water quality assessment and modeling - … long term control plan update water quality assessment...

CSS Long Term Control Plan Update

Water Quality Assessment and Modeling

City of Alexandria Department of Transportation and Environmental Services

FINAL – October 2015




Table of Contents

i

Executive Summary ................................................................................................ ES-1

USEPA Presumption Approach ............................................................................................................. ES-1

USEPA Demonstration Approach and Water Quality ............................................................................ ES-2

Waste Load Allocations ......................................................................................................................... ES-3

Section 1 Background ............................................................................................... 1-1

Section 2 Short List of Combined Sewer Control Strategies ................................. 2-1

Section 3 Modeling Methods ..................................................................................... 3-1

3.1 Hydrologic and Hydraulic Modeling .................................................................................. 3-1

3.2 Water Quality Modeling .................................................................................................... 3-1

Section 4 USEPA National CSO Policy Performance .............................................. 4-1

4.1 Presumption Approach ..................................................................................................... 4-1

4.2 Demonstration Approach ................................................................................................. 4-3

4.2.1 AlexRenew WRRF Load .................................................................................................. 4-3

4.2.2 Potomac Boundary ........................................................................................................... 4-4

4.2.3 Proportional v. Discrete Controls ...................................................................................... 4-4

4.2.4 Decay Rates ..................................................................................................................... 4-5

4.2.5 2004-2005 Climate Period................................................................................................ 4-5

4.3 Water Quality Modeling Scenarios ................................................................................... 4-6

4.3.1 Scenario 1 – Verification .................................................................................................. 4-6

4.3.2 Scenario 2 - All Modifications except Climate .................................................................. 4-6

4.3.3 Scenario 3 - All Modifications except WRRF and Climate ................................................ 4-6

4.3.4 Water Quality Scenario 1 - Verification ............................................................................ 4-7

4.3.5 Water Quality Scenario 2 – All Modifications except Climate Period ............................. 4-10

4.3.6 Water Quality Scenario 3 – All Corrections except Climate Period and WWTP Load .... 4-14

Section 5 TMDL Waste Load Allocation ................................................................... 5-1

5.1 Waste Load Allocations .................................................................................................... 5-1

5.1.1 Discrete Outfall Basis ....................................................................................................... 5-1

5.1.2 Collective Consistency ..................................................................................................... 5-2




Table of Contents

ii

5.1.3 Climate Period Considerations ......................................................................................... 5-2

Section 6 Conclusions ............................................................................................... 6-1

6.1 Presumption Approach ..................................................................................................... 6-1

6.2 Demonstration Approach ................................................................................................. 6-1

6.3 Waste Load Allocation ..................................................................................................... 6-1

Section 7 References ................................................................................................. 7-1 List of Tables

Table ES-1 Presumption Approach Performance .................................................................................................... ES-2

Table ES-2 Waste Load Allocation Performance ..................................................................................................... ES-3

Table 2-1 Combined Sewer Control Technologies Evaluated .................................................................................... 2-1

Table 2-2 Short Listed Combined Sewer Control Strategies ...................................................................................... 2-3

Table 4-1 LTCPU Alternatives Overflow Summary* ................................................................................................... 4-1

Table 4-2 Short Listed Strategies: Presumption Approach Performance ................................................................... 4-2

Table 4-3 2004-2005 AlexRenew Wet Day and Dry Day Effluent Averages for E. coli (cfu/100 mL) ......................... 4-3

Table 4-4 Extreme Storm Events included in 2004-2005 Climate Period .................................................................. 4-6

Table 4-5 Water Quality Model Scenarios ................................................................................................................. 4-7

Table 4-6 Short Listed Strategies and Water Quality Model Runs ............................................................................. 4-7

Table 4-7 Scenario 2 – All Modifications except Climate Period Processed ELCIRC-based E. coli Predictions for

2004-2005 .................................................................................................................................................. 4-10

Table 4-8 Scenario 3 – All Modifications except Climate Period and WWTP Load Processed ELCIRC-based E. coli

Predictions for 2004-2005 .......................................................................................................................... 4-14

Table 5-1 Waste Load Allocation for Combined Sewer System – Discrete Outfall Basis .......................................... 5-1

Table 5-2 Load Deficit Discrete Collective Consistency ............................................................................................. 5-2

Table 5-3 E. coli Waste Load Allocation for AlexRenew Water Resources Reclamation Facility ............................... 5-2

Table 5-4 Waste Load Allocation for COA Combined Sewer System – 2005 without Extreme Event ....................... 5-3

List of Figures

Figure 2-1 LTCPU Decision Process ......................................................................................................................... 2-2

Figure 3-1 ELCIRC Model Overview .......................................................................................................................... 3-2

Figure 4-1 Scenario 1 (Verification vs. TMDL Base: Upstream Hunting Creek – ELCIRC-predicted Daily and Monthly

E. coli Concentrations .................................................................................................................................. 4-8

Figure 4-2 Scenario 1 (Verification vs. TMDL Base: Hunting Creek Embayment – ELCIRC-predicted Daily and

Monthly E. coli Concentrations .................................................................................................................... 4-9

Figure 4-3 Scenario 2 vs. TMDL Base: Upstream Hunting Creek – ELCIRC-predicted Daily and Monthly E. coli

Concentrations ........................................................................................................................................... 4-12




Table of Contents

iii

Figure 4-4 Scenario 2 vs. TMDL Base: Hunting Creek Embayment – ELCIRC-predicted Daily and Monthly E. coli

Concentrations ........................................................................................................................................... 4-13

Figure 4-5 Scenario 3 vs. TMDL Base: Upstream Hunting Creek – ELCIRC-predicted Daily and Monthly E. coli

Concentrations ........................................................................................................................................... 4-15

Figure 4-6 Scenario 3 vs. TMDL Base: Hunting Creek Embayment – ELCIRC-predicted Daily and Monthly E. coli

Concentrations ........................................................................................................................................... 4-16




Executive Summary

ES-1

Executive Summary

A shortlist of Long Term Control Plan Update (LTCPU) combined sewer control strategies for further

evaluation has been developed in Alternative Evaluation: Ranking and Recommendation Technical

Memorandum. These strategies have been evaluated for their performance with respect to the United

States Environmental Protection Agency (EPA) Combined Sewer Overflow (CSO) Policy presumption

and demonstration approaches, meeting water quality standards and addressing the Hunting Creek Total

Maximum Daily Load (TMDL).

USEPA Presumption Approach

There are three presumption approach criteria:

POi: “No more than an average of four overflow events per year, provided that the permitting

authority may allow up to two additional overflow events per year.”

POii: “The elimination or the capture for treatment of no less than 85% by volume of the

combined sewage collected in the CSS during precipitation events on a system-wide annual

average basis”; and

POiii: “The elimination or removal of no less than the mass of the pollutants, identified as

causing water quality impairment through the sewer system characterization, monitoring, and

modeling effort, for the volumes that would be eliminated or captured for treatment under

paragraph ii.”

