NC0004308_More Information (Received)_20210111

tn
water resources I environmental consultants 3 lnnwood Circle, suite 220 • Little Rock, AR 72211 • [phone] • Fax [phone]
TECHNICAL MEMORANDUM
DATE: January 11, 2021
TO: Jason Mibroda
Alcoa Corporatio��nn�
FROM: Philip Mass&Id Nathan Siria
FTN Associates, Ltd.
SUBJECT: Estimated Assimilation of Badin Business Park Outfall 005 Discharges in
Little Mountain Creek Under Modeled Critical Conditions
FTN No. R06010-1805-001
1.0 INTRODUCTION
This memorandum presents a technical analysis of assimilation of discharges from Outfall 005 at
the Badin Business Park (BBP) facility located in Badin, North Carolina, in order to achieve
compliance with North Carolina water quality regulations. The analysis examines critical
conditions (i.e., conservative estimate based on receiving water low flow estimates to ensure the
discharge does not contribute to an excursion above water quality standards) for Little Mountain
Creek based on North Carolina Water Quality Standards and effluent now characteristics for
Outfall 005.
Outfall 005 is one of multiple outfalls permitted to discharge groundwater, stormwater, and/or fire
protection water from the BBP facility as authorized by National Pollutant Discharge Elimination
System (NPDES) Permit NC0004308. The sources of water for Outfall 005 include both
stormwater and groundwater, resulting in a continuous discharge that has a wide range of now
rates related to varying hydrologic conditions. Discharges from Outfall 005 now through a
roadside ditch (i.e., unnamed tributary) along the east side of State Highway 740 for approximately
900-1000 ft before entering Little Mountain Creek (NC Stream Index # 13-5-1-[2]) (Figure 1).
Little Mountain Creek is classified as a WS-IV (i.e., water supply). There are no US Geological
Survey (USGS) stream flow gages or published 7Q10 flow values for Little Mountain Creek;
however, the October 2015 fact sheet associated with the NPDES permit for this facility identifies
Little Mountain Creek as a zero -flow stream during critical conditions.
Regional Offices: Fayetteville, AR; Baton Rouge, LA; Chesterfield, MO • www.ftn-assoc.com • [e-mail]

Mr. Jason Mibroda
December 11, 2020
Page 2
Legend
New (shallow) drainage system
Remaining pre-2015 drains
Outfall 005 watershed
Outfall OD5
Figure 1. Map of BBP Outfall 005 and its watershed.
Z� F11-1

Mr. Jason Mibroda
December 11, 2020
Page 3
This evaluation focuses on meeting standards in Little Mountain Creek rather than the roadside
ditch that conveys discharges from Outfall 005 to Little Mountain Creek. This assumes that either
the existing roadside ditch will be deemed an "effluent channel" by the North Carolina Department
of Environmental Quality (NCDEQ), or the discharges will be conveyed from Outfall 005 to
Little Mountain Creek via a pipeline or a constructed ditch that would qualify as an "effluent
channel".
In the current permit, the monthly average limits for total cyanide and fluoride at Outfall 005 were
developed assuming zero dilution in the receiving stream during critical conditions. The North
Carolina Water Quality Standards (15A NCAC 02B .0206) state that the 7Q 10 now should be used
for determining effluent limits to protect aquatic life from chronic toxicity. The Standards also
recognize that alternative "flow strategies" may be acceptable if they provide equal or better
protection for water quality standards compared to setting effluent limits based on 7Q10
conditions. Due to the flow characteristics of Outfall 005, the protection of water quality standards
was evaluated utilizing two different modeling approaches:
1) Dilution Assessment for 7Q10 Conditions: The quantity of dilution during low -flow
conditions was evaluated using a simple steady state calculation for 7Q10 conditions (the
lowest 7-day average stream flow that is likely to occur in one out of ten years).
2) Dilution Assessment for Variable Hydrologic Conditions: Quantities of dilution were
calculated for variable hydrologic conditions by modeling Little Mountain Creek and
Outfall 005 discharges on a daily basis over a long-term 19-year period. This analysis was
conducted to assess the impact of variable hydrologic conditions on the dilution ratio and
differs from the 7Q10 flow assessment in that the statistical analysis is conducted on the
daily dilution ratios as compared to one based on a single low -flow statistic.
2.0 DILUTION ASSESSMENT FOR 7Q10 CONDITIONS
2.1 7Q10 Flow for Little Mountain Creek
For ungaged streams, such as Little Mountain Creek, a US Environmental Protection Agency
(EPA) handbook for NPDES permit writers (EPA 2018) recommends estimating a 7Q10 now
using a regional regression published by the USGS (if available for the area of interest) or
developing a regional regression using long-term flow data and watershed characteristics for
similar streams. Regional regressions for low -flow statistics in North Carolina are not published
on the USGS StreamStats website, and the development of a regional regression was considered
here to be appropriate only for USGS personnel with local hydrologic expertise.
In the absence of either a published 7Q10 value or regional regression for low -flow statistics, a
7Q10 flow was estimated for Little Mountain Creek by assuming that the 7Q10 flow per unit of
drainage area is the same between Little Mountain Creek and a nearby small stream that has a
published 7Q10 value (Dutchmans Creek near Uwharrie; USGS Gage Number 02123567). This
concept has been used by the USGS and is proposed by BBP as an "other method" per
Mtn

