2.07 5.3 6.9 7.6 6 0 5.2 10.6

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 4,600 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 11,000 --

Copper, dissolved, μg/L 3.5 2.3 1.8 2.4 5.9 6 0 1.7 7.0

Copper, total recoverable, μg/L 17.2 9.5 12.3 16.3 22.6 6 0 3.8 32.0

Lead, dissolved, μg/L 0.13 0.06 0.08 0.10 0.20 6 0 0.07 0.20

Lead, total recoverable, μg/L 16.7 10.4 9.7 16.4 22.6 6 0 2.4 32.5

Manganese, dissolved, μg/L 8.8 3.6 7.0 8.1 10.9 6 0 4.3 14.7

Manganese, total recoverable, μg/L 257 112 215 253 335 6 0 78 406

Zinc, dissolved, μg/L 7.5 3.2 4.8 6.8 10.4 6 0 4.6 11.9

Zinc, total recoverable, μg/L 69.7 41.8 41.6 68.5 92.3 6 0 13 134
77

Table A2. Statistics for concentrations and measurement values at South Platte River at Denver by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; μg/L, 

micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, number of 

“less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

06714000 South Platte River at Denver, Colorado, 2008

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.6 0.3 7.4 7.6 7.8 4 0 7.2 7.8

Specific conductance, μS/cm at 25°C 377 112 291 363 462 4 0 260 520

Hardness, total, mg/L as CaCO3 104 29 83 96 125 4 0 78.4 146

Calcium, mg/L 31.4 8.1 25.6 29.2 37.2 4 0 24.4 42.8

Magnesium, mg/L 6.26 2.25 4.77 5.66 7.75 4 0 4.26 9.47

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 267 199 105 242 428 4 0 74 510

Nitrogen, total, ammonia plus organic as N, mg/L 2.5 1.6 1.2 2.4 3.8 4 0 1.0 4.2

Nitrogen, dissolved, ammonia as N, mg/L 0.266 0.190 0.104 0.261 0.429 4 0 0.082 0.463

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 1.37 0.43 1.00 1.36 1.74 4 0 0.97 1.78

Orthophosphate, dissolved as P, mg/L 0.183 0.046 0.143 0.184 0.223 4 0 0.140 0.223

Phosphorus, total as P, mg/L 0.74 0.46 0.38 0.64 1.11 4 0 0.36 1.33

Carbon, dissolved, organic as C, mg/L 7.1 1.8 5.7 6.9 8.6 4 0 5.6 9.1

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 23,000 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 23,000 --

Copper, dissolved, μg/L 2.0 0.6 1.5 1.9 2.4 4 1 1.7 2.7

Copper, total recoverable, μg/L 28.8 26.9 9.9 20.4 47.7 4 0 7.5 67.0

Lead, dissolved, μg/L 0.12 0.04 0.09 0.11 0.14 4 0 0.07 0.17

Lead, total recoverable, μg/L 30.2 34.2 7.4 18.7 53.0 4 0 4.0 79.5

Manganese, dissolved, μg/L 4.1 4.9 0.9 2.5 7.3 4 0 0.1 11.2

Manganese, total recoverable, μg/L 509 340 221 493 795 4 0 174 873

Zinc, dissolved, μg/L 6.6 2.0 5.4 7.4 7.9 4 0 3.7 8.1

Zinc, total recoverable, μg/L 128 118 40 99 216 4 0 25 287
78

Table A2. Statistics for concentrations and measurement values at South Platte River at Denver by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; μg/L, 

micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, number of 

“less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

06714000 South Platte River at Denver, Colorado, 2009

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.5 0.3 7.5 7.5 7.8 6 0 6.6 8.0

Specific conductance, μS/cm at 25°C 509 140 394 524 620 16 0 295 721

Hardness, total, mg/L as CaCO3 140 45 101 138 165 16 0 85.6 225

Calcium, mg/L 42.4 13.8 29.6 42 50.2 16 0 26.7 67.5

Magnesium, mg/L 8.43 2.77 6.08 8.2 10.0 16 0 4.61 13.7

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 150 83 83 158 204 16 1 20.0 300

Nitrogen, total, ammonia plus organic as N, mg/L 1.4 0.53 0.84 1.5 1.9 15 0 0.81 2.3

Nitrogen, dissolved, ammonia as N, mg/L 0.076 0.089 0.021 0.034 0.123 15 2 0.010 0.300

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.94 0.42 0.67 0.80 0.99 15 0 0.55 1.94

Orthophosphate, dissolved as P, mg/L 0.152 0.079 0.103 0.12 0.168 15 0 0.068 0.368

Phosphorus, total as P, mg/L 0.46 0.15 0.32 0.47 0.61 15 0 0.22 0.67

Carbon, dissolved, organic as C, mg/L 6.0 2.2 4.6 5.3 6.3 16 0 4.3 12.2

Escherichia Coli, MPN/100 mL -- -- -- -- -- -- -- -- --

Fecal coliform, MPN/100 mL -- -- -- -- -- -- -- -- --

Copper, dissolved, μg/L 2.7 0.9 2.2 2.4 2.9 16 0 1.5 4.8

Copper, total recoverable, μg/L 13.6 6.8 7.8 13.6 19.0 16 0 4.2 26.9

Lead, dissolved, μg/L 0.11 0.08 0.06 0.09 0.11 16 0 0.04 0.31

Lead, total recoverable, μg/L 12.2 8.4 6.0 9.7 17.8 16 0 2.1 31.0

Manganese, dissolved, μg/L 6.8 7.9 0.7 2.5 12.4 16 0 0.2 23.1

Manganese, total recoverable, μg/L 272 122 168 258 348 16 0 111 490

Zinc, dissolved, μg/L 7.9 3.8 4.9 6.8 10.6 16 0 3.3 15.8

Zinc, total recoverable, μg/L 57 32 28 55 88 16 0 15 112
79

Table A2. Statistics for concentrations and measurement values at South Platte River at Denver by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; μg/L, 

micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, number of 

“less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

06714000 South Platte River at Denver, Colorado, 2010

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.5 0.3 7.4 7.5 7.6 16 0 7.0 8.0

