View
7
Download
0
Category
Preview:
Citation preview
Using TritiumUsing Tritium--Helium Helium Groundwater Age to Assess Groundwater Age to Assess Contamination Vulnerability Contamination Vulnerability
MORAN, Jean E. and HUDSON, G. Bryant
NWQMC 2006 session G3-1
This work was performed under the auspices of the U.S. Department of Energy by the University of CaliforniaLawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
The Groundwater Ambient Monitoring and Assessment program is sponsored by the State Water Resources Control Board and carried out in a collaboration between the U.S. Geological Survey and Lawrence Livermore National Laboratory
The Question• How will water quality in deep aquifers
used for public water supply be affected by vertical transport?
The Tools• Groundwater Age (Tritium-Helium method)
• Oxygen isotopes (recharge water source)
• Ultra low level VOC analysis (part per trillion detection limits)
10
100
1000
10000
100000
1950 1960 1970 1980 1990 2000Recharge Year
3 H a
t rec
harg
e (p
Ci/L
) PortlandSanta MariaOttawa
Atmospheric nuclear testing introduced a large amount of tritium into the atmosphere in the early 1960’s
Water recharged before 1950 does not contain tritium now
Assessing Groundwater VulnerabilityWater Age VOCs detected? Vulnerability
Post-modern Yes Vulnerable –sources present
Post-modern No Vulnerable – no sources
Pre-modern No Low vulnerability
Pre-modern Yes Short-circuit
Type(s) of VOCs detected, and age analysis including mixing, give additional time-of-travel information.
0
50
100
150
200
250
0.5 1 1.5 2 2.5 3 3.5 4 7.5 10 13.5 15 17.5 20 25 30 40 50 140
Tritium Concentration (pCi/L)
Freq
uenc
y
Contain post-bombanthropogenic tritium
In the range of modern-day precipitation
Post-modern componentis less than 4%
Tritium-dead samples provide an archive of pre-modern water quality
Tritium alone can be misleadingLocation Measured
Tritium (pCi/L)Calculated GW Age (yrs)
Bakersfield 1 11.1 15Bakersfield 2 11.3 34San Jose 1 12.4 11San Jose 2 12.5 42Chico 11.0 32Half Moon Bay 11.1 3.5
271 samples (25%) fall in the ambiguous range of 9 to 15 pCi/L tritium
For every tritium decay, an atom of 3He is produced
0
250
500
750
1000
0 12 24
Time (years)
Triti
um a
tom
s
0
250
500
750
1000
Hel
ium
-3 a
tom
s
nn np3H 3He
pp Beta decay
The 3He from 3H decay starts to accumulate once the water has
become groundwater
Age (years) = 18 x ln( 1 + 3He / 3H )
0 years 12 years 24 years
Dissolved noble gas concentrations and 3He/4He are measured by noble gas
mass spectrometry
Results from 150 3H-3He age measurements in Orange Co. CA
Groundwater age in years
0
200
400
600
800
1000
May-49 May-60 Apr-71 Mar-82 Mar-93Vintage (yr)
Initi
al T
ritiu
m (p
Ci/L
)
Well Data
averagedprecipitation
Many tritiated samples contain a large fraction of pre-modern water
Relative ages are most informative• Relative ages and mixing
fractions in a basin allow delineation of flow field and identification of vulnerable areas
• Absolute ages at individual wells can be difficult to interpret
Base map from Shelton et al., 2001
The GAMA program has provided the means to go from low density to basin-wide coverage
San Jose
Relative ages give bulk flow rate and identify ‘fastpaths’
VOC Detections
Data from Shelton et al., 2001
MTBE Concentrations
Groundwater Age is a Good Predictor of Contamination Vulnerability
45%
16% (Sacto, Stockton, Chico)
6% (Tulelake, Sacto, Chico)
57%
> 50 years
68%Any THM
24%PCE
67%MtBE
18% (Burney, Ft. Jones, Santa Clara County)
No VOCs
< 11 years
45%
16% (Sacto, Stockton, Chico)
6% (Tulelake, Sacto, Chico)
57%
> 50 years
68%Any THM
24%PCE
67%MtBE
18% (Burney, Ft. Jones, Santa Clara County)
No VOCs
< 11 years
43%
Coastal Basins1
N=194Central Valley Basins2 N=338
No VOCs3 105 (54%) 48 (14%)
MtBE 29 (15%) 87 (25%)
PCE 18 (9%) 166 (49%)
CHCl3 68 (35%) 252 (74%)
1
2
3
