EPA/600/R-21/006 | March 2021
www.epa.gov/emergency-response-research
United States
Environmental Protection
Agency
oEPA
Persistence of Surrogate
Radionuclides on Wastewater
Collection System Infrastructure
Office of Research and Development
Homeland Security Research Program
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EPA/600/R-21/006
March 2021
Persistence of Surrogate Radionuclides on Wastewater
Collection System Infrastructure
by
Jeffrey Szabo
U.S. Environmental Protection Agency
Cincinnati, OH 45268
Rendahandi G. Silva, Jill Webster and Lee Heckman
Aptim Federal Services, LLC
Cincinnati, OH 45204
Contract EP-C-14-012, Task Order 4-06
Office of Research and Development
Homeland Security Research Program
Cincinnati, OH 45268
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Disclaimer
The U.S. Environmental Protection Agency (EPA) through its Office of Research and
Development funded and managed the research described herein under Contract EP-C-14-012,
Work Assignment 4-06 with Aptim Federal Services, LLC. It has been subjected to the Agency's
review and has been approved for publication. Note that approval does not signify that the
contents necessarily reflect the views of the Agency. Any mention of trade names, products, or
services does not imply an endorsement by the U.S. Government or EPA. The EPA does not
endorse any commercial products, services, or enterprises.
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Foreword
The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting the
Nation's land, air, and water resources. Under a mandate of national environmental laws, the
Agency strives to formulate and implement actions leading to a compatible balance between
human activities and the ability of natural systems to support and nurture life. To meet this
mandate, EPA's research program is providing data and technical support for solving
environmental problems today and building a science knowledge base necessary to manage our
ecological resources wisely, understand how pollutants affect our health, and prevent or reduce
environmental risks in the future.
The Center for Environmental Solutions and Emergency Response (CESER) within the Office of
Research and Development (ORD) conducts applied, stakeholder-driven research and provides
responsive technical support to help solve the Nation's environmental challenges. The Center's
research focuses on innovative approaches to address environmental challenges associated with
the built environment. We develop technologies and decision-support tools to help safeguard
public water systems and groundwater, guide sustainable materials management, remediate sites
from traditional contamination sources and emerging environmental stressors, and address
potential threats from terrorism and natural disasters. CESER collaborates with both public and
private sector partners to foster technologies that improve the effectiveness and reduce the cost
of compliance, while anticipating emerging problems. We provide technical support to EPA
regions and programs, states, tribal nations, and federal partners, and serve as the interagency
liaison for EPA in homeland security research and technology. The Center is a leader in
providing scientific solutions to protect human health and the environment.
If a wide area or water system contamination event involving radionuclides occurs, large
volumes of contaminated water could be generated during clean-up activities, or during rain
events. Should this water enter a sewer system, radiological contamination could persist on
collection system infrastructure, or travel with the sewage to the wastewater treatment plant. The
extent of radionuclide persistence on sewer infrastructure materials is currently unclear. This
report contains data on the persistence of non-radioactive cesium, cobalt and strontium on sewer
system infrastructure. The report is the first step in collecting a data set that decision makers
could use to determine response actions should radiological contamination enter a sewer system.
