Phosphorus Concentrations in Stream-Water 

and Reference Samples—An Assessment of 

Laboratory Comparability

By Michael R. McHale and Dennis McChesney

Prepared in cooperation with the U.S. Environmental Protection Agency

Open-File Report 2007–1267

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

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 materials contained within this report.

Suggested citation:

McHale, M.R., and McChesney, Dennis, 2007, Phosphorus concentrations in stream-water and reference samples—

An assessment of laboratory comparability: U.S. Geological Survey Open-File Report 2007–1267, 25 p. ONLINE ONLY
iii

Contents

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

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

Objectives...............................................................................................................................................2

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

Methods...........................................................................................................................................................2

Sample Preparation..............................................................................................................................2

Statistical Methods...............................................................................................................................2

Data Comparability ........................................................................................................................................3

Conclusions.....................................................................................................................................................9

Acknowledgments.........................................................................................................................................9

References Cited............................................................................................................................................9

Figures

1–3. Graphs showing—

1. Concentrations of total phosphorus, total dissolved phosphorus, and 

soluble reactive phosphorus (orthophosphate) in 10-laboratory comparison 

of stream-water and reference samples, 2003...............................................................4

2. The coefficient of variation (CV) for total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for all stream-water 

and reference sample triplicates reported by the 10 participating laboratories......7

3. Difference between phosphorus fractions in 10-laboratory comparison of 

phosphorus concentration in stream-water and reference samples, 2003...............8

Tables

1. Laboratories participating in 2003 study of laboratory comparability, 

U.S. Environmental Protection Agency methods used, and method titles........................12

2. Samples included in the interlaboratory comparison ..........................................................13

3. Concentrations of total phosphorus, total dissolved phosphorus, and 

soluble reactive phosphorus (orthophosphate) for stream-water and reference 

samples, z-values, and ratings for the 10 laboratories involved in the 2003 

comparability study ....................................................................................................................14
iv

Conversion Factors and Abbreviations

Concentrations of chemical constituents in water are given in:

milligrams per liter (mg/L) » micrograms per gram (μg/g) ≈ parts per million (ppm)

ABBREVIATIONS USED IN THIS REPORT

BQS Branch of Quality Services

CV coefficient of variation

mg/L milligrams per liter

mL milliliters

P phosphorus

SRP soluble reactive phosphorus (orthophosphate)

SRS standard reference sample

TDP total dissolved phosphorus

TP total phosphorus

μm micrometers

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey
Abstract

In 2003, a study was conducted to evaluate the accuracy 

and precision of 10 laboratories that analyze water-quality 

samples for phosphorus concentrations in the Catskill Mountain region of New York State. Many environmental studies 

in this region rely on data from these different laboratories for 

water-quality analyses, and the data may be used in watershed modeling and management decisions. Therefore, it is 

important to determine whether the data reported by these 

laboratories are of comparable accuracy and precision. Each 

laboratory was sent 12 samples for triplicate analysis for total 

phosphorus, total dissolved phosphorus, and soluble reactive phosphorus. Eight of these laboratories reported results 

that met comparability criteria for all samples; the remaining 

two laboratories met comparability criteria for only about 

half of the analyses. Neither the analytical method used 

nor the sample concentration ranges appeared to affect the 

comparability of results. The laboratories whose results were 

comparable gave consistently comparable results throughout 

the concentration range analyzed, and the differences among 

methods did not diminish comparability. All laboratories had 

high data precision as indicated by sample triplicate results. In 

addition, the laboratories consistently reported total phosphorus values greater than total dissolved phosphorus values, and 

total dissolved phosphorus values greater than soluble reactive 

phosphorus values, as would be expected. The results of this 

study emphasize the importance of regular laboratory participation in sample-exchange programs.

Introduction

The Catskill Mountains in southeastern New York are 

part of the New York City reservoir system that supplies 

drinking water to about 9 million people. The Catskill region 

has been a focus of environmental research by federal, state, 

county, municipal, and private researchers for several decades. 

These entities use a variety of laboratories and techniques 

for water-quality analyses; thus, it is important to establish 

whether the data reported by these laboratories are of comparable accuracy and precision.

In 2003, the U.S. Geological Survey (USGS), in cooperation with the U.S. Environmental Protection Agency (USEPA)

conducted an interlaboratory comparison of phosphorus (P) 

concentrations for water samples analyzed by 10 laboratories to assess the comparability of laboratories that analyze 

chemical data collected in and around the Catskill Mountains 

of New York State. This study was part of an agricultural 

research project conducted by the U.S. Department of Agriculture, the USGS, and Cornell University that focused on P 

contamination of surface waters in the Cannonsville Reservoir 

watershed in southeastern New York State. The project used 

three laboratories: U.S. Geological Survey National Water 

Quality Laboratory in Denver, Colo., U.S. Department of 

Agriculture Agricultural Research Service in State College, 

Pa., and Cornell University Department of Agricultural Engineering in Ithaca, N.Y. The USEPA, which funded the project 

through the New York City Department of Environmental 

Protection and the Watershed Agricultural Council, required 

documentation that the three laboratories used in the project 

provide comparable P results because the data were combined 

to model P movement and transformation within the watershed. Six additional laboratories that analyze stream-water 

samples from the Catskill region were invited to participate 

in this comparison, and a 10th laboratory, the University of 

Puerto Rico, asked that their laboratory be included to evaluate 

its accuracy and precision.

Phosphorus Concentrations in Stream-Water and 

Reference Samples—An Assessment of Laboratory 

Comparability

By Michael R. McHale1 and Dennis McChesney2

1

 U.S. Geological Survey, Water Resources Discipline, 425 Jordan Road, 

Troy, NY 12180

2

 U.S. Environmental Protection Agency, Region 2, 290 Broadway, 

New York, NY 10007
2 Phosphorus Concentrations in Stream-Water and Reference Samples—An Assessment of Laboratory Comparability

Objectives

The objectives of this study were to (1) evaluate the 

performance of the 10 participating laboratories in terms 

of their reported P concentrations in terms of accuracy 

and precision, (2) identify analytical problems any of the 

participating laboratories may be having, and (3) assess 

whether the concentrations reported by the 10 laboratories 

for the same samples were comparable for three phosphorus 

components—total phosphorus (TP), total dissolved 

phosphorus (TDP), and soluble reactive phosphorus (SRP), 

sometimes referred to as orthophosphate.

Purpose and Scope

This report describes sample preparation, sample distribution, and statistical methods used to assess comparability 

among laboratories, and reports the results of the comparison.

Methods

Each laboratory was sent 12 samples, all of which were 

analyzed in triplicate for each analyte by each laboratory 

except where sample volume was insufficient, to provide a 

measure of analytical precision. The analytical methods used 

by each laboratory are summarized in table 1. The laboratories 

were randomly assigned numbers of 1 through 10; these numbers are used here for data presentation and do not coincide 

with the order in which the laboratories are listed in table 1. 

