NC0020354_Correspondence_20081023

NPDES DOCIMENT SCANNING COVER SHEET
NPDES Permit:
NC0020354
Pittsboro WWTP
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Complete File - Historical
Engineering Alternatives (EAA)
;,Correspondence
Owner Name Change
Technical Correction
Instream Assessment (67b)
Speculative Limits
Environmental Assessment (EA)
Document Date:
October 23, 2008
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Low -flow characteristics for Roberson Creek near Pittsboro...Re: Pittsboro
2oO U -S &AA
Subject: Low -flow characteristics for Roberson Creek near Pittsboro...Re: Pittsboro
From: John C Weaver <[e-mail]>
Date: Thu, 23 Oct 2008 13:00:18 -0400
To: Mike Templeton <[e-mail]>
CC: Adam Kiker <[e-mail]>, Bob Sledge <[e-mail]>, James
McKay <[e-mail]>, Tim Baldwin <[e-mail]>, John C
Weaver <[e-mail]>
All,
A quick check of the low -flow files here at the USGS North Carolina Water Science Center indicates that
low -flow characteristics for your point of interest on Roberson Creek at SCL RR near Pittsboro (station id
02097069, drainage area 10.5 sqmi) were updated in October 1999 in response to a consulting
engineering firm. This site has the previous 7Q10 and 30Q2 flow estimates at zero and 0.6 cfs,
respectively, that was mentioned in the previous email. Therefore, I'm presuming this is same location as
your specific point of interest on Roberson Creek. No known USGS records of discharge are known to
exist at this specific site, but there are other nearby locations on Roberson Creek where records of
miscellaneous measurements have been collected.
Where no or insufficient data exists to allow a site -specific low -flow analysis for a point of interest, the
low -flow characteristics are determined by assessing a range of low -flow yields (expressed as flow per
sqmi drainage area, or cfsm) at nearby locations where estimates have previously been determined.
The most recent low -flow information published for streams in Chatham County is in a basinwide low -flow
report completed in 2001. It is USGS Water -Resources Investigations Report 01-4094, "Low -flow
characteristics and discharge profiles for selected streams in the Cape Fear River Basin, North Carolina,
through 1998 " (Weaver and Pope, 2001). An online version of the report is available through
http://nc.water.usgs.gov/reports/wri014094/. The report provides the low -flow characteristics (based on
data through 1998) for continuous -record gaging stations and partial -record sites within the Cape Fear
basin. The report also provides low -flow discharge profiles (7Q10, 30Q2, winter 7Q10, and 7Q2) for the
Cape Fear River and selected tributaries within the basin.
Several points for consideration...
(1) As for the updated estimates determined in 1999, the 7Q10 was estimated at 0.03 cfs (below the
"positive flow" threshold used by NC DWQ) and the 30Q2 was estimated at 0.40 cfs. The updated
estimates were based on average of low -flow yields at three continuous -record gaging stations in vicinity of
Pittsboro.
Sta. 02097069 Roberson Creek at SCL RR near Pittsboro
Drainage area 10.5 sqmi
7Q10 = 0.03 cfs (equivalent to 0.0032 cfs)
30Q2 = 0.40 cfs (equivalent to 0.0393 cfs)
winter 7Q10 = 0.41 cfs (equivalent to 0.0401 cfs)
7Q2 = 0.19 cfs (equivalent to 0.0188 cfs)
(2) In Table 8 of the above -reference Cape Fear report, there is one downstream site on Roberson Creek
where sufficient records exist to allow for the determination of low -flow characteristics: Roberson Creek
near Seaforth (station id 02097189, drainage area 27.2 sqmi, at Secondary Road 1939) with records of 21
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Low -flow characteristics for Roberson Creek near Pittsboro...Re: Pittsboro
miscellaneous measurements obtained during the 1954-81 period.
