Method Adjustment Analyses forVA DEQ Nutrient Determinations

Introduction• 1984 until 1994

TN calculated as TKNW + NO23F(NO2F + NO3F)TP measured directly (analyzed by DCLS).

• January 1994 TN calculated as TDN + PN (analyzed by VIMS).TP calculated as TDP + PP (analyzed by VIMS).

• February 1995 to the presentDCLS adopts VIMS methods for PC, PN, PP, and TDNDCLS uses EPA method 365.2 for TDP.

• Changes resulted in step trends in both parameters.

• Adversely affects statistical analyses.

Example – TN at Station LE5.4

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004

Year

To

tal

Nit

rog

en (

mg

/L)

Pre-Method Change

Post-Method Change

Methods• 33 Tidal Monitoring Stations (1998-2003).

• 46 Pfiesteria Monitoring stations (1998-2002).

• Focus on Tidal Monitoring data only but Pfiesteria Monitoring data also analyzed.

• For CBP TP, there was an average of 33 with a min. of 11 and max. of 42 samples per station.

• For CBP TN, there was an average of 28 samples per station with a min. of 10 and max. of 75 samples per station.

• Sample collection and processing reflect historical methods used except:

change in instrumentation.

Definition of Bias• New Method – Old Method = Difference Between

Methods.

• A negative value indicates Old Method is biased high relative to the New Method i.e. consistent with historical bias.

• A positive value indicates Old Method is biased low relative to the New Method in contrast with the historical bias.

Total Nitrogen

TN - Paired Comparisons

• Difference Between Methods significantly different from zero:

Student’s t= -26.66; Prob. >| t | <0.01,

Wilcoxon Signed Rank S=-500812; Pr >|S| <0.01.

TN - Screening Analyses

• Mean difference between methods: -0.320.52.• 75% of all observations at or below 0.0 mg/L.• Old Method biased high.• There were two distinct groups of values for the differences.

-6.00

-5.00

-4.00

-3.00

-2.00

-1.00

0.00

1.00

2.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50

Mean of Both Methods (mg/L)

Dif

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nce

Bet

wee

n M

eth

od

s (m

g/L

)+ 2 Std

- 2 Std

Mean Difference

TN – Screening Analyses• First group of differences

Mean: 0.00 mg/LRange: –0.50 to 0.80 mg/LRange constant regardless of mean conc.

• Second group of differencesMean: -1.00 mg/LRange: -0.50 to –2.00Difference decreases with increasing mean conc.

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

0.00 0.50 1.00 1.50 2.00 2.50

Mean of Both Methods (mg/L)

Dif

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e B

etw

ee

n M

eth

od

s (

mg

/L)

TN - Screening Analyses

0

50

100

150

200

250

300

Difference Between Methods (mg/L)

Fre

qu

ency

• Frequency histogram confirms two groups in bias.

TN – Source of Groups?

• Procedural Effects:

– Two Monitoring Programs

– Three DEQ Offices (TRO, PRO, NRO)

• Spatial Effects

• Temporal Effects

• Environmental Effects

• Combinations

TN – Monitoring Program Effect

Mean diff. = -0.41Positive Bias

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

0.00 0.50 1.00 1.50 2.00 2.50

Mean of Both Methods (mg/L)

Diff

eren

ce B

etw

een

Met

hods

(mg/

L)

Mean diff. = -0.22 Positive Bias

CBP

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

0.00 0.50 1.00 1.50 2.00 2.50

Mean of Both Methods (mg/L)

Diff

eren

ce B

etw

een

Met

hods

(mg/

L)

Pfiesteria

TN – Collection Office Effect (CBP)

• Was collection agency (PRO, TRO, NRO) responsible?

• Grouping persists in TRO data with Old Method biased high.

• Grouping disappears in PRO data but now Old Method biased low.

• Grouping disappears in NRO data but now Old Method biased low.

NRO

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

0.00 0.50 1.00 1.50 2.00 2.50

M ean of Both M ethods (mg/L)

Dif

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nce

Bet

wee

n M

eth

od

s (m

g/L

)

`

Mean diff. =0.080.18Old Method Biased Low

PRO

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

0.00 0.50 1.00 1.50 2.00 2.50

Mean of Both Methods (mg/L)

Dif

fere

nc

e B

etw

ee

n M

eth

od

s (

mg

/L)

`

Mean diff. =0.030.33Old Method Biased Low

TRO

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

0.00 0.50 1.00 1.50 2.00 2.50

Mean of Both Methods (mg/L)

Dif

fere

nc

e B

etw

ee

n M

eth

od

s (

mg

/L)

`

Mean diff. =-0.480.45Old Method Biased High

TN – Spatial Effects on Bias (CBP)

• Negative bias at most Tidal Fresh and Oligohaline stations.

• PRO and NRO responsible for collection at most of these stations.

• Positive bias at higher salinity (mostly TRO) stations.

TN – Spatial and Temporal Effects (CBP)

• Spatial pattern persists between years.