The EPA CSO Policy for presumption control is met if any one of these three criteria is met. As shown

below, all the combined sewer control strategies selected for further consideration exceed each criterion

of the presumption approach.




Executive Summary

ES-2

Table ES-1

Presumption Approach Performance1

Strategy Description

Presumption

Option POi Overflows/Yr.

Presumption

Option POii

% Capture

Presumption

Option POiii

Equivalent Load

4 Average

(up to 6 maximum) 85% Minimum 85% Minimum

S-7

Storage Tunnel for CSO 003/4 (Hooffs Run) and Storage Tank at CSO 002 (Hunting Creek)

4 overflows/year 95% 95%

S-3a

Separate Storage Tunnels 003/4 to Hooffs Run and 002 to Hunting Creek


S-3b

Separate Storage Tunnels 003/4 to Hooffs Run and potential Outfall Relocation for 002 to the Potomac


S-1 Storage Tunnels for 002/3/4 (Hooffs Run and Hunting Creek)


USEPA Demonstration Approach and Water Quality

Under the EPA CSO Policy, a control level less than called for by the presumption approach can be

selected if it can be demonstrated that “the CSO discharges remaining after implementation of the

planned control program will not preclude the attainment of WQS (water quality standards) or the

receiving waters' designated uses or contribute to their impairment.” To demonstrate this, the LTCPU

evaluated the alternatives using the water quality modeling developed as part of the Hunting Creek

TMDL. The modeling assessment conducted concludes that with an appropriate decay rate (the rate at

which bacteria die off in the environment), all combined sewer control strategies selected for further

evaluation meet the water quality demonstration requirements of the EPA CSO Policy stated above.

1 POi, POii, and POii refer to the Presumption Options labeled i, ii, and iii in the EPA CSO policy.




Executive Summary

ES-3

Waste Load Allocations

On November 2, 2010, the Virginia Department of Environmental Quality (VDEQ) issued Bacteria

TMDLs for the Hunting Creek, Cameron Run, and Holmes Run Watersheds (Hunting Creek TMDL). All

of the combined sewer control strategies selected for further evaluation can meet the waste load

allocations of the TMDL with the following considerations:

All the combined sewer control strategies meet the TMDL wasteload allocations (WLA) for

1984. The year 1984 has been has been selected to represent a typical rainfall year for

combined sewer planning and design purposes and is used to evaluate alternatives under the

EPA CSO Policy.

For the 2004-2005 Hunting Creek TMDL climate period:

CSO Control Strategy S7 – the CSO-003/4 storage tunnel will discharge to Hooffs Run and the

CSO-002 storage tank will discharge to Hunting Creek. To meet the WLA for this alternative,

the extreme once in 40-60 year storms must be excluded from the evaluation. In addition, a

collective consistency1 allocation from AlexRenew of approximately 53% is needed.

CSO Control Strategy S3 - Separate Storage Tunnels 002 and 003/4 to Hooffs Run and Outfall

Relocation for 002 to the Potomac River or Hunting Creek. To meet the WLA the extreme

once in 40-60 year storm must be excluded from the evaluation.

CSO Control Strategy S1 – one storage tunnel that will discharge to Hooffs Run and Hunting

Creek. To meet the WLA for these alternatives, the extreme once in 40-60 year storm must be

excluded from the evaluation. In addition, a collective consistency1allocation from AlexRenew

of approximately 53% is needed.

Table ES-2

Waste Load Allocation Performance

Strategy

Meets WLA

Typical Year

1984

Meets WLA

TMDL Climate

Period 2004-2005

S-7

Hooffs Run/Hunting Creek Embayment YES

Yes with 53% Collective Consistency and No Extreme Storm

S-3a



S-3b

Hooffs Run/Potomac River YES

YES with No Extreme Storm

Collective Consistency Not Needed

S-1






Section 1

1-1

Section 1 Background

The City of Alexandria Long Term Control Plan Update (LTCPU) is being developed to determine

appropriate improvements to the City’s approved LTCP called for by the Virginia Department of

Environmental Quality (VDEQ) as described in the Regulatory Requirements Technical Memorandum.

Appropriate upgrades to the combined sewer system will be selected using a variety of factors as

described in the Evaluation Criteria Technical Memorandum. Among those evaluation criteria addressed

in this memorandum are considerations of:

Improving water quality;

Addressing water quality standards;

USEPA National CSO Policy Performance including:

Presumption and Demonstration Approach; and

Addressing the Hunting Creek TMDL.




Section 2

2-1

Section 2 Short List of Combined Sewer Control Strategies

A short list of LTCPU combined sewer control strategies has been selected as described in the Alternative

Evaluation: Ranking and Recommendation Technical Memorandum. The selection process included

looking at a long list of individual technologies, applying appropriate technology to the various outfalls

and developing technology alternatives for different parts of the City’s combined sewer system. The

individual technology alternatives identified are included in Table 2-1. Figure 2-1 provides an overview

of the process.

Table 2-1

Combined Sewer Control Technologies Evaluated

Technology Alternatives

T1-A – Relocate CSO-003 and CSO-004 to AlexRenew

T2-A Combine CSO-002, CSO-003, and CSO-004 and Relocate to AlexRenew

T3-A - Combine CSO-002, CSO-003, and CSO-004 and Relocate to the Potomac River

T4-A - Relocate CSO-002 to the Potomac River

ST002-A – CSO-002 Storage Tank

ST003/4-A CSO-003/4 Storage Tank

D002-A - CSO-002 - Disinfection

D003/4-A CSO-003/4 - Disinfection

SE002 – Separation for CSO-002

SE003/4 – Separation for CSO-003/4

GI002* – Green Infrastructure for CSO-002

GI003/4* – Green Infrastructure for CSO-003/4

*Green infrastructure cannot be implemented in an

effective manner to meet the Hunting Creek TMDL

Two scenarios were studied to size the various alternatives to reduce CSO volume and frequency to meet

the goal of the TMDL:

Scenario A: Capture and retain the CSO volume of the 5th largest storm in the typical year

(1984), for CSO outfalls 002, 003, and 004. Consistent with the presumption approach (POi) of

the National CSO Policy, which results in four overflows per year in the typical year.

Scenario B: Capture and retain the CSO volume to achieve 80% (002) and 99% (003 and 004)

bacteria reduction for the largest storm in the 2004-2005 TMDL period.

The Scenario B sizing is in strict accordance with the assumptions and requirements of the TMDL

modeling. The TMDL modeling was based on 80% control for CSO-002 and 99% control for CSO-003

and CSO-004 during each day. Alternatively, Scenario B could be achieved on an annual basis with




Section 2

2-2

reduced sizing. For example, CSO-002 could be sized to capture 100% of most of the storms, but less

than 80% of the really large storm event. As noted in the Regulatory Requirements Technical

Memorandum, the City has repeatedly raised concerns with many of the assumptions associated with the

TMDL modeling. The City believes the assumptions do not represent the actual nature of CSO impacts

or an understanding of how CSOs are typically controlled.