The laboratories participated in this study on condition that the 

results would be reported anonymously.

Sample Preparation

Each laboratory was sent stream-water samples and 

laboratory-prepared water samples in two rounds. The laboratory-prepared samples were obtained from the USGS Standard 

Reference Sample (SRS) program, an interlaboratory analytical evaluation program that includes more than 275 USGS and 

non-USGS laboratories. Program samples are prepared twice 

annually and shipped to participating laboratories, and a report 

is produced detailing results of each evaluation. The SRS 

program has been in operation since 1962.

Sample distribution for this study occurred in two rounds; 

during the first round in the spring of 2003, the USGS sent 

three SRS samples and one stream-water sample to each of 

the 10 laboratories. The SRS program sample numbers were 

N-77, which was prepared with deionized water and N-78, 

which was prepared in a natural water matrix (water collected 

from Bear Creek east of Kittredge, Colo.). A detailed description of SRS sample preparation is given in Woodworth and 

Connor (2003). Sample N-77 was shipped in a concentrated 

form for sample stability, and the laboratories were asked to 

dilute the sample by a factor of 10 before analyzing. The third 

SRS sample was a second aliquot of SRS sample N-77 that 

the laboratories were not asked to dilute; the laboratories were 

unaware that both aliquots represented the same SRS sample. 

The fourth sample was a stream-water sample collected at 

the USGS streamgage 01421618, Town Brook southeast of 

Hobart (table 2).

The USGS sent the second round of samples during 

the summer of 2003. This round consisted of eight samples 

collected from eight streams throughout the Catskill 

Mountains (table 2). These samples were filtered for TDP 

and SRP aliquots streamside through 0.4-μm cartridge 

filters and transported on ice to the USGS Water Quality 

Laboratory in Troy, N.Y., where the unfiltered TP aliquots 

were acidified with 1 mL of 4.5 normal sulfuric acid. Samples 

were refrigerated overnight and shipped on ice the next day to 

each participating laboratory. The samples for this study were 

labeled P1–P12 (table 2).

Statistical Methods

A z-value was calculated for each sample to rate the 

performance of each laboratory in relation to all other laboratories in the study. The rating for SRS samples N-77 and N-78 

(labeled P1 and P2 for this study) was based on results submitted during March 2003 by all laboratories that participated 

in the SRS sample exchange for nutrients (71 laboratories, 

including the 10 that participated in this study). The z-value 

used is analogous to a z-score, or standard score, which is 

calculated as:

z X = − 

 (1)

where

X = value of a given sample,

 = the sample mean,

and

 = standard deviation.

A nonparametric equivalent of the standard score was 

used for this comparison. The sample median was used in 

place of the sample mean, and the F-pseudosigma was used 

in place of the standard deviation. A nonparametric statistic 

was used to prevent the sample distribution from affecting the 

results. The F-pseudosigma is calculated as the sample third 

quartile minus the first quartile, divided by 1.349 (50 percent 

of the data lies within 1.349 standard deviations from the 

mean). When the F-pseudosigma was less than 5 percent of 

the sample median, 5 percent of the sample median was used 

instead. The acceptability of each analysis was rated according 

to the following criteria:
Data Comparability 3

Rating Absolute z-value

4 (Excellent) 0.00 to 0.50

3 (Good) 0.51 to 1.00

2 (Satisfactory) 1.01 to 1.50

1 (Marginal) 1.51 to 2.00

0 (Unsatisfactory) Greater than 2.00

Results with a rating of 2 to 4 are considered comparable to 

one another, and results with a rating of 1 or 0 are considered 

not comparable. It is important to note that for samples where 

there is little variation among concentrations reported by the 

laboratories, the z-values are sensitive to concentration differences to 3 decimal places, therefore, concentrations that are 

identical to 2 decimal places may have different z-values and, 

therefore, different ratings (table 3).

The method used to rate sample acceptability is the same 

as that used by the USGS program to rate laboratory performance in their SRS program. The F-pseudosigma value was 

greater than the median value for TP and TDP in sample P4. 

An F-pseudosigma value that exceeds the sample median 

indicates the z-value is invalid, and the rating is listed as NA 

(table 3).

The analytical precision for each laboratory was 

evaluated through the coefficient of variation (CV) calculated 

for each set of triplicate analyses. The coefficient of variation 

was calculated as:

CV s

x = ( ) 100

(2)

where

s = standard deviation,

and

x = arithmetic mean of the triplicate samples.

The data-quality objective for all analyses is a CV of less than 

10 percent.

Phosphorus concentration was also evaluated in terms 

of the difference between each P fraction analyzed. The TP 

concentration should be equal to or greater than the TDP 

concentration, which in turn should be equal to or greater than 

the SRP concentration. The difference between each P fraction 

was calculated for each sample sent to each laboratory to provide an indication of how accurately each laboratory analyzed 

the three fractions of phosphorus relative to one another.

Data Comparability

A total of 1,080 values would have been reported if 

the laboratories had analyzed all three phosphorus fractions 

in triplicate for each of the 12 samples, but several factors 

intervened and allowed a total of only 860 reported values 

(table 3).

•	 Laboratory 2 reported nondetectable concentrations for 

all fractions of sample P4 (table 3).

•	 Laboratory 6 did not receive samples from Round 2 

because of a shipping error and therefore, reported 

only results from Round 1.

•	 Laboratory 8 did not analyze the second round of 

samples (P5–P12) in triplicate because of a miscommunication.

•	 Laboratory 9 was unable to analyze the first round 

of samples (P1–P4) in triplicate because the sample 

volume was insufficient for their method.

•	 Laboratory 10 analyzes only for SRP (orthophosphate) 

and therefore, provided only those results.

Of the 860 values reported, 132 (15 percent) did not meet the 

comparability criteria set for the study (the absolute z-value 

was greater than 1.5). Of the three P fractions analyzed, SRP 

had the worst comparability—69 of 310 reported values 

(22 percent) did not meet the criteria. For TP, 37 of 276 

reported values (13 percent) did not meet the comparability criteria. TDP showed the best comparability—26 of 274 

reported values (9 percent) did not meet the established 

criteria. Two of the laboratories (Laboratories 2 and 6) failed 

comparability criteria for 55 percent of the values reported. 

Laboratories 2, 4, and 6 accounted for 80 percent of the data 

that did not meet comparability criteria. Three laboratories 

(Laboratories 5, 7, and 8) met comparability criteria for every 

value reported, and Laboratory 1 met the criteria for all but 

one value. Another three laboratories (Laboratories 3, 9, and 

10) had 10 or less values that did not meet the criteria.