Expressing these estimates as low -flow yields (again, flow per square mile of drainage area, or cfsm)
provides some indication of the yield that could potentially be applicable to your location. The 7Q10
low -flow yield at this site is 0.0007 cfsm and the 30Q2 low -flow yield is 0.0404 cfsm. Similarly, the winter
7Q10 and 7Q2 low -flow yields at this site are both 0.0184 cfsm. Applying these 7Q10 and 30Q2 yield
values to the drainage area at your specific point of interest results in 7Q10 and 30Q2 flow estimates at
zero flow and 0.42 cfs, respectively.
(3) As for the average annual flow, yields for this statistic at the nearby sites are 1.0 cfsm. Applying this
yield to the drainage area (10.5 sqmi) results in average annual flow estimate between 10 and 11 cfs.
(4) As you consider the above information, please be aware of the following note. The above data are
based on periods of record ending in the 1996 water year, well in advance of the drought conditions that
have occurred since publication of the report. In some North Carolina basins, the low -flow conditions
observed during the 1998-2002 and current (2007-08) droughts have resulted in lower low -flow statistics.
No formal statewide investigation has been completed to document the changes in low -flow statistics
since the drought. However, where updated analyses have been completed for selected stations, the
changes in pre -drought versus updated 7Q10 discharges have shown varying percentage changes of
decline.
Putting together the above pieces of information...
It appears appropriate to continue using the estimates provided in response to the 1999 request for your
point of interest (station id 02107069). The 7Q2 and 30Q2 low -flow yields used in the 1999 estimates and
those for the downstream site (02107189) included in the Cape Fear low -flow report can be considered
fairly comparable. However, the 7Q10 and winter 7Q10 low -flow yields at downstream site are much lower
than those used in the updated 1999 estimates. This is an issue that would require further assessment,
but the 7Q10 estimate at your point of interest is less than 0.05 cfs and should probably be considered
zero flow.
Hope this information is helpful.
Thank you.
Curtis Weaver
***********************************************************************
J. Curtis Weaver, Hydrologist, PE
USGS North Carolina Water Science Center
3916 Sunset Ridge Road
Raleigh, NC 27607
Telephone: [phone] Fax: [phone]
E-mail address — [e-mail]
Internet address -- http:llnc.water.usgs.gov/
***********************************************************************
Mike Templeton
To Tim Baldwin <[e-mail]>
2 of 5 10/23/2008 1:01 PM

Low -flow characteristics for Roberson Creek near Pittsboro...Re: Pittsboro
<[e-mail]>
10/10/2008 12:14 PM
Tim -
cc Adam Kiker <[e-mail]>, Bob Sledge
<[e-mail]>, James McKay
<[e-mail]>
Subject Re: Pittsboro
The latest figures I found for Robeson Creek at the Pittsboro discharge are a 7Q10 of zero and a 30Q2 of
0.6 cfs. Those values go back to our '91 wasteload allocation for the Town's discharge. We haven't
updated the flows because any changes in flows this low are unlikely to affect permit limits. If you are
interested in having them re -calculated to include the last 15+ years of flow data, you can request that
from Curtis Weaver at the USGS here in Raleigh, at [phone]. I believe there is a fee for the
service, and, if so, Curtis can fill you in on the details.
I will be out of the office from about midday on Monday until the 22nd. If you have any questions during
that time, please contact Bob Sledge or Jim McKay. Both are familiar with the Pittsboro permit.
-MikeT
Tim Baldwin wrote:
Thanks Mike. Hate to bother you again, but I've been digging around, and I can't seem to find the 7Q10
and 30Q2 in Robeson at the discharge location. Perhaps I am overlooking, but have been through the
Robeson TMDL report and the 319 report. Don't have any info from your last wasteload evaluation. You
wouldn't have any of this at your fingertips or top of your head, would you? It's been reported by anecdotal
discussion that the 7Q10 is zero, but haven't heard about 30Q2, nor seen any real document. Thanks
again!!
