Bronx River Pollutant Loading Model Summary

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Margaret Eaton
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1 Bronx River Pollutant Loading Model Summary Section of River Located in Bronx County Table of Contents Drainage Areas...2 Data Sets Used for Analysis...2 Land Use Statistics...3 Model Calculations...3 Results and Discussion...6 Appendix 1: CSO Loading Tables...10 Appendix 2: Drainage Map...15

2 Introduction Surface runoff quantity and quality undergo significant changes as land becomes developed, and it is important to understand how changes in land use effect the availability and integrity of local water resources. This document outlines the characteristics, calculations, and results from a modeling exercise in an attempt to estimate pollutant concentrations found in storm runoff draining into the Bronx River from portions of its watershed located within Bronx County. The model was developed by The Center for Watershed Protection (CWP) to be used by watershed managers as a tool to better understand potential sources of nutrient loading to rivers and associated water quality concerns. The model was also designed for use in the planning process for areas that are undergoing significant landscape modifications. Drainage Areas The majority of drainage area to the river in these reaches is captured by a combined sewer system that carries wastewater and stormwater, during relatively small rain events, to be treated by the local sewage treatment plant. During large rain and snowmelt events, however, the combined sewer system will reach its capacity and be forced to discharge sewage and stormwater effluent directly into the Bronx River as a combined sewer overflow (CSO). The model results reported here were generated for the five CSO drainage areas (HP-004, HP-007, HP-008, HP-009, HP-010). Combined sewers HP-008 and HP-010 do not overflow due to the ability of the combined sewer system to handle both small and large storm events without discharging to the river. Other direct storm drainage areas exist within the Bronx Zoo, Botanical Gardens, and Woodlawn Cemetery. Runoff from these areas was not considered using this modeling approach since there is no land use breakdown, required by the model, for these locations. Data Sets Used for Analysis A most useful, and necessary, exercise performed in this analysis established information regarding proportions of land use, and their associated impervious cover, found in each CSOshed. Land use data was obtained through the PLUTO system, a citywide GIS database comprised of land use and infrastructure data developed by the New York City Planning Department. A fine resolution (3 x 3 ft.) land cover data set was used to calculate impervious, grass, and tree cover statistics within these land uses. This EMERGE layer was generated by the USDA Forest Service. The raster data set is classified by cover type (tree, grass, impervious surface, or unclassified). Further descriptions of these classifications can be found in Myeong, Data on road sanding was acquired from the New York City Department of Sanitation. The model does not require information on road salting. 2

3 Other GIS data layers including road networks, watershed/sewershed delineation, and orthophotography were acquired from within The Natural Resources Group s (NYC Parks) GIS database. Land Use Statistics The data layers described above were used to calculate various land use/cover statistics. Table 1 shows general impervious characteristics of each CSO-shed. CSO Area Total Area (Acres) Impervious Area (Acres) % Impervious HP % HP % HP % HP % HP % Table 1: CSO and impervious area calculations generated from EMERGE data For model data entry, it was required that a percent impervious cover be input for each type of land use classification (i.e. high density residential, commercial, etc.) used in the calculations. Spreadsheets were created that show the results of this land use/land cover analysis for each CSO-shed. These statistics can be used to obtain an understanding of the spatial distribution of land use and land cover throughout the CSO areas draining to the Bronx River. Land use files have the breakdown of all land use and cover information generated through this analysis, as well as graphs showing the percentages of different types of land cover for each land use within the CSO areas. Model Calculations The equations and assumptions described here show the basis for the loading calculations that led to the results shown in this report. Many of the calculations used in the model are based on the Simple Method (Schueler, 1987). The Simple Method is outlined below, showing the variables and equations used for calculation. The Simple Method estimates stormwater runoff pollutant loads. The technique requires little information, including the subwatershed drainage area and impervious cover, stormwater runoff pollutant concentrations, and annual precipitation. Urban land is broken into more detailed land use categories, such as two-acre lot residential, commercial, and roadway. The model then estimates pollutant loads for chemical constituents as a product of annual runoff volume and pollutant concentrations, as: L = R C A 3