None of the laboratories showed consistently poor performance on the diluted (low-concentration) samples; of the 132 

reported values that did not meet comparability criteria, only 

69 had concentrations less than 0.05 mg/L. Of the 860 values 

reported, 512 were less than 0.05 mg/L. Therefore, low-concentration samples accounted for 60 percent of the reported 

values but represented only 52 percent of the values that failed 

the comparability criteria. Thus, laboratory performance 

seemed to have a greater effect than sample concentration on 

laboratory comparability, and laboratories that had difficulty 

meeting the criteria at low concentrations also had difficulty at 

high concentrations.

The results for the 10 laboratories that participated in 

this study were within the range reported for the laboratories 

participating in the SRS program for samples distributed and 

evaluated through the USGS (samples P1 and P3), except for 

the TDP fraction of sample P3 (fig. 1). This study generally 

found that for each sample, there were one or two laboratories 

whose reported values did not meet the comparability criteria, 

although not always the same one or two laboratories.

The laboratories performed well in terms of precision; 

only 5 of 106 sets of TP triplicates, 5 of 104 sets of TDP 

triplicates, and 3 of 120 sets of SRP triplicates did not meet 
4 Phosphorus Concentrations in Stream-Water and Reference Samples—An Assessment of Laboratory Comparability

Figure 1. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) in 

10-laboratory comparison of stream-water and reference samples, 2003. 
Data Comparability 5

Figure 1. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) in 

10-laboratory comparison of stream-water and reference samples, 2003.—Continued

Total Phosphorus

Median = 0.031
6 Phosphorus Concentrations in Stream-Water and Reference Samples—An Assessment of Laboratory Comparability

Figure 1. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) in 

10-laboratory comparison of stream-water and reference samples, 2003.—Continued
Data Comparability 7

Figure 2. The coefficient of variation (CV) for total phosphorus, total 

dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) 

for all stream-water and reference sample triplicates reported by the 10 

participating laboratories.

the data-quality objective (a coefficient of variation less than 

10 percent) (fig. 2). Even when the laboratories did not meet 

the comparability objective, they typically met the precision 

objective with the triplicate samples (table 1). In other words, 

the laboratories reported similar values for triplicate samples, 

whether or not the values were close to the median concentration for all laboratories.

The calculated differences among phosphorus fractions 

indicate that most of the laboratories consistently reported 

higher TP values than TDP values, and higher TDP values 

than SRP values, as expected (fig. 3). Reported TDP values 

that exceeded the TP value, or that were exceeded by the 

reported SRP value, were usually at concentrations less than 

0.025 mg/L. Laboratory 2 consistently reported TP values 

lower than TDP values, which may indicate that the laboratory 

has difficulty analyzing acidified samples. Only five samples 

had SRP concentrations greater than the TDP concentrations, 

and the difference in four of those was close to zero—typically 

less than 0.05 mg/L.

-
8 Phosphorus Concentrations in Stream-Water and Reference Samples—An Assessment of Laboratory Comparability

Figure 3. Difference between phosphorus fractions in 10-laboratory 

comparison of phosphorus concentration in stream-water and reference 

samples, 2003. (A) Total phosphorus minus total dissolved phosphorus, 

and (B) total dissolved phosphorus minus soluble reactive phosphorus. 

[Laboratory 10 analyzed only for soluble reactive phosphorus]
References Cited 9

Conclusions

Laboratory performance has two principal measures—

accuracy and precision. Accuracy is a measure of how closely 

a reported concentration matches the actual concentration, 

whereas precision is a measure of how well a laboratory 

can repeat results. This study was conducted to evaluate the 

accuracy and precision of 10 laboratories that analyze water 

samples for phosphorus concentrations in research programs 

in the Catskill Mountain region. The laboratories compared 

well in consideration of the different methods used. The 

actual concentrations of the samples are unknown; therefore, 

the accuracy of each laboratory’s method is also unknown. 

The median sample concentration from all of the laboratories 

is taken as the most probable value, although it is not necessarily the correct value. Thus, a laboratory could theoretically 

report a value that does not meet the comparability criteria 

and yet be the only laboratory to report the correct concentration for that sample. When the actual sample concentration is 

unknown, the median concentration is assumed to be close to 

the true value.

The differences among analytical methods used by the 

laboratories in this study did not appear to greatly affect 

laboratory performance. The laboratories whose results were 

least comparable used methods that differed from each other’s. 

The three laboratories that met comparability criteria for all 

reported values used the same method, but other laboratories that also compared well used a variety of methods. The 

concentrations of the samples did not appear to greatly affect 

laboratory comparability. The laboratories that compared well 

did so at high and low concentrations, whereas the laboratories 

that did not meet comparability criteria had difficulty at both 

high and low concentrations. These results indicate that the 

laboratories that compared well did so regardless of method or 

sample concentration. This study identified a difficulty with 

sample digestion that one laboratory experienced during the 

first round of sample analysis, but this was corrected and the 

samples were reanalyzed. The results from the reanalysis are 

included in this report, and the earlier values are excluded.

All of the laboratories showed a high level of precision. 

Only 13 of 330 sets of triplicate analyses did not meet the 

data-quality objective. Data precision was expected to be high 

because most laboratories run triplicate samples as part of 

their regular quality-control procedures. Most of the laboratories also consistently reported TP values that were equal to or 

greater than TDP values, and TDP values equal to or greater 

then SRP values. The one notable exception was TP for Laboratory 2, which may indicate that the laboratory had difficulty 

analyzing acidified samples.

The results of this study emphasize the need for laboratories to participate in regular sample-exchange programs. Internal laboratory quality-assurance and quality-control programs 

can sometimes fail to identify sources of laboratory inaccuracy 

that would become apparent in sample-exchange programs.

Acknowledgments

Thanks are extended to Merle Shockey, Charles 

Patton, Harold Ardourel, Thomas Maloney, Terry Schertz, 

Mark Woodworth, and Mary Cast of the U.S. Geological 

Survey, for their help with project design. Thanks are also 

extended to the laboratories for their participation.

References Cited

Woodworth, M.T., and Connor, B.F., 2003, Results of the 

U.S. Geological Survey’s analytical evaluation program for 

standard reference samples distributed in March 2003: 

U.S. Geological Survey Open-File Report 03–261, 114 p.

Tables 1–3
12 Phosphorus Concentrations in Stream-Water and Reference Samples—An Assessment of Laboratory Comparability

Table 1. Laboratories participating in 2003 study of laboratory comparability, U.S. Environmental Protection Agency (USEPA) methods 

used, and method titles.