Specific conductance, μS/cm at 25°C 627 308 409 537 613 17 0 294 1,390

Hardness, total, mg/L as CaCO3 163 40 138 156 176 16 0 110 251

Calcium, mg/L 48.2 10.7 42 47 53.1 16 0 34.2 75.9

Magnesium, mg/L 10.2 3.49 7.97 9.53 10.8 16 0 6.00 17.7

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 230 146 138 197 301 16 0 50 660

Nitrogen, total, ammonia plus organic as N, mg/L 2.0 0.71 1.4 1.9 2.4 16 0 0.82 3.2

Nitrogen, dissolved, ammonia as N, mg/L 0.095 0.111 0.029 0.044 0.117 16 2 0.024 0.395

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 1.415 0.712 0.93 1.27 1.5 15 0 0.71 3.44

Orthophosphate, dissolved as P, mg/L 0.149 0.113 0.084 0.109 0.166 16 0 0.046 0.487

Phosphorus, total as P, mg/L 0.56 0.22 0.40 0.53 0.71 16 0 0.19 1.04

Carbon, dissolved, organic as C, mg/L 6.2 1.6 5.0 6.1 7.2 16 0 4.1 9.8

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 110 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 170 --

Copper, dissolved, μg/L 3.3 1.1 2.4 3.1 3.7 16 0 2.0 6.2

Copper, total recoverable, μg/L 17.4 10.6 10.5 15.1 21.2 13 0 5.2 41.8

Lead, dissolved, μg/L 0.13 0.05 0.08 0.12 0.16 16 0 0.06 0.23

Lead, total recoverable, μg/L 15.0 12.9 6.7 11.3 16.5 13 0 2.1 50.2

Manganese, dissolved, μg/L 10.0 22.1 1.0 3.1 6.8 16 0 0.6 90.1

Manganese, total recoverable, μg/L 391 211 244 319 608 13 0 97 741

Zinc, dissolved, μg/L 9.5 5.2 5.4 8.6 13.3 16 0 3.4 21.3

Zinc, total recoverable, μg/L 76 49 35 70 89 13 0 22 183
80

Table A3. Statistics for concentrations and measurement values at Toll Gate Creek above 6th Avenue at Aurora by year, 2006–2010.

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; μg/L, 

micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, number 

of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

394329104490101 Toll Gate Creek above 6th Avenue at Aurora, Colorado, 2006

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.6 0.1 7.5 7.6 7.8 7 0 7.5 7.8

Specific conductance, μS/cm at 25°C 1,090 579 643 780 1,950 7 0 587 2,210

Hardness, total, mg/L as CaCO3 298 182 162 209 539 7 0 156 583

Calcium, mg/L 90 52 51 64 159 7 0 49 172

Magnesium, mg/L 17.8 12.5 8.67 12.1 34.4 7 0 8.24 37.4

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 215 260 40 105 323 6 0 21 695

Nitrogen, total, ammonia plus organic as N, mg/L 1.64 0.69 1.10 1.30 2.30 7 0 1.00 2.80

Nitrogen, dissolved, ammonia as N, mg/L 0.087 0.088 0.011 0.059 0.166 7 0 0.009 0.230

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.584 0.205 0.450 0.520 0.700 7 0 0.420 1.000

Orthophosphate, dissolved as P, mg/L 0.018 0.018 0.005 0.007 0.034 7 0 0.004 0.052

Phosphorus, total as P, mg/L 0.323 0.292 0.120 0.210 0.520 7 0 0.070 0.900

Carbon, dissolved, organic as C, mg/L 12.9 3.8 11.1 11.9 14.3 7 0 8.6 20.7

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 3,300 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 3,300 --

Copper, dissolved, μg/L 3.5 1.3 2.2 3.5 4.9 7 0 1.9 4.9

Copper, total recoverable, μg/L 12.6 9.3 7.0 8.3 16.8 7 0 6.9 32.0

Lead, dissolved, μg/L 0.127 0.105 0.04 0.12 0.18 7 1 0.04 0.31

Lead, total recoverable, μg/L 5.60 5.93 1.83 2.98 9.18 7 0 1.52 17.7

Manganese, dissolved, μg/L 93.7 147 5.0 11.7 286 7 0 4.7 330

Manganese, total recoverable, μg/L 367 229 184 365 583 7 0 128 751

Zinc, dissolved, μg/L 12.4 9.6 4.1 11.7 23.0 7 0 1.8 26.1

Zinc, total recoverable, μg/L 48 26 28 40 63 7 0 26 96
81

394329104490101 Toll Gate Creek above 6th Avenue at Aurora, Colorado, 2007

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.7 0.1 7.6 7.7 7.9 12 0 7.5 7.9

Specific conductance, μS/cm at 25°C 999 442 720 922 1,500 10 0 363 1,710

Hardness, total, mg/L as CaCO3 301 134 221 280 406 12 0 85 525

Calcium, mg/L 91 39 68 85 123 12 0 26 158

Magnesium, mg/L 18.0 8.85 12.3 16.5 24.5 12 0 4.60 32.1

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 208 205 62 110 407 12 0 10 590

Nitrogen, total, ammonia plus organic as N, mg/L 1.73 0.91 1.04 1.40 2.60 12 0 0.74 3.20

Nitrogen, dissolved, ammonia as N, mg/L 0.126 0.093 0.054 0.103 0.184 12 1 0.037 0.299

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.616 0.136 0.490 0.625 0.725 12 0 0.410 0.820

