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

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1950 1960 1970 1980 1990 2000Recharge Year

3 H a

t rec

harg

e (p

Ci/L

)

PortlandSanta MariaOttawaFremontLivermore