Supporting Information for

Watershed-Scale Impacts of Stormwater Green Infrastructure on Hydrology, Nutrient Fluxes,

and Combined Sewer Overflows in the Mid-Atlantic Region

Michael J. Pennino1*, Rob I. McDonald2, and Peter R. Jaffe1

1Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ, USA.

2Global Cities Program, The Nature Conservancy, Arlington, VA

*Corresponding Author: michael.pennino@gmail.com

Table S1. Descriptive data on area, impervious surface cover, and stormwater management for all watershed used in this study for hydrologic and water quality analysis.

USGS Site ID Location

Area (km2)

% ISC*

% SGI 2005

% SGI 2013

% SGI avg*

Cond-ition

WQ

WQ Sampling Frequency

% DP

% SM

% WP

% SF

% IT

% BR

% SW

% Other

01585100 Balt. Co. 20.6 29.8 25.5 31.2 31.2 High N 65 16 7 4 6 0 0 001585104 Balt. Co. 6.0 15.2 7.6 20.5 19.7 High N 61 1 12 20 4 0 0 101589180 Balt. Co. 0.9 18.7 15.2 15.2 15.2 High Y Weekly 53 37 0 10 0 0 0 001589197 Balt. Co. 12.5 16.1 37.5 42.3 41.1 High Y Weekly 79 13 0 5 3 0 0 001589100 Balt. Co. 7.5 37.1 16.8 19.9 19.1 High N 78 0 16 3 3 1 0 001589316 Balt. Co. 1.9 38.9 33.0 36.2 36.2 High N 91 0 8 0 0 0 0 001589317 Balt. Co. 1.1 53.0 55.7 57.8 57.8 High Y Biweekly 64 31 1 2 4 1 1 001589320 Balt. Co. 5.0 44.9 32.9 38.5 38.1 High N 77 10 6 2 1 1 0 301585090 Balt. Co. 6.6 33.1 21.4 22.5 22.4 High N 64 7 14 8 6 0 0 101589290 Balt. Co. 9.0 22.2 9.8 11.5 11.4 High Y Monthly 85 0 6 7 1 0 0 101589330 Balt. Co. 14.3 40.1 27.9 31.1 25.0 High Y Weekly 87 5 3 1 1 1 0 215839796

7 Balt. Co. 5.4 21.2 3.1 6.0 5.9 High Y Biweekly 83 0 1 4 8 1 0 2CUERE Balt. Co. 18.0 15.9 22.6 30.0 29.7 High Y Biweekly 72 1 7 11 5 3 0 2

01585200 Balt. Co. 6.0 25.6 0.7 1.6 1.2 Low N 79 0 0 11 0 0 0 1001589305 Balt. Co. 9.5 36.8 0.9 2.1 2.0 Low Y Monthly 39 15 1 30 1 10 3 101589312 Balt. Co. 0.9 31.9 3.4 3.5 3.5 Low N 100 0 0 0 0 0 0 001585225 Balt. Co. 0.5 40.5 0.0 0.0 0.0 Low Y Monthly 0 0 0 0 0 0 0 001585230 Balt. Co. 9.0 33.0 0.5 0.6 0.6 Low Y Monthly 86 0 0 14 0 0 0 001589464 Balt. Co. 5.7 19.8 0.0 0.0 0.0 Low Y Monthly 0 0 0 0 0 0 0 001644371 Mon. Co. 1.1 24.0 0.4 36.0 36.0 High N 23 0 0 57 10 9 4 001644375 Mon. Co. 3.1 38.3 54.6 57.1 57.1 High N 33 34 1 11 19 1 0 001647850 Mon. Co. 7.5 26.3 17.9 37.9 36.5 High N 23 47 22 2 1 1 0 401649190 Mon. Co. 34.3 13.7 22.6 27.2 27.1 High Y Biweekly 33 11 32 7 12 0 0 401650800 Mon. Co. 16.9 27.1 33.5 34.7 34.7 High Y Biweekly 4 56 32 1 1 0 1 4

ISC = impervious surface cover, SGI = stormwater green infrastructure, DP = Detention Pond, SM = Shallow Marsh, WP = Wet Pond, SF = sand filter, IT = Infiltration Trench or Basin, BR = Bioretention, SW = Swale, Other: For Baltimore County, Other means Dry Well, Underground Storage, Porous Pavement, Green Roofs,Oil/Grit Separator, Storm Filter, Level Spreader. For Montgomery County, Other means Dry Well, Flow Splitter, Stormfilter, Oil/Grit Separator,

Porous pavement, Rain Barrel, Green Roof, Aquafilter, Aquaswirl, Baysaver, Bayfilter, Tree Box, Underground Storage. CUERE – this stream gauge was maintained by the Center for Urban Environmental Research and Education.

