NC0023965_Speculative Limits_20020227

NPDES DOCIMENT !SCANNING: COVER SHEET
NPDES Permit:
NC0023965
Wilmington Northside WWTP
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Complete File - Historical
Engineering Alternatives (EAA)
Correspondence
Owner Name Change
Instream Assessment (67b)
Speculative Limits
Environmental Assessment (EA)
Document Date:
February 27, 2002
This document is printed on reuse paper - ignore any
content on the reYerse side

State of North Carolina
Department of Environment
and Natural Resources
Division of Water Quality
Michael F. Easley, Governor
William G. Ross, Jr., Secretary
Gregory J. Thorpe, Acting Director
February 27, 2002
Mr. Hugh Caldwell, P.E.
Director of Public Utilities
City of Wilmington
P.O. Box 1810
Wilmington, North Carolina 28402
AT:Ii1FrA
NCDENR
NORTH CAROLINA DEPARTMENT OF
ENVIRONMENT AND NATURAL RESOURCES
Subject: Speculative Limits
Wilmington Northside WWTP
NPDES No. NC0023965
New Hanover County
Dear Mr. Caldwell:
This letter is offered in response to your request for speculative effluent limits for the
expansion of the Wilmington - Northside wastewater treatment plant. The plant currently has a
permit to discharge 8 MGD of treated municipal wastewater to theePtItCape Fear River in
New Hanover County. You requested speculative limits for a potential flow increase to 16 MGD.
The current permit is in the renewal process and expired December 31, 2001 [the City is
authorized to continue to operate under this permit until the renewal permit is issued].
The Division of Water Quality (Division) has reviewed the request for expansion and
provides the following response with the speculative limits. The speculative limits presented here
are based on our understanding of the proposal and of present environmental conditions. The
Division cannot guarantee that it will issue the City an NPDES permit to expand its discharge of
treated wastewater into waters of the State. Nor can the Division guarantee that the effluent
limitations and other requirements included in any permit will be exactly as presented here. Final
decisions on these matters will be made only after the Division evaluates a formal permit
application for the City's discharge and notices the proposal for public comment.
Environmental Reviews of New Projects and Expansions:
As you are likely aware, the City will have to evaluate this project for environmental
impacts before applying for a permit modification. The environmental review should contain a
clear justification for the proposed project. It should provide an analysis of potential alternatives,
including a thorough evaluation of non -discharge alternatives. Nondischarge alternatives or
alternatives to expansion, such as spray irrigation, water conservation, or inflow and infiltration
reduction, are considered to be environmentally preferable to a surface water discharge.
In order to make sure information is complete within the environmental review, we
recommend that the City contact Milt Rhodes, Community Planner with the Local Government
Assistance Unit, at [phone], ext. 366.
1617 MAIL SERVICE CENTER, RALEIGH, NORTH CAROLINA 27699-1617 - TELEPHONE [phone]/FAX [phone]
AN EQUAL OPPORTUNITY AFFIRMATIVE ACTION EMPLOYER - 50% RECYCLED/ 10% POST -CONSUMER PAPER
VISIT US ON THE WEB AT http://h2o.enr.state.nc.us/NPDES

Mr. Hugh Caldwell, P.E.
Page 2
Review of the Water Quality Model
The Division forwarded the water quality model for the Lower Cape Fear River Basin,
prepared by your consultant, to EPA Region 1V for assessment and review. The Division's
Modeling/TMDL Unit comments are attached with this letter, which also incorporates comments
from EPA Region IV. To summarize, the Division is not confident that this model adequately
reflects conditions occurring or predicted to occur in the Lower Cape Fear River. Therefore, the
Division cannot use this model in its current form to determine limits for this expansion phase at
the Wilmington Northside Plant. Due to the extensive complexity of the 3-D model and estuary
system, the main concern with this modeling effort is the "lack of data on the Cape Fear Estuary
system." Another area of concern is that the model "consistently over -predicts dissolved oxygen in
several areas by 1 to 1.5 mg/1." However, the Division does believe the work on this model can be
used to form the basis of a TMDL, which in turn may eventually be used for evaluation of future
allocations for the City of Wilmington and other point sources. Should you have specific concerns
regarding the comments in the memo, you may contact Michelle Woolfolk (Modeling/TMDL Unit
Supervisor) directly.
Speculative Effluent Limits [refer to Table 1 below]
Based on the available information, tentative limits for the proposed discharge to Northeast
Creek in the Cape Fear River Basin are listed in Table 1. Some aspects of the speculative limits
are explained below.
BOD5 and NH3-N. Due to the continued depressed dissolved oxygen levels in the lower Cape Fear
River and the impaired water body status, the Division must continue to recommend the strategy
specified in the Cape Fear River Basinwide Water Quality Plan. Specifically, the Division will
continue to recommend summer limits of 5 mg/1 BOD5 (monthly average) and 1 mg/1 NH3-N
(monthly average).
Chlorine. The Division requires chlorine limits and dechlorination for all new or expanding
dischargers proposing the use of chlorine for disinfection. The process of chlorination/
dechlorination, or an alternate form of disinfection such as ultraviolet, should allow the facility to
comply with the total residual chlorine limit of 28 ug/1 (this is based on an acute toxicity value
and is independent of dilution).
Toxicity Testing and Toxicant Parameters. Estimation of toxicant parameters will not be given with
this speculative letter, but will be submitted as appropriate with the draft NPDES permit. The
Division will take into consideration the modeling efforts performed to determine potential dilution
ratios. Staff of the Division will contact you or your consultant if further information is needed
prior to drafting the expansion permit.
Table 1. Speculative Limits for the Expansion of the Wilmington Northside Plant to 16 MGD.
,�.T,-e:Lr�.f c'xYx-''3' .�-' s_.....4,p ri'>
&�a'a:�<r!v ,.raw`-+0.F.'ror1 aOif
_t..T3ist rs'• -.i..,.1/4,4..wa_a.:
.Flow (MGD)
16
16
BOD5 (mg/I)
5
10
NH3-N (mg/I)
1
2
DO (mg/1) daily avg.
6 minimum
6 minimum
Fecal col form (# / 100m1)
200
200
TSS (rng/1)
30
30
Total Residual Chlorine
lug/1)
28
28
NOTE: Monthly averages are presented above unless stated otherwise.