Figure 2-1

LTCPU Decision Process

CSO Technologies Screening

(43 Technologies)

Preliminary Alternatives Evaluation

(12 Site Specific Alternatives)

CSO Control Strategies

(9 Strategies)

Ranking and

Scoring

Short List of CSO Control

Strategies




Section 2

2-3

The alternatives were then combined into nine combined sewer control strategies, which were then

evaluated and subsequently ranked in the Alternative Evaluation: Ranking and Recommendation

Technical Memorandum. As a result of the ranking a short list of combined sewer control strategies was

selected for further evaluation and engineering analysis. The short list of strategies is presented in Table

2-2.

Table 2-2

Short Listed Combined Sewer Control Strategies

Strategy Description Receiving Water

S-7 Storage Tunnel for 003/4 and Tank at 002 Hooffs Run and Hunting Creek

S-3a Separate Storage Tunnels 002 and 003/4 Hooffs Run and Hunting Creek

S-3b Separate Storage Tunnels 002 and 003/4 and Outfall Relocation for 002 to the Potomac

Hooffs Run and Potomac River

S-1 Storage Tunnels for 002/3/4 Hooffs Run and Hunting Creek




Section 3

3-1

Section 3 Modeling Methods

To address the water quality performance of the short list of combined sewer control strategies, two sets

of models are used as follows:

Hydrologic and Hydraulic Modeling; and

Water Quality Modeling.

3.1 Hydrologic and Hydraulic Modeling

The LTCPU uses software that is based on EPA's Storm Water Management Model (SWMM) to estimate

the quantity and quality of CSO. SWMM is used throughout the world for planning, analysis, and design

related to stormwater runoff, combined and sanitary sewers, and other drainage systems in urban areas.

SWMM is a dynamic hydrology-hydraulic water quality simulation model. It is used for single event or

long-term (continuous) simulation of runoff quantity and quality from the entire sewer service drainage

area. SWMM tracks the quantity and quality of runoff made within each sub catchment. It tracks the

flow rate, flow depth, and quantity of water in each pipe segment. SWMM is used in the LTCPU to

simulate the addition of storage, treatment, pumps, and regulators to the combined sewer system. The

details of CSO modeling are described in the Hydrologic and Hydraulic Modeling Plan dated January

2015.

3.2 Water Quality Modeling

Water quality modeling for the LTCPU is based on a model developed by the Virginia Institute for

Marine Sciences (VIMS) to develop Bacteria TMDLs for the Hunting Creek, Cameron Run, and Holmes

Run Watersheds (VDEQ 2010).

The VIMS Euler-Lagrangian Circulation (ELCIRC) model was used for the assessment of water quality

in tidally-impacted waters of Cameron Run, Hooffs Run, and the Hunting Creek Embayment. The

ELCIRC model is a continuous simulation model and it was applied to assess LTCPU strategies for 2004-

2005, the same time period used for the VDEQ Hunting Creek Bacteria TMDL modeling previously

conducted by VIMS. The ELCIRC model grid contains 4,550 nodes which encompass all of the tidal

waters in the Hunting Creek and a portion of the mainstem Potomac River upstream and downstream

(from Bellevue in the North to Belle Haven/New Alexandria in the South) as shown in Figure 3-1.




Section 3

3-2

Figure 3-1

ELCIRC Model Overview




Section 3

3-3

The bacteria loading sources that are assessed in ELCIRC are:

Municipal Separate Storm Sewer System (MS4) and nonpoint source edge-of stream (EOS),

and wildlife direct deposition loads;

Potomac River loads;

City of Alexandria CSO loads; and

Alexandria Renew Enterprises (AlexRenew) Water Resources Recovery Facility (WRRF)

loads.

MS4 and nonpoint source EOS loads, and direct deposition of bacteria to streams by wildlife in the

Hunting Creek watershed are represented by the final TMDL scenario condition (83% to 98% reduction

for EOS, 50% reduction for wildlife) for the LTCPU alternatives evaluation. These loading sources were

all determined with the Hydrologic Simulation Program in FORTRAN-based watershed model (HSPF

model) during the TMDL study and are utilized without modification. Potomac River loads are

represented by upstream and downstream tidal boundary conditions which are varied from TMDL

assumptions as described in Section 4, and Chesapeake Bay watershed model runoff loads that are

unmodified from those used in the final TMDL scenario (i.e., associated bacteria concentrations are set at

the water quality standards). The CSO loads are represented by LTCPU-specific flow and load reduction

alternatives simulated with the SWMM model. The AlexRenew discharges are simulated as time-varying

by the SWMM model to represent the TMDL-allocated design flow of 66 mgd2 during dry weather and

flows of up to approximately 105 mgd during wet weather conditions.

For the LTCPU alternatives evaluation, the ELCIRC model-predicted impacts of these loading sources on

bacteria levels in tidal Hunting Creek were post-processed following the time- and spatial-averaging

protocols employed for the TMDL study modeling effort. The ELCIRC post-processing also required use

of the VDEQ bacteria translator equation used for the TMDL. This was needed because the modeled

instream fecal coliform concentrations must be converted in order to evaluate predicted conditions against

the applicable monthly geometric mean water quality standard for E. coli.

2 66 mgd was allocated in the Hunting Creek TMDL as 54 mgd for the average discharge flowrate at AlexRenew

with an additional 12 mgd allocated for future growth.




Section 4

4-1

Section 4 USEPA National CSO Policy Performance

As described in the Regulatory Requirements Technical Memorandum, the EPA CSO Policy has two

approaches for the development of CSO abatement alternatives. These are the presumption approach and

the demonstration approach. Both approaches are evaluated under this LTCPU.

4.1 Presumption Approach

The presumption approach is met if any one of the following criterion are met:

POi-Presumption Option i – “No more than an average of four overflow events per year,

provided that the permitting authority may allow up to two additional overflow events per

year”;

POii- Presumption Option ii – “The elimination or the capture for treatment of no less than

85% by volume of the combined sewage collected in the CSS during precipitation events on a

system-wide annual average basis”; and

POiii- Presumption Option iii – “The elimination or removal of no less than the mass of the

pollutants, identified as causing water quality impairment through the sewer system

characterization, monitoring, and modeling effort, for the volumes that would be eliminated or

captured for treatment under paragraph ii.”.

The performance of the alternative technologies individually applied to the outfalls is presented in Table

4-1. As shown all alternatives meet and exceed one or more of the presumption criteria with the

exception of Green Infrastructure.