From: Mike Templeton [mailto: [e-mail]]
Sent: Friday, October 03, 2008 5:17 PM
To: Tim Baldwin
Subject: Re: Pittsboro
Tim - The WLA in this case limits the amount of phosphorus Pittsboro can discharge to the creek, not
the amount eventually delivered to the lake. So the 146 kg is, in fact, a discharge allocation, and the
delivered values will be less once we apply the TF for one discharge location or the other. Here is a link
to the Robeson Cr. TMDL, and the WLA is described on p. 49 of the report (p. 56 of the PDF
document). - Mike T
Tim Baldwin wrote:
Thanks Mike ... just one clarification. The WLA and transport factor for phosphorus. If the amount
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Low -flow characteristics for Roberson Creek near Pittsboro...Re: Pittsboro
delivered to the lake must be less than 146 kg over that 213 day period, and the transport factor is 82%,
wouldn't that mean that the amount discharged could be up to 146 / 0.82 = 178 kg at the discharge
location?
From: Mike Templeton[mailto:[e-mail]
Sent: Thursday, October 02, 2008 10:28 AM
To: Tim Baldwin
Subject: Re: Pittsboro
Hi, Tim - The time does fly by, doesn't it? Here is info I sent Les after our meeting on the Pittsboro
expansion last November.
First, here are the proposed allocations figures from the Jordan Reservoir nutrient strategy.
TN: 27,514 lb/yr delivered allocation (at the reservoir)
That translates to 36,202 lb/yr discharge allocation at Robeson Creek outfall (76% transport factor) or
27,792 lb/yr at the Haw River near US64 (99% TF)
TP: 3,731 lb/yr delivered allocation
Equivalent to 4,551 lb/yr discharge at Robeson Creek (82% TF), 3,769 lb/yr at the Haw location(99%
TF)
The WLA for the Robeson Creek is given in the TMDL report as a seasonal limit: 146 kg P in the
summer (Apr - Oct, 213 days). That makes it about 1.51b/day, or 0.24 mg/L at 0.75 MGD.
The Robeson Creek WLA was expressed as a discharge load. There was no need in that case to apply
limits as delivered loads, since Pittsboro was the only discharger affected.
Hope this helps.
- Mike T
Michael E. Templeton, P.E.
Point Source Branch/ NPDES
Surface Water Protection Section
NCDENR • DWQ Raleigh, NC
Tel: [phone]
Fax: [phone]
Tim Baldwin wrote:
Hi Mike ... long time no time
Say, our stakeholders group is involved again with the draft EIS. I am rekindling the discussions we had
on maintaining flows in Robeson Creek with very highly treated water. My recollection is that the Robeson
phosphorus TMDL was 144 kg in the seven warm months, and I presume that is delivered to the lake. I
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Low -flow cfiaracteristics for Roberson Creek near Pittsboro...Re: Pittsboro
am looking to refresh my memory on the attenuation/delivery factors for both nitrogen and phosphorus for
the current Pittsboro WWTP location. I seem to recall a factor of 88%, but can't recall if that is accurate
and for TN or TP. Can you help? Thanks!
Tim Baldwin
Director of Total Water Management
McKIM&CREED
ENGINEERS SURVEYORS PLANNERS
1730 Varsity Drive Suite 500
Raleigh, NC 27606
919.233.8091
919.233.8031 fax
http://www.mckimcreed.com
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company. Finally, the recipient should check this e-mail and
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accepts no liability for any damage caused by any virus
transmitted by this e-mail.
This e-mail and any files transmitted with it are confidential
and intended solely for the use of the individual or entity to
whom they are addressed. If you have received this e-mail in
error please notify the system manager. Please note that any
views or opinions presented in this e-mail are solely those of
the author and do not necessarily represent those of the
company. Finally, the recipient should check this e-mail and
any attachments for the presence of viruses. The company
accepts no liability for any damage caused by any virus
transmitted by this e-mail.
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