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



## Appendix B-10: Analysis of Land Disturbance Index Computation Approach for FDEP’s Benchmark WBIDsTSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-1 

### Appendix B-10. Analysis of Land Dist urbance Index Computation Approach for FDEP’s Benchmark WBIDs Florida Department of Environmental Protectio n’s (FDEP) approach for computing criteria on the basis of benchmark sites relies on scr eening sites (and waterbody IDs (WBID)) using the Stream Corridor Land Disturbance Index (LDI). The Stream Corridor LDI computed by FDEP is based on tabulating land use with in a 100 meter buffer surrounding al l streams upstream of a site for 10 kilometers, tracing all stream paths. Such tabulations are used to create an area-weighted LDI for the site (and WBID). Sampling locatio ns (sites) were aggregated by FDEP into waterbodies, identified by a uni que WBID. Sites with sufficiently low LDI scores (< 2.0) were identified as potential sites for inclusion into a benchmark WBID. The purpos e of this analysis is to determine, for the 135 nutrient benchmark WB IDs identified by FDEP , whether alternative land inclusion definitions for computing an area -weighted LDI explain inter-WBID variability in total nitrogen (TN) and total phosphorus (TP), sp ecifically high nutrient va lues. This analysis does not consider the general applicab ility of the Stream Corridor LDI. For this analysis, two alternate land inclusion rules were develope d. These rules are listed below.
 Drainage basin LDI. LDI computed from all dr ainage areas within Florida upstream of the most downstream site within a WBID. 4
 Near-field watershed LDI. LDI computed from all drainage areas (at the 12-digit HUC level) in Florida within a 10 kilomete r straight-line radius to the most downstream site within a WBID. 

### Geographic Information System Anal ysis and Quality Control Checks The drainage basin rule includes the entire upland watershed but was necessarily limited to lands in Florida because there were not similarly classified land cover data from Georgia and Alabama. Thus, there is a potential that si gnificant portions of the upland watershed were excluded from the analysis. The near-field watershed rule wa s focused on the closer portion of the watershed and had less influence from differe nces in land cover data availabi lity. Florida’s stream corridor LDI was originally calculated using land cove r data from 1995. Since FDEP’s original estimation, 2004 land use data became available. To address the issue of whether there was an effect of using the 1995 land use for computing th e Stream Corridor LDI versus more recent land cover data, LDIs were computed using both the 1995 and 2004 land cover data. Although more recent land cover data were available for some wa ter management districts, land cover data from 2004 were chosen since these data were availabl e statewide. The following provides an overview of the GIS computation steps: 1. Land use data as stored by FDEP or indi vidual Water Management Districts, Florida Digital Elevation Model (DEM) data, Nati onal Hydrography Dataset (NHD), watershed (12-digit HUC) data, and Florida’s WBID da ta were downloaded and integrated into a geographic information system (GIS) environment.

4 Because of WBID hydrology, two WBIDs (2120A_ and 3489_) did not have a most downstream site—thus two sites in each of these WBIDs were maintain ed in this analysis.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-2 2. The most downstream site within a WBID was se lected visually on the basis of the list of benchmark sites and WBIDs provided by FDEP staff. 3. The downstream site was snapped to the nearest NHD stream segment and a manually delineated watershed was created by vi sual inspection of the DEM data. 4. Using available GIS tools, the near-field wa tershed and drainage basin boundaries were created in a manner consistent with the above definitions. 5. Land use was tabulated for each watershed a nd made available for post processing to compute the LDI. The GIS work was distributed amon g contractor and FDEP staff. FDEP staff processed most of the sites in the St. John’s River Water Management District, while contract or staff processed the remainder. Inherent analyst- to-analyst differences in ma nual watershed delineation can contribute to differences in resu lts. Also, FDEP staff had more direct knowledge of sites that would tend to lead to more informed delinea tions. Additional differences include FDEP’s processing of land cover data in vector format vers us contractor processing of land cover data in raster format. To verify work between the an alysts, some sites were processed by both the contractor and FDEP. Quality control steps were used to verify the comparability of the work between analysts and utility of the combined work product (see Table B10-1 and Figure B10-1). The results of the quality control checks indicat e that land cover results from FDEP and the contractor can be combined and used for the present purpose. 

