Quantifying Apparent Groundwater Ages
Near Managed Aquifer Recharge
Operations using 35S as an Intrinsic Tracer
Jordan F. Clark* and Stephanie H. Urióstegui†
Department of Earth Science, University of California, Santa Barbara, CA USA
Richard K. Bibby and Bradley K. Esser
Lawrence Livermore National Laboratory, Livermore, CA USA
Gideon Tredoux
CSIR Natural Resources and the Environment (retired), Stellenbosch, South Africa
Andrew L. Herczeg
CSIRO Land and Water (retired), Glen Osmond, South Australia 5064, Australia
*[email protected]
†Also at Lawrence Livermore National Laboratory
What is 35S?
• Sulfur has five naturally occurring isotopes:
Four are stable (32S, 33S, 34S, and 36S) and one
is radioactive (35S)
• 35S has a very short half life (t1/2 = 87.4 day or
0.24 yr)
• 35S exists in nature only because it is produced
in the upper atmosphere by cosmic rays
35S
life cycle
Upper Atmosphere
Cosmic Rays + Ar
35S
35S
(by spallation)
rapidly oxidizes to 35SO2
35SO
2
is converted to H235SO4
H235SO
Troposphere
H235SO4 enters water cycle as dissolved sulfate [35SO4]
35S
Land Surface
35Cl
+ b-
t1/2 = ~ 87 d
Basic 35S Geochemistry
1. Naturally produced in the upper atmosphere
 Intrinsic or environmental tracer
2. Radioactive with a short, ~87 d, half life
 Removed naturally from water systems after ~1.0 y
 Can be used to date groundwater
3. Deposited to ground via both wet and dry
deposition
4. Why apply it to MAR operations?
 Half life is appropriate for managing these facilities
 True for the State of California
Indirect Potable Reuse Requirements for
California, USA



Originally proposed in 1978 as part of “Wastewater
Reclamation Criteria, Title 22, Division 4, Chapter 3 of the
California Code of Regulations and Environmental Health”
The Department of Health Services (DHS) original set the
criteria for planned indirect potable reuse projects
ca. 1995, the rules specified:
1) Pre-recharge treatment for DOC and nitrogen removal for
reclaimed wastewater sources
2) Dilution requirements with higher quality source waters
3) A minimum subsurface retention time of 6 months for pathogen
removal based on the work of Yates et al. (1985)
4) A minimum horizontal distance of 500 ft (~150 m) between
recharge location and potable production
Groundwater Recharge Requirements



Originally proposed in 1978 as part of “Wastewater
Reclamation
Title 22, Division
4, Chapter
ThereCriteria,
were problems
with these
rules: 3 of the
California Code of Regulations and Environmental Health”
The
of Health
Services
(DHS)
original set
1)Department
While there
was good
reason
to specify
thethe
criteria for planned indirect potable reuse projects
subsurface retention time of 6 months, there
ca. 1995, the rules specified:
was no way to determine it.
1) Pre-recharge treatment for DOC and nitrogen removal
2) Dilution requirements with higher quality source waters
Many MAR
sites retention
in California
3)2)A minimum
groundwater
time of 6pre-date
months forthe
pathogen
1978 statute.
removal
4) A minimum horizontal distance of 500 ft (~150 m) between
recharge location and potable production
Groundwater Replenishment Recharge Regulations (2014)
Methods to Determine Log Reduction of Virus (LRV)
Planning and Engineering Report Effort vs. LRV
Method
General
Accuracy
Formula
(Darcy Law)
Poor
3-D model
Intrinsic
(Environmental)
Tracer
Added
(Deliberate)
Tracer
Fair
Better
Best
Effort Level
Log Virus
per month
Travel
Time
Limited info
on Aquifer
0.25
2.0 y
0.50
1.0 y
0.67
0.75 y
1.0
0.5 y
Substantial
info on
Aquifer
Quantify
Existing
Indicators
Track added
Tracer
35S
Study Objectives
1. Determine if 35S dating is possible near MAR sites
a. All previous 35S measurements from high elevation,
low [SO4] headwater streams
Studies show 35S activities are very low in the environment.
b. Need to improve existing analytical procedure.
With improved procedure, we found detectable 35S near MAR sites.
2. Can groundwater apparent ages be estimated using a
simple piston flow model? No
3. Is there any value in 35S data? Yes but with reservations
4. Conclusions
35S is detectable in MAR source and ground waters.
