TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

Appendix B-7: Distribution Statistics for Streams and Rivers Using 

U.S. EPA's Nutrient Watershed Regions 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

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Appendix B-7. Distribution Statistics for Streams and Rivers Using U.S. 

EPA's Nutrient Watershed Regions 

The United States Environmental Protection Agency’s (U.S. EPA) Nutrient Criteria Technical 

Guidance Manual: Rivers and Streams (USEPA 2000) describes several general approaches, 

with certain options, that can be used to develop numeric nutrient criteria for streams. One line of 

evidence, referred to as the reference condition approach, for developing candidate endpoints is 

to evaluate distributional statistics from benchmark sites, typically using the higher end of the 

distribution (e.g., 75th percentile, 90th percentile). Another line of evidence is to evaluate data 

from all sites, not simply benchmark sites, where it is most common to emphasize the lower end 

of the distribution (e.g., 25th percentile). For further information and justification for the use of 

the reference condition approach, see the introduction of Appendix B6. 

This appendix documents the U.S. EPA’s analyses of benchmark sites and all stations using U.S. 

EPA nutrient regions1

. Although the 25th and 75th percentiles were the recommended and most 

commonly considered percentiles in U.S. EPA guidance, numerous percentiles are presented in 

this document for consideration as they might be used for other purposes. This appendix also 

provides a comparison of dissolved oxygen (DO) data to 3 mg/L and 5 mg/L. 

Data Processing Steps 

The overall approach was to generally replicate the approach that had been implemented by the 

Florida Department of Environmental Protection (FDEP) in previous analyses used for 

evaluating benchmark sites including data aggregation, computational approaches, and the like. 

 FDEP provided an update to the All Streams data to EPA. The updated data addressed an 

error with DO data (the previous DO analysis found that DO % saturation was mixed with DO 

concentration data). That data set also includes sample time and temperature. This file 

contains 270,024 records for 8,614 sites—many of the additional records are associated with 

water temperature samples with no corresponding nutrient data. FDEP also provided a 

supplemental data set for agency 21FLGW that included 16,971 records. 

 Under the analyses described in Appendix B6, each site had been attributed with a 

corresponding waterbody ID (WBID). In a related analysis, U.S. EPA developed a revised set 

of nutrient regions, corresponding more consistently to drainage basins and assigned each 

WBID to an U.S. EPA nutrient region. With this information, each site was attributed to an 

U.S. EPA nutrient region. 

 Total nitrogen (TN) was computed as the sum of nitrite (NO2) plus nitrate (NO3) and Total 

Kjeldahl Nitrogen (TKN) only when both component parameters were reported. Chl. a data 

are for corrected chl. a. 

 

1

 U.S. EPA conducted a statistical distribution analysis for 3 Nutrient Watershed Regions (NWRs - Panhandle, Peninsula, and 

Bone Valley). Since that analysis, U.S. EPA identified a fourth NWR (North Central) and is proposing a regionalization scheme 

reflecting those 4 NWRs. A statistical distribution analysis for the 4 NWRs would reflect the same approach taken for the 3 

NWRs. 
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 One of the FDEP outlier analysis steps for benchmark sites was to remove observations 

greater than the upper adjacent value.2 This approach was abandoned in lieu of a simple 

screening threshold to remove erroneous data. The screening thresholds of 15 mg/L total 

phosphorus (TP), 100 mg/L TN, 1,000 mg/L chlorophyll, and 20 mg/L DO were selected. A 

total of 115 samples were removed from further consideration. Further justification of this 

screening approach is provided in Appendix B6. 

 Upon filtering the data for sites within one of the U.S. EPA nutrient regions and having data 

of interest (i.e., TN, TP, DO, or chlorophyll), 89,672 samples remained. The DO results from 

these samples are used in the Dissolved Oxygen Comparison to 3 mg/L and 5 mg/L section of 

this appendix. 

 Data were organized by Region-WBID-Year and observation counts were made in a manner 

identical to that used by FDEP’s analysis of benchmark sites. That is, whenever the TP 

observation count for a particular Region-WBID-Year was less than four, all TP observations 

for that Region-WBID-Year were dropped. A similar process for TN, DO, and chlorophyll 

was implemented. 