### Relationship of Alternative LD I Approaches to TN and TP The LDIs computed from the above approach were merged with TN and TP summary data which had been computed as the geometric mean of benchmark WBID-Years

. Thus, the initially selected set of 135 WBID LDIs includes multiple WBID-Year valu es computed for TN and TP. The remainder of the discussion in this section is drawn from observati ons taken from Figures B10-2 through B10-12. Near-field watershed and drainage basin LDIs using 1995 and 2004 land use data were plotted as a function of Stream Corridor LDI (Figure B 10-2). Of the 1995 near-f ield watershed and drainage area LDIs, 63% and 61% were less than two, respectively. They decreased to 55% and 53% for the 2004 land cover data, re spectively. More than 92% of a ll computed LDIs were less than three. There is a general increasing trend in each panel, and the correlation coefficients ranged from 0.39 for the 2004 drainage area LDI to 0.52 for the 1995 and 2004 near-field LDIs. TP and TN were plotted as a function of St ream Corridor LDI (Figure B10-3). The most noticeable high concentrations associated with high LDI values were TP concentrations for WBID 1653_. This graphic provid es evidence that WBID 1653_ s hould not be included in the benchmark site distribution, especially given the high LDI values for this site. The same TP and TN data were plotted after removing WBID 1653_ (Figure B10-4). Figures B10-5 through B10-8 do not include data from WBID 1653_. TN and TP were plotted as a f unction of alternative LDI approach es (Figures B10-5 and B10-6). TN and TP were positively correl ated (significance level > 95%) w ith each of the LDIs shown inTSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-3 Figures B10-4, B10-5, and B10-6. Correlation coefficients are lis ted below. TN and TP were most strongly correlated with the near field LDI. TN TP Stream Corridor LDI 0.121 0.343 Near-field LDI (1995) 0.264 0.392 Drainage Area LDI (1995) 0.194 0.241 Near-field LDI (2004) 0.224 0.353 Drainage Area LDI (2004) 0.159 0.224 From that analysis, which was developed to expl ore potential explanations for elevated nutrients among benchmark sites, three WBIDS in partic ular exhibited higher TP concentrations associated with increased near-field LDI va lues (Figure B10-6). WBID 3401_ in the North Central region had multiple observations exceed ing 0.45 mg/L. Each of the other two WBIDs (1807_ and 1811_) had one observation exceeding 0.45 mg/L. Because those observations were also associated with higher ne ar-field LDIs, it cannot be concluded whether these data are from WBIDs with naturally high occurring leve ls of TP or because of higher LDI. To additionally explore the potential effect of extra-corridor land cover in explaining nutrient excursions, TN and TP were plotted as a func tion of the incremental difference between the alternative LDIs and the Stream Co rridor LDI (e.g., 2004 near-field LDI minus Stream Corridor LDI , Figures B10-7 and B10-8). In each of the la nd cover relationships with TN and the nearfield relationship for TP, a statistically si gnificant model can be de veloped; however, the R2 values were less than 10%. The TP in the Bone Valley and North Central regions was further investigated in light of the above discussion that highlights three WBID s in the Bone Valley region (1653_, 1807c_, and 1811_) and one WBID in the North Central region (3401_). Bone Valley TP (including WBID 1653_) is pl otted as a function of the alternative LDI approaches and the Stream Corridor LDI, in cluding WBID 1653_ (Figure B10-9) and the incremental difference between th e alternative LDIs and the St ream Corridor LDI (Figure B10- 10). Similarly, North Central TP is plotted in Figures B10-11 and B10-12. These figures underscore the observations made earlier in relation to Figures B10-3 and B10-6. 

