PROGRAMS AND PLANS--Phosphorus Methods and the Quality of Phosphorus Data

In Reply Refer To:                                  July 13, 1992
Mail Stop 412


Subject:   PROGRAMS AND PLANS--Phosphorus Methods and the Quality
                               of Phosphorus Data


In 1990 and 1991, two changes were implemented at the National
Water Quality Laboratory (NWQL) for the analysis of phosphorus.
Both changes improved the quality of data produced, with resulting
implications for the use and interpretation of results.  The
purposes of this memorandum are to:

1.  Document the chronology of phosphorus methods used at the
2.  Assess the magnitude of bias produced by the historic
    phosphorus methods;
3.  Discuss the implications for data interpretation of the bias
    in historic phosphorus data; and
4.  State the process for rigorous documentation of new methods at
    the NWQL and inclusion of field quality control (QC) samples in
    water-quality programs and projects.


Since 1973, the chronology of phosphorus methods at the NWQL
has been:
                              Digestion         Measurement
  Period        Method          Step                Step

1973 -      I-2600/I-4600    potassium         orthophosphate
April 30,   without a        persulfate and    by phosphoantimonyl-
1990        dilution SOP     sulfuric acid;    molybdenum blue
                             30 min. at 121oC  (mixed reagent
                             in an autoclave   procedure) Murphy
                             at 15-20 psi      & Riley (1962)

May 1,      I-2600/I-4600    same as above     same as above
1990 -      with a dilution
Sept. 30,   SOP

Since       I 2610/I-4610    Kjeldahl reagents  orthophosphate
Oct. 1,                      (sulfuric acid     by phosphoantimonyl-
1991                         and potassium      molybdenum blue
                             sulfate); 15 min.  (separate reagent
                             at 375oC in a      procedure) Murphy
                             Kjeldahl block     & Riley (1962)

U.S. Geological Survey TWRI method I-2600/I-4600 was initially
developed based on U.S. Environmental Protection Agency (U.S. EPA)
method 365.1.  For the digestion step, both methods rely on
oxidation of polyphosphate and organic phosphorus compounds to
orthophosphate by persulfate ions and sulfuric acid.  During
digestion, the conversion of polyphosphates occurs more readily
than conversion of organic phosphorus compounds that occur in
samples either as part of organic particles or attached to mineral

Apparently, sometime after 1973, during a method rewrite, a
miscalculation or typographical error was introduced into method
I-2600/I-4600.  The error resulted in the use of low concen-
trations of persulfate and sulfuric acid for the digestion step.
The error went unnoticed until 1989, when the Ohio District began
splitting samples and submitting aliquots to both the NWQL and the
Heidelberg College Water-Quality Laboratory (HCWQL).  For certain
samples, the NWQL method resulted in lower total phosphorus
concentrations than the HCWQL method.  The differences were
especially notable on samples with suspended-sediment
concentrations exceeding 50 mg/L.

During early 1990, studies at the NWQL confirmed that method
I-2600/I-4600 caused reagent-limiting, incomplete digestions
(total phosphorus concentrations were therefore biased low) on
samples having high concentrations of suspended sediment.  It was
not determined whether the reagent limitation resulted from high
concentrations of suspended sediment, high concentrations of
associated organic carbon, or both.  The studies did indicate that
the reagent limitation could be overcome for most high phosphorus
samples if "off scale" samples--those found to have total
phosphorus concentrations exceeding 1.0 mg/L--were diluted tenfold
(1 + 9) prior to redetermination.  Previously, the extent of
dilution for off scale samples was discretionary.  Accordingly,
on May 1, 1990, an SOP for tenfold dilution of off scale samples
(>1.0 mg/L total phosphorus) was implemented for method
I-2600/I-4600.  The SOP also required that all samples appearing
"--particularly colored or sediment laden are diluted by a factor
of 2-5 fold at the analyst's discretion--" prior to digestion and

Method I-2600/I-4600 with the added SOP was the official method
for phosphorus analysis at the NWQL for the period May 1, 1990
through September 30, 1991.