Mr. Jason Mibroda
December 11, 2020
Page 4
15A NCAC 02B .0206(e). The Dutchmans Creek stream gage is located approximately 5.4 miles
east-southeast of BBP (Figure 2) and its drainage area of 3.44 square miles is similar to the
drainage area of Little Mountain Creek at State Highway 740 (5.62 square miles as measured using
the USGS StreamStats website). The Dutchmans Creek published 7Q10 now value is 0.04 cubic
feet per second (cfs) (USGS 2016); multiplying this value by the ratio of drainage areas (5.62/3.44)
yields an estimated 7Q10 flow for Little Mountain Creek at State Highway 740 of 0.065 cfs
(i.e., 29 gallons per minute [gpm]).
B.v_ -m �RI
d 0.5 f
'-e� New Lmtlm
Little Mountain
Creek on upstream Outfall 005
side of Hwy 740 5.6 �qm, 46 acres10.072 sq mi
E dm
r
Al damvle % c•_,., .
.am
4=
,G
4
Dutchmans Creek
(USGS gage
02123567)
2.4 sq mi
5 Miles - Drainage Area Outlet I] Watersheds Q Outfall 005 Watershed
1
Figure 2. Location of Dutchmans Creek stream gage and watershed.
2.2 Discharge Flow Rate for 7Q10 Conditions
Outfall 005 flows continuously but is expected to have minimal or no flow during 7Q10
conditions. Flow rates at Outfall 005 were less than 10 gpm during parts of July and August 2020.
During the same period, stream flow rates at central North Carolina USGS gages were generally
at or above normal values, which indicates that these two months were not abnormally dry. For
7Q10 conditions, the flow rate at Outfall 005 is expected to be at least as low as the values
measured during August 2020. Therefore, it was considered conservative to use 10 gpm as the
estimated Outfall 005 flow rate for 7Q10 conditions.
2.3 Dilution Ratio for 7Q10 Conditions
With an upstream flow rate of 29 gpm in Little Mountain Creek and an effluent now rate of
10 gpm, the dilution ratio in Little Mountain Creek is 2.9 to 1 (i.e., 2.9 gallons of upstream now
per gallon of effluent from Outfall 005) for 7Q10 conditions.
Mtn

Mr. Jason Mibroda
December 11, 2020
Page 5
3.0 DILUTION ASSESSMENT FOR VARIABLE HYDROLOGIC CONDITIONS
3.1 General Approach
The previous method makes certain assumptions to determine a dilution ratio under predefined
critical conditions, demonstrated with the 7Q10 analysis, but these assumptions may not account
for critical conditions under varying hydrologic conditions. Because the Outfall 005 flow rate
increases greatly for storm events, as does the upstream flow rate in Little Mountain Creek albeit
with a different response time, the dilution ratio necessary to protect aquatic life from chronic
toxicity may be more complex than demonstrated with a simple 7Q10 analysis.
This variability in hydrologic conditions was evaluated using a long-term daily water balance for
a period of 19 years. For each day, values were estimated for the Little Mountain Creek flow rate
and the Outfall 005 flow rate to calculate the dilution ratio in Little Mountain Creek.
3.2 Estimation of Daily Flow Rates for Little Mountain Creek
Daily flow rates for Little Mountain Creek at State Highway 740 were estimated using the USGS
compatible watershed method as described in Section 2.1. The flow each day in Little Mountain
Creek was estimated as the flow in Dutchmans Creek multiplied by the ratio of drainage areas
(5.62 / 3.44). This approach was considered to be more accurate than simulating daily hydrology
(i.e., runoff, infiltration, shallow groundwater, and evapotranspiration) in Little Mountain Creek
due to its variability in land use, vegetation, and soil characteristics throughout the watershed.
The period of record for the USGS daily flow data for Dutchmans Creek was October 1985 —
September 2004 (19 years). The long-term daily water balance was carried out for the entire period
of record for which the flow data were available. The long-term daily water balance was not an
attempt to simulate actual events during 1985 — 2004; it was simply an analysis using the
hydrologic conditions for that time period.
3.3 Estimation of Daily Flow Rates for Outfall 005
The daily flows at Outfall 005 were estimated using simplified versions of calculations that are in
the Generalized Watershed Loading Functions (GWLF) watershed model (Haith and Shoemaker
1987). Estimated flow data for Outfall 005 are used for this analysis in favor of historical flow data
because: a) historical data are not representative of the current drainage system in the Outfall 005
watershed and included other sources of water prior to 2007, and b) flows were only measured at
discrete intervals during each month as required for discharge monitoring reports (DMRs).
The modeled daily flow at the outlet of the watershed (i.e., at Outfall 005) is the sum of
contributions from surface runoff and seepage from shallow subsurface moisture. Surface runoff
was computed from daily precipitation data and using the Soil Conservation Service (SCS) curve
number method. Rainfall that is not intercepted by vegetative canopy (where applicable) and does
not become surface runoff is assumed to infiltrate into the ground and adds to the amount of
shallow subsurface moisture that is stored below ground. The fraction of the shallow subsurface
Mtn