• Difference between methods higher in 2002 ( 0.00 mg/l).

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

Dif

fere

nc

e B

etw

ee

n M

eth

od

s (

mg

/L)

Y2002 Y2003

TN – Temporal Effects (CBP - TRO)

• Two groups observed appear to be two different time periods.

• Old Method is biased high for both time periods.

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

0.00 0.50 1.00 1.50 2.00 2.50

Mean of Both Methods (mg/L)

Dif

fere

nce B

etw

een

Meth

od

s (

mg

/L)

Year 1998-2002 Year 2003Mean diff. =-0.130.30Old Method Biased High

Mean diff. =-0.650.43Old Method Biased High

TN – Environmental Effects (CBP - TRO)

Date Month Salinity Salinity2 Temperature

Difference -0.71;<0.01 -0.11;0.02 -0.12;0.01 -0.26;<0.01 -0.20;<0.01

Depth pH CHL a TSS PC

Difference -0.08;0.07 -0.08;<0.09 -0.22;<0.01 0.16;<0.01 0.13;<0.01

Shown are Pearson’s |R| and associated p values. All correlations based on > 400 observations except CHL a (221).

• Several significant correlations but none explain patterns observed.

• Other potential predictors?

TN – Environmental Effects (CBP - TRO)

• Prior to 2003, difference was much higher even in high salinities.

• For 2003, values of difference at 0 salinities were higher.

• Significant but slight correlation with salinity (|R|=-0.12;p<0.01).

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

0.0 5.0 10.0 15.0 20.0 25.0 30.0

Salinity (mg/L)

Diff

eren

ce B

etw

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Met

hods

(mg/

L)

Year 2002 Year 2003

TN – Component Variables (CBP – TRO)

0

0.5

1

1.5

2

2.5

3

2002 2003 2004

TKNW

0

0.5

1

1.5

2

2.5

3

2002 2003 2004

TDN

0

0.5

1

1.5

2

2.5

3

2002 2003 2004

NO23F

0

0.5

1

1.5

2

2.5

3

2002 2003 2004

PN

TN – Component Variables (CBP - TRO)

• Plots of component variables indicate TKNW as problem.

• Plot of bias confirms this observation.

• Correlation analysis revealed no apparent environmental causes.

-1.6-1.4-1.2

-1-0.8-0.6-0.4-0.2

00.20.40.6

0 0.5 1 1.5 2 2.5

Mean of Both Methods (mg/L)

Dif

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s (

mg

/L)

TKNW < 0.8 TKNW >= 0.8

TN – Component Variables (All Data)

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

1998 1999 2000 2001 2002 2003 2004

Year

Dif

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s (m

g/L

)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

1998 1999 2000 2001 2002 2003 2004

Year

TK

NW

(m

g/L

l)

TN – Component Variables (CBP - TRO)

• When TKNW < 0.8 then Old Method bias is negative.

• When TKNW >=0.8 then Old Method bias is positive.

-3.00

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

1998 1999 2000 2001 2002 2003 2004

TKNW >=0.8 TKNW < 0.8

TKNW – Environmental Effects (All Data)

0

50

100

150

200

250

300

350

400

1998 1999 2000 2001 2002 2003 2004

Year

TS

S (

mg

/L)

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

1998 1999 2000 2001 2002 2003 2004

Year

PC

(m

g/L

)

0

20

40

60

80

100

120

140

160

180

1998 1999 2000 2001 2002 2003 2004

Year

CH

L a

(u

g/L

)

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

1998 1999 2000 2001 2002 2003 2004

Year

Sal

init

y (p

pt)

|R| = 0.25

|R| = 0.29

|R| = NS

|R| = 0.08

TN - Conclusions• Overall significant difference between methods with Old

Method biased high relative to the New Method.

• Two groups of differences in the data set.

• Two groups were not due to differences between monitoring programs or DEQ Offices.

• Old Method was biased low at most Tidal Fresh and Oligohaline stations and at low concentrations of TKNW.

• Old Method was biased high when concentrations of TKNW were high.

• No clear effect of environmental factors on results.

TN - Conclusions• Method correction development is not recommended

using these data

– Bias in Old Method low at Tidal Fresh/Oligohaline areas

– Bias is inconsistent in all other areas

– Bias changes in relation to TKNW

– No apparent environmental predictor of this change

– Changes in instrumentation are still at issue.

TN - Recommendations

• Use Blocked Seasonal Kendall until questions are answered.

• No other recommendations…

• Are there additional analytical approaches that might be useful for exploring these data?

• Are there any explanations for the patterns observed?

Total Phosphorus

TP - Paired Comparisons (CBP Only)

• Mean difference between methods significantly different from zero: Student’s t=8.00; Prob. >| t | <0.01 Wilcoxon Signed Rank S value =219432; Prob. >|S| <0.01

• Mean difference between methods = 0.020.035 mg/L.