Gregory Sayles, Director
Center for Environmental Solutions and Emergency Response
111
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Table of Contents
Disclaimer ii
Foreword iii
List of Figures and Tables iv
Abbreviations and Acronyms vi
1.0 Introduction 1
2.0 Materials and Methods 2
2.1 Description of the pilot scale wastewater collection system setup 2
2.2 Conditioning of infrastructure coupons 4
2.3 Contamination and sampling of SETBC 5
2.4 Determination of metals adhered to coupon surfaces 5
2.5 Other unchlorinated secondary treated effluent parameters 5
2.6 Quality control and data quality 6
2.6.1 Data quality 7
2.6.2 Deviations 7
3.0 Results and Discussion 8
3.1 Unchlorinated secondary effluent water quality 8
3.2 Evaluation of cesium and cobalt adhesion to infrastructure materials 8
3.3 Evaluation of strontium adhesion to infrastructure materials 8
3.4 Discussion, implications and future research 11
4.0 Conclusions 13
5.0 References 14
List of Figures and Tables
Figure 1: Schematic of the secondary effluent test bed channels (SETBC) (left) including the
overall orientation of the pipes, source of secondary effluent, injection ports and location of flow
sensor. The picture (left) shows two of the pipes and their layout 3
Figure 2: Coupons of collection system infrastructure. From top to bottom: high-density
polyethylene, brick, rubber, concrete, iron (uncorroded), clay, PVC 4
Figure 3: Strontium recovered from collection system infrastructure materials. One experiment
lasted for 28 days (top) and one lasted 5 days (bottom) 9
Table 1: Quality Control Data Objectives 6
Table 2: Unchlorinated Secondary Treated Effluent 5-day, BOD, TSS and NFb Data 8
Table 3: Strontium Extracted from Uncontaminated Conditioned Control Coupons (Top Rows)
and Those Soaked in Acid to Extract More Cesium (Bottom Rows) 10
iv
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Executive Summary
Contamination from a radiological dispersal device, improvised nuclear device or nuclear power
plan accident could be widespread. Cleanup activities or precipitation events could result in
radioactive contamination entering a wastewater or stormwater collection system. In this study,
collection system infrastructure materials such as brick, clay, concrete, high density
polyethylene, iron, polyvinyl chloride (PVC), and rubber were conditioned in wastewater
flowing through six 6-inch (15.2 cm) diameter PVC pipes for two months. Conditioning allowed
biofilms to form and wastewater solids to accumulate on the coupons surface. Subsequently,
non-radioactive cesium chloride, cobalt chloride and strontium chloride were injected into the
wastewater flow, and persistence on the infrastructure coupons (excised sample materials) was
determined over time. Flow in each channel was approximately 50 gallons per minute (189.2
liters per minute), and each non-radioactive salt was spiked into the flow at 5 mg/L. This setup
was designed to determine if non-radioactive surrogates for radionuclides would adhere to or
persist on common collection system infrastructure materials.
Results for cesium and cobalt showed that metal adhesion to conditioned infrastructure materials
was undetectable. Strontium was detected on concrete coupons at levels above the method
detection limit. However, further analyses of the concrete suggested that strontium in the
concrete matrix was being detected, not strontium from the contaminant injection. Although the
data suggests that none of the surrogate radionuclides adhered to or persisted on collection
system infrastructure materials, research with real radionuclides at lower concentration levels
could confirm these results.
v
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Abbreviations and Acronyms
BOD biochemical oxygen demand
CESER Center for Environmental Solutions and Emergency Response
cm2 square centimeter
Co cobalt
Cs cesium
EPA United States Environmental Protection Agency
FEMA Federal Emergency Management Agency
gpm gallon(s) per minute
HDPE high density polyethylene
HSPD Homeland Security Presidential Directive
IAEA International Atomic Energy Agency
IND improvised nuclear device
L liter(s)
m meter(s)
MDL method detection limit
mg milligram(s)
mgd million gallons per day
min minute(s)
|iS micro Siemens
mL milliliter(s)
MPN most probable number
MSDGC Metropolitan Sewer District of Greater Cincinnati
NA not applicable
NH3 ammonia
NIST National Institute of Standards and Technology
PVC polyvinyl chloride
QA quality assurance
QAPP quality assurance project plan
QC quality control
RDD radiological dispersal device
RPD relative percent difference
SETBC secondary effluent test bed channels
Sr strontium
T&E Test and Evaluation
TSS total suspended solids
TV true value
vi
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1.0 Introduction
During the last two decades, there has been an increased focus on the potential impacts of
radiological materials that could be used in intentional attacks or resulting from accidental
contamination in the United States. For example, the Federal Emergency Management Agency
(FEMA) uses 15 all-hazards National Planning Scenarios as a basis for assessing national
preparedness and implementing Homeland Security Presidential Directive 8: National
Preparedness (HSPD-8) (FEMA 2003). The National Planning Scenarios encompass
cyberthreats, natural disasters, pandemics and attacks with chemical, biological or radiological
materials. The radiological scenarios focus on radiological dispersal devices (RDD) and on
improvised nuclear devices (IND). In the case of an RDD, radiological material is dispersed
with conventional explosives, while an IND is the detonation of a nuclear device. In both cases,
the impacts of these devices could be significant if they were detonated in an urban area (USEPA
2007)
Significant research has been undertaken to understand the impacts of RDD or IND detonations
in urban environments (Biancotto et al. 2020, Lee et al. 2010, Regens et al. 2007, USEPA 2007).