 TP, TN, and chlorophyll parameters were transformed by taking the natural logarithm (e.g., 

LN(TP), LN(TN)). Log transformation converts data that are positively skewed to more 

symmetrically (closer to normally) distributed data. DO data from benchmark sites were 

reviewed for distribution. It was found that DO data did not follow a log-normal distribution, 

and were therefore not transformed.3

 The mean of transformed TN, TP, and chlorophyll data and raw DO data were computed for 

each Region-WBID-Year. The values resulting from these data processing steps are used in 

the distribution statistics portion of this report. 

DO Comparison to 3 mg/L and 5 mg/L 

Using the raw DO data, the number of samples with observations less than 3 mg/L and 5 mg/L 

were computed. Also, the number of WBIDs with at least one observation less than 3 mg/L and 

5 mg/L were computed. Table B7-1 presents the results for benchmark and non-benchmark sites, 

organized by U.S. EPA nutrient region. Figures B7-1 present B7-2 these results graphically. The 

following observations are made: 

 At benchmark sites, from 20% to 28% of DO observations were less than 5 mg/L by U.S. 

EPA nutrient region. Overall, 25% of benchmark DO observations were less than 5 mg/L. At 

non-benchmark sites, 16% of Panhandle observations were less than 5 mg/L, while the other 

two regions ranged from 38% to 39%. Overall, 36% of non-benchmark DO observations were 

less than 5 mg/L. 

 At benchmark sites, from 4% to 11% of DO observations were less than 3 mg/L by U.S. EPA 

nutrient region. Overall, 8% of benchmark DO observations were less than 3 mg/L. At nonbenchmark sites, 6% of Panhandle observations were less than 3 mg/L while the other two 

 

2

 The upper adjacent value is defined as the largest observation that is less than or equal to the upper quartile plus 1.5 x 

Interquartile Range. 

3

 The raw DO data at benchmark sites had an Anderson-Darling test statistic of 20.3 while the natural logarithm transformed data

had an Anderson-Darling test statistic of 431.2. The data were not normally or log-normally distributed; however, the raw data 

deviated less; thus the DO data were processed without transformation. 
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regions ranged from 16% to 17%. Overall, 15% of non-benchmark DO observations were less 

than 3 mg/L. 

 At benchmark WBIDs, 100%, 61%, and 95% of WBIDs from the Bone Valley, Panhandle, 

and Peninsula respectively had at least one observation less than 5 mg/L, overall 82% of 

benchmark WBIDs had at least one observation less than 5 mg/L. At non-benchmark WBIDs, 

83%, 43%, and 86% of WBIDs from the Bone Valley, Panhandle, and Peninsula respectively 

had at least one observation less than 5 mg/L, overall 74% of non-benchmark WBIDs had at 

least one observation less than 5 mg/L. 

 At benchmark WBIDs, 60%, 35%, and 78% of WBIDs from the Bone Valley, Panhandle, and 

Peninsula respectively had at least one observation less than 3 mg/L, overall 61% of 

benchmark WBIDs had at least one observation less than 3 mg/L. At non-benchmark WBIDs, 

67%, 26%, and 68% of WBIDs from the Bone Valley, Panhandle, and Peninsula respectively 

had at least one observation less than 3 mg/L, overall 57% of non-benchmark WBIDs had at 

least one observation less than 3 mg/L. 

Distribution Statistics 

Using the Region-WBID-Year mean of the natural logarithm transformed TP, TN, and 

chlorophyll data and the Region-WBID-Year mean of the raw DO that were computed as 

described in the Data Processing Steps section of this document, the mean and standard 

deviation were estimated by region and across all regions. These summary statistics were used to 

compute theoretical distributions assuming a lognormal distribution for TP, TN, and chlorophyll 

and a normal distribution for DO (see Tables B7-2 through B7-5). These distributions were 

plotted in Figures B7-3 through B7-6. The following results were observed: 

Total Phosphorus 

 At all sites, the 25th percentile TP for the Bone Valley, Panhandle, and Peninsula nutrient 

regions was 0.174 mg/L, 0.018 mg/L, and 0.068 mg/L. Across all regions, the 25th percentile 

TP at all sites was 0.059 mg/L. 