Mr. Hugh Caldwell, P.E.
Page 3
Again, although the current model cannot be used for the development of speculative limits
for this expansion phase, the information will be used to develop the basis of the TMDL for the
Lower Cape Fear River. Please contact Dave Goodrich at (919) 733 — 5083, ext. 517 or Michelle
Woolfolk, ext. 505 if you need additional information or clarification.
Sincerely,
cc: Central Files, w/attachment
Wilmington Regional Office, Water Quality Section, w/attachment
PDES file, w/attachment
Michelle Woolfolk, Modeling/ TMDL Unit
Tim Wool, EPA Region IV
Coleen Sullins

NCDENR - Division of Water Quality
Water Quality Section
Interoffice Memorandum
January 8, 2002
TO: Bill Reid, Point Source Branch
Dave Goodrich, NPDES Unit
FROM: Michelle Woolfolk, Modeling/TMDL
RE: Review of the 3-Dimensional EFDC Water Quality Model of the Lower Cape Fear River
and Its Estuary Report -
The modeling unit and EPA Region IV have completed our review of the modeling report for the
Cape Fear Estuary. As you know, the Cape Fear Estuary is on the 2000 North Carolina 303(d)
List of impaired waters due to low dissolved oxygen. Therefore, a TMDL will be required for
this estuary. We would prefer to have one estuary model to guide all management decisions,
including TMDL development and permitting. However, we are not prepared to endorse this
model, as currently configured, for TMDL purposes. •
An over-riding concern for this modeling effort is the lack of data on the Cape Fear Estuary
system. A portion of the data needed is described in Section 7 of the report. Although certain
water quality parameters are monitored on a regular basis by both DWQ and the Lower Cape
Fear River Basin Association, these data are often insufficient to drive complex, mechanistic
models. As such, it is difficult to determine if the model is sufficiently parameterized and
calibrated to describe the assimilative capacity of the Cape Fear Estuary. It is also difficult to
determine the adequacy of the calibration period and selection of critical conditions. Following
additional data collection, the model can be built upon to form the basis of a TMDL. We have
the following comments to make regarding the Cape Fear River and Estuary Model using EFDC:
General Comments:
• We agree with the selection of EFDC as the modeling framework for the Cape Fear River
Estuary. This is a well -documented model that has been successfully applied to other waters
throughout the country. Reproduction of portions of the EFDC manual within the report is
not needed.
• A thorough review of the data and data sources used to derive model parameters and inputs
should be presented. This data summary can also be used to support model selection and the
dimensionality. Although some data summarization is provided in scattered locations
throughout the report, it should be consolidated.
♦ The report should clarify whether BOD or carbon is the primary simulation parameter in the
estuary. If there is no gradient in the system, and the point and natural conditions are similar,
the model may not provide insight to the competing pollutant sources in the estuary.

Specific Comments:
• Additional text is needed to describe the dimensionality of the estuary, including data to
support the need for multiple dimensions. The current application of EFDC in three
dimensions is questionable since there doesn't appear to be a strong dissolved oxygen
gradient based on the data presented. Given the large amount of energy moving through the
estuary and the relative narrowness of the channel, multiple dimensions would not appear to
be needed.
• Verify that hydrodynamic model calibration plots are from the same model run. There seems
to be some discrepancy between plots.
• The report should provide water surface elevation plots at multiple locations in the estuary.
Currently, the only plot provided is at the NOAA Tide Gauge in Wilmington. The
performance of the model in the upper estuary is unknown. We recognize .that the lack of
such comparisons may be due to a lack of monitoring data.
• The report and/or model should address the processes causing over -prediction of salinity at
station 4 and under -prediction at stations 3 and 3.5. The salinity prediction at Station 8,
which is in the vicinity of the discharges, indicates that there is excess energy forced up the
river.
• Additional information is needed to describe how the point source BOD load was converted
to a carbon load. The same is true for the nutrient loads to the water quality model.
• A description of the analysis conducted to justify collapsing the water quality grid from four
to two vertical layers should be presented. Breaking the vertical layers into two equal
volumes could cause a larger impact on dissolved oxygen in the surface water from the
underlying layer.
• The report should describe a dissolved oxygen calibration objective and whether the model
meets this objective. Currently, the model consistently over -predicts dissolved oxygen in
several areas by 1 to 1.5 mg/L. In fact, as pointed out during the presentation, the SOD
sensitivity model simulations fit the observed data better than the calibrated simulation.
• We agree with the inclusion of inputs from surrounding marshes/wetlands to the estuary.
Conceptually, this would allow the model to simulate the impact from those adjacent
systems. However, additional field study would be needed in order to quantify the impact on
the estuarine system.
• An analysis of loading from all inputs, point and nonpoint, should be presented. A loading
analysis would illustrate the relative contribution from the upstream boundary, the point
sources, the marsh/wetlands, and the downstream boundary.
cc: Boyd DeVane, Planning Branch
Milt Rhodes, Local Gov't Assistance Unit
Tim Wool, USEPA Region IV
9