Laboratory USEPA 

methods used Method title

U.S. Geological Survey National Water Quality 

Laboratory

365.1 Phosphorus (All forms, colorimetric, automated, ascorbic acid)

U.S. Department of Agriculture Agricultural Research 

Service Laboratory at the University of Pennsylvania

365.3/365.4 Phosphorus (All forms, colorimetric, ascorbic acid, two reagent)

Phosphorus (Total, colorimetric, automated, block digester AA II)

Cornell University Department of Agricultural 

Engineering

365.1 Phosphorus (All forms, colorimetric, automated, ascorbic acid)

New York State Department of Health Wadsworth 

Laboratory

365.2/365.3 Phosphorus (All forms, colorimetric, ascorbic acid, single reagent)

Phosphorus (All forms, colorimetric, ascorbic acid, two reagent)

New York City Department of Environmental 

Protection

365.1 Phosphorus (All forms, colorimetric, automated, ascorbic acid)

Columbia Analytical Services 365.1 Phosphorus (All forms, colorimetric, automated, ascorbic acid)

Upstate Freshwater Institute 365.2 Phosphorus (All forms, colorimetric, ascorbic acid, single reagent)

Severn Trent Laboratories 365.2 Phosphorus (All forms, colorimetric, ascorbic acid, single reagent)

Academy of Natural Sciences 365.1 Phosphorus (All forms, colorimetric, automated, ascorbic acid)

University of Puerto Rico 365.1 Phosphorus (All forms, colorimetric, automated, ascorbic acid)
Table 2 13

Table 2. Samples included in the interlaboratory comparison.

[USGS, U.S. Geological Survey]

Sample number Sample description

Round 1, April 2003

P1 Standard Reference Sample N-78

P2 Standard Reference Sample N-77 undiluted

P3 Standard Reference Sample N-77 diluted by a factor of 10

P4 USGS streamgage Town Brook southeast of Hobart, N.Y. (Gage 01421618)

Round 2, July 2003

P5 USGS streamgage Town Brook southeast of Hobart, N.Y. (Gage 01421618)

P6 Small stream draining farmland within the Town Brook Watershed

P7 Small stream draining farmland within the Town Brook Watershed

P8 Small stream draining a farm within the Cannonsville Reservoir Watershed

P9 USGS streamgage West Branch Delaware River upstream from Delhi, N.Y. (Gage 01421900)

P10 USGS streamgage Little Delaware River near Delhi, N.Y. (Gage 01422500)

P11 USGS streamgage East Brook east of Walton, N.Y. (Gage 01422747)

P12 West Branch of the Delaware River at Beerston, N.Y.
14 Phosphorus Concentrations in Stream-Water and Reference Samples—An Assessment of Laboratory Comparability

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

[Conc., concentrations are in milligrams per liter (mg/L); ND, below detection limit; --, no value reported. Note that for samples where there is little variation 

among concentrations reported by the laboratories, the z-values are sensitive to concentration differences to 3 decimal places, therefore, concentrations that are 

identical to 2 decimal places may have different z-values and, therefore, different ratings]

Sample P1

Laboratory

Total phosphorus

Sample median = 0.64 mg/L

F-pseudosigma = 0.069

Total dissolved phosphorus

Sample median = 0.64 mg/L

F-pseudosigma = 0.028

Soluble reactive phosphorus

Sample median = 0.632 mg/L

F-pseudosigma = 0.019

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.61 0.47 4 0.63 0.19 4 0.64 0.17 4

1 0.60 0.59 3 0.63 0.47 4 0.64 0.02 4

1 0.60 0.56 3 0.63 0.31 4 0.65 0.36 4

2 0.73 1.28 2 0.76 3.66 0 0.62 0.46 4

2 0.72 1.12 2 0.75 3.47 0 0.63 0.21 4

2 0.73 1.31 2 0.76 3.72 0 0.61 0.74 3

3 0.63 0.15 4 0.62 0.63 3 0.68 1.44 2

3 0.64 0.01 4 0.64 0.00 4 0.67 1.12 2

3 0.64 0.01 4 0.63 0.31 4 0.67 1.12 2

4 0.64 0.01 4 0.64 0.09 3 0.63 0.14 4

4 0.63 0.12 4 0.63 0.34 3 0.63 0.27 4

4 0.64 0.04 4 0.64 0.06 3 0.63 0.08 4

5 0.62 0.28 4 0.64 0.03 4 0.65 0.43 4

5 0.62 0.28 4 0.64 0.09 4 0.64 0.27 4

5 0.62 0.28 4 0.64 0.03 4 0.64 0.17 4

6 0.71 1.01 2 0.67 0.94 3 0.64 0.17 4

6 0.71 1.01 2 0.70 1.88 1 0.68 1.44 2

6 0.72 1.15 2 0.64 0.00 4 0.69 1.76 1

7 0.67 0.36 4 0.63 0.41 4 0.63 0.17 4

7 0.65 0.17 4 0.66 0.59 3 0.64 0.08 4

7 0.66 0.24 4 0.66 0.66 3 0.63 0.21 4

8 0.64 0.07 4 0.65 0.16 4 0.63 0.02 4

8 0.62 0.33 4 0.64 0.06 4 0.63 0.21 4

8 0.61 0.50 3 0.63 0.34 4 0.62 0.40 4

9 0.51 1.94 1 0.52 3.66 0 0.64 0.14 4

9 -- -- -- -- -- -- -- -- --

9 -- -- -- -- -- -- -- -- --

10 -- -- -- -- -- -- 0.57 2.20 0

10 -- -- -- -- -- -- 0.59 1.28 2

10 -- -- -- -- -- -- 0.60 1.22 2
Table 3 15

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

—Continued

Sample P2

Laboratory

Total phosphorus

Sample median = 0.65 mg/L

F-pseudosigma = 0.028

Total dissolved phosphorus

Sample median = 0.65 mg/L

F-pseudosigma = 0.025

Soluble reactive phosphorus

Sample median = 0.60 mg/L

F-pseudosigma = 0.020

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.64 0.46 4 0.64 0.42 4 0.60 0.07 4

