NC0004308_More Information (Received)_20210112

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water resources
TECHNICAL MEMORANDUM
DATE: January 12, 2021
TO: Jason Mibroda
Alcoa Corporation
FROM: Philip Massir r nd athan Siria
FTN Associates, Ltd.
SUBJECT: Proposed solution for achieving compliance at Badin Business Park Outfall 005
FTN No. R06010-1805-001
1.0 INTRODUCTION
This memorandum presents an alternatives analysis and proposes a solution for achieving compliance with
North Carolina water quality regulations for discharges from Outfall 005 at the Badin Business Park (BBP)
facility located in Badin, NC. It also discusses current site conditions to include a summary of investigation
and remediation activity in support of the alternatives analysis.
This memorandum is subsequent to and makes reference to a memorandum dated January 11, 2021
("previous memo'; FTN 2021) which described methods employed to evaluate assimilation of discharges
from Outfall 005 to Little Mountain Creek.
2.0 SITE CONDITIONS
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. Discharges from Outfall 005 are fed from a system of interconnected storm
drainpipes and drainage channels. The discharge from Outfall 005 flows into and 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 4 13-5-1-[2]) (Figure 1).
Groundwater in proximity to the Outfall 005 storm drain may at times contain elevated concentrations of
fluoride and total cyanide — although no free or available cyanide is detected. This issue is currently being
addressed with oversight from the North Carolina Department of Environmental Quality (DEQ) Hazardous
Waste Section. Under certain conditions, deeper sections of the storm drain piping interface with
groundwater. Investigations have demonstrated that infiltration of groundwater into the storm drain is the
primary mechanism that transports fluoride and total cyanide to Outfall 005. Remedial activity to date has
focused on isolating these groundwater contributions.
Regional Offices: Fayetteville, AR; Baton Rouge, LA; Chesterfield, MO • www.ftn-assoc.com • [e-mail]

Mr. Jason Mibroda
January 12, 2021
Page 2
Figure 1. Map of BBP Outfall 005 and its watershed.
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2.1 Site Investigations
Over the years, multiple investigations have been undertaken in support of achieving full compliance with
the site discharge permit. These investigations were intended to identify potential sources contributing to
the elevated fluoride and cyanide concentrations. Early investigations included a review of operational
history and engineering drawings; collection of groundwater, surface water, soil, and sediment samples;
and video inspections of the drainage system. More recent investigation activity looked to refine existing
knowledge by:
Assessing for the presence of unknown sources or pathways for constituents to enter the storm drain
by performing an electromagnetic geophysical evaluation,
2. Characterizing shallow subsurface soils and groundwater through excavation of test pits and
collection of environmental samples in response to the geophysics evaluation,
Assessing the storm drain contributions to Little Mountain Creek through continual monitoring of
flow rates in multiple storm drain locations and in Little Mountain Creek, and
4. Assessing the fluoride and cyanide contribution to the storm drain at multiple locations in response
to storm events of varying intensity and duration.
These recent investigations have confirmed that groundwater infiltration into the storm drain is the primary
mechanism for transporting fluoride and cyanide to Outfall 005. More specifically the elevated
concentrations are sourced primarily from the eastern section of the drainage system. The consistent flow
from the eastern section varies relative to fluctuations in the seasonal groundwater elevation with the least
amount of flow observed during the dry summer months. Further, the investigations demonstrated that
fluoride concentrations are consistent, typically in the range of 1 to 3 mg/L. Fluoride concentrations tend
to be on the low end of the range during storm events but rebound upon the cessation of precipitation.
2.2. Site Improvements to Reduce Sources
Multiple site improvements have been undertaken to reduce the contribution of fluoride and cyanide at
Outfall 005. As noted, the primary mechanism that transports fluoride and cyanide to Outfall 005 is water
infiltrating into the ground and then draining to the outfall through the underground drainage system.