Orthophosphate, dissolved as P, mg/L 0.03 0.026 0.009 0.02 0.05 12 1 0.003 0.077

Phosphorus, total as P, mg/L 0.460 0.421 0.120 0.290 0.700 12 0 0.070 1.38

Carbon, dissolved, organic as C, mg/L 9.5 1.7 8.6 9.1 10.8 12 0 6.1 12.7

Escherichia Coli, MPN/100 mL -- -- -- -- -- 2 0 330 3,300

Fecal coliform, MPN/100 mL -- -- -- -- -- 2 0 330 3,300

Copper, dissolved, μg/L 2.3 1.0 1.7 2.1 2.6 12 0 1.0 4.5

Copper, total recoverable, μg/L 14.9 11.0 6.7 10.2 25.4 12 0 3.0 32.8

Lead, dissolved, μg/L 0.11 0.08 0.07 0.09 0.12 12 1 0.06 0.34

Lead, total recoverable, μg/L 10.4 10.5 2.85 5.09 19.7 12 0 0.71 28.3

Manganese, dissolved, μg/L 69.0 55.3 29.9 49.9 112 12 0 2.8 165

Manganese, total recoverable, μg/L 432 302 202 276 715 12 0 111 940

Zinc, dissolved, μg/L 7.8 2.5 5.9 8.1 9.2 2 0 4.1 12.6

Zinc, total recoverable, μg/L 56.1 46.8 23.4 34.7 101 12 0 9.3 132

Table A3. Statistics for concentrations and measurement values at Toll Gate Creek above 6th Avenue at Aurora by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; μg/L, 

micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, number 

of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 
82

394329104490101 Toll Gate Creek above 6th Avenue at Aurora, Colorado, 2008

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.6 0.3 7.4 7.5 8.0 6 0 7.4 8.0

Specific conductance, μS/cm at 25°C 1,080 244 905 992 1,220 6 0 860 1,510

Hardness, total, mg/L as CaCO3 265 98 186 228 381 6 0 172 396

Calcium, mg/L 79.6 29.2 55.6 69.5 115 6 0 51 117

Magnesium, mg/L 16.1 6.21 11.4 13.3 22.8 6 0 10.8 25.0

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 152 188 46 97 102 5 0 31 484

Nitrogen, total, ammonia plus organic as N, mg/L 1.62 1.03 0.78 1.50 1.80 6 0 0.63 3.50

Nitrogen, dissolved, ammonia as N, mg/L 0.331 0.204 0.159 0.300 0.462 6 0 0.110 0.655

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.723 0.161 0.580 0.695 0.830 6 0 0.570 0.970

Orthophosphate, dissolved as P, mg/L 0.021 0.014 0.010 0.021 0.028 6 0 0.003 0.044

Phosphorus, total as P, mg/L 0.242 0.192 0.110 0.215 0.230 6 0 0.070 0.610

Carbon, dissolved, organic as C, mg/L 11.1 6.1 6.1 9.3 15.6 6 0 5.3 20.9

Escherichia Coli, MPN/100 mL -- -- -- -- -- -- -- -- --

Fecal coliform, MPN/100 mL -- -- -- -- -- -- -- -- --

Copper, dissolved, μg/L 1.7 0.5 1.3 1.8 2.1 6 0 1.1 2.3

Copper, total recoverable, μg/L 9.2 7.2 5.0 7.3 9.7 6 0 2.9 23.1

Lead, dissolved, μg/L 0.08 0.04 0.05 0.08 0.11 6 2 0.05 0.12

Lead, total recoverable, μg/L 5.61 5.70 2.93 3.38 5.70 6 0 1.38 16.9

Manganese, dissolved, μg/L 114 45.2 91.5 113 130 6 0 50.9 187

Manganese, total recoverable, μg/L 377 371 159 242 351 6 0 149 1,120

Zinc, dissolved, μg/L 7.95 3.72 6.5 8.45 11.1 6 0 1.5 11.7

Zinc, total recoverable, μg/L 39.5 31.4 23.6 27.1 45.5 6 0 13.4 100

Table A3. Statistics for concentrations and measurement values at Toll Gate Creek above 6th Avenue at Aurora by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; μg/L, 

micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, number 

of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 
83

394329104490101 Toll Gate Creek above 6th Avenue at Aurora, Colorado, 2009

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.7 0.2 7.6 7.6 7.8 4 0 7.5 7.9

Specific conductance, μS/cm at 25°C 798 131 695 765 900 4 0 692 970

Hardness, total, mg/L as CaCO3 207 59 161 222 252 4 0 130 254

Calcium, mg/L 63 19 49 67 78 4 0 38 79

Magnesium, mg/L 12.1 2.87 9.86 12.6 14.4 4 0 8.52 14.9

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 156 106 66 164 246 4 0 42 254

Nitrogen, total, ammonia plus organic as N, mg/L 1.52 0.57 0.86 1.80 1.90 3 0 0.86 1.90

Nitrogen, dissolved, ammonia as N, mg/L 0.085 0.085 0.010 0.067 0.177 3 0 0.010 0.177

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.379 0.017 0.360 0.388 0.390 3 0 0.360 0.390

Orthophosphate, dissolved as P, mg/L 0.022 0.013 0.011 0.018 0.036 3 0 0.011 0.036