 At benchmark sites, the 75th percentile TP for the Bone Valley, Panhandle, and Peninsula 

nutrient regions was 0.643 mg/L, 0.063 mg/L, and 0.178 mg/L. Across all regions, the 75th

percentile TP at benchmark sites was 0.149 mg/L. 

Total Nitrogen 

 At all sites, the 25th percentile TN for the Bone Valley, Panhandle, and Peninsula nutrient 

regions was 0.97 mg/L, 0.45 mg/L, and 0.88 mg/L. Across all regions, the 25th percentile TN 

at all sites was 0.79 mg/L. 

 At benchmark sites, the 75th percentile TN for the Bone Valley, Panhandle, and Peninsula 

nutrient regions was 1.20 mg/L, 0.76 mg/L, and 1.36 mg/L. Across all regions, the 75th

percentile TN at benchmark sites was 1.19 mg/L. 
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Chlorophyll 

 At all sites, the 25th percentile chlorophyll for the Bone Valley, Panhandle, and Peninsula 

nutrient regions was 0.98 μg/L, 0.60 μg/L, and 1.08 μg/L. Across all regions, the 25th

percentile chlorophyll at all sites was 0.92 μg/L. 

 At benchmark sites, the 75th percentile chlorophyll for the Bone Valley, Panhandle, and 

Peninsula nutrient regions was 1.78 μg/L, 1.33 μg/L, and 1.52 μg/L. Across all regions, the 

75th percentile chlorophyll at benchmark sites was 1.47 μg/L. 

Dissolved Oxygen 

 At all sites, the 25th percentile DO for the Bone Valley, Panhandle, and Peninsula nutrient 

regions was 4.55 mg/L, 5.54 mg/L, and 4.41 mg/L. Across all regions, the 25th percentile DO 

at all sites was 4.56 mg/L. 

 At benchmark sites, the 75th percentile DO for the Bone Valley, Panhandle, and Peninsula 

nutrient regions was 7.19 mg/L, 7.75 mg/L, and 6.89 mg/L. Across all regions, the 75th

percentile DO at benchmark sites was 7.19 mg/L. 

Quality Control Check 

Raw data from benchmark sites in the Bone Valley were extracted from the database. 

Distribution statistics for the data were computed using a separate procedure and found to be 

consistent with those computed from the primary algorithms used to prepare the results presented 

in this document. 
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Table B7-1. Number of DO Observations Less than 3 mg/L or 5 mg/L and WBIDs with at Least 

One Observation Less than 3 or 5 mg/L, organized by U.S. EPA Nutrient Region 

Benchmark Sites/WBIDs 

DO Observations WBIDs 

U.S. EPA 

Nutrient 

Region Total 

Observations 

Observations 

< 5 mg/L 

% 

Observations 

< 5 mg/L 

Total 

WBIDs 

WBIDs with 

one observ. 

< 5 mg/L 

% WBIDs 

with one 

observ. < 5 

mg/L 

Bone Valley 294 62 21% 5 5 100% 

Panhandle 3,712 746 20% 51 31 61% 

Peninsula 5,375 1,527 28% 76 72 95% 

Total 9,381 2,335 25% 132 108 82% 

Non-benchmark Sites/WBIDs 

DO Observations WBIDs 

U.S. EPA 

Nutrient 

Region 

Total 

Observations 

Observations 

< 5 mg/L 

% 

Observations 

< 5 mg/L 

Total 

WBIDs 

WBIDs with 

one observ. 

< 5 mg/L 

% WBIDs 

with one 

observ. < 5 

mg/L 

Bone Valley 13,222 5,184 39% 332 275 83% 

Panhandle 8,105 1,301 16% 424 181 43% 

Peninsula 47,989 18,283 38% 844 722 86% 

Total 69,316 24,768 36% 1,600 1,178 74% 

Benchmark Sites/WBIDs 

DO Observations WBIDs 

U.S. EPA 

Nutrient 

Region 

Total 

Observations 

Observations 

< 3 mg/L 

% 

Observations 

< 3 mg/L 

Total 

WBIDs 

WBIDs with 

one observ. 