Beginning October 1, 1991, a Kjeldahl digestion method--similar to
U.S. EPA method 365.4--as modified by Jirka and others (1976) and
Bowman and Delfino (1982)--was implemented at the NWQL for both
phosphorus and nitrogen.  The new phosphorus method
(I-2610/I-4610) completely replaced method I-2600/I-4600.  The
digestion step in method I-2610/I-4610 involves very rigorous
conversion--through reduction and hydrolysis--of polyphosphates
and organic phosphorus compounds to orthophosphate using sulfuric
acid and potassium sulfate at 375oC.  Because of the very rigorous
digestion, method I-2610/I-4610 avoids either a reagent limitation
or an interference with reagents by samples having high
concentrations of suspended sediment, organic carbon, or
associated constituents.  For these reasons, no dilution step is
required for analysis of phosphorus in any type of sample.  Method
I-2610/I-4610 was implemented at the NWQL because it: (a) improved
data quality, and (b) markedly reduced sample handling and analyst


The best information for assessing the possible effect of the
pre-May 1, 1990, application of method I-2600/I-4600 on USGS
phosphorus measurements stems from samples collected at seven
Illinois stream sites from 1986 through 1989.  A total of 147
samples were collected at the Illinois sites during the period.
All samples were collected by USGS personnel, split, and then
analyzed for total and dissolved phosphorus by both the Illinois
EPA (IEPA) and the USGS.  All IEPA measurements used the U.S. EPA
Persulfate Method 365.1 (the method on which USGS I-2600/I-4600
was originally based).  Analyses of suspended sediment and total
organic carbon (TOC) were also available for 126 and 77 of the
samples, respectively.

Differences in total phosphorus concentrations by the two methods
(computed as IEPA measurement minus USGS measurement) are examined
here for: (a) several concentration ranges of total phosphorus,
suspended sediment, and TOC (Table 1), and (b) geographic
(station) location (Table 2).  In addition, differences in
dissolved phosphorus concentrations are compared for selected
ranges of concentrations (Table 3).  Each comparison is based on
analysis of the comparative data by the Wilcoxon Signed Rank
Test--a nonparametric procedure for determining if medians of two
populations are statistically different.  For this memorandum, a
probability (p) of 2 0.05 was selected as representing a
statistically significant difference.

In general, USGS total phosphorus values are lower than those
of IEPA with the largest and most statistically significant
differences in samples having high concentrations of total
phosphorus, suspended sediment, and TOC (Table 1).  For the full
total phosphorus concentration range of 0.05 to 2.0 mg/L, a highly
significant (p <0.001) median difference of 0.01 mg/L is detected.
For samples with relatively high phosphorus concentrations (above
0.2 mg/L), highly significant differences ranging from 0.01 to
0.03 mg/L are detected for the concentration ranges examined.  At
concentrations below 0.2 mg/L, the median difference of 0.01 mg/L
is not statistically significant.  Larger negative bias is
observed in USGS total phosphorus measurements for samples with
high concentrations of suspended sediment.  Statistically
significant median differences of 0.04 and 0.06 mg/L occur for
samples with suspended sediment concentrations above 75 mg/L and
100 mg/L, respectively.  For samples with high concentrations
of both total phosphorus (>0.2 mg/L) and suspended sediment
(>100 mg/L), statistically significant median differences of 0.09
and 0.18 mg/L are observed.  Although fewer TOC measurements are
available for statistical analysis, the negative bias in USGS
total phosphorus measurements again is larger for samples with
high TOC concentrations.

Although the results show some ambiguity, the effect of varying
quantities of suspended sediment and TOC on the measurement bias
of USGS total phosphorus concentrations is also seen in Table 2.
Statistically significant median differences in total phosphorus
are observed for the Little Wabash River, Rock River, and Spoon
River sites, which have high median concentrations of suspended
sediment, together with high percentages of particulate phase
phosphorus (low DP/TP ratios).  The two stations predominantly
influenced by point sources and having moderate to low suspended
sediment concentrations--the Illinois River and Sangamon River
sites--display no significant bias and have an estimated median
difference of zero.  These sites have relatively high
concentrations of total phosphorus, but nearly 75 percent is in
the dissolved phase.