4 Where: L = Annual load (lbs) R = Annual runoff (inches) C = Pollutant concentration (mg/l) A = Area (acres) = conversion factor For bacteria, the conversion factor is modified, so that the loading equation is: L = 1.03*10-3 R C A Where: L = Annual load (billion colonies) R = Annual runoff (inches) C = Bacteria concentration (#/100ml) A = Area (acres) 1.03*10-3 = conversion factor Annual runoff (R) is calculated as a product of annual rainfall and a runoff coefficient (determined by land use). R = P P j Rv Where: R = Annual runoff (inches) P = Annual rainfall (inches) P j = Fraction of annual rainfall events that produce runoff (typically 0.9) Rv = Runoff coefficient The runoff coefficient (Rv) is calculated based on impervious cover in the subwatershed as: Rv = Ia Where: Ia = Fraction of impervious surface (CWP, WTM documentation) Loading rates were established from five sources found within the boundaries of the combined sewersheds. Descriptions of the calculations for each source are listed below: 4

5 Urban Land: Land use types found in the watershed were grouped into one of five categories High Density Residential, Multi-Family Residential, Commercial, Roadway, or Industrial. Data was entered into the Simple Method equation to get the annual loading rate. These loads were then reduced by an amount based on the data specified for Existing Management Practices, simulating the effect that these practices would be expected to have on reducing pollutant inputs. CSO s: This source is also calculated using the Simple Method, with a slight variation since the sewer system does have a certain capacity to handle storms before overflowing to the river. In this case, a capacity of 0.1in was assumed as the amount of rain the combined system could handle before an overflow. This value should be readjusted after further information regarding this capacity is obtained. The median rain event for the watershed was assumed to be 0.2in., and should be investigated further to provide a more realistic value. Information regarding typical concentrations found in a combined sewer discharge was acquired from the Department of Environmental Protection via Hydroqual Inc. Illicit Connections: The default values from CWP were used for illicit connections that may exist in the watershed. No information was available to make a judgement that would have been any more accurate. Channel Erosion: Loading from this source was calculated by using an assumed sediment loading rate for an urban stream (500 lbs/acre/year) and back-calculating the natural loading rate by subtracting known (or estimated) sources of sediment that are input from the surrounding watershed. Vacant Lots: Annual loading rates from vacant lots were required for entry to the model. Loading rates were assumed to be similar to those from forested land, as given in the CWP model. These rates were multiplied by the acreage of vacant lots found in the watershed to generate these results. Runoff Concentrations Source TN (mg/l) TP (mg/l) TSS (mg/l) FC (MPN/100ml) Urban Land CSO s ,400,000 Loading Rates TN (lb/acre) TP (lb/acre) TSS (lb/acre) FC (# billion/acre) Vacant Lots Table 2: Runoff concentrations and loading rates used for model calculations 5

6 Results and Discussion The majority of the work performed in regard to model development was done in the land use/cover analysis described above, and used as input to the Primary Sources sheet of the CWP model. The CWP default values for runoff concentrations for the individual land use classes was held in place. Model results would be more accurate if these concentrations were updated with literature values that would better characterize runoff from this type of land use. Secondary Sources and Existing Management Practices data was obtained via the following sources and set of assumptions: Secondary Sources Number of dwelling units: GIS database/pluto system Nutrients in urban land: no input Septic systems: assumed no septic in the CSO-sheds Active construction: assumed no construction (requiring erosion control plan) SSO s: assumed no SSO s CSO s: Overflow data and concentrations of typical overflow obtained from Hydroqualmedian storm event was assumed (discussed later) Illicit connections: used CWP default values Channel erosion: chose method 1 (discussed later) Lawns: no input Road sanding: contacted Dept. of Sanitation- no sand is used, only salt Non-point source water inputs: no input Vacant lots: area used from land use calculations- used same conc. values as other land uses Existing Management Practices Educational program implementation and effectiveness was assumed Erosion and sediment control: no input Street sweeping: 80% of roadways were assumed to have been swept, parking lot acreage was taken from land use calculations 6