One often overlooked aspect of this research is understanding how radionuclides could interact
with sewer collection system infrastructure. Radionuclides could enter a sewer system due to
remediation activities such as washing down contaminated road and building surfaces. Water
runoff from cleanup activities should be collected and then treated or disposed of, but accidental
releases of contained water are possible. Furthermore, if a rain event were to occur after an RDD
or IND event, it might be difficult to prevent all runoff from entering a sanitary or stormwater
sewer system. Furthermore, should a nuclear power plant accident occur, such as the 2011
incident in Fukushima, Japan, radionuclides could be spread over a wide area. Collection
systems would be vulnerable to radiological nuclear power plant contamination from
remediation activities or rain events. Should radionuclides enter a sewer system, persistence on
collection system infrastructure or contamination at the treatment plant could pose a long term
health risk for workers, and potentially disrupt the wastewater treatment process.
Research has been conducted to determine the fate of biological agents on water infrastructure
surfaces, including drinking water pipes (LeChevallier et al. 1988, De Beer et al. 1994, Chu et al.
2003, Emtiazi et al. 2004, Szabo et al. 2007, Miller et al. 2015) and wastewater collection
infrastructure (USEPA 2017). The persistence of radionuclides has also been examined on
drinking water pipe surfaces (Szabo et al. 2009, USEPA 2016, USEPA 2018). The purpose of
this study was to fill a research gap and to produce data on the persistence of water-soluble
radionuclide surrogates on collection system infrastructure. Small sections of collection system
infrastructure material (brick, clay, concrete, HDPE, iron, PVC, and rubber) were conditioned
over two months in flowing wastewater, which allowed for formation of wastewater biofilms and
accumulation of solids from the wastewater flow. The coupons were then exposed to non-
radioactive salts of cesium (Cs), cobalt (Co) and strontium (Sr). These compounds are common
homeland security radionuclides of concern (USEPA 2007). Subsequently, persistence of these
radionuclides on the infrastructure coupons (excised sample materials) over time was
determined.
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2.0 Materials and Methods
2.1 Description of the pilot scale wastewater collection system setup
Experiments were conducted at the United States Environmental Protection Agency's (EPA's)
Test and Evaluation (T&E) Facility in Cincinnati, Ohio. The pilot scale system used was the
secondary effluent test bed channels (SETBC) (Figure 1). The SETBC consists of six 6-inch
(15.2 cm) diameter PVC pipes arranged horizontally with the upper section of the pipe removed,
which allowed access to the open channel flow in the pipes. Unchlorinated secondary treated
effluent was pumped from the adjacent Metropolitan Sewer District of Greater Cincinnati's
(MSDGC) 100 million gallons per day (mgd) (378.5 liters per day) Mill Creek wastewater
treatment plant and into a manifold where flow was distributed to each pipe. Before reaching the
SETBC, untreated wastewater had undergone primary settling, treatment with activated sludge,
and secondary settling. The secondary treated effluent was diverted to the SETBC before
chlorination.
Although secondary treated effluent is lower in solids and biological activity than raw sewage, it
is more consistent in quality and more amenable to pumping through the SETBC channels. Flow
rates in each of the six SETBC pipes was adjusted to approximately 50 gallons per minute (gpm)
(189.2 liters per minute). Each of the six PVC pipes has its own flow control valve, two sections
of fabricated horizontal open grids to mount test material coupons, an injection port, and a flow
monitoring sensor and a data logger. In addition, the system also consists of two sets of pH,
conductivity, and temperature monitoring sensors. One-inch (2.54 cm) diameter coupons were
cut from seven unused collection system infrastructure materials (brick, clay, concrete, HDPE,
iron, PVC, and rubber) were secured to metal bars which spanned the section cut from the top of
pipe (Figure 2). Using this setup, the coupons were mounted to the horizontal grids and inserted
into the unchlorinated secondary effluent flow.
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Secondary Effluent from MSDGC
Flow Rate Display Panel
Coupon Holding Grid
Section B
Section A
Legend
Injection ports
Secondary effluent flow controlling valves
Secondary effluent drainage port
Secondary effluent open/close and flow control
Flow sensors
Pipes
Figure 1: Schematic of the secondary effluent test bed channels (SETBC) (left)
including the overall orientation of the pipes, source of secondary effluent, injection
ports and location of flow sensor. The picture (left) shows two of the pipes and their
layout.