### Conclusions and Recommendations This analysis was limited to reviewing LDI co mputations for the 135 nutrient benchmark WBIDs that were identified by FDEP using the Stream Co rridor LDI and further screened as documented elsewhere. Thus, the below conclusions and reco mmendations cannot be used to infer the general applicability of the Stream Corridor LDI beyond the WBIDs evaluated in this document.
 For the 135 WBIDs evaluated, near-field LDIs were better predictors of TN and TP than the Stream Corridor LDI. However, the linear re gression models developed using the data presented in Figures B10-7 and B10-8, while st atistically significant, provided little meaningful benefit ( R 2 < 10%) in comparison to the Stream Corridor LDI.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-4  On the basis of the data presented in Figur e B10-3 (and reemphasized in Figures B10-9 and B10-10), it is recommended to delete WB ID 1653_ from the benchmark data set.
 On the basis of the data presented in Fi gure B10-6 (and reemphasized in Figures B10-9 through B10-12), two WBIDs in the Bone Va lley (1807c_ and 1811_) and one WBID in the North Central region (3401_) have higher TP observations associated with higher near-field LDIs. It cannot be concluded th at these data are from WBID s with naturally high occurring levels of TP or from higher LDI. In light of the competing interest to (1) identify benchmark WBIDs that are not affected, or at most, mini mally affected by anthropogenic effects, and (2) sample size, there are three likely approach es that can be consid ered for these three WBIDs. o Compute benchmark criteria, excluding the data from all three WBIDs. Such an approach might lead to sample size concerns in the Bone Valley region. o Compute benchmark criteria including the da ta from all three WBIDs. That approach maintains the sample size in the Bone Va lley region but draws into question whether WBIDs that are more than minimally impaired WBIDs are included. o Remove the North Central region WBID (3401_) from the analysis but maintain the two Bone Valley WBIDs. Such an approach eliminates the concern that more than minimally impaired WBIDs are included in the North Central Region and would allow the North Central Region to be handled in a manner similar to other regions. That would not be the case for the Bone Valley region. The technical concerns related to following one of the above approaches that maintains a WBID that is potentially more than minimally impaired can be addressed by choo sing a lower percentile for the criteria. Table B10-1. GIS Quality Control Checks and Results # Check Result 1 Comparison of watershed size using rasterized tabulations and those computed from watershed polygons. Six sites, fully contained in Florida were analyzed. The greatest error between the land use tabulation and the polygon coverage was 0.05%. 2 Comparison of land use summary between rasterized land use tabulations (contractor algorithm) and use of vectorized tabulations (FDEP algorithm). For site 21FLWQSP_ESC012GS, the FDEP approach found 38 different land covers, while the contractor algorithm found 37. The FDEP approach included land cover 1210 of 5.7 m 2 . For 29 of the land cover categories, the difference in total land cover between the FDEP and contractor algorithm was less than 1%. For the eight land cover categories with more than 1% difference, the total land area associated with these lands (and 1210 discussed earlier) was 66.9 acres, 0.39% of the total polygon area. 3 Comparison of 5 m and 10 m raster tabulations. Eleven (11) sites were evaluated. A total of 441 site/land cover categories were tabulated. For the site/land cover categories with an error greater than 1%, the total land cover for these areas was 0.18% of the total land area summarized.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-5 # Check Result 4 Comparison of LDI computed from rasterized land cover tabulations (contractor algorithm) and use of vectorized tabulations and LDI values computed by FDEP. For site 21FLCEN_20010824, the FDEP delineated watershed was 751 acres larger than the contractor delineated watershed. Because of the small watershed size, that represented a 4.8% difference. Nevertheless the LDI from the FDEP and contractor analysis was 2.584 and 2.599, respectively. 5 Compare computed LDIs in this analysis The near-field watershed and drainage basin LDIs using 2004 data were plotted as a function of LDIs computed using 1995 data (see Figure B10-1). The four highlighted sites were reexamined within t he GIS. It was confirmed that the noticeable differences in LDI could be accounted for in land cover changes present in the source data.

0.0 1.0 2.0 3.0 4.0 5.0 6.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 1995 Land Use2004 Land Use Near Field Drainage Area

Points with "+" and "o" on top of each other indicate that near
field and drainage area watersheds are identical. See LDI Exception s

Figure B10-1. Near-field waters hed and drainage basin LDIs using 2004 land cover data as a function of LDIs computed using 1995 land cover data.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-6 Stream Corridor LDI 5 4 3 2 1

2.5 2.0 1.5 1.0 5 4 3 2 1 2.5 2.0 1.5 1.0 Near Field LDI(1995) Drainage Area LDI(1995)

Near Field LDI(2004) Drainage Area LDI(2004)

Figure B10-2. Near-field waters hed and drainage basin LDIs using 1995 and 2004 land use data as a function of Stream Corridor LDI.

