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



## Appendix B-8: Extrapolating Natural Background Nutrient Concentrations

(Chapter 12 from USEPA. 2009. Proposed Methods and Approaches for Developing Numeric Nutrient Criteria for Florida’s Inland Waters . United States Environm ental Protection Agency, Office of Water, Office of Science and Technol ogy, Health and Ecological Criteria Division, Washington, DC.)TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

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### Appendix B-8. Extrapolating Natural Background Nutrient Concentrations
Dodds and Oakes (2004) proposed an approach to predict natural background, or reference nutrient concentrations in streams. A multiple regression model is developed to determine the relationship between nutrient con centrations and human land uses in the watershed. Reference nutrient concentrations in the stream water co lumn are extrapolated from the intercept of predicted nutrient concentrations, where human la nd use is zero. Other similar applications have used additional natural predictors and allowed for some acceptable disturbance (Soranno et al. 2008). Dodds and Oakes (2004) original approach required development of a multiple regression model using all human land uses as explanatory va riables, and extrapolating those land uses to zero altered land cover a nd 100% natural land cover. For this analysis, many of the sites only have Florida Landscape Developm ent Intensity Index (LDI) scor es available, so simple regressions could not be performed.

Natural land cover of Florid a includes extensive wetlands . Southeastern wetlands and bottomland forests contribute large amounts of di ssolved organic compounds to surface water and shallow groundwater. The organic compounds also include organic nitrogen compounds, and southeastern wetland waters ofte n have high organic nitrogen.

Human-altered land uses contribute mo re inorganic nitrogen (ammonium (NH 4 + ), nitrate (NO 3 ), and nitrite (NO 2 )) to runoff from domestic sewage, animal water, and fertilizer s applied to crop and lawns. Inorganic nitrogen, NO 3 in particular, is also tr ansported long distance in ground water. These complications lead us to predict that the percent of alte red land cover will be positively correlated with dissolved inorganic ni trogen (DIN), but that a significant correlation between land use and total nitrogen (TN) was less likely.



### Methods Description Two separate data sets were used to model the relationships between land use and nutrient concentrations in streams. One data set was provided by the Florida Department of Environmental Protection (FDEP), the Macroinv ertebrate metrics and chemistry datasheet – Stream file2. This file included LDI and instantaneous measurement of nutrient concentrations in stream water. LDI score was calculated based on la nd uses in a catchment or corridor, and each land use category was weighted by a coefficient and the total sc ores were summarized as an index. The minimum coefficient fo r all land use categories was one so the smallest LDI expected where no human disturbance was introduced in the stream watershed was one. The sites comprised the entire gradient of human distur bance from LDI score range of 1.0 (completely undisturbed) to 8.82 (heavily altered). However, the LDI values were based on 100 m stream corridor calculation on e ach side of stream (not the watershed). Site delineation for 100 benchmark stations was performed. Land use/land cover and LDI of near-fie ld and drainage area based on 1995 and 2004 land use layers were calc ulated. Nutrient measurements for these stations were extracted from a data set provid ed by FDEP and were averaged (geometric mean) for each station/year. Statistical analysis wa s performed using open source software - R.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

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### Stream Corridor LDI vs. Nutrients FDEP developed stream corridor LD I scores for streams in their gradient study sites (388 sites). The relationships between these LDI and instanta neous measurement of nutrient concentrations in these streams were examined first. The results (Fi gure B8-1) showed that although a significant relationship existed between tota l phosphorus (TP) and LDI and between total inorganic nitrogen (TIN) and LDI, the models e xplained little of the variance in nutrient concentrations ( R2 =0.07 and 0.08, respectively, outliers TP > 1mg/L and TIN > 2mg/L were excluded). Due to the variability, the predicted/ex trapolated natural nutrient concentrations had very wide prediction in tervals (Figure B8-1).

As expected, the relationship between TN and LDI was not significant. Relations between nutrients and LDI scores in each nutrient region we re plotted in separated graphs (Figure B8-2). Although most of the observed relationships were significant, due to variability, it was hard to extrapolate nutrient endpoints from these weak patterns.



### Near-field and Drainage Area Land Uses vs. Nutrients

The LDI scores between 1995 and 2004 da ta were strongly correlated ( R2 =0.84). However, the slope of the regression between 1995 and 2004 was 1.09, indicating that LDI has increased by 9% over the ten year period (Figure B8-3). It is unknown if this reflec ts a 9% increase in landscape development or if land cover measur ement methods may have changed between 1995 and 2004.

As shown in Table B8-1, LDIs calculated from both near-field and dr ainage area were only weakly correlated with TN and TIN concentrati ons, similar to those observed in corridor land use relations shown in Figure B8-1. No matter wh ich land use layers and delineation methods are used, organic nitrogen concentrations were alwa ys positively correlated with wetland percent, and negatively correlated with forest percent. In the meantime, both TN and TP concentrations were negatively correlated with percent forest. NO 3 and TIN appeared positively correlated with urban land use and negatively correlated with wetlands.

Selected nutrient variables were plotted verse 200 4 drainage area percen t land cover (Figure B8- 4). Based on the patterns observed, it seems TN concentrations in Florida streams were confounded by high organic nitroge n input from wetland areas, as expected. TIN and NO 3 concentrations were correlated with urban land use but this was characterized by a generally nonlinear response.

Candidate nutrient criteria c ould not be reliably derived us ing the Dodds and Oakes (2004) methods as applied here with the available data.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B8-3 Table B8-1. Correlations between Nutrient Variables and Percent Land Use and LDITSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B8-4 Figure B8-1. TN and TIN concentrations vs. LD I in all streams. The 80% confidence (inner dashed lines) and prediction (outer dashed lines) limits were plotted.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

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B8-6 Figure B8-2. TP and TIN concentrations vs. LDI in different nutrient regions. The 80% confidence (inner dashed lines) and prediction (outer dashed lines) intervals were plotted as well.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B8-7 Figure B8-3. Relationships between near-field and drainage area LDI in 1995 vs. those in 2004. Solid lines are regression limits and dashed lines are 1:1 lines.TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

B8-8 Figure B8-4. Nutrient variables vs. percent 2004 drainage areas land cover. LOESS regression lines and their confidence intervals were also plotted.