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From top to bottom: high-density
polyethylene, brick, rubber,
concrete, iron (uncorroded), clay,
PVC
Figure 2: Coupons of collection system infrastructure. From top to bottom: high-density
polyethylene, brick, rubber, concrete, iron (uncorroded), clay, PVC.
2.2 Conditioning of infrastructure coupons
Collection system infrastructure coupons were conditioned using unchlorinated secondary
effluent pumped from an adjacent 100 million gallons per day (mgd) wastewater treatment plant.
In order to disinfect the SETBC between experiments, the system was drained and sprayed with
undiluted bleach between the tests and then flushed with unchlorinated secondaiy effluent.
Similarly, coupons were wiped with undiluted bleach prior to conditioning. Coupons were
randomly placed amongst the six channels, but each material was represented in equal numbers
in each channel. Each coupon was mounted so that the face of the coupon was parallel to the
direction of the water flow. Coupons were conditi oned in unchlorinated secondary treated
effluent for two months with flow at approximately 50 gpm in each channel.
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2.3 Contamination and sampling of SETBC
After conditioning, the second phase of the experiment focused on the adhesion of Cs, Co and Sr
to the conditioned infrastructure material coupons. Contamination experiments with Cs, Co and
Sr were conducted separately, and each experiment was conducted in duplicate for a total of six
injection experiments. Before contamination, two coupons (conditioned uncontaminated
controls) of each material were removed from the SETBC and sampled for the metal being used
in each test. Injection solutions (1000 mg/L) of cesium, cobalt, and strontium were prepared
separately by dissolving chloride salts of Cs, Co, and Sr in deionized water. Metal salt solutions
were injected through the injection ports in each pipe (Figure 1) for one minute at 1,000 mL/min
using a pre-calibrated peristaltic pump into each pipe to achieve a target metal concentration of 5
mg/L in the flow. Coupons of each material were harvested in pairs after injection in order to
determine persistence. Sampling timeframes varied and are further discussed in the results.
2.4 Determination of metals adhered to coupon surfaces
After the coupons were removed from the SETBC, their surfaces were sampled to determine the
concentration of adhered Cs, Co and Sr. Each coupon was washed with 5 mL of concentrated
trace metal grade nitric acid. Each acid extract was diluted in deionized water if needed, filtered
through a 0.45 |im filter and stored 4.0 ± 0.5 °C prior to analysis. Samples were analyzed
separately to determine the metal concentration either by a 240Z Graphite Furnace Atomic
Absorption (Agilent, Santa Clara, CA) for cesium or an Optima 2100 DV Inductively Coupled
Plasma-Optical Emission Spectrometer (PerkinElmer, Waltham, MA) for cobalt and strontium
using EPA Methods 200.9 (USEPA 1994) and 6010B (USEPA 1996), respectively.
Method detection limits (MDL) for Co and Sr were 100 |ig/L and 5 |ig/L in the acidified extract
solution, respectively. The MDL for Cs using atomic adsorption was not determined, and the
lowest reportable value was 20 |ig/L, which was the lowest point on the calibration curve. Data
reported here is in the form of the mass of surrogate radionuclide recovered from the surface of
the coupon (|ig/cm2). Coupon surface area exposed to water flow was 10.7 cm2 (1.7 in2), which
includes the front and back of the coupon. Once the metal concentration in solution was known,
the concentration was multiplied by the solution volume so the total mass was known. The total
mass was then divided by 10.7 cm2 to get the amount of adhered metal recovered from the coupon.
This resulted in the lowest reportable values of 9.1 |ig/cm2, 45.6 |ig/cm2, and 2.3 |ig/cm2 for Cs,
Co and Sr, respectively.
2.5 Other unchlorinated secondary treated effluent parameters
Coliform/Escherichia coli levels were measured via Colilert-18 Quanti-tray 2000 (Idexx
Laboratories, Westbrook, ME). Briefly, unchlorinated secondary treated effluent samples were
serially diluted in pH 7.2 Butterfields phosphate buffer (Fisher Scientific, Waltham, MA), plated
and incubated according to the manufacturer's instructions (35° C). The pH, temperature and
specific conductance were measured by online sensors. Conductivity, temperature and pH were
measured with a GF Signet 2820 series sensor (George Fisher/GF Piping Systems, Schaffhausen,
Switzerland). Water quality parameters were measured continuously over the course of
approximately one year, which was the time frame in which the experiments took place. Sensors
were maintained and calibrated according to the manufacturer's instructions. Data was reported
at 2-minute intervals. MSDGC provided weekly data on 5-day biochemical oxygen demand
5
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(BOD), total suspended solids (TSS) and ammonia (NH3). BOD, TSS, and NH3 were analyzed
according to methods 5210, 2540, and 4500 in Standard Methods for the Examination of Water
and Wastewater (Baird et al. 2017). The weekly MSDGC data spanned a one year period when
the experiments were taking place.