1 0.63 0.78 3 0.65 0.17 4 0.60 0.10 4

1 0.62 1.03 2 0.64 0.32 4 0.59 0.37 4

2 0.72 2.31 0 0.81 4.72 0 0.60 0.03 4

2 0.71 1.99 1 0.77 3.65 0 0.60 0.00 4

2 0.74 3.09 0 0.73 2.29 0 0.60 0.03 4

3 0.65 0.00 4 0.64 0.45 4 0.63 1.00 2

3 0.65 0.00 4 0.64 0.45 4 0.64 1.33 2

3 0.65 0.00 4 0.65 0.14 4 0.63 1.00 2

4 0.66 0.36 4 0.65 0.23 4 0.60 0.00 4

4 0.67 0.53 3 0.64 0.35 4 0.60 0.07 4

4 0.66 0.18 4 0.65 0.20 4 0.60 0.13 4

5 0.64 0.43 4 0.66 0.08 4 0.60 0.07 4

5 0.63 0.57 3 0.65 0.02 4 0.60 0.10 4

5 0.63 0.64 3 0.66 0.05 4 0.61 0.23 4

6 0.65 0.00 4 0.53 3.83 0 0.59 0.50 3

6 0.66 0.36 4 0.65 0.14 4 0.60 0.00 4

6 0.64 0.36 4 0.64 0.45 4 0.56 1.33 2

7 0.69 1.49 2 0.66 0.17 4 0.60 0.17 4

7 0.67 0.85 3 0.68 0.78 3 0.61 0.20 4

7 0.68 0.92 3 0.68 0.72 3 0.61 0.17 4

8 0.67 0.53 3 0.68 0.63 3 0.63 0.97 3

8 0.64 0.32 4 0.66 0.05 4 0.62 0.70 3

8 0.64 0.25 4 0.65 0.14 4 0.62 0.80 3

9 0.60 1.70 1 0.66 0.02 4 0.00 19.84 0

9 0.60 1.70 1 -- -- -- -- -- --

9 -- -- -- -- -- -- -- -- --

10 -- -- -- -- -- -- 0.58 0.73 3

10 -- -- -- -- -- -- 0.58 0.67 3

10 -- -- -- -- -- -- 0.57 0.87 3
16 Phosphorus Concentrations in Stream-Water and Reference Samples—An Assessment of Laboratory Comparability

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

—Continued

Sample P3

Laboratory

Total phosphorus

Sample median = 0.065 mg/L

F-pseudosigma = 0.014

Total dissolved phosphorus

Sample median = 0.065 mg/L

F-pseudosigma = 0.0044

Soluble reactive phosphorus

Sample median = 0.060 mg/L

F-pseudosigma = 0.0037

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.066 0.06 4 0.067 0.40 4 0.061 0.30 4

1 0.065 0.07 4 0.065 0.11 4 0.061 0.30 4

1 0.066 0.00 4 0.066 0.07 4 0.060 0.11 4

2 0.075 0.58 3 0.053 2.86 0 0.044 4.36 0

2 0.086 1.35 2 0.057 1.87 1 0.044 4.36 0

2 0.081 1.03 2 0.055 2.36 0 0.047 3.54 0

3 0.062 0.30 4 0.059 1.39 2 0.059 0.27 4

3 0.063 0.23 4 0.062 0.72 3 0.060 0.00 4

3 0.064 0.16 4 0.063 0.49 4 0.060 0.00 4

4 0.066 0.02 4 0.059 1.39 2 0.060 0.00 4

4 0.069 0.19 4 0.064 0.27 4 0.059 0.27 4

4 0.064 0.16 4 0.062 0.72 3 0.060 0.00 4

5 0.061 0.37 4 0.062 0.72 3 0.060 0.00 4

5 0.061 0.37 4 0.062 0.72 3 0.061 0.27 4

5 0.060 0.44 4 0.062 0.72 3 0.061 0.27 4

6 0.133 4.66 0 0.123 13.00 0 0.066 1.64 1

6 0.127 4.24 0 0.127 13.89 0 0.062 0.55 3

6 0.130 4.45 0 0.124 13.22 0 0.069 2.45 0

7 0.066 0.03 4 0.066 0.22 4 0.060 0.00 4

7 0.066 0.02 4 0.067 0.43 4 0.061 0.16 4

7 0.067 0.03 4 0.066 0.20 4 0.060 0.03 4

8 0.068 0.14 4 0.066 0.07 4 0.058 0.55 3

8 0.066 0.00 4 0.065 0.04 4 0.058 0.60 3

8 0.066 0.01 4 0.064 0.22 4 0.058 0.52 3

9 0.046 1.41 2 0.068 0.58 3 0.064 1.20 2

9 -- -- -- -- -- -- -- -- --

9 -- -- -- -- -- -- -- -- --

10 -- -- -- -- -- -- 0.056 0.98 3

10 -- -- -- -- -- -- 0.057 0.71 3

10 -- -- -- -- -- -- 0.058 0.60 3
Table 3 17

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

—Continued

Sample P4

Laboratory

Total phosphorus

Sample median = 0.018 mg/L

F-pseudosigma = 0.032

Total dissolved phosphorus

Sample median = 0.015 mg/L

F-pseudosigma = 0.030

Soluble reactive phosphorus

Sample median = 0.013 mg/L

F-pseudosigma = 0.0044

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.018 0.00 4 0.014 0.04 4 0.012 0.01 4

1 0.017 0.03 4 0.015 0.00 4 0.012 0.06 4

1 0.016 0.06 4 0.015 0.01 4 0.012 0.01 4

2 ND ND ND ND ND ND ND ND ND

2 ND ND ND ND ND ND ND ND ND

2 ND ND ND ND ND ND ND ND ND

3 0.018 0.00 4 0.014 0.04 4 0.011 0.30 4

3 0.018 0.00 4 0.014 0.04 4 -- -- --

3 -- -- -- -- -- -- -- -- --

4 0.019 0.03 4 0.013 0.07 4 0.012 0.08 4

4 0.025 0.22 4 0.018 0.10 4 0.010 0.53 3

4 0.014 0.12 4 0.013 0.07 4 0.012 0.08 4

5 0.013 0.15 4 0.013 0.07 4 0.014 0.37 4

5 0.012 0.19 4 0.013 0.07 4 0.013 0.15 4

5 0.012 0.19 4 0.013 0.07 4 0.012 0.08 4

6 0.095 2.40 0 0.090 2.52 0 0.031 4.19 0

6 0.097 2.46 0 0.092 2.58 0 0.026 3.07 0

6 0.098 2.49 0 0.090 2.52 0 0.030 3.97 0

7 0.018 0.00 4 0.016 0.04 4 0.012 0.06 4

7 0.018 0.00 4 0.016 0.02 4 0.012 0.03 4

7 0.018 0.00 4 0.015 0.01 4 0.012 0.06 4

8 0.018 0.00 4 0.014 0.04 4 0.011 0.35 4

8 0.018 0.00 4 0.015 0.01 4 0.011 0.39 4

8 0.016 0.05 4 0.015 0.01 4 0.011 0.28 4

9 0.017 0.02 4 -- -- -- 0.016 0.78 3

9 -- -- -- -- -- -- -- -- --

9 -- -- -- -- -- -- -- -- --

10 -- -- -- -- -- -- 0.018 1.27 2

10 -- -- -- -- -- -- 0.015 0.57 3

10 -- -- -- -- -- -- 0.016 0.71 3
18 Phosphorus Concentrations in Stream-Water and Reference Samples—An Assessment of Laboratory Comparability