Therefore, these site improvements focused primarily on the subsurface drainage system, including:
• Jet cleaning of storm water piping to remove residual materials,
• Slip lining of the storm drain piping to reduce potential infiltration,
• Installation of a shallow storm water system to reduce potential infiltration,
• Earthwork and asphalt paving to shed surface water and reduce water infiltration to the shallow
groundwater, and
• Abandonment/isolation of existing deep storm drain piping through slurry wall placement and
installation of pipe seals and grouting.
These improvements have resulted in a reduction in concentrations of fluoride and cyanide at Outfall 005
but not to a degree to consistently comply with the existing discharge limits. This is in part because the
infiltration pathway has been impeded but not completely removed. These improvements can be observed
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from statistics for daily maximum concentrations reported on discharge monitoring reports (DMRs), where
the concentrations observed since August 2019 are lower than the same statistics for the four years prior to
August 2019 (Table 1).
Table 1. Comparison of Recent Fluoride and Cyanide Concentrations to Past Results.
Time Period
Fluoride (mg/L)
Total Cyanide (µg/L)*
Median
Average
Median
Average
August 2015 — July 2019
2.9
2.95
18
28.5
August 2019 — October 2020
2.1
1.98
< 5
8.0
* Cyanide statistics were computed using a value of 5 µg/L for values below the reporting limit. A different analytical method
that more accurately measures cyanide was used starting in October 2020.
3.0 WATER MANAGEMENT ALTERNATIVES
Alternatives for improving water quality at Outfall 005 continue to be identified and explored. The
foundation of these alternatives has been twofold: manage the source and/or manage the discharge. Many
of the improvements to date have looked to manage the source of fluoride and cyanide through location and
removal or by removing the pathway to the storm sewer.
Although source removal continues to be pursued where practical, more recent alternatives have focused
towards managing the discharge. Some alternatives considered but dismissed as impractical are:
Piping the discharge to the Yadkin River — would require a large pipe several miles long with an
elevation rise of more than 100 ft to cross a ridge;
Fluoride treatment plant — technology not practical to treat < 2 ppm with episodic flows; and
Implementing a hydrograph-controlled release (HCR; temporarily storing effluent as needed and
discharging it when there is sufficient assimilative capacity in Little Mountain Creek). Impractical
as a result of limited storage availability.
Alternatives retained for consideration include pumping some of the Outfall 005 discharge to Badin Lake
(via Outfall 012) where the assimilative capacity and ability to maintain criteria is greater when compared
to discharging to Little Mountain Creek. The location of Outfall 012 in relation to Outfall 005 is shown on
Figure 2. Three variations of this alternative have been evaluated and are as follows:
Scenario A: Isolate water with elevated concentrations of fluoride and/or cyanide in the Outfall 005
watershed and pump that water at rates up to 40 gpm to Outfall 012 to be discharged to Badin Lake
utilizing the existing diffuser.
Scenario B: Pump water from Outfall 005 at rates up to 40 gpm to Outfall 012 to be discharged to
Badin Lake utilizing the existing diffuser. Modify the permit limits at Outfall 005 based on dilution
in Little Mountain Creek to accommodate discharges of effluent flows in excess of 40 gpm.
Scenario C: Pump water from Outfall 005 to Badin Lake at rates up to 180 gpm. Modify the permit
limits at Outfall 005 based on dilution in Little Mountain Creek to accommodate discharges of
effluent flows in excess of 180 gpm. Install new pump, pipe, and diffuser system for Outfall 012.
Calculate new mixing zone boundaries for Outfall 012 and modify the permit limits accordingly.
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The 40 gpm pumping threshold for Scenarios A and B is based on the existing capacity of Outfall 012,
which is 47 gpm and the basis for the current Outfall 012 mixing zone boundaries and permit limits.
Currently, Outfall 012 flows only in response to rainfall events, leaving unused discharge capacity between
rainfall events.
Outfall 011 ADOutfall 012 Diffuser
r
Figure 2. Location of Outfalls 005 and 012 at Badin Business Park facility.
3.1 Scenario A
Scenario A assumes that 1) it would be possible to isolate water with concentrations exceeding the current
permit limits at Outfall 005, and 2) the flow rate of that water would not exceed 40 gpm.