2.6 Quality control and data quality
Quality control (QC) samples for the contaminant reference method included continuing
duplicate samples, controls and laboratory blanks. The data quality objectives for each of these
quality control samples are provided in Table 1. The acceptable ranges limit the error introduced
into the experimental work. All analytical methods operated within the QC requirements for
controls and laboratory blanks, and unless otherwise noted in the Deviations (Section 2.6.2), all
data quality objectives in Table 1 were met.
Table 1: Qualif
y Contro
Data Objectives
Measurement
Matrix
QA/QC Check
Frequency
Acceptance
Criteria
Corrective Action
Temperature
Water
NIST certificate
expiration date
Before each use
Within expiration
data
Replace/recalibrate
thermometer
BOD
Water
Calibration Check
Every batch
Within 30% true
value
Rerun samples
Blank
Every batch
Sample is
compared against
blank
NA
Duplicate
Every 10 samples
(bench)
± 30% RPD
Discard data point,
repeat experiment if
insufficient data
points
pH
Water
Initial calibration
Calibration check
Before each use
Every 10 samples
±0.1 pH units
±0.1 pH units
Check standard
buffers for
contamination, check
electrode for
electrolyte, replace
probe if required
Ammonia
Water
Calibration Check
Every batch
Within 30% true
value
Rerun samples
Blank
Every batch
Sample is
compared against
blank
NA
Duplicate
Every 10 samples
(bench)
± 30% RPD
Discard data point,
repeat experiment if
insufficient data
points
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Measurement
Matrix
QA/QC Check
Frequency
Acceptance
Criteria
Corrective Action
TSS
Water
Calibration Check
Prior to analysis,
every 10 samples,
and at the end of
the batch
±10% TV
Recalibrate and/or
reanalyze affected
samples.
Duplicates
Once per batch or
every 10 samples
RPD<20%
Repeat analysis on
the same sample; if
sample volume does
not allow, choose
another sample and
document
accordingly
E. coli
Sterile
buffer /
water
Positive control
using stock
Once per batch
±10 fold of the
stock
Investigate laboratory
technique. Re-analyze
the stock and change
if necessary
E. coli
Sterile
buffer /
water
Negative control
using sterile buffer
/ sterile water
Once per batch
0 MPN / plate
Investigate laboratory
technique. Re-analyze
the sterile buffer /
sterile water and
change if necessary
Equipment
Calibration
Initially
R2> 0.995
Re-calibrate
Initial calibration
verification
Once immediately
after calibration
±10 % of the
actual
concentration
Re-calibrate
Metals (Cs, Co,
10%
Laboratory check
blanks
One per batch
< instrument
detection limit
Re-run blank
Sr)
nitric
water
Matric Spike
One per batch of
20 Samples
± 20 % recovery
Re-run spike,
Re-prepare Spike
Lab Duplicate
One per batch of
20 samples
< 10% RPD
Re-run duplicate
Re prepare duplicate
Continuing
Calibration
Verification
Every 10 sample
± 15 % recovery
Re-prepare QCs
Re run affected
samples
BOD, biochemical oxygen demand; MPN, Most Probable Number; NA, Not Applicable; NIST, National Institute of
Standards and Technology; QA, quality assurance; QC, quality control; RPD, relative percent difference; TSS, total
suspended solids; TV, true value.
2.6.1 Data quality
At least 10% of the data acquired during the evaluation were audited. The data were traced from
the initial acquisition, through analysis, to final reporting, to ensure the integrity of the reported
results. All calculations performed on the data undergoing the audit were checked. No significant
adverse findings were noted in this audit.
2.6.2 Deviations
Coliforms/E. coli measurements were not in the original quality assurance project plan (QAPP)
developed for this study. The QC criteria for coliforms/E.coli measurement is described in Table
1 and the methodology in section 2.5.