Table 3 compares results for dissolved phosphorus.  Although
estimated median differences between IEPA and USGS data range from
-0.02 to 0.01, these differences are not statistically significant
(p 2 0.05) for any of the concentration ranges examined.  We
believe the lack of statistically significant bias in the USGS
dissolved phosphorus data (in contrast to the total phosphorus
data) results from a relative lack of suspended sediment and
organic carbon in the analyzed filtrates.

I-2610/I-4610 AS COMPARED TO METHOD I-2600/I-4600

Charles Patton and Earl Truitt of the NWQL developed method
I-2610/I-4610 for phosphorus and Kjeldahl nitrogen.  As part of
the method proveout, duplicate analyses were run on 1,572 samples
to compare phosphorus results from this new method to results from
method I-2600/I-4600 with the addition of the sample dilution SOP
(three- to fivefold for colored and/or sediment laden samples;
tenfold for off scale samples).  The first set of duplicates was
run on the 417 samples received at the NWQL during April 1991 for
which Districts requested both nitrogen and phosphorus analyses.

Because of District concerns about the change to a new method,
another set of duplicates was run on 1,155 samples received at the
NWQL during the period July-September 1991.  This set included:
(a) the approximately 100 NASQAN samples that arrived for analyses
during the period, and (b) about 1,050 samples for which Districts
specifically requested phosphorus to be analyzed by both the new
and old methods.

Prior to statistical analysis, both the April and July-September
data sets were sorted by sample type into four groups:  unfiltered
and filtered surface water; and unfiltered and filtered ground

Each group of data produced by the two analytical methods was then
compared using: (a) all samples within each group, and (b)
specific concentration ranges of phosphorus within each group.
The comparisons were again based on the Wilcoxon Signed Rank Test
to determine if medians of the two populations are statistically
different.  The test results--shown in Tables 4 and 5--compare the
median differences found by subtracting estimated medians of
phosphorus measurements by method I-2600/I-4600 from estimated
medians of measurements by method I-2610/I-4610.  Thus, a positive
bias means that the new method produced higher concentrations than
the old method.

For surface water (Table 4), median phosphorus concentration for
the inclusive concentration ranges showed a positive bias during
both time periods of 0.02 milligrams per liter (mg/L) for
unfiltered samples and about 0.01 mg/L for filtered samples.  For
the discrete concentration ranges tested, the positive bias
increased progressively with phosphorus concentration for both the
unfiltered and filtered samples in each time period.  For example,
for the unfiltered samples during July-September, the bias was
0.003 for the 0.00-0.15 mg/L range, 0.03 for the 0.15-0.30 mg/L
range, and 0.3 mg/L for the 2.00-4.50 mg/L range.  The bias
was statistically significant in 22 of the 24 cases.  The two
exceptions were for filtered (dissolved) samples: (a) the
0.22-0.93 mg/L range during April, and (b) the 0.00-0.15 mg/L
range during July-September.  The trend noted in bias--increasing
positive bias with increasing phosphorus concentration--probably
reflects reagent limitation remaining in method I-2600/I-4600 even
when: (a) colored and/or sediment laden samples were diluted two
to fivefold prior to initial determination, and (b) off scale
samples were diluted tenfold prior to redetermination.  As noted,
the trend was observed for filtered, as well as unfiltered
samples.  For the unfiltered samples, the major cause of the
suspected reagent limitation can be rationalized as resulting from
incomplete digestion of phosphorus compounds associated with
progressively increasing concentrations of organic and mineral
particles in suspended sediment.  For filtered samples, perhaps
the trend results from phosphorus associated with progressively
increasing amounts of colloidal-size organic and mineral particles
in the filtrates.  We considered the possibility that some of the
bias in both unfiltered and filtered samples might result from the
somewhat different techniques used for colorimetrically measuring
the orthophosphate (see page 1 for descriptions of the measurement
step).  The possibility was rejected, because instrument
calibrations would have compensated for such differences in
analytical results.

Ground-water samples comprised roughly 5 and 10 percent of the
July-September and April samples, respectively.  The results of
statistical testing for ground-water samples (Table 5) showed

1.  In all comparisons, the estimated median bias is below
    0.01 mg/L--the estimated method detection limit for

2.  In general, the estimated median bias values are lower than
    observed for surface-water samples.

3.  For certain comparisons, the bias is negative, rather than

4.  Statistically significant differences between the two methods
    are observed in only two of the seven cases.

5.  No difference exists in bias for the filtered as compared to
    unfiltered samples.

Although supportive data are lacking, Charles Patton believes that
the small differences in total phosphorus concentrations between
the two methods results from low concentrations of suspended
sediment and/or organic carbon in the samples.  For such samples,
method I-2600/I-4600 would have little or no reagent limitation
and, hence, would produce concentrations very similar to those
from method I-2610/I-4610.