7 Impervious cover disconnection: assumed (20% residential, 10% business) Structural stormwater management practices: no input Riparian buffer: estimated from Bronx River data (discussed later) Catch-basin cleaning: assumed all impervious area drained to catch basins that were cleaned semi-annually Marina pumpouts: assumed no marina pumpout stations The model has the ability to predict future runoff conditions with development and increased management. These data were not entered into any of the spreadsheet calculations. Results for existing conditions were generated with the data available and assumptions used for input. Table 3 shows a summary of loading rates from this analysis, a breakdown of this data by source is shown in Appendix 1 for each CSO-shed. Area Impervious TN TP TSS Fecal Coliform CSO (acres) (%) (lb/year) (lb/year) (lb/year) (billion/year) HP , , ,190 HP-007 1, ,617 1, ,700 1,785,689 HP , , ,329 HP , , ,834 HP , ,650 86,758 Table 3: Summary of pollutant loading (total nitrogen, total phosphorus, total suspended solids, fecal coliform) from each combined sewer outfall draining to the Bronx River The stormwater and associated sewage from HP-007 ranks highest for all annual loading constituents targeted. This CSO-shed has the largest contributing area along with a high percentage of impervious cover. HP-004 and HP-009 show moderate loading rates due to their smaller size. Combined sewer outfalls HP-008 and HP-010 do not overflow, and this is apparent in the relatively low loading rates calculated above. It is unlikely that these results accurately simulate observed conditions in the CSO areas, due to the following reasons: Runoff nutrient, TSS, and FC concentrations for individual land uses are all equal. However, it would be expected that variable pollutant runoff concentrations between these land uses exist. Changing these concentration values would alter model results from the urban land source. Channel erosion: within the actual CSO-shed there is no channel erosion taking place. Values from this source could be summarized into the total load to the river only once (not 7

8 for each CSO-shed) in order to simulate channel erosion occurring upstream from the CSO outfalls. Perhaps the most central issue in this model application is the fact that there seems to be a redundancy of loading rates occurring in the calculations. The model assumes the existence of an outlet point (stream channel) to which the runoff water drains. In the cases examined here, the outlet point is the CSO outfall. In reality, the CSO discharge information used on the Secondary Sources sheet is comprised of each runoff source used as input for the rest of the model, resulting in a source being contributed twice to the total annual loads summary. The source is added once through the Primary Sources sheet and then again as a CSO on the Secondary Sources sheet. The assumed input data to the model (particularly nutrient/bacteria runoff concentrations from the various land uses, the median storm event used for CSO discharges, and concentration data for these overflows) has a high potential for inaccuracy. If these data are verified in some fashion, and are proved to be accurate then it may be said that this model can be used to give a relatively accurate description of annual total phosphorus, total nitrogen, total suspended solids, and fecal coliform loading. The model was developed to predict loading rates from sub-urban land undergoing development. This application involved an ultra-urban area that has already been developed. Further complication of the model assumptions arose as the watershed areas used for each analysis did not encompass a stream channel, rather, were comprised of the land area draining to a combined sewer outfall. The most accurate way that pollutant loading from these areas can be estimated is by obtaining monitoring data for combined sewer overflows in regards to nutrient, TSS, and FC concentrations as well as the volume of overflow. Taking the product of average concentrations of each CSO and the volume of the discharge and summing over the course of a year would give a good representation of what is entering the Bronx River at these locations. Management efforts, such as stormwater control and treatment practices, would serve to reduce these loading rates. Estimated efficiencies of these practices could be used to determine the reduction in load (both pollutant and water volume) to the combined system enhancing the water quality of the river in its tidal reach, estuarine environment, and the East River. A water quality modeling study and report to characterize CSO loading to the Bronx River was generated by Hydroqual Inc. and is currently under review by the Department of Environmental Protection. Results from this report may support or refute the results of this analysis. In either case, evaluation and comparison of the two methods would serve to provide a clearer understanding of the data and processes involved. No input for future development or future management practices was given to the model to predict future loading. This was made under the assumptions that no plans exist for any significant land use modifications in this area and that stormwater management options remain a topic of discussion between the agencies involved in implementation. 8