A 40 gpm pump would be installed in the Outfall 005 watershed at a location where water with elevated
concentrations could be captured and pumped to the existing Outfall 012 sump. Water that is not pumped
to the Outfall 012 sump would continue to discharge through Outfall 005 and must meet the existing permit
limits at Outfall 005 in order for this scenario to be feasible.
3.2 Scenario B
Scenario B consists of installing a 40 gpm pump at or upstream of Outfall 005 and operated primarily
during June — October (the months when there is less assimilative capacity in Little Mountain Creek).
The system would be operated such that when there is 40 gpm or less of flow at Outfall 005, all of the water
at Outfall 005 would be pumped to the Outfall 012 sump (i.e., no discharge at Outfall 005). However, when
volumetric flows exceed the 40 gpm, 40 gpm of the flow would be pumped to the Outfall 012 sump and
the _remainder of the flow would discharge through Outfall 005. Permit limits for Outfall 005 would be
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calculated based on the amount of dilution in Little Mountain Creek during conditions when Outfall 005
would be discharging. This assessment follows the same methodology that was discussed in the previous
memo dated January 11, 2021.
This scenario assumes that either the existing roadside ditch between Outfall 005 and Little Mountain Creek
will be deemed an "effluent channel' by DEQ, 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'.
3.2 Scenario C
Scenario C is conceptually similar to Scenario B, except that Scenario C would include:
1. Pumping from Outfall 005 to Badin Lake up to 180 gpm instead of 40 gpm,
2. Modifications to Outfall 012 (new pipe, pumping system, and diffuser) to increase the capacity of
the Outfall 012 diffuser from 47 gpm to at least 180 gpm, and
Determining a new mixing zone for Outfall 012 in Badin Lake and seek modified permit limits for
Outfall 012.
As with Scenario B, permit limits for Outfall 005 would be modified based on the dilution in Little
Mountain Creek when discharges occur; and the same assumptions regarding the roadside ditch would
apply.
4.0 EVALUATION OF PUMPING SCENARIOS
Scenarios A, B, and C were evaluated using the recent investigation data discussed previously in this
memorandum and the long-term daily water balance assessment that was described in the previous memo.
As a reminder, the water balance was used to calculate the dilution ratio (the ratio of upstream flow in Little
Mountain Creek to effluent flow from Outfall 005) for each day over a 19-year period in order to account
for a wide range of hydrologic conditions. The investigation data was used to refine the water balance
calculations and to help locate and quantify sources of water with elevated pollutant concentrations within
the Outfall 005 drainage area.
Scenario A was eliminated from consideration after field data showed that more than 40 gpm of water with
elevated concentrations discharge from Outfall 005 during the wet seasons of spring and winter. This water
primarily originates and flows along a single subsurface drainage pipe on the eastern side of the Outfall 005
drainage area.
Scenarios B and C would provide equal or better protection of water quality standards when compared to
setting effluent limits based on 7Q 10 conditions (i.e., the lowest 7-day average stream flow that is likely to
occur in one out of ten years and the basis for determining effluent limits to protect aquatic life under North
Carolina Regulation ([15A NCAC 02B .0206; see previous memo]). Neither Scenario B nor Scenario C
would discharge at Outfall 005 when there is less than 40 gpm or 180 gpm, of water draining to the outfall,
respectively.
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To assess the protectiveness of each Scenario, the long-term daily water balance was used to calculate the
percentage of time that chronic criteria for fluoride and cyanide would be met in Little Mountain Creek
considering different effluent concentrations at Outfall 005. The results of this assessment are shown on
Figure 3; also shown for reference are the result considering no pumping to Badin Lake.
100%
m
90%
E
80%
.3
g 70%
u 60%
50%
u
�o
+s 40%
E
30%
0
20%
`v
a
10%
0% _�_
Cyanide (µg/L): 5
Fluoride (mg/L): 1.80
------------------------ -
--------------------------- -------- -----------------
Effluent concentrations that allow chronic criteria to be
met 98.8% of the time:
14.8 µg/L cyanide, 5.1 mg/L fluoride (180 gpm pumping)
13.1 µg/L cyanide, 4.6 mg/Lfluoride (40 gpm pumping)
8.0 µg/L cyanide, 2.8 mg/Lfluoride (no pumping) ------.