To summarize, two recent changes at the NWQL have affected
analytical results for phosphorus.  The first was a SOP change
within method I-2600/I-4600 on May 1, 1990, that defined a set
dilution procedure designed to ameliorate reagent limitation
problems in samples having high concentrations of phosphorus,
suspended sediment, and/or organic carbon.  The second is the
replacement of method I-2600/I-4600 with method I-2610/I-4610 on
October 1, 1991.

Before May 1, 1990, no SOP existed within method I-2600/I-4600 to
dilute: (a) colored and/or sediment laden samples, or (b) off scale
total phosphorus (> 1.0 mg/L) samples.  For the former, there was
no SOP to determine: (a) which samples arriving at the NWQL for
total phosphorus analysis were colored or sediment laden enough to
warrant dilution, or (b) volumetrically, how much to dilute the
selected samples.  Likewise, no SOP existed for the dilution of off
scale dissolved phosphorus samples.  Instead, dilution procedures
were left to each analyst's discretion and, during the period 1980-
1990, over 25 analysts worked on the phosphorus line.  In addition,
NWQL had no system for recording whether individual samples were
diluted and, if so, the selected dilution factor.  Moreover, the
data produced by the NWQL in 1990 to compare phosphorus
measurements in high sediment samples by method I-2600/I-4600--with
and without the dilution SOP--are insufficient in number to support
statistical analysis.

As noted, Tables 4 and 5 present the statistical results for
comparing phosphorus measurements by method I-2610/I-4610 and
method I-2600/I-4600 (with the dilution SOP).  Additional data on
the samples--such as concentrations of suspended sediment and
organic carbon--are not available to enable adjustments for the
detected biases.

Tables 1-5 provide only a fragmentary picture of the bias in
historic USGS phosphorus data.  Tables 1-3 provide some insight to
bias in USGS data produced prior to May 1, 1990, but relative to
IEPA method 365.1, not to the subsequent USGS methods.  Tables 4
and 5 show the estimated bias in the USGS data between the periods
May 1, 1990 through September 30, 1991 and post October 1, 1991.
For total phosphorus in surface waters, the combined results show:
(a) a negative bias in USGS data produced prior to May 1, 1990,
relative to IEPA data, and (b) a negative bias in USGS data
produced during May 1, 1990 through September 31, 1991 relative to
USGS data produced since using method I-2610/I-4610.  Please note
that the Illinois results provide only limited insight about the
possible biases in USGS data produced prior to May 1, 1990.  How
well these results describe bias in samples from other U.S. rivers
is uncertain due to the physical and chemical differences that may
exist--such as the concentrations of sediment-bound phosphorus and
organic carbon, and the nature and concentration of the suspended

For dissolved (filtered) phosphorus in surface waters, Table 3
indicates that method I-2600/I-4600 (without the dilution SOP) did
not produce a statistically significant bias relative to IEPA method
365.1.  In contrast, the USGS methods comparison study on samples
from across the country (Table 4) indicate that method I-2600/I-4600
(with the dilution SOP) caused statistically significant negative
biases in dissolved phosphorus relative to method I-2610/I-4610.
The bias for all filtered samples is 0.01 mg/L, compared to a bias
of 0.02 mg/L for all unfiltered samples.  However, the bias for
unfiltered samples increases progressively with phosphorus
concentration, and the individual biases for the high concentration
ranges are similar to those for the comparable concentration ranges
for total phosphorus.  We believe that given the negative observed
biases in the methods comparison study (Table 4), an even larger
negative bias probably exists in USGS dissolved phosphorus produced
before May 1, 1990.  Perhaps the lack of bias in the Illinois
filtered sample data and the presence of bias in the study comparing
the USGS methods result from the digestion step in method
I-2610/I-4610 being more rigorous (resulting in a more complete
conversion of organic phosphorus compounds in the sample to
orthophosphate) than in IEPA method 365.1.  The OWQ will investigate
this possibility by statistically comparing split sample phosphorus
data now being produced by the IEPA and the USGS (using method
I-2610/I-4610).  Results will be communicated in a future OWQ Tech

