Summary of City of Raleigh, Neuse
River WWTF Five-Year Evaluation of
Variance From Groundwater
Regulations
Groundwater and Wastewater Committee
Informational Item
July 8, 2015
Jon Risgaard, Wastewater Branch
Rick Bolich, Raleigh Regional Office
Background
• City of Raleigh’s Neuse River WWTP
o Located at 8500 Battle Bridge Rd., Raleigh,
NC
o Maximum capacity of 60 mgd
o has an NPDES discharge permit, nondischarge permit (residuals and spray
irrigation of reclaimed water.
Neuse
River
NPDES
Outfall
Wastewater
Treatment
Plant
Site Map
2005
3
Background
• 1980 – City began residuals application.
o Currently have 1,439 acres permitted for
land application
• Groundwater monitoring revealed nitrate
concentrations above 02L standard beyond
the compliance boundary in several wells.
• Raleigh paid $73,936 fine in 2002 for overapplication of residuals leading to
groundwater quality violations beyond
compliance boundary & suspended
residuals application.
Background
• NCAC 02L .0106 Requires “active treatment” of
groundwater contamination beyond compliance
boundary
• The EMC approved Raleigh’s variance request in
2009, allowing monitored natural attenuation for
groundwater.
o Deduct estimated excess groundwater nitrate
concentration from NPDES permit allocation.
o Evaluate the effectiveness of overall
remediation strategy every 5 years
5- Year Variance Report
• Elements of Approved Groundwater
Remediation Strategy:
1. Groundwater Containment System;
2. Monitored Natural Attenuation with Nitrate Debit
to NPDES Outfall Permit;
3. Constructed Subsurface Flow Treatment
Wetlands; and
4. Off-site Riparian Buffer Restoration.
5- Year Report
Monitored Natural Attenuation Alternatives
Variance requires evaluation of treatment strategy to determine if
additional treatment technologies are feasible.
The report evaluated three additional remedial alternatives to MNA
including:
• Enhanced in-situ Biodenitrification
• Enhanced Flushing
• Phytoremediation methods.
The report shows that these treatment technologies are either not
feasible for the site or are cost-prohibitive.
5- Year Report
Groundwater Containment System
• Groundwater capture and containment system for fields 50 and
500.
• Prevent the migration of groundwater to decommissioned private
water supply wells.
• Groundwater containment system captured estimated 100% of the
groundwater emanating from field 500 and 50% for field 50.
Year
Flow to
NRWWTP
(Million
Gallon)
Average NO3
in Flow to
NRWWTP
(mg/l)
NO3 to
NRWWTP
(lb)
2008
6.73
31
1,756
2009
11.71
26
2,558
2010
13.63
29
3,245
2011
12.89
29
3,168
2012
13.34
29
3,172
2013
15.53
27
3,467
2014
1.31
28
305
Total
75.15
17,670
Provided by City of Raleigh
Constructed
Subsurface Flow
Wetlands
• 3 wetland cells
• Approved variance
predicted removal of 28,500
to 42,800 pounds of nitrogen
annually
• The three wetlands removed
approximately 1,000 lbs
(total) of nitrogen since
constructed.
• Nitrogen concentrations in
the stream have declined
faster than had been
expected.
Provided by City Of Raleigh
5- Year Report
Off-site Riparian Buffer Restoration
• Restoration of Butlers Branch site in
Craven County, NC (Completed in 2010)
• Removal of nonpoint sources of pollution associated
with agricultural practices
• Reduction of sediment and siltation within on-site
and downstream receiving wasters
• Promotion of floodwater attenuation, and
• Providing terrestrial wildlife habitat
• Estimated 4,191 pounds of nitrogen offset
per year.
5- Year Report
Monitored Natural Attenuation
• Monitored Natural Attenuation (MNA) is the remediation activity
for the remainder of the site
• Includes triennial sampling of 73 monitoring stations.
• Trend directions and magnitudes for all monitoring stations
are shown in the 5-year report (page 28).