Mr. Jason Mibroda
December 11, 2020
Page 6
moisture that seeps out of the ground each day is defined as the recession coefficient (whose value
is specified by the user). Shallow subsurface moisture is also consumed by evapotranspiration (in
vegetated areas), which was estimated as the long-term average lake evaporation for each day of
the year multiplied by a user -specified evapotranspiration coefficient for each month of the year.
The initial estimates of the curve numbers and other coefficients were calibrated by applying these
calculations to a recent time period when continuous flow data were recorded at Outfall 005. Using
the water balance to simulate Outfall 005 flows from late November 2019 through March 2020
showed that total volumes of flow measured at Outfall 005 were slightly higher than total volumes
of rainfall on the watershed. Therefore, a source of groundwater from outside the watershed was
added to the calculations and calibrated based on observed flows at Outfall 005. The groundwater
contribution varies by month but is limited by the accuracy and availability of recorded flow data
(November 2019 to March 2020). This groundwater contribution yields a conservative estimate of
flow at Outfall 005 in that it maintains a 10 gpm minimum for the reasons discussed in Section 2.2.
After adjusting the initial estimates of curve numbers, recession coefficient, evapotranspiration
coefficients, and external groundwater inputs, a reasonable match was obtained between predicted
and observed flows for Outfall 005 (Figure 3). The calibrated input values were then used for
calculating daily Outfall 005 flows associated with the daily rainfall for the 19-year period
mentioned above (October 1985 — September 2004).
1000
10
---------------------- ---- -- - _
-
-------- ------------------------------ - ---;------------------------------------------------------------- ------ ----,---------- —Predicted —Observed
--------------',--------------- -------------}`------------------------------------------------------------- - ---`,-------------------------------,'------------------------------
--- ------------------------------`--------------'---------------
i
------ -- ----------------- --- - r- -_--- ---_-_ --- ---- ----- -- - ----- ------ - ---- ----------------- ----- - --
--- ----� - - -- r--------- ----- -- - --------- - ------------- - - ----------------- --------------------------- ------ -- - ------------ ----
-- ---- ---- - ---- - -----i ----- -- ------ -
i
11/23/19 12/03/19 12/13/19 12/23/19 1/02/20 1/12/20 1/22/20 2/01/20 2/11/20 2/21/20 3/02/20 3/12/20 3/22/20
Figure 3. Predicted and observed daily average flows at Outfall 005 for calibration period.
3.4 Dilution Ratios in Little Mountain Creek
The dilution ratio for each day of the 19-year period was calculated as the upstream flow in
Little Mountain Creek divided by the flow at Outfall 005. The calculated dilution ratio varied over
several orders of magnitude over the course of the 19-year period (Figure 4), with a median value
of 20.6 (i.e., 20.6 gallons of upstream water per gallon of effluent from Outfall 005). The calculated
dilution ratio was 3.6 or more for 90% of the time, 2.1 or more for 95% of the time, and 0.5 or
more 99% of the time. Out of 19 years (6,939 days), there were only 14 days for which the
Mtn