Table 4-1

LTCPU Alternatives Overflow Summary*

Based on overflows during the Typical Year 1984

Presumption Option POi POii POiii

Alternative # of

Overflows per Year

% Capture* Equivalent Load

Removed

T1-A – Relocate CSO-003 and CSO-004 to AlexRenew

3 96.9% 96.9%

T2-A Combine CSO-002, CSO-003, and CSO-004 and Relocate to AlexRenew

4 95.4% 95.4%

T3-A - Combine CSO-002, CSO-003, and CSO-004 and Relocate to the Potomac River

4 95.4% 95.4%

T4-A - Relocate CSO-002 to the Potomac River 4 94.2% 94.2%

ST002-A – CSO 002 Storage Tank 4 94.2% 94.2%




Section 4

4-2

Presumption Option POi POii POiii

ST003/4-A CSO 003&004 Storage Tank 4 96.9% 96.9%

D002-A - CSO 002 - Disinfection 53 59.6% 96.0%3

D003/4-A CSO 003/004 - Disinfection 60 78.9% 97.9%

SE002 – Complete Separation CSO 002 0 N/A N/A4

SE003/4 – Complete Separation CSO 003/004 0 N/A N/A

GI002 – Green Infrastructure for CSO 002 41 67.6% 67.6%

GI003/4– Green Infrastructure for CSO 003/004 43 85.9% 85.9%

These technologies were considered and assembled in to a list of nine (9) combined sewer control

strategies; these strategies were then ranked against the evaluation criteria and a short list of three (3)

strategies were recommended for further consideration. The combined sewer control strategies have the

following estimated performance with respect to the presumption criteria:

Table 4-2

Short Listed Strategies: Presumption Approach Performance

Strategy Description

Presumption

Option POi Overflows/Yr.

Presumption

Option POii

% Capture

Presumption

Option POiii

Equivalent Load

4 Average

(up to 6 maximum) 85% Minimum 85% Minimum

S-7

Storage Tunnel for CSO 003/4 (Hooffs Run) and Storage Tank at CSO 002 (Hunting Creek)


S-3a

Separate Storage Tunnels 003/4 to Hooffs Run and 002 to Hunting Creek


S-3b

Separate Storage Tunnels 003/4 to Hooffs Run and potential Outfall Relocation for 002 to the Potomac


S-1 Storage Tunnels for 002/3/4 (Hooffs Run and Hunting Creek)


3 Equivalent Load Capture for disinfection based on 100% of flow captured and one log kill for remaining load 4 Separation meets the POi criteria for overflows per year; however, E.coli from stormwater will remain.




Section 4

4-3

4.2 Demonstration Approach

Under the EPA CSO policy, a control level less than called for by the presumption approach can be

selected if it can be demonstrated that “the CSO discharges remaining after implementation of the

planned control program will not preclude the attainment of water quality standards or the receiving

waters' designated uses or contribute to their impairment.” To demonstrate this, the LTCPU evaluated

the alternatives using the water quality modeling described above. These modeling methods were

developed for the Hunting Creek TMDL. As described in the Regulatory Requirements Technical

Memorandum, there are five assumptions in the Hunting Creek TMDL that need to be assessed to address

the CSO Policy demonstration approach definition stated above. These assumptions include the

following:

AlexRenew WRRF Load;

Potomac River Boundary;

Proportional versus Discrete Controls;

Decay Rates; and

2004-2005 Hunting Creek TMDL Climate Period.

4.2.1 AlexRenew WRRF Load

The Hunting Creek TMDL includes the following statement about the AlexRenew discharge:

“In tidal Hunting Creek, two additional conservative assumptions were made. First, the

concentration of the source responsible for the largest volume of water entering tidal Hunting

Creek, ASA’s WWTP [AlexRenew WRRF], was set at the fecal coliform equivalent of its monthly

E. coli permit limit, 126 cfu/100 ml, which is also the geometric mean water quality criterion.

The WWTP load used in the TMDL greatly overstates the load discharged by the AlexRenew WRRF. To

evaluate the demonstration approach for the LTCPU, actual load from the treatment plant has been

analyzed. The following table includes statistics for the treatment plant E. coli discharge during the 2004-

2005 Hunting Creek TMDL assessment period.

Table 4-3

2004-2005 AlexRenew Wet Day and Dry Day Effluent Averages for E. coli (cfu/100 mL)

Wet Days Dry Days

Max 30-Day Average

Max 30-Day Geomean

Max 30-Day Average

Max 30-Day Geomean

Average 7.13 1.50 2.38 1.36

Standard Deviation

8.59 0.48 1.37 0.25




Section 4

4-4

As shown the plant performance is much better than the performance assumed in the TMDL scenario and

its’ associated allocations of bacteria load.

4.2.2 Potomac Boundary

The Hunting Creek TMDL includes the following statement about the bacterial load from the upstream

Potomac:

“…TMDL scenarios for tidal Hunting Creek were developed based on the principle that the tidal

drainage to Hunting Creek had to meet water quality standards without significant dilution from

the Potomac River.” (Section 5.1)… “The concentrations at the boundaries of the model domain

in the Potomac River were held at the fecal coliform equivalent of the E. coli geometric mean

standard of 126 cfu/100 ml” (Section 5.1)

For the purposes of evaluating the actual expected E. coli load under the demonstration approach, the tidal

Potomac River boundary concentrations upstream and downstream of the Hunting Creek Embayment

were specified based upon the current DC CSO LTCP selected scenario (CDMod-2c) upon approval for

use by DC Water (DC Water 2014). The DC LTCP scenario used the EPA-approved Dynamic Estuary

Model (DEM) to simulate bacteria conditions in the Potomac River for both fecal coliform and E. coli

(DCWASA 2001). The DEM water quality simulation represented full implementation of the selected

scenario to control CSO discharges to predict projected future conditions of Potomac River bacteria levels

during average, wet and dry years (1988-1990) which are different than the selected climate period for the

Alexandria LTCPU study. Given the climate period difference and that DEM modeling results are

unavailable for the LTCPU climate period, the tidal Potomac boundary conditions were specified based

on the average bacteria concentrations predicted by DEM over the 3-year simulation period. For fecal

coliform, the form simulated by ELCIRC for the Hunting Creek TMDL study which is later translated to

E. coli, the upstream (DEM node 129) and downstream (DEM node 18) tidal Potomac River boundary

conditions are 54 cfu/100 mL and 25 cfu/100 mL, respectively.

4.2.3 Proportional v. Discrete Controls

The Hunting Creek TMDL includes the following statement about CSO controls:

“Reductions in CSO bacteria loads were simulated by keeping the simulated bacteria

concentration at the outfall’s baseline level, but proportionately reducing flows on each day an

overflow occurs. In other words, a 50% reduction in CSO loads was implemented by reducing

flows by 50% for each overflow event.” (Section 5.2.2)

The TMDL modeled all controls on bacteria loading sources proportionally. If the WLA for stormwater

calls for a 90 percent reduction, each storm event is reduced by 90%. CSO loads are modeled the same

way. While most stormwater and other controls may be well represented by this proportional approach,

the alternative CSO controls selected for further study under the LTCPU eliminate all loads from small

storms and allow a limited number of large storms to continue with some varied control depending on the

storm size. In evaluating the demonstration approach proportional controls are used where appropriate

(such as for stormwater) and discrete controls will be used for storage and conveyance options for CSOs.