Stream Corridor LDI 2.5 2.0 1.5 1.0 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 2.5 2.0 1.5 1.0 2.5

2.0

1.5

1.0

0.5

0.0 GeoMeanTP GeoMeanTN

Figure B10-3. TP (GeoMeanTP, mg/L) and TN (G eoMean TN, mg/L) as a function of Stream Corridor LDI. Includes WBID 1653_ with TP values greater than 0.75 mg/L. WBID 1653_TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-7 Stream Corridor LDI 2.00 1.75 1.50 1.25 1.00 0.6 0.5 0.4 0.3 0.2 0.1 0.0 2.00 1.75 1.50 1.25 1.00 2.5

2.0

1.5

1.0

0.5

0.0 GeoMeanTP GeoMeanTN

Figure B10-4. TP (GeoMeanTP, mg/L) and TN (G eoMean TN, mg/L) as a function of Stream Corridor LDI. Excludes WBID 1653_ with TP values greater than 0.75 mg/L.GeoMeanTN 2.5 2.0 1.5 1.0 0.5

4 3 2 1 2.5 2.0 1.5 1.0 0.5
4 3 2 1 Near Field LDI(1995) Drainage Area LDI(1995)

Near Field LDI(2004) Drainage Area LDI(2004)

Figure B10-5. TN as a function of the alternative LDI approaches.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-8 Figure B10-6. TP as a function of the altern ative LDI approaches with selected WBIDs highlighted.GeoMeanTN 2.5 2.0 1.5 1.0 0.5

3 2 1 0 2.5 2.0 1.5 1.0 0.5 3 2 1 0 1995_NF - LDI 1995_DA - LDI

2004_NF - LDI 2004_DA - LDI

Figure B10-7. TN as function of the incremental difference between the a lternative LDIs and the Stream Corridor LDI.GeoMeanTP 0.60 0.45 0.30 0.15 0.00

4 3 2 1 0.60 0.45 0.30 0.15 0.00 4 3 2 1 Near Field LDI(1995) Drainage Area LDI(1995)

Near Field LDI(2004) Drainage Area LDI(2004) 1807 3401

1811TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-9GeoMeanTP 0.60 0.45 0.30 0.15 0.00

3 2 1 0 0.60 0.45 0.30 0.15 0.00 3 2 1 0 1995_NF - LDI 1995_DA - LDI

2004_NF - LDI 2004_DA - LDI

Figure B10-8. TP as function of the incremental difference between the a lternative LDIs and the Stream Corridor LDI.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-10GeoMeanTP 1.2 0.9
0.6 0.3 0.0 54321

54321 1.2 0.9 0.6
0.3 0.0 54321 Near Field LDI(1995) Drainage Area LDI(1995) Stream Corridor LDI

Near Field LDI(2004) Drainage Area LDI(2004) 1653_ 1807C_ 1811_ 1978_ WBID

Figure B10-9. TP as a function of the alternativ e LDI approaches and the Stream Corridor LDI for the Bone Valley region.GeoMeanTP 1.2 0.9 0.6 0.3 0.0

3.02.52.01.51.00.50.0 1.2 0.9 0.6 0.3 0.0 3.02.52.01.51.00.50.0 1995_NF - LDI 1995_DA - LDI

2004_NF - LDI 2004_DA - LDI 1653_ 1807C_ 1811_ 1978_ WBID

Figure B10-10. TP as function of the incrementa l difference between the alternative LDIs and the Stream Corridor LDI for the Bone Valley region.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B10-11GeoMeanTP 0.60
0.45
0.30
0.15
0.00 4321

4321 0.60
0.45
0.30
0.15
0.00 4321 Near Field LDI(1995) Drainage Area LDI(1995) Stream Corridor LDI

Near Field LDI(2004) Drainage Area LDI(2004) 3504_ 3504A_ 3506_ 3506B_ 3530_ 3578_ 3598_ 3605_ 3605D_ 3641_ 3324_ 3341A_ 3341B_ 3351_ 3368_ 3388_ 3401_ 3448_ 3477_ WBID

Figure B10-11. TP as a function of the alternative LDI approaches for the North Central region.GeoMeanTP 0.60 0.45 0.30 0.15 0.00

2.0 1.5 1.0 0.5 0.0 0.60 0.45 0.30 0.15 0.00 2.0 1.5 1.0 0.5 0.0 1995_NF - LDI 1995_DA - LDI

2004_NF - LDI 2004_DA - LDI 3504_ 3504A_ 3506_ 3506B_ 3530_ 3578_ 3598_ 3605_ 3605D_ 3641_ 3324_ 3341A_ 3341B_ 3351_ 3368_ 3388_ 3401_ 3448_ 3477_ WBID

Figure B10-12. TP as function of the incrementa l difference between the alternative LDIs and the Stream Corridor LDI for the North Central region.