For ground-water samples, no split sample data are available to
examine the possibility of bias in phosphorus measurements made
prior to May 1, 1990.  The results in Table 5 indicate that little
or no negative bias exists in phosphorus data from ground-water
samples between the periods of May 1, 1990 through September 30,
1991, versus post October 1, 1991.  However, the number of ground-
water samples in the methods comparison study was fairly small.

As a result of the described situation, projects should recognize
the following limitations in using historical (prior to
October 1, 1991) USGS phosphorus data:

Total Phosphorus in Surface-Water Samples

1. Total phosphorus data produced prior to October 1, 1991, tend
to be biased low.  This is especially true for samples having
high concentrations of particulate phosphorus, suspended
sediment, and organic carbon.  It is likely that the negative
bias is larger in the pre-May 1, 1990 data than in the data
produced between May 1, 1990 through September 30, 1991.  There
is no general way to make scientifically defensible corrections
to the data for either period.

2. Because the noted bias increases in samples having high
concentrations of particulate phosphorus, suspended sediment,
and organic carbon, estimates of annual loads of total
phosphorus based on data produced prior to May 1, 1990, are
likely to have a sizable negative bias.  Negative bias is also
likely in loads estimated from total phosphorus data produced
from May 1, 1990 through September 30, 1991.

3. Interpretations of total phosphorus data across the time
boundaries of May 1, 1990 and October 1, 1991 should be
avoided.  In the case of trend testing for concentrations,
or worse, for estimated total annual loads, artificial upward
trends could result because of the negative bias in total
phosphorus measurements prior to May 1, 1990, and through
September 30, 1991.

4. During statistical analysis, individual projects may be able to
adjust for the bias in total phosphorus data, provided suitable
historic split-sample data are available.  The scientific
integrity of bias-adjusted statistical estimates depends on the
availability of split-sample data that are representative of
the geohydrologic conditions of project sites.  Ideally, split-
sample data should exist for: (a) the observed ranges of
discharges and the concentrations of particulate phosphorus,
suspended sediment, and organic carbon; (b) the time period of
interest; and (c) the sites of interest.  In situations where
site-specific data are not available, but geohydrologic
characteristics are similar to those of the Illinois case
study, the Illinois data may be useful for making approximate
adjustments of bias for the pre-May 1990 period.  However,
large errors may be associated with such bias adjustments.
Under no condition should bias adjustments be made to the
individual data values in computer storage or in hard copy.
The Branch of Systems Analysis may be contacted with questions
about the use of statistical methods for bias adjustment.

Dissolved Phosphorus in Surface-Water Samples

5.  The same cautions cited in items 1,2, and 3 for total
phosphorus generally apply to dissolved phosphorus.

6. As for total phosphorus, corrections of the bias in dissolved
phosphorus data can be made for individual projects that
possess the proper split sample data for the sites and time
period in question (see item 4 above).

Total and Dissolved Phosphorus in Ground-Water Samples.

7. Despite uncertainties, we believe that historic USGS phosphorus
data for ground water have little or no negative bias.  This
belief is based on : (a) the limited results in Table 5, and
(b) the fact that most ground-water samples have low
concentrations of suspended sediment and relatively low
(compared to surface waters) concentrations of organic carbon.


This memorandum underlines the need for: (a) rigorous documen-
tation and review of new methods and SOPs at the NWQL, and (b) use
of field QC samples in all water-quality programs and projects.

In 1990, the Division instituted a revised review process for
implementing new analytical methods at the NWQL which requires:
1.  Documentation of new methods in an Open-File Report,
2.  Review of the manuscript by at least one analyst from another
    agency, and
3.  Sign off of new methods by the Chief, Office of Water Quality.

In addition, the Inorganics Program at the NWQL has established an
approach for implementation of new SOPs that includes:
documentation of plans; request for supervisor approval; review by
supervisors; decision whether the change should be made or not;
decision whether the change is major enough to warrant comparison
testing; and record keeping on the chronology of changes.  The
Organics Program will soon implement a similar approach.