While apparent ages are difficult to determine, 35S
activities can be used to identify a component of < 1 yr
Improved Analytical Procedure
Doi:10.1021/acs.analchem.5b00584; 2015, Vol. 87, 6064-6070
Basic Steps:
1) 2 hr spin w/20 gr of Amberite Resin & ~20 L filtered sample.
2) [SO4] washed off of Amberlite Resin w/ 5% NaCl solution.
3) BaSO4 precipitation and collection from saline wash solution
4) After drying, BaSO4 mixed with Insta-Gel Plus scintillation cocktail.
5) 35S activities measured on a Quantulus 1220 Ultra-Low Liquid
Scintillation Spectrometer (optimized for a specific energy window).
Two well known MAR sites chosen to test
and develop the 35S methodology
Atlantis MAR Location
South Africa
MAR began: 1977
35S field campaign August 2010
Kraemer Basin Orange County
Water District, CA, USA
MAR began: 1988
35S field campaign 2012-2013
Atlantis MAR
Pond 7
Monitoring Well
226c
Production Well
G34025
Field Laboratory
Field Laboratory: 2
Field Laboratory: 3
Field Laboratory allowed
us to return to the USA
with small bottles rather
than 20 L containers.
First Result of Atlantis Study
PW G30966
Groundwater vs Surface Water
Surface Water sample is much darker
Resin must be collecting other anions
Have not identify the but suspect that
they are organic.
Analytical Procedure Changed
Extracted saline [SO4] solution passed
through a 2nd column containing
activated carbon prior to the BaSO4
precipitation step
Split Structure
Atlantis 35S Results: MAR Source Water
Split Structure:
12.8 ± 0.8 mBq/L
Pond 12:
16.2 ± 0.9 mBq/L
Pond 7:
13.8 ± 0.8 mBq/L
Atlantis 35S Result: Nearby Well Samples
All groundwater samples had detectable 35S
Activity Range: ~15 to ~6 Bq/L
35S Apparent Ages,
t (wks)
Assumed Very Simple Piston Flow
æ Aö
t = 1 ln ç ÷
l è A0 ø
l = 0.0554 wk -1
A0 = source water acivity
Results from Atlantis:
Analytical method needs to include the addition of
an activated carbon column
Field Laboratory could easily be setup and only
small volume samples needed to be transported to
the counting laboratory
35S
is detectable in the MAR setting
However, saw no systematic relationship between
apparent 35S age and horizontal flow distance
Kraemer Basin 35S Experiment
• Encouraged by the Atlantis results, a second field study
was initiated at OCWD’s Kraemer Basin
• This site was chosen because:
– Close to UCSB
• Proximity allowed time series measurements
– Long history of travel time experiments
• T/3He apparent age survey
• Dissolved Gas (SF6 & 136Xe) Exiperiments conducted in
1998 & 2008
– Hydraulic Connections Known
Kraemer Basins Gas Tracer Results
Oct 1998 (LLNL) Exp
SCWC-PBF3/4
Kraemer Basin
La Jolla Basin
AM-7
SCWC-PLJ2
KBS-1
KBS-3
for ~ 7 days
Recharge Rate = ~120 cfs
KB-1
AM-49
AMD-1
AM-48/48A
January 2008 Exp
AM-10
AM-9
136Xe
d18O for ~ 45 days
AMD-10
AMD-12
AM-8
AM-44
AMD-11
SF6 for ~14 days
AM-13
AM-14
Recharge Rate = ~70 cfs
Monitoring Well
Production Well
0
1
Kilometers
SAR
[SF6] = 66 pmol/L
Detect. Limit = 0.5 pmol/L
OCWD MAR Source Water
Kraemer Basin and 6 other locations
2 End
Member
Mixing
Kraemer
Basin
Samples
With no Decay
Groundwater
Replenishment
System Water
Mixing and Decay
OCWD MAR Groundwater:
35S analyzed along Northern flow path down
gradient from Kraemer Basin
OCWD MAR Groundwater
35S Apparent Age (wks) Time Series
35S
Study Conclusions
1. Created an improved batch method for LSC 35S
a. For MAR sites, activated carbon column must be
added to remove color impurities.
b. Method is portable and can be used in field
laboratories, allowing for small samples to be
sent/transported to the counting laboratory.
2. 35S detected in almost all of the MAR samples collected
at the Atlantis and OCWD MAR sites.
3. Source variability is large both spatially and temporally
making groundwater dating very difficult and uncertain.
4. While apparent ages are uncertain, 35S activities can be
used to identify a component of < 1 yr