Section 4

4-5

For this water quality assessment, a control level of no more than 4 overflows in 1984 (the Scenario A

sized alternatives) was used based on storage or tunnel technology alternatives.

4.2.4 Decay Rates

The Hunting Creek TMDL includes the following statement about the decay rates for the tidal waters:

“…the simulation which uses a decay rate of 0.1/day…”

The Hunting Creek TMDL was developed with a model (ELCIRC) that was calibrated to the available

data by turning the decay rate down to an extremely low level (0.1/day). Typical literature-based

minimum bacteria decay rates are nearly an order of magnitude higher, (Chapra 1997; Thomann and

Meuller 1987; Thomann 1972). Modelers conducting the calibration have indicated they used the very

low decay rate to account for multiple undetermined factors understanding that the exceptionally low

decay rate adjusted for what was likely a higher load than modeled. As commented on during the TMDL

development, the calibration should have also adjusted load to obtain a more realistic decay rate. While

the VDEQ approach with the very low decay rate may provide a good calibration fit to observed data, it

renders the model impractical for evaluating real-world CSO controls because other sources of bacteria

loading are likely under-represented.

Rather than simply employing a literature-based bacteria decay rate for the LTCPU study modeling

needs, a site-specific rate based on the calibrated DEM water quality model that was used in the EPA-

approved DC CSO LTCP to simulate both fecal coliform and E. coli levels in the tidal Potomac River.

Bacteria die-off was modeled for that effort with a first-order decay rate of 1.5/ day with no temperature

correction (DCWASA 2001). On December 31, 2014, EPA approved an update to DC’s Potomac River

Bacteria TMDL (USEPA 2014) which utilizes the findings from the DEM modeling and converts the

original fecal coliform TMDL allocations to E. coli allocations. Therefore, selection of this higher

alternate decay rate for the Alexandria LTCPU is supported from both TMDL and CSO LTCP

perspectives, both of which EPA has approved for the District of Columbia (DDOE) and DC Water.

4.2.5 2004-2005 Climate Period

The climate period chosen by VDEQ includes a series of storms with atypical frequency and magnitude.

While the TMDL guidance calls for choosing a critical period, the inclusion of extreme wet weather for

the evaluation of recreational standards is not reasonable. While there is no specific regulation specifying

the appropriate frequency for recreational standards (E.coli) there are examples to guide an appropriate

selection. For example, typical frequencies for toxics criteria (such as lead, copper, and zinc) are 3 years.

Dissolved oxygen criteria are not applied below the 7 day 10 year low flow, a condition that typically

occurs once in four years. New Jersey has used a return frequency of once in 3 years for the development

of TMDLs for recreational standards. Of particular concern is the inclusion of the October 2005 wet

weather event. As shown in Table 4-3, the October 2005 storm was a very rare event with a 43-year to a

67-year recurrence.




Section 4

4-6

Table 4-4

Extreme Storm Events included in 2004-2005 Climate Period

Year Event Rainfall

(in.)

Duration (hrs.)

NOAA IDF

Return Frequency1

Weibull Return Frequency2

2005 Largest Storm (October) 7.30 39 43-year 67-year storm 1 Return frequency interpolated from the Alexandria IDF curves developed in Atlas 14, Volume 2, Version 3 2 Weibull Return Period based on 40 years used in the Typical Year Selection TM (1974-2013) 3 A letter was sent to VDEQ on April 15, 2015 detailing the City’s concerns with the 2004-2005 climate period.

For the purposes of assessing the demonstration approach, the LTCPU has included the entire 2004-2005

Hunting Creek TMDL assessment period.

4.3 Water Quality Modeling Scenarios

The water quality modeling scenarios have a number of variables including the following:

4.3.1 Scenario 1 – Verification

The verification scenario was run to demonstrate that the model used in the LTCP update matched the

model used in the Hunting Creek TMDL. This scenario uses the same climate period as the TMDL

(2004-2005), the same CSO frequency as was modeled for the actual conditions of 2004-2005, the

AlexRenew WRRF at an effluent concentration equal to the VPDES Permit (126 cfu/100 mL of E.coli),

upstream loads equal to the TMDL uncontrolled year and the TMDL decay rate (0.1 per day). This

scenario run indicated the model used in the update matched the previous model.

4.3.2 Scenario 2 - All Modifications except Climate

This scenario used the 2004-2005 climate period and the following modifications:

CSO discharges were reduced to four per year based on the infrastructure sizing used in the

preliminary CSO Control Strategies, rather than the percent reduction used in the TMDL;

The WRRF E. coli discharge was revised to estimate actual performance in wet and dry

conditions;

The upstream loads were modeled using the HSPF model on a daily basis;

The Potomac Boundary was set to the DC LTCP estimates; and

The decay rate was set to the DEM decay rate of 1.5 per day.

4.3.3 Scenario 3 - All Modifications except WRRF and Climate

This scenario is similar to Scenario 2 except the WWRF was run at the permit discharge limit of 126

cfu/100 mL.

These three scenarios are summarized in Table 4-5.




Section 4

4-7

Table 4-5

Water Quality Model Scenarios

Scenario Year CSO Frequency WRRF

Concentration Upstream

Loads Potomac Boundary

Decay Rate

1 Verification 2004-2005 TMDL 126 E. coli

(195 Fecal coli) TMDL 126 TMDL

2 All

Modifications except Climate

2004-2005 New CSO Controls

Condition (4 in Typical Year)

Wet Weather Average / Dry

Weather Average HSPF

1988, 89, 90 DC LTCP Average

(no other TMDLs Implemented)

DEM Decay Rate

3

All Modifications except WRRF and Climate

2004-2005 New CSO Controls

Condition (4 in Typical Year)

126 E. coli (195 Fecal coli)

HSPF

1988, 89, 90 DC LTCP Average

(no other TMDLs Implemented)

DEM Decay Rate

As shown in Table 4-6, at this stage of the LTCPU, only the S-7, S-3a, and S-1 strategies with discharge

to Hooffs Run and the Hunting Creek Embayment are modeled. The S-3b strategy with discharge to

Hooffs Run and the Potomac River can be expected to perform better than the other short listed strategies

due to the increased assimilative capacity of the Potomac River.