Table S2. Hydrology, water quality, and CSO metric descriptions, expected effects of SGI, and percent difference between watersheds with SGI and watersheds without SGIMetric Description Peak Runoff (mm/day)

Mean annual value from each hydrograph peak

Baseflow (m3/s) Mean baseflow discharge estimated by excluding rain producing hydrographs

Peak Frequency Mean frequency of peaks above 3 monthly median

Volume-to-Peak Ratio (hrs)

hydrograph volume divided by peakflow discharge

Hydrograph Duration (hrs)

Time from start to end of hydrograph

CV of Runoff Coefficient of variation for entire year of instantaneous runoff

NO3- Load

(kg/ha/yr)Based on 2011, 2012, and 2013 LOADEST estimates

TN Load (kg/ha/yr)

Based on 2011, 2012, and 2013 LOADEST estimates

PO4-3 Load

(kg/ha/yr)Based on 2011, 2012, and 2013 LOADEST estimates

TP Load (kg/ha/yr)

Based on 2011, 2012, and 2013 LOADEST estimates

Avg # CSOs Based on 2011, 2012, 2013, and 2014 DC CSO data

Total CSO Duration (hrs)

Based on 2011, 2012, 2013, and 2014 DC CSO data

CSO Volume (mg)

Based on 2011, 2012, 2013, and 2014 DC CSO data

% Difference is between watersheds with and watersheds without SGI. Negative % difference means metrics for the watersheds with SGI are less than for the watersheds without SGI.* indicates that the effect of SGI is significant based on ANOVA at p < 0.05

Table S3. % Difference between analyzing hydrology data using 1 vs. 15 minute interval or 5 vs. 15 minute interval data.

Peak Runoff (mm/day)

Baseflow (m^3/s) *1000

Freq. Peakflow

Vol-to-Peak Ratio /10

Hydrograph Length

CV Runoff *10 OVERALL

% Difference 10.1 ± 2.4 6.6 ± 1.4 1.5 ± 0.7 5.1 ± 0.7 5.8 ± 1.0 1.7 ± 0.5 5.1 ± 3.1

Table S4. Ranges and averages (± standard errors) for all the USGS watersheds in Baltimore County and Montgomery County

N All Sites

Shed Area Range (km2)

Mean Area (km2)

Median Area (km2)

% ISC Range

Mean % ISC

Median % ISC

% SGI Range

Mean % SGI

Median % SGI

Baltimore County

36 0.3 - 808

86 ± 32

13.40 - 53

19.5 ± 2.8

19.20 - 58

13.7 ± 2.8

5.2

Montgomery County

14 0.8 – 264

40 ± 19

9.5 0.4 - 38

12 ± 3.2

8.90 - 57

19.7 ± 4.9

17.8

Both Counties

50 0.3 - 808

73 ± 23

12.00 - 53

17 ± 2.2

15.60 - 58

15.6 ± 2.4

12.3

ISC = impervious surface cover, SGI = Stormwater Green Infrastructure, CSO = Combined Sewer Overflows. % SGI is based on 2013 data.

Table S5. Sensitivity Analysis for impact of change %SGI cutoff for high vs. low SGIMetric 3% SGI 5% SGI 10% SGIPeak Runoff (mm/day)

0.03 0.02 0.01

Baseflow (m3/s) 0.62 0.58 0.78Peak Frequency 0.05 0.02 0.02Volume-to-Peak Ratio (hrs)

0.03 0.02 0.00

Hydrograph Duration (hrs)

0.03 0.01 0.00

CV of Runoff 0.01 0.01 0.03

NO3- Export (kg/ha/yr) NA 0.07 0.36

TN Export (kg/ha/yr) NA 0.10 0.51PO4

-3 Export (kg/ha/yr)

NA NA 0.45

TP Export (kg/ha/yr) NA 0.39 0.76

Avg # CSOs 0.60 0.82 0.78Total CSO Duration (hrs)