• Monitoring results used to evaluate predictive model of
groundwater flows to Neuse River, and Nitrogen debits in
NPDES permit.
5- Year Report
Monitored Natural Attenuation
• Monitored Natural Attenuation (MNA)
• Updated Model to improve accuracy based on:
• Refining the model grid from 60x60 feet to 20x20 feet.
• Additional Nitrate sampling results at various depths
• Water percolation from the soil zone
• Nitrate fate and transport in the vadose zone
• The report concludes that MNA shows reducing nitrogen
concentrations at and within the compliance boundary
5- Year Report
Monitored Natural Attenuation
Year
Predicted N Load to Neuse River (in Excess of 10
mg/l )
Debit in NPDES
Permit (lb/yr)
Variance Model (lb/yr)
Updated Model (lb/yr)
2008
115,402
75,576
120,058
2009
111,748
76,958
117,183
2010
107,719
77,791
114,246
2011
103,487
77,671
110,370
2012
99,146
77,602
106,196
2013
94,804
76,878
102,022
2014
90,462
75,172
97,549
2015
86,186
73,807
93,276
2016
82,020
71,956
89,002
2017
77,965
69,744
84,828
2018
73,954
67,490
80,754
2019
70,119
64,856
76,679
2020
66,350
62,734
72,803
2021
62,691
60,459
69,026
2022
59,165
57,804
65,250
5- Year Report
Monitored Natural Attenuation
Year
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
Predicted N Load to Neuse River (in Excess
of 10 mg/l )
Variance Model
(lb/yr)
115,402
111,748
107,719
103,487
99,146
94,804
90,462
86,186
82,020
77,965
73,954
70,119
66,350
62,691
59,165
Updated Model
(lb/yr)
75,576
76,958
77,791
77,671
77,602
76,878
75,172
73,807
71,956
69,744
67,490
64,856
62,734
60,459
57,804
Debit in
NPDES Permit
(lb/yr)
120,058
117,183
114,246
110,370
106,196
102,022
97,549
93,276
89,002
84,828
80,754
76,679
72,803
69,026
65,250
Note - The updated model predicts higher N load to Neuse River than the variance
model starting in year 2024 and beyond.
5- Year Report
Monitored Natural Attenuation
Provided by City Of Raleigh
5- Year Report
Monitored Natural Attenuation -- Variance Model
160,000
Variance Model Constant N at 10 mg/l in
Recharge
Variance Model Total N Load Minus
Load at 10 mg/l
140,000
Annual Loading to Neuse River and Tributaties,Lb/Yr
Debit Values from Permit
120,000
100,000
80,000
60,000
40,000
20,000
0
1/1/80
12/31/89
1/1/00
12/31/09
1/1/20
12/31/29
1/1/40
12/31/49
Provided by City Of Raleigh
5- Year Report
Monitored Natural Attenuation – Updated Model
160,000
Updated Model Total N Load
Updated Model VS2DT Source Max 10
mg/l in Recharge
Updated Model Total N Load Minus
Load at 10 mg/l
Annual Loading to Neuse River and Tributaties,Lb/Yr
140,000
120,000
100,000
80,000
60,000
40,000
20,000
0
1/1/80
12/31/89
1/1/00
12/31/09
1/1/20
12/31/29
1/1/40
12/31/49
Provided by City Of Raleigh
5- Year Report
Monitored Natural Attenuation
Provided by City Of Raleigh
Permitted Total N Allocation to Neuse vs.
Discharge and Debit in Permit
Provided by City Of Raleigh
Staff Assessment
• Remediation activities appear to be working as intended based on 5 yrs
worth of data.
• Support continued monitoring and evaluating the current corrective
action plan and variance using current monitoring wells and surface
water sampling locations.
• Support continued monitoring and evaluating the groundwater
containment system.
• Support continued use of the original debit schedule from the September
2010 approved Corrective Action Plan for the next NPDES permit cycle
(to be issued from 2015 to 2020)
• Use the 2019 Five-Year Evaluation to further verify the groundwater
model and establish debits schedule for NPDES permits beyond 2022.