------- —180 gpm pumping during Jun -Oct
—40 gpm pumping during Jun -Oct
-------------
—No pumping
-- - ----------`------- ----- -- - --- - --- ------ -------------------
10 15 20 25
3.51 5.22 6.93 8.64
Outfall 005 Concentrations (µg/L cyanide or mg/L fluoride)
Figure 3. Percent of time that chronic criteria will be met in Little Mountain Creek with different
pumping scenarios and different effluent concentrations.
30
10.35
As shown on Figure 3, there is little difference between the results for pumping 40 gpm versus pumping
180 gpm. For example, with effluent concentrations of 15 µg/L total cyanide and 5.22 mg/L fluoride,
increasing the pumping rate from 40 gpm to 180 gpm provides only about a 0.5% increase in the percent
of time that chronic water quality criteria would be maintained in Little Mountain Creek.
The analysis presented in the previous memorandum identified that discharge limits based on the 7Q 10
analysis would be expected to result in water quality standards being maintained 98.8% of the time in Little
Mountain Creek. Under Scenario C, effluent concentrations of 14.8 µg/L total cyanide and 5.1 mg/L
fluoride would demonstrate a level of protection for chronic water quality criteria that would be equal to
that of the 7Q10 analysis. Under Scenario B, the effluent concentrations to show equivalent protection
would be reduced to 13.1 µg/L total cyanide and 4.6 mg/L fluoride. Under a no pumping scenario, the
allowable effluent concentrations to show equivalent protection would be reduced to 8.0 µg/L total cyanide
and 2.8 mg/L fluoride.
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5.0 SELECTION OF PROPOSED SOLUTION
As noted above, Scenario A was eliminated from consideration based on 2019-2020 field data. Either
Scenario B or Scenario C would be technically feasible, but Scenario C provides minimal benefits over
Scenario B even though it is more complex and would take longer to implement. Scenario C would require
construction of a larger pipeline and diffuser in Badin Lake to accommodate the almost 4 times as much
flow as the current capacity of Outfall 012.
Based on the evaluation described above, Scenario B, with slight modification, is the proposed solution for
achieving compliance for fluoride and cyanide at Outfall 005. The components of this proposed solution
include:
• Install a 40 gpm pump at or upstream of Outfall 005, to be operated during the months of
June -October;
• Construct a pipeline to convey water from Outfall 005 to the Outfall 012 sump;
• Proposed monthly average limits for Outfall 005: 13.1 µg/L total cyanide, 4.6 mg/L fluoride;
• Proposed daily maximum limits for Outfall 005: 46.6 µg/L total cyanide, 24 mg/L fluoride (same
as current limits); and
• Satisfy NCDEQ requirements regarding conveyance of Outfall 005 discharges to Little Mountain
Creek such that permit limits for Outfall 005 will be based on dilution in Little Mountain Creek
rather than dilution in the roadside ditch.
The modifications referenced above would relocate the pumping location to intercept water from the eastern
section of the Outfall 005 system to focus on diverting water with elevated concentrations to Outfall 012
and allowing non -impacted water to continue to discharge through Outfall 005.
If you have any questions regarding this technical memorandum, please feel free to call me or Nathan Sin
at [phone].
R:AWP_FILES\06010-1805-001\CORRESPONDENCE\2021-01-12 FTN TO ALCOA-PART 2 ESTIMATED ASSIMILATION OF OUTFALL 005\2021-01-12 FTN TO ALCOA�
ESTIMATED ASSIMILATION OF OUTFALL 005.DOCX
6.0 REFERENCES
FTN. 2021. Estimated Assimilation of Badin Business Park Outfall 005 discharges in Little Mountain
Creek Under Modeled Critical Conditions. Memorandum from Philip Massirer and Nathan Sin a,
FTN Associates, Ltd. to Jason Mibroda, Alcoa Corporation. January 12, 2021.
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