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

—Continued

Sample P5

Laboratory

Total phosphorus

Sample median = 0.031 mg/L

F-pseudosigma = 0.0070

Total dissolved phosphorus

Sample median = 0.028 mg/L

F-pseudosigma = 0.031

Soluble reactive phosphorus

Sample median = 0.020 mg/L

F-pseudosigma = 0.0020

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.032 0.22 4 0.031 0.11 4 0.020 0.00 4

1 0.030 0.07 4 0.026 0.04 4 0.020 0.10 4

1 0.031 0.11 4 0.030 0.07 4 0.020 0.14 4

2 0.011 2.81 0 0.072 1.44 2 0.035 7.27 0

2 0.015 2.24 0 0.068 1.31 2 0.036 7.76 0

2 0.013 2.53 0 0.068 1.31 2 0.031 5.35 0

3 0.030 0.10 4 0.028 0.01 4 0.017 1.20 2

3 0.029 0.30 4 0.027 0.03 4 0.018 0.96 3

3 0.031 0.01 4 0.028 0.00 4 0.018 1.06 2

4 0.036 0.77 3 0.034 0.21 4 0.032 5.96 0

4 0.037 0.92 3 0.036 0.27 4 0.035 7.20 0

4 0.033 0.34 4 0.032 0.15 4 0.030 4.87 0

5 0.023 1.09 2 0.023 0.15 4 0.019 0.43 4

5 0.023 1.09 2 0.024 0.11 4 0.019 0.43 4

5 0.023 1.09 2 0.023 0.15 4 0.019 0.43 4

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

7 0.032 0.14 4 0.027 0.00 4 0.020 0.19 4

7 0.032 0.24 4 0.025 0.10 4 0.019 0.29 4

7 0.031 0.01 4 0.025 0.07 4 0.019 0.29 4

8 0.030 0.04 4 0.027 0.01 4 0.018 0.92 3

8 -- -- -- -- -- -- -- -- --

8 -- -- -- -- -- -- -- -- --

9 0.033 0.36 4 0.026 0.04 4 0.021 0.72 3

9 0.033 0.36 4 0.028 0.00 4 0.022 0.92 3

9 0.032 0.26 4 0.028 0.00 4 0.022 0.82 3

10 -- -- -- -- -- -- 0.019 0.53 3

10 -- -- -- -- -- -- 0.020 0.00 4

10 -- -- -- -- -- -- 0.020 0.14 4
Table 3 19

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

—Continued

Sample P6

Laboratory

Total phosphorus

Sample median = 0.21 mg/L

F-pseudosigma = 0.017

Total dissolved phosphorus

Sample median = 0.19 mg/L

F-pseudosigma = 0.038

Soluble reactive phosphorus

Sample median = 0.17 mg/L

F-pseudosigma = 0.0093

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.20 0.05 4 0.18 0.10 4 0.17 0.14 4

1 0.21 0.03 4 0.19 0.05 4 0.17 0.39 4

1 0.21 0.03 4 0.18 0.11 4 0.17 0.39 4

2 0.20 0.44 4 0.24 1.46 2 0.18 1.07 2

2 0.20 0.09 4 0.24 1.41 2 0.19 1.39 2

2 0.20 0.56 3 0.24 1.25 2 0.19 1.39 2

3 0.21 0.09 4 0.19 0.00 4 0.17 0.43 4

3 0.21 0.50 4 0.19 0.08 4 0.17 0.21 4

3 0.21 0.32 4 0.19 0.10 4 0.17 0.11 4

4 0.27 3.90 0 0.19 0.08 4 0.18 0.90 3

4 0.27 3.66 0 0.19 0.10 4 0.18 1.18 2

4 0.27 3.60 0 0.19 0.05 4 0.18 1.04 2

5 0.20 0.62 3 0.19 0.03 4 0.17 0.00 4

5 0.19 0.68 3 0.19 0.00 4 0.17 0.00 4

5 0.19 0.74 3 0.19 0.00 4 0.17 0.00 4

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

7 0.20 0.44 4 0.18 0.18 4 0.17 0.00 4

7 0.20 0.62 3 0.17 0.36 4 0.17 0.43 4

7 0.20 0.33 4 0.19 0.05 4 0.17 0.00 4

8 0.22 1.08 2 0.19 0.13 4 0.18 0.21 4

8 -- -- -- -- -- -- -- -- --

8 -- -- -- -- -- -- -- -- --

9 0.22 0.61 3 0.18 0.29 4 0.19 2.14 0

9 0.22 0.91 3 0.19 0.03 4 0.18 0.54 3

9 0.22 0.91 3 0.17 0.47 4 0.18 0.96 3

10 -- -- -- -- -- -- 0.10 7.47 0

10 -- -- -- -- -- -- 0.11 7.24 0

10 -- -- -- -- -- -- 0.11 6.93 0
20 Phosphorus Concentrations in Stream-Water and Reference Samples—An Assessment of Laboratory Comparability

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

—Continued

Sample P7

Laboratory

Total phosphorus

Sample median = 0.049 mg/L

F-pseudosigma = 0.014

Total dissolved phosphorus

Sample median = 0.046 mg/L

F-pseudosigma = 0.033

Soluble reactive phosphorus

Sample median = 0.040 mg/L

F-pseudosigma = 0.0047

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.047 0.14 4 0.044 0.05 4 0.041 0.11 4