Phosphorus, total as P, mg/L 0.193 0.200 0.040 0.120 0.420 3 0 0.040 0.420

Carbon, dissolved, organic as C, mg/L 7.2 1.5 6.0 7.0 8.4 4 0 5.6 9.0

Escherichia Coli, MPN/100 mL -- -- -- -- -- -- -- -- --

Fecal coliform, MPN/100 mL -- -- -- -- -- -- -- -- --

Copper, dissolved, μg/L 2.1 0.7 1.6 2.2 2.6 4 0 1.2 2.8

Copper, total recoverable, μg/L 10.3 5.4 5.8 11.2 14.8 4 0 3.8 14.9

Lead, dissolved, μg/L 0.08 0.04 0.06 0.08 0.11 4 0 0.05 0.13

Lead, total recoverable, μg/L 6.64 4.20 3.16 7.34 10.1 4 0 1.72 10.2

Manganese, dissolved, μg/L 49.5 22.0 32.8 53.7 66.2 4 0 20.3 70.1

Manganese, total recoverable, μg/L 315 174 173 338 456 4 0 103 480

Zinc, dissolved, μg/L 5.5 2.6 4.0 4.4 7.0 4 0 3.8 9.4

Zinc, total recoverable, μg/L 41.5 26.5 19.7 43.7 63.3 4 0 10.6 68.0

Table A3. Statistics for concentrations and measurement values at Toll Gate Creek above 6th Avenue at Aurora by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; μg/L, 

micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, number 

of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed]
84

394329104490101 Toll Gate Creek above 6th Avenue at Aurora, Colorado, 2010

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.6 0.1 7.6 7.6 7.7 4 0 7.5 7.7

Specific conductance, μS/cm at 25°C 1,757 464 1,360 1,740 2,155 4 0 1,320 2,230

Hardness, total, mg/L as CaCO3 579 178 425 576 733 4 0 418 746

Calcium, mg/L 177 53 131 177 223 4 0 129 226

Magnesium, mg/L 33.3 11.2 23.7 32.9 43.0 4 0 23.2 44.4

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 102 83 44 79 160 4 0 33 219

Nitrogen, total, ammonia plus organic as N, mg/L 1.35 0.45 1.05 1.20 1.65 4 0 0.99 2.00

Nitrogen, dissolved, ammonia as N, mg/L Fewer than 3 uncensored values 4 2 0.040 0.047

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.868 0.343 0.585 0.805 1.150 4 0 0.580 1.280

Orthophosphate, dissolved as P, mg/L 0.009 0.002 0.008 0.009 0.010 4 0 0.007 0.010

Phosphorus, total as P, mg/L 0.208 0.131 0.125 0.160 0.290 4 0 0.110 0.400

Carbon, dissolved, organic as C, mg/L 11.5 1.9 10.5 10.6 12.5 4 0 10.4 14.4

Escherichia Coli, MPN/100 mL -- -- -- -- -- -- -- -- --

Fecal coliform, MPN/100 mL -- -- -- -- -- -- -- -- --

Copper, dissolved, μg/L 2.7 0.56 2.25 2.6 3.15 4 0 2.2 3.4

Copper, total recoverable, μg/L 7.0 3.8 4.9 5.5 9.2 4 0 4.3 12.7

Lead, dissolved, μg/L 0.06 0.01 0.06 0.06 0.07 4 0 0.05 0.08

Lead, total recoverable, μg/L 3.06 2.60 1.64 1.88 4.48 4 0 1.54 6.95

Manganese, dissolved, μg/L 29.5 17.7 19.3 22.3 39.6 4 0 17.5 55.8

Manganese, total recoverable, μg/L 326 238 201 217 450 4 0 185 682

Zinc, dissolved, μg/L 10.4 4.4 7.2 9.2 13.7 4 0 6.9 16.4

Zinc, total recoverable, μg/L 32.0 16.4 19.3 29.6 44.7 4 0 15.8 52.9

Table A3. Statistics for concentrations and measurement values at Toll Gate Creek above 6th Avenue at Aurora by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; μg/L, 

micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, number 

of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed]
85

Table A4. Statistics for concentrations and measurement values at Sand Creek at mouth near Commerce City by year, 2006–2010.

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; 

μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, 

number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

394839104570300 Sand Creek at mouth near Commerce City, Colorado, 2006

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.7 0.1 7.6 7.8 7.8 12 0 7.5 7.9

Specific conductance, μS/cm at 25°C 907 606 508 632 1,325 12 0 424 2,020

Hardness, total, mg/L as CaCO3 218 126 136 163 284 12 0 113 455

Calcium, mg/L 65.5 35.2 42.6 51.2 82.9 12 0 35.4 131

Magnesium, mg/L 13.2 9.24 7.30 8.65 18.8 12 0 6.06 30.9

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 252 199 86 190 347 12 0 56 685

Nitrogen, total, ammonia plus organic as N, mg/L 2.4 1.0 1.7 2.1 3.0 12 0 1.1 4.4

Nitrogen, dissolved, ammonia as N, mg/L 0.072 0.076 0.011 0.049 0.106 12 3 0.010 0.267

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 1.12 0.58 0.76 0.91 1.19 12 0 0.67 2.55

Orthophosphate, dissolved as P, mg/L 0.141 0.099 0.081 0.133 0.157 12 0 0.027 0.405

Phosphorus, total as P, mg/L 0.68 0.26 0.53 0.60 0.82 12 0 0.38 1.25

Carbon, dissolved, organic as C, mg/L 11.7 4.3 7.8 10.8 13.8 12 0 7.2 19.2

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 4,900 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 4,900 --

Copper, dissolved, μg/L 3.2 0.9 2.5 2.9 3.8 12 0 2.1 5.0

Copper, total recoverable, μg/L 17.2 9.7 9.7 13.8 22.9 12 0 8.0 39.3

Lead, dissolved, μg/L 0.19 0.09 0.13 0.16 0.26 12 0 0.07 0.34

Lead, total recoverable, μg/L 12.8 9.53 4.21 12.2 14.9 12 0 3.67 35.9

Manganese, dissolved, μg/L 53.0 100 0.8 1.3 55.7 12 0 0.5 304

Manganese, total recoverable, μg/L 478 284 287 405 602 12 0 144 1,240

Zinc, dissolved, μg/L 11.0 5.9 6.8 8.4 14.8 12 0 3.8 21.1

Zinc, total recoverable, μg/L 73.0 36.9 52.5 67.0 78.5 12 0 22.0 166
86

Table A4. Statistics for concentrations and measurement values at Sand Creek at mouth near Commerce City by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; 

μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, 

number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

394839104570300 Sand Creek at mouth near Commerce City, Colorado, 2007

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.7 0.2 7.6 7.7 7.8 16 0 7.4 7.9

Specific conductance, μS/cm at 25°C 734 270 595 667 848 14 0 330 1,340

Hardness, total, mg/L as CaCO3 186 69.2 151 171 214 16 0 74.8 363

Calcium, mg/L 56.5 20.0 46.0 52.5 65.2 16 0 23.2 106

Magnesium, mg/L 11.0 4.69 8.69 9.75 12.8 16 0 4.09 23.7

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 304 266 123 236 374 16 0 72.0 966

Nitrogen, total, ammonia plus organic as N, mg/L 2.0 0.87 1.4 1.7 2.5 16 0 1.1 3.7

Nitrogen, dissolved, ammonia as N, mg/L 0.134 0.086 0.050 0.133 0.192 16 0 0.019 0.286

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.81 0.29 0.59 0.73 1.00 16 0 0.41 1.41

Orthophosphate, dissolved as P, mg/L 0.158 0.155 0.068 0.104 0.160 16 0 0.041 0.627

Phosphorus, total as P, mg/L 0.73 0.41 0.44 0.61 0.87 16 0 0.34 1.65

Carbon, dissolved, organic as C, mg/L 10.6 4.5 7.8 9.1 11.7 15 0 5.8 23.6

Escherichia Coli, MPN/100 mL -- -- -- -- -- 2 0 790 4,900

Fecal coliform, MPN/100 mL -- -- -- -- -- 2 0 790 2,200

Copper, dissolved, μg/L 2.9 1.6 1.8 2.3 3.7 6 0 1.2 6.0

Copper, total recoverable, μg/L 21.2 13.5 11.5 16.4 25.4 16 0 6.5 51.1

Lead, dissolved, μg/L 0.19 0.13 0.11 0.15 0.25 16 0 0.07 0.57

Lead, total recoverable, μg/L 18.0 14.7 6.21 11.3 22.7 16 0 4.99 50.3

Manganese, dissolved, μg/L 18.9 20.0 3.4 8.8 31.3 16 0 0.5 59.4

Manganese, total recoverable, μg/L 490 346 259 331 620 16 0 177 1,270

Zinc, dissolved, μg/L 12.5 7.2 7.9 11.3 15.4 16 0 2.8 31.1

Zinc, total recoverable, μg/L 92.6 74.0 40.7 60.3 113 16 0 30.1 252
87

Table A4. Statistics for concentrations and measurement values at Sand Creek at mouth near Commerce City by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; 

μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, 

number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

394839104570300 Sand Creek at mouth near Commerce City, Colorado, 2008 

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.6 0.3 7.4 7.5 7.8 10 0 7.3 8.0

Specific conductance, μS/cm at 25°C 826 206 630 800 945 10 0 575 1,140

Hardness, total, mg/L as CaCO3 194 37.8 167 189.5 205 10 0 143 262

Calcium, mg/L 58.3 11.1 49.7 57.8 61.2 10 0 43.6 77.6

Magnesium, mg/L 11.7 2.63 9.23 11.4 13.4 10 0 8.24 16.6

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 488 474 98 233 822 10 0 77 1,360

Nitrogen, total, ammonia plus organic as N, mg/L 3.7 3.1 1.6 1.9 6.0 8 0 1.2 9.5

Nitrogen, dissolved, ammonia as N, mg/L 0.232 0.155 0.130 0.214 0.340 8 0 0.021 0.472

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 1.07 0.31 0.91 0.94 1.33 8 0 0.63 1.54

Orthophosphate, dissolved as P, mg/L 0.094 0.048 0.058 0.094 0.106 8 0 0.039 0.193

Phosphorus, total as P, mg/L 1.00 0.85 0.36 0.52 1.82 8 0 0.27 2.36

Carbon, dissolved, organic as C, mg/L 11.6 4.5 9.0 11.0 12.4 10 0 6.1 21.1

Escherichia Coli, MPN/100 mL -- -- -- -- -- -- -- -- --

Fecal coliform, MPN/100 mL -- -- -- -- -- -- -- -- --

Copper, dissolved, μg/L 2.9 3.4 1.6 1.8 2.1 10 0 1.5 12.5

Copper, total recoverable, μg/L 25.0 21.1 7.9 11.5 47.2 10 0 7.2 54.8

Lead, dissolved, μg/L 0.15 0.11 0.10 0.11 0.14 10 0 0.09 0.45

Lead, total recoverable, μg/L 25.8 24.9 5.62 9.72 55.0 10 0 4.71 58.5

Manganese, dissolved, μg/L 48.0 43.8 2.8 47.8 66.1 10 0 0.9 132.0

Manganese, total recoverable, μg/L 1,200 1,190 280 416 1,970 10 0 226 3,210

Zinc, dissolved, μg/L 9.2 6.2 4.7 8.4 9.2 10 0 3.9 25.5

Zinc, total recoverable, μg/L 151 188 33.0 50.7 211 10 0 27.2 625
88

Table A4. Statistics for concentrations and measurement values at Sand Creek at mouth near Commerce City by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; 

μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, 

number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

394839104570300 Sand Creek at mouth near Commerce City, Colorado, 2009

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.37 0.1 7.3 7.35 7.45 4 0 7.3 7.5