Table 4-6

Short Listed Strategies and Water Quality Model Runs

Strategy Description Receiving Water Comments

S-7 Storage Tunnel for 003/4 and Tank at 002

Hooffs Run & Hunting Creek Modeled In this Memorandum

S-3a

Separate Storage Tunnels 002 and 003/4 and Outfall Relocation for 002 to the Potomac

Hooffs Run & Hunting Creek Modeled in this Memorandum

S-3b

Separate Storage Tunnels 002 and 003/4 and Outfall Relocation for 002 to the Potomac

Hooffs Run and Potomac River Expected to perform better than modeled in this memorandum

S-1 Storage Tunnels for 002/3/4 Hooffs Run & Hunting Creek Modeled In this Memorandum

4.3.4 Water Quality Scenario 1 - Verification

Scenario 1 is a verification application of the ELCIRC model to ensure that the final Hunting Creek E.

coli TMDL scenario could be repeated and post-processed to effectively match what was presented in the

TMDL report. The final TMDL scenario was re-run using the existing model input files from the TMDL

study and generated hourly fecal coliform predictions for 2004 and 2005 at each of the 4,550 ELCIRC

model grid nodes. The grid nodes encompassing the two VDEQ TMDL spatial assessment areas:




Section 4

4-8

“Upstream Hunting Creek” (above the GW Parkway) and “Hunting Creek Embayment” (below the GW

Parkway) were identified and provided by the ICRBP for use in the LTCPU study. Daily average results

at every grid node were computed, averaged over each assessment area and then translated from a fecal

coliform concentration to E. coli using the VDEQ bacteria translator formula. The daily average E. coli

concentrations for each assessment area were then used to compute monthly geometric mean E. coli

concentrations for comparison to the monthly water quality standard. The 2004-2005 monthly geometric

mean E. coli concentrations for the LTCPU verification scenario were computed to be within <1 cfu/100

mL of those presented for the final TMDL scenario in the Hunting Creek TMDL report (Table C-11,

TMDL Scenario 10-T). The insignificant deviations from the individual monthly values presented in the

TMDL report can be attributed to independent summarization and slight differences that might arise from

the number of significant digits tracked through each post-processing step.

As an additional check on the verification scenario, the following two figures present the results in similar

fashion to those presented in TMDL report Figures 5-3 and 5-4 for the Upstream Hunting Creek and

Hunting Creek Embayment assessment areas, respectively. Daily E. coli predictions and daily rainfall are

also presented on these figures for reference to illustrate the impact of wet weather events on instream

bacteria levels in Hunting Creek waters due to both CSO and stormwater runoff.

Figure 4-1

Scenario 1 (Verification vs. TMDL Base: Upstream Hunting Creek – ELCIRC-predicted

Daily and Monthly E. coli Concentrations




Section 4

4-9

Figure 4-2

Scenario 1 (Verification vs. TMDL Base: Hunting Creek Embayment – ELCIRC-predicted

Daily and Monthly E. coli Concentrations

The computed verification scenario monthly geometric mean E. coli concentrations effectively match

those presented in the TMDL report and the above time-series presentation of these results also matches

those in the TMDL report. These results confirm that the LTCPU verification scenario simulation with

ELCIRC reproduces the original final Hunting Creek bacteria TMDL scenario. All subsequent LTCPU

scenario predictions generated by the ELCIRC model are post-processed in identical fashion to this

verification scenario.

The following can be observed from Figures 4-1 and 4-2 about the TMDL Control WLA and LA:

These figures represent the modeling and control assumptions of the TMDL;

The Monthly Geometric Mean Water Quality Standard for E. coli is not exceeded; however,

there are many months when the daily E.coli is relatively high;

In addition to the monthly geometric mean criterion, the Virginia WQS include the following:

“If there are insufficient data to calculate monthly geometric means in freshwater, no more

than 10% of the total samples in the assessment period shall exceed 235 E. coli CFU/100 ml.”

This 10% criterion does not apply because the monthly geometric mean is shown to be attained

with sufficient data. However the modeling indicates that the water quality standard criterion




Section 4

4-10

of 10% exceedance of 235 counts per 100 ml in a 30 day period (on a month by month basis) is

not exceeded. July 2004 and October 2005 include 3 daily exceedances of the 235.

The WQS and VDH uses 235 counts per ml as a beach advisory criterion as follows:

“For beach advisories or closures, a single sample maximum of 235 E. coli CFU/100 ml in

freshwater”

There are 15 days upstream of Hunting Creek and 22 days in the Hunting Creek Embayment

when the daily E.coli is above the 235 VDH criterions for beach advisory. These could be

considered days when if there were beaches, such beaches would be closed. These advisory

conditions are not violations of the WQS. The modeling indicates such beach advisories would

be called for with or without the existence of the CSOs. As with the 10% criteria, exceedance

of the 235 on a single day does not indicate the water quality standards are not met.

4.3.5 Water Quality Scenario 2 – All Modifications except Climate Period

Scenario 2 incorporates all of the modifications discussed under the Demonstration Approach section

above with the exception of the climate period. CSO loads for this LTCPU scenario represent the

response of specific CSO controls to rainfall events as simulated in the SWMM model rather than the

simple scaling of the CSO loading time series that was done for the Hunting Creek TMDL. This scenario

also incorporates the actual AlexRenew WRRF bacteria concentrations (based on wet- and dry-weather

effluent monitoring data), DEM-based tidal Potomac River boundary condition, and the DEM-based

bacteria decay rate of 1.5/day. Collectively, these modifications better reflect expected future site-

specific load and environmental planning conditions in support of a Demonstration Approach to CSO

control evaluation for the LTCPU study. All other ELCIRC model input specifications were left

unchanged from the TMDL scenario. The purpose of Scenario 2 is to evaluate whether a given level of

CSO control would be sufficient to comply with the E. coli water quality standard under representative

future conditions after full implementation based on the demonstration approach of the EPA CSO Policy.

The post-processed results from ELCIRC for Scenario 2 are tabulated below and indicate that this

scenario would be in compliance with the monthly geometric mean water quality standards for E. coli

through all months during 2004 and 2005 for both the Upstream Hunting Creek and Hunting Creek

Embayment assessment areas.

Table 4-7

Scenario 2 – All Modifications except Climate Period

Processed ELCIRC-based E. coli Predictions for 2004-2005

2004

Monthly E. coli Geomean

(cfu/100 mL) 2005


(cfu/100 mL)

Upstream Hunting Creek

Hunting Creek Embayment



January 4.8 2.0 January 23.2 6.2

February 14.8 3.3 February 12.1 4.0

March 9.9 3.0 March 13.0 5.7

April 7.2 4.6 April 7.3 7.4




Section 4

4-11

2004


(cfu/100 mL) 2005


(cfu/100 mL)





May 12.3 3.4 May 21.7 12.8

June 20.1 7.3 June 6.4 4.3

July 42.9 12.0 July 24.7 7.4

August 42.3 11.6 August 5.4 5.1

September 22.9 11.8 September 3.0 4.2

October 11.1 7.2 October 54.4 11.0

November 8.9 6.8 November 8.7 2.4

December 6.7 3.3 December 22.4 2.4

The Scenario 2 results are also presented in the following figures for comparison with the TMDL base

condition monthly E. coli geometric mean concentrations and to illustrate the daily average bacteria

predictions for each of the two assessment areas.