Regarding field QC, the present system of Standard Reference Water
Samples and Blind Samples did not detect the type of methodolog-
ical error (reagent limitation) which produced low total
phosphorus results, most notable on samples having high suspended
sediment concentrations.  Instead, as noted, the low total
phosphorus results and, subsequently, the error in method, were
detected by statistical interpretation of data from split samples
generated when aliquots of the same field samples were analyzed by
another laboratory.  Available Standard Reference Water Samples
for nutrients do not contain solids because reference samples must
be stable for long time periods, and inclusion of solids tends to
destabilize the nutrients, thereby significantly reducing shelf

The Office of Water Quality is preparing QC guidelines to define the
minimal percentage of various types of QC samples (for example,
replicate samples, split samples, blanks, spikes, etc.) to include
in projects for different combinations of objectives, sample
matrices (water, sediment, tissue), and classes of chemical
constituents.  The intent is for projects to produce enough QC data
to determine the quality of their environmental data and to identify
and correct problems quickly.  In addition, the Branch of Quality
Assurance (BQA) is developing the capability to synthesize and
interpret QC data from all Division programs to identify and resolve
problems that may exist, but can not be detected by individual
programs or projects reviewing their own data.

Details on: (a) the QC guidelines, and (b) Division-wide analysis of
QC data will be provided in future Tech Memos from the OWQ and BQA.


Bowman, G.T., and Delfino, J.J., 1982, Determination of total
Kjeldahl nitrogen and total phosphorus in surface waters and
wastewaters: Journal of the Water Polution Control
Federation, v. 54, p. 1324-1330.

Jirka, A.M., Carter, M.J., May, Dorothy, and Fuller, F.D., 1976,
Ultramicro semiautomated method for simultaneous determi-
nation of total phosphorus and total Kjeldahl nitrogen in
wastewaters: Environmental Science and Technology, v. 10,
n. 10, p. 1038-1044.

Murphy, J., and Riley, J.T., 1962, A modified single solution
method for the determination of phosphate in natural waters:
Analytica Chimica Acta, v. 27, p. 31-36.

                                  David A. Rickert
                                  Chief, Office of Water Quality


Key Words:  Methods, National Water Quality Laboratory,

This memorandum does not supersede any previous Office of Water
Quality Technical Memorandum.

Distribution:  A, B, S, FO, PO

Table 1. Wilcoxon Signed Rank test results for seven stream sites comparing
differences in total phosphorus (P) measurements by Illinois EPA (IEPA)
method 365.1 and USGS method I-2600/I-4600 (without the dilution SOP)

                USGS                    Number     total P
IEPA            suspended    IEPA       of         (IEPA-USGS)
(mg/L)          (mg/L)      (mg/L)      samples      (mg/L)      p

0.05 - 2.00                             147         0.010    <0.001*
0.05 - 0.20                              42         0.010     0.255
0.20 - 0.30                              30         0.010     0.021*
0.30 - 0.40                              34         0.030     0.044*
0.40 - 2.00                              42         0.020     0.015*
                10 - 4,000              126         0.010    <0.001*
                10 - 25                  28         0.010     0.577
                25 - 50                  27         0.005     0.476
                50 - 75                  24         0.010     0.251
                75 - 100                  9         0.038     0.028*
               100 - 4,000               36         0.061     0.004*
                              3 - 25     77         0.010     0.006*
                              3 - 7      38         0.010     0.171
                              7 - 25     39         0.020     0.015*
                             10 - 25     15         0.090     0.054
0.20 - 2.00    100 - 4,000               29         0.090     0.003*
0.30 - 2.00    100 - 4,000               20         0.180     0.005*
* Significant for alpha=0.05

Table 2. Station Wilcoxon Signed Rank test results for seven stream sites
comparing differences in total phosphorus measurements by IEPA method 365.1
USGS method I-2600/I-4600 (without the dilution SOP)

                                   Median                    Median
                          Median   USGS       Median         total P
                  Number  IEPA     suspended  IEPA           difference
Station Name      of      total P  sediment   TOC     DP/TP* (IEPA-USGS)
(USGS ID #)       samples (mg/L)   (mg/L)     (mg/L)    %     (mg/L)      p