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3.0 Results and Discussion
3.1 Unchiorinated secondary effluent water quality
Unchlorinated secondary effluent temperature varied from 13-27 °C depending on the season in
which the experiment took place, but averaged 22 °C. The pH varied from 6.7 to 7.4, with an
average value of 7.0. Conductivity ranged from 760 to 2000 |iS, with an average value of 1,500
|iS across all experiments. Coliform and E. coli levels were measured on four occasions and
ranged from 20,000 to 150,000 most probable number (MPN)/100 ml and 1,300 to 7,500
MPN/100 mL, respectively. MSDGC provided data on BOD, TSS, and NH3, which is
summarized in Table 2.
Table 2: Unchlorinated Secondary Treated Effluent 5-day, BOD, TSS and NH3 Data
BOD
TSS
nh3
(mg/L)
(mg/L)
(mg/L)
Average
3.35
6.02
0.75
Standard Deviation
1.78
2.52
0.67
Maximum
Observed
10.0
14.0
2.86
Minimum Observed
2.00
2.50
0.10
Number of Samples
48
61
31
BOD, biochemical oxygen demand; TSS, total suspended solids
3.2 Evaluation of cesium and cobalt adhesion to infrastructure materials
Contamination experiments with cesium and cobalt were conducted in duplicate. Contamination
injections were conducted the same across experiments with a one-minute injection that achieved
5 mg/L of the metal in the flow. For cesium, sampling of coupons occurred for 28 days after
injection in the first test. Across all infrastructure materials, cesium was not detected in the
coupon extract over the course of the 28 days of sampling. Because of this result, the duplicate
test was conducted for 5 days. No cesium was detected in the coupon extract during the 5-day
sampling period. Since no cesium was detected on the any infrastructure material, experiments
with cobalt were limited to 5 days of post-injection sampling. Like cesium, no cobalt was
detected in any coupon extract across all infrastructure materials.
3.3 Evaluation of strontium adhesion to infrastructure materials
Like cesium and cobalt, strontium tests were conducted in duplicate. The first strontium test
(Test 1) was conducted for 28 days (Figure 3, top). In the first test, Sr was detected above the
MDL during one sampling event on concrete coupons (day 21). On concrete, the Sr
concentration varied from 0.45 to 2.32 |ig/cm2, with the 2.32 |ig/cm2 data point being the only
one above the MDL. The second strontium sampling experiment (Test 2) was conducted for 5
days (Figure 3, bottom). Similar to the first test, strontium was detected in all coupon types, but
only samples from concrete coupons were above the MDL. These concentrations ranged from
3.7 to 6.3 |ig/cm2 (Figure 3). Note that concentration values were determined for sample points
below the MDL and are shown in Figure 3, but it cannot be stated that these values are greater
than zero with 99% confidence.
-------
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7.0
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5.0
4.0
3.0
2.0
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- HDPE A Brick
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-------
Table 3: Strontium Extracted from Uncontaminated Conditioned Control Coupons (Top
Rows) and Those Soaked in Acid to Extract More Cesium (Bottom Rows) i
Material
Brick
Clay
Concrete
HDPE
Iron
PVC
Rubber
Uncontaminated conditioned control (ng/cm2), n=2
Test 1
(28 day)
Mean
0.26
0.18
1.03
0.37
0.33
0.21
0.18
Range
0.14
0.02
0.30
0.39
0.02
0.00
0.01
Test 2
(5 day)
Mean
0.10
0.04
5.88
0.22
0.16
0.14
0.11
Range
0.13
0.00
1.90
0.20
0.00
0.07
0.03
Uncontaminated conditioned control (ng/cm2), n=4
All Tests
Mean
0.18
0.11
3.45
0.30
0.24
0.18
0.15
Standard
Deviation
0.12
0.08
2.91
0.20
0.09
0.05
0.04
Uncontaminated
coupons soaked in acid (ng/cm2), n=3
Mean
4.65
6.43
97.02
2.17
10.54
21.16
3.40
Standard
Deviation
0.30
0.27
3.08
0.01
0.33
1.19
0.05
HDPE. high density polyethylene. Note: Values in bold are above the MDL. Values in normal font are below the
MDL
In Test 1, all control samples are below the MDL. However, during the 28 day collection period,
three concrete samples were above the range of the control (days 3, 4 and 21) and three were
below (days 7, 10 and 28). In Test 2, only strontium extracted from concrete was above the
MDL. In this case, all samples harvested after introduction of strontium into the test bed
channels fall within the range of the control, except for the last sample on day 5 that is below
(Figure 3). All concrete samples analyzed for strontium during both duplicate tests (four total)
fall within the range of the standard deviation of the "All Test" pre-injection conditioned control
samples, except the sample from Test 1, day 10, which was less.