1 0.051 0.13 4 0.044 0.07 4 0.041 0.28 4

1 0.048 0.11 4 0.046 0.01 4 0.042 0.36 4

2 0.068 1.30 2 0.088 1.25 2 0.055 3.21 0

2 0.072 1.58 1 0.092 1.37 2 0.054 3.00 0

2 0.066 1.16 2 0.094 1.43 2 0.052 2.57 0

3 0.044 0.39 4 0.042 0.13 4 0.040 0.11 4

3 0.046 0.27 4 0.043 0.10 4 0.040 0.00 4

3 0.047 0.19 4 0.043 0.10 4 0.040 0.02 4

4 0.140 6.34 0 0.058 0.35 4 0.046 1.37 2

4 0.154 7.31 0 0.059 0.38 4 0.047 1.51 1

4 0.148 6.90 0 0.060 0.41 4 0.050 2.06 0

5 0.037 0.86 3 0.041 0.15 4 0.039 0.21 4

5 0.037 0.86 3 0.041 0.15 4 0.039 0.21 4

5 0.037 0.86 3 0.040 0.18 4 0.039 0.21 4

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

7 0.047 0.15 4 0.043 0.08 4 0.039 0.19 4

7 0.050 0.07 4 0.053 0.21 4 0.039 0.13 4

7 0.048 0.12 4 0.045 0.05 4 0.040 0.09 4

8 0.048 0.07 4 0.049 0.07 4 0.038 0.43 4

8 -- -- -- -- -- -- -- -- --

8 -- -- -- -- -- -- -- -- --

9 0.053 0.26 4 0.046 0.01 4 0.045 1.07 2

9 0.052 0.19 4 0.047 0.03 4 0.045 1.07 2

9 0.051 0.14 4 0.047 0.01 4 0.045 1.13 2

10 -- -- -- -- -- -- 0.030 2.23 0

10 -- -- -- -- -- -- 0.028 2.57 0

10 -- -- -- -- -- -- 0.028 2.58 0
Table 3 21

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

—Continued

Sample P8

Laboratory

Total phosphorus

Sample median = 0.066 mg/L

F-pseudosigma = 0.015

Total dissolved phosphorus

Sample median = 0.049 mg/L

F-pseudosigma = 0.051

Soluble reactive phosphorus

Sample median = 0.034 mg/L

F-pseudosigma = 0.0055

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.050 1.08 2 0.041 0.16 4 0.027 1.35 2

1 0.050 1.10 2 0.044 0.11 4 0.027 1.31 2

1 0.053 0.91 3 0.046 0.07 4 0.026 1.51 1

2 0.066 0.03 4 0.212 3.22 0 0.045 2.01 0

2 0.064 0.16 4 0.210 3.18 0 0.046 2.19 0

2 0.066 0.03 4 0.204 3.06 0 0.048 2.55 0

3 0.063 0.20 4 0.047 0.04 4 0.034 0.07 4

3 0.064 0.16 4 0.048 0.02 4 0.035 0.18 4

3 0.067 0.03 4 0.049 0.01 4 0.035 0.18 4

4 0.067 0.04 4 0.052 0.06 4 0.041 1.32 2

4 0.069 0.17 4 0.050 0.02 4 0.041 1.32 2

4 0.064 0.16 4 0.056 0.14 4 0.043 1.67 1

5 0.055 0.75 3 0.043 0.12 4 0.034 0.00 4

5 0.054 0.82 3 0.043 0.12 4 0.034 0.00 4

5 0.054 0.82 3 0.043 0.12 4 0.034 0.00 4

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

7 0.072 0.39 4 0.051 0.03 4 0.034 0.04 4

7 0.068 0.13 4 0.046 0.07 4 0.034 0.04 4

7 0.068 0.08 4 0.046 0.06 4 0.034 0.00 4

8 0.073 0.43 4 0.050 0.01 4 0.034 0.00 4

8 -- -- -- -- -- -- -- -- --

8 -- -- -- -- -- -- -- -- --

9 0.076 0.66 3 0.052 0.06 4 0.041 1.26 2

9 0.075 0.60 3 0.054 0.09 4 0.041 1.26 2

9 0.077 0.73 3 0.058 0.17 4 0.041 1.29 2

10 -- -- -- -- -- -- 0.021 2.45 0

10 -- -- -- -- -- -- 0.019 2.67 0

10 -- -- -- -- -- -- 0.020 2.49 0
22 Phosphorus Concentrations in Stream-Water and Reference Samples—An Assessment of Laboratory Comparability

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

—Continued

Sample P9

Laboratory

Total phosphorus

Sample median = 0.057 mg/L

F-pseudosigma = 0.025

Total dissolved phosphorus

Sample median = 0.036 mg/L

F-pseudosigma = 0.040

Soluble reactive phosphorus

Sample median = 0.022 mg/L

F-pseudosigma = 0.0036

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.056 0.04 4 0.036 0.01 4 0.022 0.00 4

1 0.057 0.00 4 0.035 0.02 4 0.022 0.19 4

1 0.058 0.04 4 0.035 0.02 4 0.022 0.19 4

2 0.088 1.26 2 0.088 1.29 2 0.031 2.42 0

2 0.094 1.50 1 0.090 1.34 2 0.031 2.42 0

2 0.096 1.58 1 0.090 1.34 2 0.032 2.70 0

3 0.057 0.02 4 0.033 0.08 4 0.020 0.66 3

3 0.054 0.12 4 0.034 0.06 4 0.021 0.47 4

3 0.057 0.00 4 0.034 0.05 4 0.020 0.52 3

4 0.058 0.06 4 0.041 0.13 4 0.030 2.24 0

4 0.055 0.06 4 0.042 0.15 4 0.030 2.07 0

4 0.062 0.22 4 0.048 0.30 4 0.030 2.07 0

5 0.044 0.51 3 0.030 0.15 4 0.022 0.06 4

5 0.044 0.51 3 0.031 0.12 4 0.022 0.06 4

5 0.044 0.51 3 0.031 0.12 4 0.022 0.06 4

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

7 0.056 0.02 4 0.036 0.01 4 0.023 0.22 4

7 0.054 0.09 4 0.034 0.04 4 0.023 0.22 4

7 0.054 0.11 4 0.034 0.06 4 0.023 0.22 4

8 0.056 0.01 4 0.037 0.03 4 0.022 0.06 4

8 -- -- -- -- -- -- -- -- --

8 -- -- -- -- -- -- -- -- --

9 0.058 0.04 4 0.037 0.02 4 0.026 1.16 2

9 0.058 0.06 4 0.037 0.02 4 0.026 1.10 2

9 0.056 0.02 4 0.037 0.01 4 0.026 1.10 2

10 -- -- -- -- -- -- 0.018 1.03 2

10 -- -- -- -- -- -- 0.019 0.95 3

10 -- -- -- -- -- -- 0.019 0.90 3
Table 3 23

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

—Continued

Sample P10

Laboratory

Total phosphorus

Sample median = 0.024 mg/L

F-pseudosigma = 0.0087

Total dissolved phosphorus

Sample median = 0.022 mg/L

F-pseudosigma = 0.022

Soluble reactive phosphorus

Sample median = 0.016 mg/L

F-pseudosigma = 0.0019

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.024 0.09 4 0.020 0.10 4 0.015 0.16 4