Specific conductance, μS/cm at 25°C 691 185 552 692 829 4 0 470 908

Hardness, total, mg/L as CaCO3 188 52.2 148 192 228 4 0 123 245

Calcium, mg/L 57.0 15.6 45.2 58.3 69.0 4 0 37.7 73.9

Magnesium, mg/L 11.0 3.29 8.50 11.1 13.4 4 0 6.91 14.7

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 242 98 185 200 299 4 0 180 388

Nitrogen, total, ammonia plus organic as N, mg/L 2.3 0.48 2 2.45 2.6 4 0 1.6 2.7

Nitrogen, dissolved, ammonia as N, mg/L Fewer than 3 uncensored values 4 2 0.090 0.123

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.52 0.16 0.44 0.46 0.61 4 0 0.42 0.76

Orthophosphate, dissolved as P, mg/L 0.059 0.043 0.024 0.054 0.094 4 0 0.017 0.112

Phosphorus, total as P, mg/L 0.61 0.15 0.50 0.66 0.72 4 0 0.39 0.74

Carbon, dissolved, organic as C, mg/L 9.7 3.7 7.3 8.5 12.2 4 0 6.7 15.1

Escherichia Coli, MPN/100 mL -- -- -- -- -- -- -- -- --

Fecal coliform, MPN/100 mL -- -- -- -- -- -- -- -- --

Copper, dissolved, μg/L 4.6 2.5 3.0 4.7 6.2 4 0 1.5 7.5

Copper, total recoverable, μg/L 18.3 4.8 14.6 19.0 22.1 4 0 12.2 23.1

Lead, dissolved, μg/L 0.46 0.23 0.31 0.46 0.61 4 0 0.18 0.74

Lead, total recoverable, μg/L 30.3 12.2 20.6 29.7 40.0 4 0 17.2 44.7

Manganese, dissolved, μg/L 134 174 35.2 67.0 233 4 0 10.6 391

Manganese, total recoverable, μg/L 466 128 368 472 565 4 0 311 609

Zinc, dissolved, μg/L 570 415 300 438 841 4 0 237 1,170

Zinc, total recoverable, μg/L 3,360 1,950 1,680 3,340 5,040 4 0 1,530 5,230
89

Table A4. Statistics for concentrations and measurement values at Sand Creek at mouth near Commerce City by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; 

μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, 

number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

394839104570300 Sand Creek at mouth near Commerce City, Colorado, 2010

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.42 0.13 7.3 7.4 7.5 5 0 7.3 7.6

Specific conductance, μS/cm at 25°C 862 507 552 686 950 5 0 423 1,700

Hardness, total, mg/L as CaCO3 175 47.8 155 170 190 5 0 115 245

Calcium, mg/L 53.4 13.8 48.0 52.0 57.9 5 0 35.9 73.4

Magnesium, mg/L 10.1 3.26 8.52 9.86 10.9 5 0 6.17 15.0

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 557 208 445 530 734 5 0 284 790

Nitrogen, total, ammonia plus organic as N, mg/L 3.3 0.98 2.5 3.3 4.2 5 0 2.2 4.4

Nitrogen, dissolved, ammonia as N, mg/L Fewer than 3 uncensored values 5 3 0.011 0.018

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 0.5 0.03 0.49 0.49 0.52 5 0 0.46 0.53

Orthophosphate, dissolved as P, mg/L 0.056 0.026 0.035 0.069 0.072 5 0 0.022 0.082

Phosphorus, total as P, mg/L 1.07 0.36 0.77 1.09 1.38 5 0 0.63 1.46

Carbon, dissolved, organic as C, mg/L 8.9 3.9 6.4 8.2 8.7 5 0 5.8 15.5

Escherichia Coli, MPN/100 mL -- -- -- -- -- -- -- -- --

Fecal coliform, MPN/100 mL -- -- -- -- -- -- -- -- --

Copper, dissolved, μg/L 1.7 0.6 1.2 1.6 2.3 5 0 1.2 2.4

Copper, total recoverable, μg/L 29.3 12.4 19.2 25.4 38.9 5 0 17.3 45.6

Lead, dissolved, μg/L 0.12 0.09 0.07 0.10 0.12 5 0 0.04 0.28

Lead, total recoverable, μg/L 33.2 18.0 19.2 31.1 39.5 5 0 15.8 60.5

Manganese, dissolved, μg/L 31.86 45.22 2.7 7.8 39 5 0 1.8 108

Manganese, total recoverable, μg/L 905 332 623 988 1,080 5 0 514 1,320

Zinc, dissolved, μg/L 90.4 84.5 22.2 80.9 120 5 0 10.2 219

Zinc, total recoverable, μg/L 1,467 1,708 528 562 1,360 5 0 436 4,450
90

Table A5. Statistics for concentrations and measurement values at South Platte River at Henderson by year, 2006–2010.

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; 

μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, 

number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 

06720500 South Platte River at Henderson, Colorado, 2006

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.5 0.2 7.3 7.5 7.6 7 0 7.1 7.7

Specific conductance, μS/cm at 25°C 524 46 495 526 540 7 0 459 608

Hardness, total, mg/L as CaCO3 132 9.8 126 129 140 7 0 119 149

Calcium, mg/L 39.7 3.0 37.8 38.3 42.2 7 0 36.0 44.6

Magnesium, mg/L 8.04 0.68 7.45 8.18 8.48 7 0 7.06 9.07

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 176 76 66 218 254 7 0 47 350

Nitrogen, total, ammonia plus organic as N, mg/L 2.9 0.9 2.2 2.9 3.6 7 0 1.4 4.2

Nitrogen, dissolved, ammonia as N, mg/L 0.840 0.550 0.650 0.690 1.12 7 0 0.141 1.92

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 2.58 0.70 1.85 2.38 3.24 7 0 1.75 3.49

Orthophosphate, dissolved as P, mg/L 0.510 0.232 0.346 0.407 0.767 7 0 0.326 0.909