Table 3 also includes a section titled "uncontaminated coupons soaked in acid". Data for this
section was collected by either grinding or shredding one gram of the clean (not conditioned in
secondary effluent), uncontaminated collection system material into the smallest pieces possible.
These pieces were then microwave digested using EPA method 3051A (USEPA 2007a) using a
Mars Xpress microwave digester (CEM Corporation, Matthew, NC), and the liquid extract
analyzed for strontium. This process was performed in triplicate. For each material, more
strontium was extracted compared to the rinsing process used during contamination experiments.
However, the rinsing method was designed to remove adhered strontium, not strontium in the
material itself. The most strontium was associated with concrete, which suggests that there is
strontium in the cement or mortar material that make up the concrete coupon that could leach
into the rinse solution. Note that data from this section of Table 3 was first normalized by the
mass of material samples, but then extrapolated to the area of the coupon for comparability to the
control samples.
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Taken together, the data suggests that strontium detected on the concrete coupons came from the
concrete itself, and was not due to adhesion of the strontium to the coupons. It cannot be ruled
out that some strontium adhered to the coupons, but any amount was small enough that it could
not be distinguished from the inherent strontium extracted from the coupon itself. It should be
noted that all concrete coupons were cut from concrete sewer pipe, but samples used in Test 1
and Test 2 came from two different concrete pipes. Assuming strontium detected during
sampling came from the pipe itself, it is possible that the different pipes contained a different
mix of cement and mortar, which affected the inherent strontium levels. This would explain the
overall difference in the amount of strontium detected in the coupon between tests. However, it
should be noted that most strontium detected in Test 1 was below the MDL.
3.4 Discussion, implications and future research
The data presented in the previous sections indicate that cesium and cobalt were not detectable
on the collection system infrastructure coupons used in this study. In particular, no results were
recorded for cesium on the coupons, even below the method detection limit. Similar results
were found with strontium, but results were often recorded even though they were below the
detection limit. However, data from the pre-contamination control samples and the ground
coupon materials digested in nitric acid suggest that the strontium found on the coupons was not
from the injected strontium, but from the material itself. It is possible that some cesium, cobalt
or strontium was present on the coupons, but the instrumentation used in this study was not
sensitive enough to detect them. It should also be noted that the data in this study is applicable to
straight runs of pipe. The coupons used in this study are not designed to replicate pipe bends or
elbows.
Two similar studies have been carried out on the bench scale using iron, concrete, copper and
PVC surfaces in chlorinated drinking water and non-radioactive salts of cesium, strontium and
cobalt (Szabo et al. 2009, USEPA 2016) In both studies, cesium did not persist on any material,
or was undetectable. Strontium and cobalt were found to persist on concrete surfaces. Cobalt
was found to persist on iron (Szabo et al. 2009) and concrete (USEPA 2016), but this was due to
cobalt forming an insoluble precipitate when in contact with chlorinated tap water, which
deposited on the coupon surfaces. Strontium was found to be persistent on concrete in USEPA,
2016. However, in that study, strontium was deposited directly onto the coupon surface and
allowed to soak in in the absence of flow, which likely lead to greater persistence. In the study
reported here, wastewater flow was continuously present in the pilot scale SETBC, which is
closer to resembling flow in a real wastewater system and makes adhesion more difficult.
Finally, drinking water is a different matrix than wastewater, which may account for differences
in the results.
When considering the implications of the results presented above, the typical uses of the
radioactive forms of the metals used this study should be considered. Radioactive cobalt-60 is
commonly used in solid metallic form in instruments ranging from medical devices (e.g.,
blood/tissue irradiators, brachytherapy, teletherapy) to level gauges and food sterilization
devices. Strontium-90 is used in medical devices, thickness gauges, level measurements, and
automatic control processes. However, one of its most common historical uses was in
radioisotope thermal electric generators, which are used to generate electricity and can contain
tens of thousands of curies of activity. However, strontium is often used in devices as strontium
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titanate, in a ceramic form, or as a metal foil (EPA 2007, IAEA 2000).