< 3 mg/L 

% WBIDs 

with one 

observ. < 3 

mg/L 

Bone Valley 294 13 4% 5 3 60% 

Panhandle 3,712 151 4% 51 18 35% 

Peninsula 5,375 597 11% 76 59 78% 

Total 9,381 761 8% 132 80 61% 

Non-benchmark Sites/WBIDs 

DO Observations WBIDs 

U.S. EPA 

Nutrient 

Region 

Total 

Observations 

Observations 

< 3 mg/L 

% 

Observations 

< 3 mg/L 

Total 

WBIDs 

WBIDs with 

one observ. 

< 3 mg/L 

% WBIDs 

with one 

observ. < 3 

mg/L 

Bone Valley 13,222 2,237 17% 332 223 67% 

Panhandle 8,105 470 6% 424 112 26% 

Peninsula 47,989 7,512 16% 844 578 68% 

Total 69,316 10,219 15% 1,600 913 57% 
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Table B7-2. Distributional Statistics Computed from the Natural Logarithm Transformed TP Data 

Observations Distribution Statistics 

Estimated from 

Natural Logtransformed Data Percentiles Computed from Mean a

Deviation Back Transformed to Re

Parameter 

U.S. EPA Nutrient Region 

Number of 

WBIDYears Number of WBIDs Mean Std Dev 10th %tile 25th %tile 50th %tile 75th %tile 

Bone Valley 739 194 -1.1240 0.9289 0.099 0.174 0.325 0.608 Panhandle 600 190 -3.2549 1.1335 0.009 0.018 0.039 0.083 Peninsula 2,915 634 -2.1235 0.8336 0.041 0.068 0.120 0.210 

TP at All Sites 

All Regions 4,254 1,018 -2.1095 1.0774 0.030 0.059 0.121 0.25

Bone Valley 21 5 -1.0155 0.8505 0.122 0.204 0.362 0.643 Panhandle 198 51 -3.3516 0.8715 0.011 0.019 0.035 0.063 Peninsula 407 75 -2.3196 0.8802 0.032 0.054 0.098 0.178 

TP at Benchmark Sites 

All Regions 626 131 -2.6023 1.0394 0.020 0.037 0.074 0.149 

Table B7-3. Distributional Statistics Computed from the Natural Logarithm Transformed TN Data 

Observations Distribution Statistics 

Estimated from 

Natural Logtransformed Data Percentiles Computed from Mean a

Deviation Back Transformed to Re

Parameter 

U.S. EPA Nutrient Region 

Number of 

WBIDYears Number of WBIDs Mean Std Dev 10th %tile 25th %tile 50th %tile 75th %tile 

Bone Valley 627 182 0.2534 0.4174 0.75 0.97 1.29 1.71 Panhandle 546 166 -0.3793 0.6328 0.30 0.45 0.68 1.05 Peninsula 2,684 593 0.1737 0.4406 0.68 0.88 1.19 1.60 

TN at All Sites 

All Regions 3,857 941 0.1084 0.5099 0.58 0.79 1.11 1.57 

Bone Valley 16 5 0.0489 0.1949 0.82 0.92 1.05 1.20 Panhandle 184 48 -0.6243 0.5185 0.28 0.38 0.54 0.76 Peninsula 372 74 0.0129 0.4355 0.58 0.76 1.01 1.36 

TN at Benchmark Sites 

All Regions 572 127 -0.1911 0.5474 0.41 0.57 0.83 1.19 
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Table B7-4. Distributional Statistics Computed from the Natural Logarithm Transformed Chlorophyll Data 

Observations Distribution Statistics 

Estimated from 

Natural Logtransformed Data Percentiles Computed from Mean a

Deviation Back Transformed to Re

Parameter 

U.S. EPA Nutrient Region 

Number of 

WBIDYears Number of WBIDs Mean Std Dev 10th %tile 25th %tile 50th %tile 75th %tile 

Bone Valley 466 170 0.7504 1.1498 0.49 0.98 2.12 4.60 Panhandle 458 172 0.0324 0.8101 0.37 0.60 1.03 1.78 Peninsula 1,792 517 0.9672 1.3194 0.48 1.08 2.63 6.41 

Chlorophyll at All Sites 

All Regions 2,716 859 0.7724 1.2660 0.43 0.92 2.16 5.08 

Bone Valley 14 4 0.1487 0.6343 0.51 0.76 1.16 1.78 Panhandle 123 34 -0.1972 0.7102 0.33 0.51 0.82 1.33 Peninsula 201 66 -0.1911 0.9088 0.26 0.45 0.83 1.52 