9 References Myeong, Nowak, Hopkins and Brock. Urban cover mapping using digital, high-spatial resolution aerial imagery. Urban Ecosystems, 5: , Schueler, T Controlling urban runoff: a practical manual for planning and designing urban BMPs. Metropolitan Washington Council of Governments. Washington, DC. The Center for Watershed Protection. Watershed Treatment Model Documentation. Ellicott City, MD. The Natural Resources Group: NYC Parks and Recreation Todd McDonnell August 25,

10 Appendix 1 HP-004 Loading Summary Existing Loads Area TN TP TSS Fecal Coliform (acres) lb/year lb/year lb/year billion/year URBAN SOURCES Urban Land 477 4, , ,318 Active Construction SSOs CSOs , ,842 Illicit Connections ,209 69,842 Channel Erosion ,102 - Marinas Road Sanding Point Sources Vacant Lots , RURAL SOURCES Rural Land Forest Livestock Open Water TOTAL LOAD 516 4, , ,190 Storm Load 4, , ,349 Non-Storm Load ,209 69,842 10

11 HP-007 Loading Summary Existing Loads Area TN TP TSS Fecal Coliform (acres) lb/year lb/year lb/year billion/year URBAN SOURCES Urban Land 1,252 11,927 1, , ,200 Active Construction SSOs CSOs , ,980 Illicit Connections , ,257 Channel Erosion ,232 - Marinas Road Sanding Point Sources Vacant Lots , RURAL SOURCES Rural Land Forest Livestock Open Water TOTAL LOAD 1,305 12,617 1, ,700 1,785,689 Storm Load 12,307 1, ,308 1,627,433 Non-Storm Load , ,257 11

12 HP-008 Loading Summary Existing Loads Area TN TP TSS Fecal Coliform (acres) lb/year lb/year lb/year billion/year URBAN SOURCES Urban Land 201 1, , ,838 Active Construction SSOs CSOs Illicit Connections ,435 Channel Erosion ,042 - Marinas Road Sanding Point Sources Vacant Lots RURAL SOURCES Rural Land Forest Livestock Open Water TOTAL LOAD 213 1, , ,329 Storm Load 1, , ,894 Non-Storm Load ,435 12

13 HP-009 Loading Summary Existing Loads Area TN TP TSS Fecal Coliform (acres) lb/year lb/year lb/year billion/year URBAN SOURCES Urban Land 391 3, , ,384 Active Construction SSOs CSOs , ,227 Illicit Connections ,159 Channel Erosion ,304 - Marinas Road Sanding Point Sources Vacant Lots RURAL SOURCES Rural Land Forest Livestock Open Water TOTAL LOAD 404 3, , ,834 Storm Load 3, , ,674 Non-Storm Load ,159 13

14 HP-010 Loading Summary Existing Loads Area TN TP TSS Fecal Coliform (acres) lb/year lb/year lb/year billion/year URBAN SOURCES Urban Land 139 1, ,076 64,559 Active Construction SSOs CSOs Illicit Connections ,175 Channel Erosion ,107 - Marinas Road Sanding Point Sources Vacant Lots RURAL SOURCES Rural Land Forest Livestock Open Water TOTAL LOAD 144 1, ,650 86,758 Storm Load 1, ,383 64,583 Non-Storm Load ,175 14

15 Appendix 2 15

Watershed Treatment Model (WTM) 2013 Documentation

Watershed Treatment Model (WTM) 2013 Documentation Funding Provided By: US EPA Office of Wetlands Oceans and Watersheds Altria Foundation Cooperative Institute for Coastal and Estuarine Environmental Technology