Should cobalt-60 or strontium-90 be used in an RDD, it would likely be in a form that has low
solubility in water. If pieces of radioactive cobalt or strontium were dispersed and then washed
into a collection system, it would likely be in insoluble pieces that may settle out or be carried in
the wastewater flow (USEPA 2007). Any dissolved fraction would be small, although it is
possible to solubilize cobalt and strontium into a soluble salt. The insolubility of radioactive
cobalt and strontium source material coupled with the fact that neither was detected on collection
system infrastructure surfaces in this study when in their soluble form indicate that it is unlikely
that either would persist on collection system infrastructure.
Unlike radioactive colbalt and strontium, cesium-137 is commonly used as a soluble chloride salt
in medical devices, sterilizers and industrial gauges (IAEA 2000). Should it be used in a RDD,
the soluble salt would be dispersed, and could dissolve in wastewater or stormwater flowing
through a collection system (USEPA 2007). However, it was not detected on any of the
infrastructure surfaces when introduced into flowing wastewater at 5 mg/L. For cesium chloride,
5 mg/L translates to activity in the water of 0.44 Ci/L (4.4x 1011 pCi/L), with the equivalent of 1
Ci injected over the course of 1 minute using the injection conditions in this study. The isotope
and amount of activity in a RDD are impossible to know and could take many different forms.
However, past scoping studies have shown that the metal concentrations used in this study are
feasible (USEPA 2007).
Finally, as noted earlier, it is possible that some contaminant adhered to the coupons, but, in this
study, the analytical instrumentation was not sensitive enough or else the extraction methods was
not efficient enough to detect it. This could be resolved by introducing the actual radionuclide
into the SETBC and measuring residual activity on the surface of the coupon. One recent study
did use cesium-137 as the contaminant in a bench scale drinking water system with concrete,
copper and PVC surfaces (USEPA 2018). Cesium-137 was introduced at 10 |iCi/L (concrete)
and 100 |iCi/L (PVC and copper). This translates to 1.14><10"7 mg/L (10 |iCi/L) and 1.14><10"6
mg/L (100 |iCi/L), which is less than the levels used in this study. Activity on the coupons was
measured using a sodium iodide spectroscopy system and a liquid scintillation counter. The
results showed that after 24 hours of stagnant exposure to dissolved cesium, there was transient
adhesion of cesium to PVC and concrete, but 91% and 93%, respectively, were removed after the
coupons were flushed with clean water. Further flushing and use of decontamination agents
removed over 99% of the adhered cesium.
A key difference between these studies was that 24 hours of stagnant contact between the cesium
and infrastructure coupons was used in USEPA, 2018, whereas coupon to exposure cesium was
only one minute with constant flow in the SETBC wastewater system. The fact that residual
cesium activity was detected on the coupons was likely due to this extended initial contact time.
However, even after flushing and decontamination with chemical cleaning agents, some residual
activity was still detected on the coupons. In the future, it would be beneficial to repeat the
experiments described in this report with radioactive cesium, cobalt, and/or strontium. Although
large scale persistence is unlikely, research with real radionuclide would determine if a small
fraction does adhere and persist on the coupons. Still, the results of this study suggest that any
cesium, cobalt or strontium washed into a wastewater collection system would not persist on the
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infrastructure, with dissolved material traveling with the flow to a combined sewer outfall or
wastewater treatment plant. Also, as noted above, not all radionuclides could enter a sewer
system in soluble form. The fate of insoluble radioactive particles in sewer collection system is a
topic that deserves further study.
4.0 Conclusions
The primary conclusions from this study are as follows:
• Cesium, cobalt and strontium salts injected into a pilot scale wastewater collection
system using secondary treated effluent were not detectable on common collection
system infrastructure materials. It should be noted that non-radioactive salts were used
and were analyzed by atomic adsorption and inductively coupled plasma.
• Strontium was the only metal detected above the MDL. However, analyses of the
strontium content of the control (pre-injection) coupons suggested that detectable
strontium came from the coupon itself and did not adhere after injection of strontium into
the wastewater flow.
• Past studies examining the persistence of radioactive cesium on infrastructure materials
in a drinking water environment did find some residual activity after flushing the surfaces
with clean water and other decontamination techniques. Repeating the experiments
presented in this report with real radionuclides would help determine if a small fraction of
cesium, strontium or cobalt do adhere to the infrastructure coupons, but could not be
detected with the analytical techniques used in this study.
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