Chlorophyll at Benchmark Sites All Regions 338 104 -0.1792 0.8324 0.29 0.48 0.84 1.47 Table B7-5. Distributional Statistics Computed from the Untransformed DO Data 

Observations Distribution Statistics 

Estimated from 

Untransformed Data 

Percentiles Computed from Mean a

Deviation of Untransformed 

Parameter 

U.S. EPA Nutrient Region 

Number of 

WBIDYears Number of WBIDs Mean Std Dev 10th %tile 25th %tile 50th %tile 75th %tile 

Bone Valley 873 231 5.6296 1.5955 3.58 4.55 5.63 6.71 Panhandle 654 193 6.7056 1.7327 4.49 5.54 6.71 7.87 Peninsula 3,081 644 5.4887 1.6054 3.43 4.41 5.49 6.57 

DO at All Sites 

All Regions 4,608 1,068 5.6881 1.6747 3.54 4.56 5.69 6.82 

Bone Valley 22 5 6.2736 1.3584 4.53 5.36 6.27 7.19 Panhandle 212 46 6.6771 1.5850 4.65 5.61 6.68 7.75 Peninsula 443 74 5.9580 1.3744 4.20 5.03 5.96 6.89 

DO at Benchmark Sites 

All Regions 677 125 6.1935 1.4787 4.30 5.20 6.19 7.19 
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% Observations

Total

Pen

Pan

BV

<>BM

BM

<>BM

BM

<>BM

BM

<>BM

BM

0 10 20 30 40 0 10 20 30 40

< 3 mg/L < 5 mg/L

Figure B7-1. Percent of benchmark (BM) and non-benchmark (<>BM) DO observations less 

than 3 mg/L and 5 mg/L organized by U.S. EPA nutrient region. 

% WBIDs

Total

Pen

Pan

BV

<>BM

BM

<>BM

BM

<>BM

BM

<>BM

BM

0 10080604020 0 10080604020

< 3 mg/L < 5 mg/L

Figure B7-2. Percent of benchmark (BM) and non-benchmark (<>BM) WBIDs with at least one 

DO observation less than 3 mg/L and 5 mg/L organized by U.S. EPA nutrient region. 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

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0.0

0.3

0.6

0.9

1.2

0 10 20 30 40 50 60 70 80 90 100

PERCENTILES

TP (MG/L)

Bone Valley (All-Sites)

Panhandle (All-Sites)

Peninsula (All-Sites)

All Regions (All-Sites)

Bone Valley (Benchmark)

Panhandle (Benchmark)

Peninsula (Benchmark)

All Regions (Benchmark)

Figure B7-3. TP distribution statistics. 

0.0

1.0

2.0

3.0

0 10 20 30 40 50 60 70 80 90 100

PERCENTILES

TN (MG/L)

Bone Valley (All-Sites)

Panhandle (All-Sites)

Peninsula (All-Sites)

All Regions (All-Sites)

Bone Valley (Benchmark)

Panhandle (Benchmark)

Peninsula (Benchmark)

All Regions (Benchmark)

Figure B7-4. TN distribution statistics. 
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0.0

3.0

6.0

9.0

12.0

15.0

0 10 20 30 40 50 60 70 80 90 100

PERCENTILES

Chlorophyll (UG/L)

Bone Valley (All-Sites)

Panhandle (All-Sites)

Peninsula (All-Sites)

All Regions (All-Sites)

Bone Valley (Benchmark)

Panhandle (Benchmark)

Peninsula (Benchmark)

All Regions (Benchmark)

Figure B7-5. Chlorophyll distribution statistics. 

0.0

2.0

4.0

6.0

8.0

10.0

0 10 20 30 40 50 60 70 80 90 100

PERCENTILES

DO (MG/L)

Bone Valley (All-Sites)

Panhandle (All-Sites)

Peninsula (All-Sites)

All Regions (All-Sites)

Bone Valley (Benchmark)

Panhandle (Benchmark)

Peninsula (Benchmark)

All Regions (Benchmark)

Figure B7-6. DO distribution statistics.