Little Wabash R
(3381495)            21    0.27      74        12     40       0.020
Rock River
(5446500)            14    0.25      67         -     41       0.010
Spoon River
(5570000)            12    0.14      86         -     43       0.017
Embarras River
(3345500)            17    0.17     138         5     50       0.020    0.103
Big Muddy River
(5599500)            21    0.25      68         -     39       0.010    0.322
Illinois River
(5543500)            43    0.44      26         7     75       0.000    0.285
Sangamon River
(5583000)            19    0.31      43         -     74       0.000    0.776
* Ratio of IEPA dissolved phosphorus to total phosphorus
** Significant for alpha=0.05

Table 3. Wilcoxon Signed Rank test results for seven stream sites
comparing differences in dissolved phosphorus measurements by
IEPA method 365.1 and USGS method I-2600/I-4600 (without the
dilution SOP)

                                dissolved P
IEPA dissolved P    Number      (IEPA-USGS)
 (mg/L)           of samples      (mg/L)        p

 0.01 - 1.30         147         0.000        0.532
 0.01 - 0.10          46        -0.005        0.157
 0.10 - 0.20          38         0.005        0.075
 0.20 - 0.30          25         0.000        0.093
 0.30 - 1.30          26        -0.015        0.253

Table 4.  Wilcoxon signed rank results for surface-water
samples comparing differences in phosphorus measurements
by USGS methods I-2610/I-4610 and I-2600/I-4600 (with the
dilution SOP)

                                 Median total P
I-2600/I-4600      Number        (I-2610/I-4610-
total P              of          I-2600/I-4600)
(mg/L)             samples           (mg/L)            p

April samples

0.00 - 3.00         319              0.020         0.000*
0.00 - 0.15         209              0.013         0.000*
0.15 - 0.30          64              0.033         0.000*
0.30 - 0.50          24              0.044         0.006*
0.50 - 3.00          25              0.056         0.011*

July-September Samples

0.00 - 4.50         616              0.020         0.000*
0.00 - 0.15         358              0.003         0.000*
0.15 - 0.30         108              0.031         0.000*
0.30 - 0.50          63              0.048         0.000*
0.50 - 0.70          21              0.064         0.000*
0.70 - 1.00          27              0.080         0.000*
1.00 - 2.00          27              0.151         0.000*
2.00 - 4.50          18              0.301         0.000*
                                Median dissolved P
I-2600/I-4600      Number       (I-2610/I-4610-
dissolved P          of         I-2600/I-4600)
(mg/L)             samples          (mg/L)            p

April samples

0.00 - 0.93          46              0.012         0.001*
0.00 - 0.21          40              0.011         0.002*
0.22 - 0.93           6              0.039         0.529

July-September Samples

0.00 - 4.00         506              0.006         0.000*
0.00 - 0.15         387              0.001         0.146
0.15 - 0.30          48              0.022         0.000*
0.30 - 0.50          21              0.043         0.000*
0.50 - 0.70          12              0.067         0.038*
0.70 - 1.00          12              0.090         0.038*
1.00 - 2.00          16              0.150         0.001*
2.00 - 4.00          16              0.208         0.001*
* Significant for alpha = 0.05.

Table 5.  Wilcoxon signed rank test results for ground-water
samples comparing differences in phosphorus measurements by
USGS methods I-2610/I-4610 and I-2600/I-4600 (with the
dilution SOP)

                               Median total P
I-2600/I-4600      Number      (I-2610/I-4610-
total P              of        I-2600/I-4600)
(mg/L)             samples         (mg/L)            p

April Samples

0.00 - 3.25         20             0.003           0.184
0.00 - 0.20         19             0.004           0.062

July-September Samples

0.00 - 1.65         43            -0.003           0.169
0.00 - 0.23         40            -0.005           0.017*

                               Median dissolved P
I-2600/I-4600      Number      (I-2610/I-4610-
dissolved P          of        I-2600/I-4600)
(mg/L)             samples         (mg/L)            p

April Samples

0.00 - 0.70         32             0.008           0.030*
0.00 - 0.21         30             0.008           0.060

July-September Samples

0.00 - 0.34         18            -0.001           0.918
* Significant for alpha = 0.05