Phosphorus, total as P, mg/L 1.03 0.26 0.9 1.04 1.22 7 0 0.57 1.43

Carbon, dissolved, organic as C, mg/L 9.0 2.6 6.2 9.2 11.9 7 0 6.2 12.5

Escherichia Coli, MPN/100 mL -- -- -- -- -- -- -- -- --

Fecal coliform, MPN/100 mL -- -- -- -- -- -- -- -- --

Copper, dissolved, μg/L 3.0 0.8 2.3 2.8 3.3 7 0 2.3 4.6

Copper, total recoverable, μg/L 21.3 10.0 13.1 22.8 28.4 7 0 9.5 37.6

Lead, dissolved, μg/L 0.26 0.06 0.20 0.28 0.33 7 0 0.18 0.34

Lead, total recoverable, μg/L 16.7 9.6 7.4 17.2 22.1 7 0 7.0 32.9

Manganese, dissolved, μg/L 51.3 26.1 20.8 56.6 76.9 7 0 17.5 79.6

Manganese, total recoverable, μg/L 350 150 220 397 412 7 0 155 600

Zinc, dissolved, μg/L 16.5 3.2 13.1 17.8 19.1 7 0 11.4 20.3

Zinc, total recoverable, μg/L 89 38 67 91 105 7 0 39 158
91

06720500 South Platte River at Henderson, Colorado, 2007

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.38 0.13 7.3 7.4 7.4 6 0 7.2 7.6

Specific conductance, μS/cm at 25°C 396 24 383 402 415 6 0 355 419

Hardness, total, mg/L as CaCO3 107 7.2 102 109 112 6 0 95 115

Calcium, mg/L 31.6 2.0 30.1 32.4 33.0 6 0 28.2 33.5

Magnesium, mg/L 6.82 0.55 6.55 6.79 7.27 6 0 5.99 7.53

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 154 88 98 132 156 6 0 80 324

Nitrogen, total, ammonia plus organic as N, mg/L 1.7 0.69 1.3 1.5 2.3 6 0 0.96 2.8

Nitrogen, dissolved, ammonia as N, mg/L 0.242 0.074 0.217 0.267 0.295 6 0 0.107 0.298

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 1.34 0.20 1.18 1.30 1.38 6 0 1.16 1.72

Orthophosphate, dissolved as P, mg/L 0.208 0.047 0.165 0.203 0.225 6 0 0.164 0.290

Phosphorus, total as P, mg/L 0.637 0.283 0.41 0.535 0.9 6 0 0.37 1.07

Carbon, dissolved, organic as C, mg/L 9.8 3.8 6.6 8.9 14.0 6 0 6.2 14.4

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 7,900 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 13,000 --

Copper, dissolved, μg/L 3.3 0.8 2.7 3.1 4.0 6 0 2.5 4.4

Copper, total recoverable, μg/L 25.1 10.7 15.8 25.6 27.4 6 0 13.0 43.2

Lead, dissolved, μg/L 0.20 0.04 0.19 0.22 0.22 6 0 0.13 0.23

Lead, total recoverable, μg/L 32.4 23.1 13.8 27.5 45.4 6 0 9.1 70.9

Manganese, dissolved, μg/L 48.8 23.8 31.0 44.1 57.9 6 0 25.9 89.9

Manganese, total recoverable, μg/L 308 142 172 291 451 6 0 149 493

Zinc, dissolved, μg/L 21.8 2.9 19.9 21.8 23.6 6 0 17.6 26.0

Zinc, total recoverable, μg/L 135 54 86 141 145 6 0 71 224

Table A5. Statistics for concentrations and measurement values at South Platte River at Henderson by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; 

μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, 

number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 
92

06720500 South Platte River at Henderson, Colorado, 2008

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.2 0.3 7.0 7.2 7.5 6 0 6.6 7.5

Specific conductance, μS/cm at 25°C 527 167 510 572 650 6 0 205 651

Hardness, total, mg/L as CaCO3 140 13.7 128 139 151 6 0 124 159

Calcium, mg/L 41.5 3.8 39.0 41.1 43.8 6 0 36.8 47.2

Magnesium, mg/L 8.76 1.13 7.86 8.69 10.00 6 0 7.33 10.00

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 210 187 62 162 284 6 0 44 545

Nitrogen, total, ammonia plus organic as N, mg/L 4.05 2.69 2.5 3 4 6 0 2.4 9.4

Nitrogen, dissolved, ammonia as N, mg/L 1.26 1.00 0.614 0.785 1.63 6 0 0.578 3.14

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 1.99 0.54 1.68 1.83 2.18 6 0 1.46 2.98

Orthophosphate, dissolved as P, mg/L 0.671 0.246 0.445 0.652 0.911 6 0 0.374 0.993

Phosphorus, total as P, mg/L 1.39 0.75 0.92 1.10 1.53 6 0 0.86 2.85

Carbon, dissolved, organic as C, mg/L 13.9 10.4 8.1 9.4 14.6 6 0 7.6 34.4

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 23,000 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 23,000 --

Copper, dissolved, μg/L 2.4 0.4 2.3 2.6 2.7 6 0 1.7 2.7

Copper, total recoverable, μg/L 25.4 17.4 10.5 23.8 27.4 6 0 9.4 57.5

Lead, dissolved, μg/L 0.23 0.07 0.18 0.22 0.30 6 0 0.16 0.32

Lead, total recoverable, μg/L 23.5 17.9 7.5 21.3 30.7 6 0 6.3 54.0

Manganese, dissolved, μg/L 176 288 26.9 52.8 140 6 0 25.2 757

Manganese, total recoverable, μg/L 639 602 282 432 596 6 0 252 1,840

Zinc, dissolved, μg/L 15.1 4.6 11.7 15.4 17.0 6 0 9.0 21.9

Zinc, total recoverable, μg/L 105 77 45 90 113 6 0 42 252

Table A5. Statistics for concentrations and measurement values at South Platte River at Henderson by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; 

μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, 

number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 
93

06720500 South Platte River at Henderson, Colorado, 2009

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.4 0.2 7.2 7.4 7.5 10 0 7.0 7.6

Specific conductance, μS/cm at 25°C 578 110 505 580 702 10 0 365 710

Hardness, total, mg/L as CaCO3 151 27.6 133 149.5 165 10 0 100 193

Calcium, mg/L 45.1 8.0 40.2 45.3 49.2 10 0 30.2 57.5

Magnesium, mg/L 9.25 1.82 8.12 8.86 10.4 10 0 6.0 11.8

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 138 66 94 126 179 10 2 53 250

Nitrogen, total, ammonia plus organic as N, mg/L 2.3 0.61 2.0 2.3 2.9 9 0 1.2 3.1

Nitrogen, dissolved, ammonia as N, mg/L 0.479 0.319 0.230 0.416 0.717 9 1 0.167 1.10

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 2.26 1.06 1.86 2.06 2.13 9 0 0.96 4.64

Orthophosphate, dissolved as P, mg/L 0.384 0.162 0.297 0.367 0.451 9 0 0.152 0.725

Phosphorus, total as P, mg/L 0.83 0.19 0.70 0.73 0.96 9 0 0.63 1.18

Carbon, dissolved, organic as C, mg/L 7.2 2.5 5.4 5.9 8.7 10 0 5.0 12.0

Escherichia Coli, MPN/100 mL -- -- -- -- -- -- -- -- --

Fecal coliform, MPN/100 mL -- -- -- -- -- -- -- -- --

Copper, dissolved, μg/L 3.1 0.7 2.7 3.0 3.5 0 0 2.2 4.6

Copper, total recoverable, μg/L 19.0 8.6 12.6 17.9 22.1 10 0 9.1 33.5

Lead, dissolved, μg/L 0.21 0.11 0.13 0.19 0.29 10 0 0.07 0.41

Lead, total recoverable, μg/L 16.5 10.0 11.4 14.1 17.4 10 0 5.1 36.0

Manganese, dissolved, μg/L 32.9 26.8 3.8 31.3 56.0 10 0 0.6 77.4

Manganese, total recoverable, μg/L 314 125 231 309 368 10 0 138 580

Zinc, dissolved, μg/L 20.4 10.2 14.4 16.5 23.6 10 0 11.4 45.8

Zinc, total recoverable, μg/L 108 84 57 74 150 10 0 32 308

Table A5. Statistics for concentrations and measurement values at South Platte River at Henderson by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; 

μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, 

number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed] 
94

06720500 South Platte River at Henderson, Colorado, 2010

Multiple censoring level statistics Range of uncensored values

Property or constituent Mean SD 25% Median 

50% 75% n nlt Minimum Maximum

pH, standard units 7.2 0.2 7.1 7.2 7.3 5 0 7.0 7.4

Specific conductance, μS/cm at 25°C 789 375 540 563 1,130 5 0 454 1,260

Hardness, total, mg/L as CaCO3 176 48.4 148 161 226 5 0 118 226

Calcium, mg/L 50.58 12.15 44.1 47.8 62.1 5 0 35.2 63.7

Magnesium, mg/L 12.0 4.46 9.16 10.1 16.3 5 0 7.27 17.2

Suspended solids, residue on evaporation at 105 

degrees Celsius, mg/L 225 102 150 196 286 5 0 122 370

Nitrogen, total, ammonia plus organic as N, mg/L 4.1 2.1 2.8 3.8 6.0 5 0 1.6 6.4

Nitrogen, dissolved, ammonia as N, mg/L 1.475 1.627 0.193 0.495 3.12 5 0 0.19 3.38

Nitrogen, dissolved, nitrite plus nitrate as N, mg/L 2.09 0.77 1.56 1.97 1.98 5 0 1.52 3.42

Orthophosphate, dissolved as P, mg/L 0.584 0.461 0.262 0.306 1.02 5 0 0.184 1.15

Phosphorus, total as P, mg/L 1.31 0.31 1.20 1.39 1.48 5 0 0.84 1.68

Carbon, dissolved, organic as C, mg/L 6.5 1.2 5.8 7.0 7.4 5 0 4.8 7.6

Escherichia Coli, MPN/100 mL -- -- -- -- -- 1 0 310 --

Fecal coliform, MPN/100 mL -- -- -- -- -- 1 0 310 --

Copper, dissolved, μg/L 3.5 1.8 2.1 2.4 5.3 5 0 2.0 5.5

Copper, total recoverable, μg/L 40.9 23.7 16.1 43.5 63.2 3 0 16.1 63.2

Lead, dissolved, μg/L 0.20 0.11 0.11 0.16 0.27 5 0 0.10 0.35

Lead, total recoverable, μg/L 35.0 15.4 17.4 42.0 45.7 3 0 17.4 45.7

Manganese, dissolved, μg/L 11.6 6.5 8.1 12.1 17.2 5 0 2.6 18.2

Manganese, total recoverable, μg/L 599 215 366 642 789 3 0 366 789

Zinc, dissolved, μg/L 16.4 11.1 8.3 9.9 27.9 5 0 6.7 29.1

Zinc, total recoverable, μg/L 166 75 79 201 217 3 0 79 217

Table A5. Statistics for concentrations and measurement values at South Platte River at Henderson by year, 2006–2010.—Continued

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; ml, milliliter; CaCO3, calcium carbonate; mg/L, milligrams per liter; 

μg/L, micrograms per liter; mm, millimeter; N, nitrogen; P, phosphorus; C, carbon; SD, standard deviation; n, number of samples; nlt, 

number of “less thans” or censored data; %, percentile; MPN, most probable number; --, not computed]