0.43 0.59 0.42

CSO Volume (mg) 0.99 0.97 0.95

Table S6. Significant relationships (at p < 0.05) for Metrics with different SGI typesMetric %

Total SGI % DP % SM % WP % SF % IT % BR

% SW % CB

Peak Runoff (mm/day)

0.02 0.03 0.04 0.21 0.62 0.37 0.52 0.54 NA

Baseflow (m3/s)

0.49 0.64 0.61 0.69 0.52 0.30 0.55 0.73 NA

Peak Frequency

0.00 0.19 0.14 0.99 0.00 0.00 0.00 0.00 NA

Volume-to-Peak Ratio (hrs)

0.00 0.25 0.16 0.39 0.01 0.00 0.10 0.09 NA

Hydrograph Duration (hrs)

0.00 0.14 0.01 0.12 0.03 0.01 0.09 0.08 NA

CV of Runoff

0.00 0.28 0.01 0.34 0.02 0.01 0.03 0.03 NA

NO3- Export

(kg/ha/yr)0.60 0.34 0.27 0.34 0.51 0.44 0.48 0.18 NA

TN Export (kg/ha/yr)

0.52 0.47 0.14 0.47 0.25 0.29 0.13 0.18 NA

PO4-3 Export

(kg/ha/yr)0.85 0.0905

*0.01 0.23 0.729* 0.1065* 0.2511

*0.10 0.85

TP Export (kg/ha/yr)

0.75 0.26 0.47 0.41 0.75 0.12 0.99 0.73 NA

Avg # CSOs 0.16 0.73 NA NA 0.31 0.85 0.66* 0.70 0.22Total CSO Duration (hrs)

0.14 0.41 NA NA 0.50 0.52 0.70* 0.80 0.48

CSO Volume (mg)

0.34 0.77 NA NA 0.59 0.79 0.43* 0.90 0.32

DP = detention pond, SM = shallow marsh, WP = wet pond, SF = sand filter, IT = infiltration trench/basin, BR = bioretention, SW = swale, CB = catch basin and stormceptor and stormfilter. *For CSO data, BR is only SGI type with negative relationship with CSO metrics. *For PO4

-3 these are the only ones with negative relationship.

Table S5. P-values for multilinear regression and ANOVAs comparing data with and without outlier values.

Regression Regression ANOVA ANOVAMetric p-value all data p-value without

Outlierp-value all data p-value without

OutlierNO3- Load (kg/ha/yr)

0.6044 0.91746 0.067 0.155

TN Load (kg/ha/yr)

0.517 0.77708 0.103 0.23

PO4-3 Load (kg/ha/yr)

0.851 0.851 NA NA

TP Load (kg/ha/yr)

0.513 0.513 0.385 0.684

Figure S1. Combined sewer overflow locations (black dots), sewersheds (tan polygons), stormwater green infrastructure locations (green dots), and city boundaries (grey) for Washington, DC and Philadelphia, PA.

Figure S2. Comparison of discharge on water quality sampling dates with entire discharge record during water quality sampling

Figure S3. Effects of stormwater green infrastructure (SGI) on hydrology metrics: a) peak runoff, b) mean baseflow, c) frequency of peak flow events, d) volume-to-peak ratio, e) hydrograph duration, and f) coefficient of variation for runoff. Comparing Baltimore County and Montgomery County. Data is for average of 2011, 2012, 2013..

Figure S4. Effect of stormwater green infrastructure on a) NO3-, b) TN, c) PO4

3-, and d) TP loads. Comparing Baltimore County and Montgomery County (data from average of 2011,2012 and 2013). Circles represent Baltimore County watersheds and Triangles represent Montgomery County Watersheds.

Figure S5. Flow Normalized Concentrations

Figure S6. Total Annual Water Yield: total annual runoff, baseflow and stormflow

Figure S7. Changes in annual runoff over time at two USGS sites in this study.

Figure S8. Effect of stormwater green infrastructure on a) frequency, b) duration, and c) volume of CSO events.

Figure S9. (a) Comparison of CSO metrics for sewersheds with and without stormwater green infrastructure, (b) Comparison of % change in CSO metrics for shewersheds with no change in SGI and sites with >5% change in SGI over a 10 year period. These figure contains data from DC only. Panel A: N = 23 for Low SGI and N = 20 for high SGI. Panel B: N = 8 for control sites and N = 5 for SGI sites. Error bars are standard errors of the mean.