1 0.023 0.24 4 0.024 0.10 4 0.015 0.11 4

1 0.025 0.01 4 0.022 0.01 4 0.016 0.00 4

2 0.031 1.02 2 0.050 1.25 2 0.025 5.03 0

2 0.038 2.10 0 0.058 1.61 1 0.026 5.57 0

2 0.031 1.02 2 0.056 1.52 1 0.025 5.03 0

3 0.024 0.01 4 0.020 0.10 4 0.012 1.71 1

3 0.022 0.43 4 0.021 0.02 4 0.013 1.66 1

3 0.023 0.24 4 0.020 0.06 4 0.013 1.55 1

4 0.038 2.10 0 0.026 0.18 4 0.023 3.78 0

4 0.040 58.85 0 0.027 0.23 4 0.021 2.74 0

4 0.042 2.72 0 0.024 0.10 4 0.020 2.36 0

5 0.018 1.00 2 0.019 0.13 4 0.015 0.32 4

5 0.017 1.16 2 0.018 0.17 4 0.014 0.86 3

5 0.017 1.16 2 0.018 0.17 4 0.014 0.86 3

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

7 0.024 0.07 4 0.024 0.08 4 0.016 0.43 4

7 0.025 0.04 4 0.022 0.00 4 0.017 0.59 3

7 0.021 0.53 3 0.022 0.02 4 0.016 0.43 4

8 0.022 0.33 4 0.020 0.10 4 0.013 1.39 2

8 -- -- -- -- -- -- -- -- --

8 -- -- -- -- -- -- -- -- --

9 0.025 0.09 4 0.022 0.01 4 0.017 0.59 3

9 0.025 0.09 4 0.022 0.02 4 0.017 0.54 3

9 0.025 0.15 4 0.022 0.00 4 0.017 0.54 3

10 -- -- -- -- -- -- 0.015 0.27 4

10 -- -- -- -- -- -- 0.015 0.40 4

10 -- -- -- -- -- -- 0.014 0.76 3
24 Phosphorus Concentrations in Stream-Water and Reference Samples—An Assessment of Laboratory Comparability

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

—Continued

Sample P11

Laboratory

Total phosphorus

Sample median = 0.023 mg/L

F-pseudosigma = 0.0094

Total dissolved phosphorus

Sample median = 0.012 mg/L

F-pseudosigma = 0.075

Soluble reactive phosphorus

Sample median = 0.0085 mg/L

F-pseudosigma = 0.0016

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.021 0.22 4 0.015 0.04 4 0.008 0.06 4

1 0.021 0.24 4 0.014 0.03 4 0.009 0.25 4

1 0.020 0.32 4 0.015 0.04 4 0.008 0.43 4

2 0.033 1.08 2 0.112 1.33 2 0.020 7.12 0

2 0.029 0.65 3 0.114 1.36 2 0.020 7.12 0

2 0.040 1.82 1 0.124 1.49 2 0.019 6.50 0

3 0.015 0.81 3 0.011 0.02 4 0.005 2.35 0

3 0.016 0.73 3 0.011 0.02 4 0.005 1.92 1

3 0.015 0.79 3 0.011 0.02 4 0.005 2.10 0

4 0.116 9.89 0 0.015 0.04 4 0.009 0.02 4

4 0.117 10.00 0 0.018 0.08 4 0.010 1.21 2

4 0.111 9.36 0 0.014 0.03 4 0.010 1.21 2

5 0.016 0.73 3 0.010 0.03 4 0.007 0.93 3

5 0.017 0.62 3 0.010 0.03 4 0.007 0.93 3

5 0.017 0.62 3 0.010 0.03 4 0.007 0.93 3

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

7 0.030 0.73 3 0.013 0.01 4 0.008 0.50 4

7 0.026 0.37 4 0.012 0.00 4 0.007 0.99 3

7 0.027 0.40 4 0.012 0.01 4 0.008 0.56 3

8 0.017 0.63 3 0.011 0.02 4 0.007 0.93 3

8 -- -- -- -- -- -- -- -- --

8 -- -- -- -- -- -- -- -- --

9 0.025 0.23 4 0.012 0.00 4 0.009 0.00 4

9 0.023 0.02 4 0.012 0.00 4 0.009 0.00 4

9 0.023 0.02 4 0.012 0.00 4 0.009 0.56 3

10 -- -- -- -- -- -- 0.009 0.29 4

10 -- -- -- -- -- -- 0.009 0.25 4

10 -- -- -- -- -- -- 0.009 0.42 4
Table 3 25

Table 3. Concentrations of total phosphorus, total dissolved phosphorus, and soluble reactive phosphorus (orthophosphate) for 

stream-water and reference samples, z-values, and ratings for the 10 laboratories involved in the 2003 comparability study.

—Continued

Sample P12

Laboratory

Total phosphorus

Sample median = 0.025 mg/L

F-pseudosigma = 0.015

Total dissolved phosphorus

Sample median = 0.019 mg/L

F-pseudosigma = 0.0073

Soluble reactive phosphorus

Sample median = 0.013 mg/L

F-pseudosigma = 0.0018

Conc. z-value Rating Conc. z-value Rating Conc. z-value Rating

1 0.027 0.14 4 0.018 0.05 4 0.013 0.00 4

1 0.025 0.01 4 0.019 0.00 4 0.014 0.17 4

1 0.025 0.00 4 0.019 0.07 4 0.013 0.00 4

2 0.052 1.76 1 0.031 1.66 1 0.025 6.52 0

2 0.044 1.24 2 0.035 2.21 0 0.022 4.83 0

2 0.056 2.02 0 0.027 1.12 2 0.025 6.52 0

3 0.025 0.01 4 0.017 0.29 4 0.010 1.85 1

3 0.023 0.12 4 0.017 0.19 4 0.011 1.57 1

3 0.025 0.00 4 0.018 0.05 4 0.011 1.57 1

4 0.033 0.53 3 0.022 0.44 4 0.016 1.23 2

4 0.032 0.46 4 0.024 0.71 3 0.017 1.95 1

4 0.036 0.72 3 0.020 0.16 4 0.016 1.59 1

5 0.016 0.57 3 0.016 0.38 4 0.011 1.35 2

5 0.016 0.57 3 0.016 0.38 4 0.012 0.79 3

5 0.016 0.57 3 0.016 0.38 4 0.012 0.79 3

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

6 -- -- -- -- -- -- -- -- --

7 0.026 0.07 4 0.019 0.01 4 0.011 1.24 2

7 0.024 0.06 4 0.017 0.23 4 0.013 0.45 4

7 0.023 0.11 4 0.017 0.25 4 0.013 0.45 4

8 0.022 0.20 4 0.017 0.22 4 0.011 1.35 2

8 -- -- -- -- -- -- -- -- --

8 -- -- -- -- -- -- -- -- --

9 0.024 0.07 4 0.019 0.04 4 0.014 0.56 3

9 0.024 0.07 4 0.019 0.04 4 0.014 0.45 4

9 0.024 0.07 4 0.019 0.00 4 0.014 0.45 4

10 -- -- -- -- -- -- 0.013 0.22 4

10 -- -- -- -- -- -- 0.013 0.03 4

10 -- -- -- -- -- -- 0.014 0.12 4
For more information concerning the research in this report, contact:

Rafael W. Rodriguez, Director

U.S. Geological Survey

New York Water Science Center

425 Jordan Road

Troy, NY 12180-8349

or visit our Web site at:

http://ny.water.usgs.gov