Mr. Jason Mibroda
December 11, 2020
Page 7
predicted daily dilution was zero; all of these occurred within a 3-month period during a severe
drought. In general, the calculated dilution ratios tended to be highest during wet periods and
lowest during extended dry periods.
3
0
0
0
O
0
3
0
u
0
0
1000
100
10
0.1
O\O1`ah o\o10 O`0101 O`O1e O`O1? O`O1? O\O1\�'ti o\o1\p�'L O\O1\9"I O\O1e O\O1e O\O1e 0\01e o\61\p�'b O`O�? O\O� f O\O1\O'ti O`O1\O.y O`O1e O\O10
1 1 ^ - 1 1 1 1 1 1 1 1 'Y 1 1 1 1 '1 '1 1
Figure 4. Predicted daily dilution ratios in Little Mountain Creek for 19-year period.
4.0 SUMMARY
As noted in Section 1.0, the North Carolina Water Quality Standards states that the 7QI O low -flow
statistic should be used for determining effluent limits to protect aquatic life from chronic toxicity
but the Standards also allow for alternative "flow strategies" if they provide equal or better
protection for water quality. For the ungaged Little Mountain Creek, which lacks a published 7Q10
value or a published regional regression for low -flow statistics, a 7Q10 flow was estimated
utilizing the drainage area ratio method. Utilizing Dutchmans Creek, a 7Q10 flow for
Little Mountain Creek was estimated at 0.065 cfs (29 gpm) and the percent of time that the
upstream flow would be equal to or greater than the 7Q10 was calculated to be 98.8% (the now
was equal to or greater than the 7Q10 on 6,856 out of 6,939 days in Dutchmans Creek). In other
words, permit limits based on the 7Q10 analysis would be expected to result in water quality
standards being maintained 98.8% of the time in Little Mountain Creek.
As supporting evidence, a second modeling approach was evaluated as an alternative flow strategy
to the 7Q10 analysis by using a long-term daily water balance to estimate dilution ratios in
Little Mountain Creek each day over a 19-year period so that the analysis would cover a wide
range of hydrologic conditions. Under the modeled assumptions, the median calculated dilution
ratio was 20.6 but varied over several orders of magnitude. The calculated dilution ratio was 3.6 or
more for 90% of the time, 2.1 or more for 95% of the time, and 0.5 or more 99% of the time.
Mtn

Mr. Jason Mibroda
December 11, 2020
Page 8
When compared to the 7Q10 assessment, the predicted dilution ratio that would provide equal or
better protection for water quality standards, that results in water quality standards being
maintained 98.8% of the time, was 0.6 (i.e., 0.6 gallons of upstream flow in Little Mountain Creek
per gallon of discharge from Outfall 005). This dilution ratio is significantly lower than the value
of 2.9 from the 7Q10 analysis because: a) the long-term daily water balance takes into account the
variability of both effluent flows and upstream flows and the differences in how the effluent flows
and upstream flows respond to rainfall, and b) the 7Q10 flow is a statistic based only on the single
lowest 7-day average flow each year, whereas the long-term daily water balance takes into account
all of the low -flow occurrences throughout the year. This effect is amplified by the conservative
assumption of a consistent groundwater contribution of 10 gpm throughout the driest periods.
These calculated dilution ratios and the known upstream concentrations within Little Mountain
Creek can be used to calculate discharge concentrations for Outfall 005 that would allow chronic
criteria for fluoride (1.8 mg/L) and total cyanide (5.0 µg/L) to be maintained in Little Mountain
Creek. For these calculations, the upstream concentrations in Little Mountain Creek were assumed
to be 0.09 mg/L of fluoride and 0 µg/L of cyanide. Using the dilution ratio of 2.9 obtained from
the 7Q10 analysis, the effluent concentrations to maintain chronic criteria would be 6.76 mg/L for
fluoride and 19.5 µg/L for cyanide. However, the long-term daily water balance dilution ratio of
0.6 yields effluent concentrations of only 2.8 mg/L for fluoride and 8.0 µg/L for total cyanide.
These dilution ratios and allowable effluent concentrations represent current conditions without
implementation of additional water management measures. A second memorandum will present
alternatives that have been evaluated by BBP, as well as dilution ratios and allowable effluent
concentrations for the selected alternative.
If you have any questions regarding this technical memorandum, please feel free to call me or
Nathan Siria at [phone].
5.0 REFERENCES
EPA. 2018. Low Flow Statistics Tools: A How -To Handbook for NPDES Permit Writers.
EPA-833-B-18-001. US Environmental Protection Agency, Office of Water. Accessed
online at https://www.epa.gov/sites/production/files/2018-
11/documents/low— flow —stats tools —handbook.pdf
Haith, D.A. and L.L. Shoemaker. 1987. Generalized Watershed Loading Functions for Stream
Flow Nutrients. Water Resources Bulletin 23(3):471-476. Accessed online at
https:Honlinelibrary.wiley.com/doi/abs/10. I I I I/j.1752-1688.1987.tb00825.x
USGS. 2016. Low Flow Characteristics and Flow -Duration Statistics for Selected USGS
Continuous Record Streamgaging Stations in North Carolina Through 2012. Scientific
Investigations Report 2015-5001, Version 1. 1, March 2016. Prepared by J. Curtis
Weaver. Accessed online at https:Hpubs.usgs.gov/sir/2015/5001/pdf/sir2Ol550011j.Jp//df
R:\WP FILES\06010-1805-001\CORRESPONDENCE\2021-01-11 FIN TO Alcoa -Part 1 Achieving Compliance at Outfall 005\2021-01-11 FTN to Alcoa -Part 1 Achieving Compliance at Outfall 005.docxi
Mtn