1 Includes Santa Clara Valley, Basins of San Mateo County (San Mateo Plain, Westside Basin and Coastside Basin), Livermore-Amador Basin, and Niles Cone (Fremont, CA)
2 Includes Bakersfield, Chico and surrounding northern Sacramento Valley, Sacramento, and San Joaquin County urban areas (Stockton, Lodi, and Manteca); wells from areas outside of alluvial basins are not included on this table
3 Samples had <RL for all compounds listed in Table 1
There is a large contrast in VOC occurrence in Coastal versus Central Valley Basin wells
PCE is found in pre-modern water, especially in the Central Valley
PCE Occurrence
0 10 20 30 40 50 60 70 80
Chico (62)
Northern Sacramento Valley (123)
Sacramento (108)
Modesto** (40)
Fresno** (50)
Bakersfield (43)
San Joaquin County (95)
Santa Clara Valley (124)
Los Angeles/Orange County* (178)
Niles Cone (16)
San Mateo County (22)
Livermore Valley (22)
Livermore Valley (22)Coastal AquifersCentral Valley
*Shelton et al., 2001**Wright et al., 2004
Groundwater ages can be used by water managers
Dictate the frequency and extent of monitoring
Wells with ages e.g., < 10 years should be monitored more frequently and for more compounds, including emerging contaminantsExpend fewer resources for testing wells that will likely remain ‘non-detect’ at DHS reporting levels
Source area protection & remediation
‘Radial distance’ method of delineating source area is more relevant for wells with young ages
Location VOCs detected
Calculated GW Age (yrs)
Bakersfield 1 TCE, PCE, THMs
15
Bakersfield 2 THMs 34
San Jose 1 THMs 11
San Jose 2 none 42
Livermore 1 MtBE, TCE, THMs
18
Livermore 2 THMs > 50
Ages and relative contamination vulnerability can inform managers and regulators about which wells to “pay attention to”
Coastside Basin
Santa Clara ForebayLA/OC Forebay
Bakersfield – unlined canals
East side of Livermore Valley
Niles Cone
Fort Jones & Burney
Llagas Basin
Highly vulnerable areas identified using tritium-helium groundwater ages
In Bakersfield, some production wells have mean ages of less than 2 years
Very low tritium concentrations over a wide area bode poorly forwater supply and opportunities for artificial recharge
Sacramento
Using groundwater age to evaluate recharge scenariosProtecting recharge areas
Widely dispersed recharge areas in which young ages are observed over a wide area (Llagas, Livermore, Bakersfield) Distant recharge area in which deep-screened drinking water well has a recharge area different from shallow aquifer (Chico)
Evaluating potential artificial recharge scenarios
entraining contaminants from a multitude of point sources in recharge areas (Santa Clara, Los Angeles, Orange County)may take water a long time to reach deep aquifer (Sacramento, Chico)
Llagas Basin, Santa Clara County
Looking to the future…• Other dissolved gases have
untapped potential in groundwater investigations
• 85Krypton– Simple source function, no
subsurface sources – Covers same age range as tritium
• Depth-specific sampling• Assessing BMPs and
remediation schemes• Examining water quality
changes during groundwater banking and aquifer storage and recovery
Contact Information
Groundwater Ambient Monitoring and Assessment (GAMA) Program
California State Water Resources Control Board
www.swrcb.ca.gov/gamaJean Moran moran10@llnl.gov
Sample set is mainly long-screened production wells
• Advantages– You get a good,
clean sample (both dissolved gases and low level VOCs)
– Characteristics are usually known
– It’s the water people are drinking
• Disadvantages– Vertical
information is smeared
– ‘Signal’ is diluted– Flow field is altered– It’s the water
people are drinking
Fremont wells have narrow age distributions; some Livermore wells have older water components
10
100
1000
10000
100000
1950 1960 1970 1980 1990 2000Recharge Year
3 H a
t rec
harg
e (p
Ci/L
)
PortlandSanta MariaOttawaFremontLivermore
Recommended