Section 4

4-12

Figure 4-3

Scenario 2 vs. TMDL Base: Upstream Hunting Creek – ELCIRC-predicted Daily and

Monthly E. coli Concentrations




Section 4

4-13

Figure 4-4

Scenario 2 vs. TMDL Base: Hunting Creek Embayment – ELCIRC-predicted Daily and


The following can be observed from Scenario 2:

Monthly geometric mean and the inapplicable 10% exceedance criteria in the Virginia Water

Quality Standards for E. coli are not exceeded. Water Quality Standards are met.

The scenario establishes the selected CSO Control Strategies meet the Demonstration Approach

for evaluating CSO controls, even with inclusion of the extreme October 2005 storm in the

evaluation.

Only relatively large rainfall events result in conditions where beach advisories might be issued

if there were beaches.

The upstream of Hunting Creek daily average predictions in Figure 4-3 indicate a total of 12

storms over two years that result in short term E. coli values above the VDH beach advisory

criterion of 235 cfu/100 ml. Eleven of these daily peaks are influenced by CSO overflow.

However, the February 2004 peak occurred due to stormwater alone. There was no CSO for

that event. From this it can be concluded that it is very likely that the stormwater alone

contributes significantly to the other 13 wet weather event impacts which elevate instream

bacteria levels in Hunting Creek waters. It is likely that these days would exceed the 235

criterion even if the CSOs were eliminated because stormwater alone would result in E.coli

above 235. This indicates that a higher level of CSO control (above the 4 overflow per/typical




Section 4

4-14

year) is unlikely to return any discernible water quality benefit even with the very high level of

stormwater and wildlife control called for in the TMDL. As noted above single day

exceedances of the 235 criterion are not violations of the WQS.

While instream E. coli levels may be greatly elevated for short periods due to CSO and

stormwater loads, these impacts are not long-lasting and still allow the level of CSO control in

this scenario to meet the water quality standard.

The monthly geometric mean values are very low. This is result of the high level of stormwater

control required in the TMDL and the intermittent CSO occurrence with the controls under

consideration. Only relatively large rainfall events result conditions where beach advisories

might be issues if beaches were present.

Because the monthly geometric mean values are so low, it is likely that the changes in the

boundary condition are not needed to demonstrate meeting the USEPA National CSO Policy

Demonstration Approach for water quality standards. Confirmation of this could be

accomplished with further modeling if needed.

4.3.6 Water Quality Scenario 3 – All Corrections except Climate Period and WWTP Load

This scenario is the same as Scenario 2, except the AlexRenew Load is run at 195 cfu/100 mL (as fecal

coliform, which translates to the monthly WQS of 126 cfu/100 mL as E. coli). The results are shown on

Table 4-8 and the following figures.

Table 4-8

Scenario 3 – All Modifications except Climate Period and WWTP Load

Processed ELCIRC-based E. coli Predictions for 2004-2005

2004


(cfu/100 mL) 2005


(cfu/100 mL)





January 15.8 6.3 January 49.6 12.0

February 34.1 8.2 February 25.7 7.7

March 24.2 7.1 March 27.0 11.0

April 19.4 9.2 April 21.1 11.7

May 27.0 7.4 May 52.1 20.2

June 42.6 12.7 June 17.4 8.4

July 81.2 20.5 July 45.9 11.6

August 88.1 20.0 August 15.1 8.6

September 50.0 18.7 September 10.4 7.9

October 25.8 12.0 October 113.0 20.6

November 21.1 11.9 November 21.4 5.9

December 18.5 7.7 December 40.7 6.4




Section 4

4-15

Figure 4-5

Scenario 3 vs. TMDL Base: Upstream Hunting Creek – ELCIRC-predicted Daily and





Section 4

4-16

Figure 4-6

Scenario 3 vs. TMDL Base: Hunting Creek Embayment – ELCIRC-predicted Daily and


Based on this Water Quality Scenario, the AlexRenew WRRF load does not impact the attainment of

water quality standards. Observations are similar to Water Quality Scenario 2.




Section 5

5-1

Section 5 TMDL Waste Load Allocation

5.1 Waste Load Allocations

On November 2, 2010, VDEQ issued Bacteria TMDLs for the Hunting Creek, Cameron Run, and Holmes

Run Watersheds (Hunting Creek TMDL). The Hunting Creek TMDL includes a number of flexible

implementation aspects that are described in Regulatory Requirements Technical Memorandum. In this

memorandum, the waste load allocations (WLA) are examined as follows:

Discrete Outfall Basis;

Collective Consistency; and

Climate Period Considerations.

Note that waste loads are presented for the strategies that discharge excess flow to Hooffs Run and

Hunting Creek (S-1, S-3a, and S-7) as well as the strategy that discharges excess flow to Hooffs Run and

the Potomac River (S-3b).

5.1.1 Discrete Outfall Basis

Actual WLAs in cfu/year are shown on Table 5-1 for combined sewer outfalls along with the performance

of the selected combined sewer control strategies. As shown, the combined sewer control strategies

selected for further consideration meet the WLA in 1984 (typical year), but do not meet the WLA in the

2004-2005 TMDL Climate Period.

Table 5-1

Waste Load Allocation for Combined Sewer System – Discrete Outfall Basis

Discharge Outfall

Wasteload Allocation (cfu/year)

Selected Alternatives Performance

Typical Year - 1984 TMDL Climate Period 2005

Load (cfu/year)

Meets Allocation?

Load (cfu/year)

Meets Allocation?

Hooffs Run/Hunting

Creek Embayment

(S-7, S-3a, S-1)

002 6.26E+13 2.48E+13 Yes 2.07E+14 No

003/004 1.61E+12 7.90E+12 No 1.14E+14 No

Total 6.42E+13 3.27E+13 Yes 3.22E+14 No

Hooffs Run/Potomac

River

(S-3b)

002 6.26E+13 0 Yes 0 Yes

003/004 1.61E+12 7.90E+12 No 1.14E+14 No

Total 6.42E+13 7.90E+12 Yes 1.14E+14 No




Section 5

5-2

5.1.2 Collective Consistency

Collective consistency evaluates the WLA by examining the total load allocated to the three CSOs and to

the AlexRenew WRRF as a single allocation. The demonstration approach water quality modeling

described above indicates that evaluating water quality with AlexRenew at the VPDES permit limit

(Scenario 3) or at actual performance (Scenario 2) both result in the protection of water quality. The

application of collective consistency does not “preclude the attainment of water quality standards or the

receiving waters' designated uses or contribute to their impairment.” As shown above, all the alternatives

meet the WLA in the typical year. The 2005 year assessment indicates that following additional load is

needed to meet the WLA:

Table 5-2

Load Deficit Discrete Collective Consistency

Combined Sewer Control Strategy Total WLA Load Deficit

S-7, S-3a, and S-1

Hooffs Run/Hunting Creek Embayment 6.42E+13 3.22E+14 -2.58E+14

S-3b

Hooffs Run/Potomac River 6.42E+13 1.14E+14 -4.98E+13

The AlexRenew WLA is shown as follows:

Table 5-3

E. coli Waste Load Allocation for AlexRenew Water Resources Reclamation Facility

Permit Number

Permit Type

Design Flow (MGD)

Permit Concentration (cfu/100mL)


VA0025160 Municipal 54 126 9.40E+13

*Allocation for the Future Growth of Point Sources: 2.10E+13

Total: 1.15E+14

*Future allocation is based on an additional 12 mgd (66 mgd total)

There is insufficient load available from AlexRenew to address strategies S-7, S-3a, and S-1. For the S-

3b strategy, approximately 43% of the total AlexRenew allocation is needed. The average discharge

concentration for this is approximately 71 cfu/100ml. This is substantially above the historic performance

of the AlexRenew facilities.

5.1.3 Climate Period Considerations

As described above, the Hunting Creek TMDL assessment period includes the October 7, 2005 extreme

storm of 7.30 inches over 39 hours that is well outside of the normal range of water quality assessment for

CSO control and, in particular, for recreational use standards. The WLA is examined here with this

extreme storm removed.




Section 5

5-3

Table 5-4

Waste Load Allocation for COA Combined Sewer System – 2005 without Extreme Event

Combined Sewer Control Strategy Outfall


TMDL Climate Period 2005 – No Extreme Storm

Load (cfu/year)

Meets Allocation?

Hooffs Run/Hunting Creek Embayment

(S-7, S-3a, S-1)

002 6.26E+13 8.70E+13 No

003/004 1.61E+12 3.65E+13 No

Total 6.42E+13 1.23E+14 No

(Yes with CC)

Hooffs Run/Potomac River

(S-3b)

002 6.26E+13 0 Yes

003/004 1.61E+12 3.65E+13 No

Total 6.42E+13 3.65E+13 Yes

As shown the Strategy S-3b meets the total combined CSO allocation with no need for collective

consistency. As noted above, combining the allocation is shown to be acceptable by the Water Quality

Modeling. Strategies S-1, S-3a, and S-7 meet the WLA with an AlexRenew collective consistency of

53%. This would require a plant performance of 60 counts/100ml. As shown above, the plant is

consistently and order of magnitude below this value.




Section 6

6-1

Section 6 Conclusions

It has been shown that for the shortlisted CSO strategies there are paths to meet the National CSO Policy

and Virginia water quality standards through the presumption approach and the demonstration approach.

The 2010 Hunting Creek TMDL is also addressed.

6.1 Presumption Approach

Each of the shortlisted CSO Strategies was evaluated against the National CSO Policy Presumption

Approach using the typical year. All shortlisted strategies meet all three of the presumption criteria

including:

There are no more than 4 overflows per year meeting the requirements of the National CSO

Policy (POi);

There is a 95-96% capture meeting the requirements of the National CSO Policy (POii;) and

There is a 95-96% capture of bacteria meeting the National CSO Policy requirements of POiii.

6.2 Demonstration Approach

The demonstration approach requires that “the CSO discharges remaining after implementation of the

planned control program will not preclude the attainment of WQS (water quality standards) or the

receiving waters' designated uses or contribute to their impairment.” By making reasonable assumptions

it has been demonstrated that with the strategies evaluated, the remaining overflows meet this

demonstration approach. The reasonable assumptions are:

Using the discrete controls of the overflows as defined in the shortlisted strategies rather than

proportional controls assumptions of the TMDL;

Using the EPA-approved DC CSO LTCP in lieu of the 2010 Hunting Creek decay rate. Model

runs with the Hunting Creek decay rate cannot realistically model the discrete CSO controls;

and

Using Potomac River boundary conditions based on data and implementation of the DC Water

LTCP in lieu of setting the river boundary to the Water Quality Standard5.

6.3 Waste Load Allocation

The short listed Control Strategies all meet the 2010 Hunting Creek Waste Load Allocation for the typical

year. The largest storm in the 2004-2005 climate period is a 67-year event. This single storm accounts

for approximately 38 percent of the load under the shortlisted control strategies. Typical water quality

assessment includes recurrence periods of 3-5 years. It can be reasonably concluded that the extreme 67-

year event should be excluded when performing the water quality analysis. It has been shown that by

5Analysis of the model runs indicate that WQS would likely be met even with the boundary conditions set to the

Water Quality Standard (126 cfu/100 mL. However, model runs testing the boundary conditions set to the Water

Quality Standard with discrete controls and the 1.5/day decay rate have not been run to confirm this.




Section 6

6-2

utilizing collective consistency6 the waste load allocation can be met for the TMDL climate period with

the extreme 67-year event excluded.

6 Collective Consistency evaluates the WLA by examining the total load allocated to the three CSOs and to the

AlexRenew WRRF as a single allocation.




Section 7

7-1

Section 7 References

Chapra 1997. Surface Water-quality Modeling by Steven C. Chapra. McGraw-Hill, New York, NY. 1997.

DCWASA 2001. Study Memorandum LTCP-6-5: Potomac River Model Documentation. Prepared for

District of Columbia Water and Sewer Authority. Prepared by Greeley and Hansen Engineering Program

Management Consultant-III Project Team. August 2001.

DC Water 2014. Approval of a City of Alexandria request to use results from the Potomac River DEM

modeling done in support of consent decree modifications, specifically scenario CDMOD-2c. Email

response from Carlton M. Ray, Director, DC Clean Rivers Project. November 14, 2014.

Thomann and Mueller 1987. Principles of surface water quality modelling and control. by Robert V.

Thomann and John A. Mueller. New York, Harper and Row. 1987.

Thomann 1972. Systems Analysis and Water Quality Management by Robert V. Thomann. McGraw-Hill

Book Company, New York, NY.

USEPA 2014. Decision Rationale – 2014 E. coli Bacteria Allocations and Daily Loads for the Potomac

River and Tributaries, TMDL Revision, District of Columbia. United States Environmental Protection

Agency, Region III. December 31, 2014.

VDEQ 2010. Bacteria TMDLs for the Hunting Creek, Cameron Run, and Holmes Run Watersheds.

Submitted by Virginia Department of Environmental Quality. Prepared by Interstate Commission on the

Potomac River Basin. November 2, 2010.

Greeley and Hansen LLC 5301 Shawnee Road, Suite 400

Alexandria, VA 22312 571.581.3000

www.greeley-hansen.com

http://www.greeley-hansen.com/

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