• 75% of all differences at or above 0.00.

TP - Screening Analyses (CBP Only)

• Mean difference between methods: 0.020.035 mg/L.

• Old TP Method biased low relative to New TP Method up to mg/L.

• This conflicts with historical pattern in the data.

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Mean of Both Methods (mg/L)

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s (m

g/L

)

+ 2 Std

- 2 Std

Mean Difference

TP - Conclusions• Significant difference between methods but Old

Method biased low relative to the New Method.

• Opposite of pattern in historical data.

• Spatial, temporal or environmental effects do not explain the difference between current and historical bias.

• Difference may be due to a change in instrumentation and/or other procedures.

• Current bias may be due to difference in accuracy between methods at high levels of TSS.

TP – Recommendations

• Data are not conducive to method adjustment.

• Blocked Seasonal Kendall for TP until additional studies are available.

• Another paired study?– Use old instrumentation or more trouble than it

is worth?– Do we control for season or not?– Do we control locations, TSS, salinity or not?– Are there other effects?

CSSP Data

• Collected at Three Stations

– Two Tidal Fresh stations PMS10 and Wilcox Wharf– One Mesohaline station CB4.4.

• May ’95 through June ’04.

• 328 total observations.

• Surface measurements only.

CSSP TP Data – Screening

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18

Mean of Both Methods (mg/L)

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)

+ 2 STD

- 2 STD

Mean

• Mean difference between methodssignificantly different from zero:

Student’s t: t value =3.79; Prob. >| t |=0.0002

Wilcoxon Signed Rank:S value =-4076; Prob. >|S|=0.0002

• Mean difference between methods = - 0.010.036 mg/L.

• 70% of all differences at or below 0.00 with 25% = 0.00.

• No significant effect on results by removing outliers.

• Data appear to be comparable with slight positive bias for Old Method.

CSSP TP Data – Station PMS10 Only• Mean difference between methods

NOT significantly different from zero:

Student’s t: t value =-0.968; Prob. >| t |=0.3367

Wilcoxon Signed Rank:S value =119; Prob. >|S|=0.1741

• Mean difference between methods = - 0.010.064 mg/L.

• Bias of Old Method positive.

• 70% of all differences at or above 0.00 with 29% = 0.00.

• Three probable outliers.

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18

Mean of Both Methods (mg/L)

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+ 2 STD

Mean

- 2 STD

CSSP TP Data – Station PMS10 Only

• Remove outliers and there is a significant difference:

Student’s t: t value =2.562; Prob. >| t |=0.013

Wilcoxon Signed Rank:S value =264; Prob. >|S|=0.0180

• Mean Difference between methods= 0.010.015 mg/L.

• With outliers removed, bias for Old Method is negative.

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

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0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18

Mean of Both Methods (mg/L)

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+ 2 STD

Mean

- 2 STD

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

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0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18

Mean of Both Methods (mg/L)

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CSSP TP Data – Station CB4.4 Only

• Mean difference between methods significantly different from zero:

Student’s t: t value =2.562; Prob. >| t |=0.013

Wilcoxon Signed Rank:S value =264; Prob. >|S|=0.0180

• Mean Difference between methods= -0.010.025 mg/L.

• No significant effect on results by removing outliers.

+ 2 STD

Mean

- 2 STD

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

Mean of Both Methods (mg/L)

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)

CSSP TP Data – Wilcox Wharf

• Mean difference was significantly difference from zero:

Student’s t: t value =-3.44; Prob. >| t |=0.0013

Wilcoxon Signed Rank:S value =261; Prob. >|S|=0.0015

• Mean Difference between methods= -0.010.021 mg/L.

CSSP - Conclusions• Data indicate a slight positive bias in Old Method.

• Bias was consistently positive at all stations except at PMS10 when outliers were removed.

• Bias was very small (–0.01 mg/L) and does not change in relation to concentration.

• Data are not compatible with a method correction.

DCLS Instrument Comparison Studies

• Data were part of a larger study comparing 8 variables between the Technicon AA and Skalar AA:

– 94 total phosphorus samples– 176 PO4F samples.

• Documentation indicates:– Study Conducted in the Summer of 1997 – No information on collection locations.

DCLS Instrument Comparison for TP

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Mean of Both Methods (mg/L)

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• Mean difference was NOT significantly different from zero:Student’s t: t value =-1.58; Prob. >| t |=0.1173Wilcoxon Signed Rank:S value =196.5; Prob. >|S|=0.0155.

• Mean difference =0.000.01 with most values around 0.00 mg/L.

• No bias for the Old Method.

DCLS Instrument Comparison for PO4F

• Mean difference between methodssignificantly different from zero:

Student’s t: t value =-11.58; Prob. >| t | <0.0001

Wilcoxon Signed Rank:S value =-6341; Prob. >|S| <0.0001.

• Mean difference was -0.010.01 mg/L

• 83% percent of all observations were at or below zero.

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Mean of Both Methods (mg/L)

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