Age-dating of Groundwater

Lecture at Washington University, St. LouisApril 11, 2007Publication # UCRL-PRES-229859

ByM. Lee Davisson Lawrence Livermore National Laboratory

Helps answer:

How much is there?

How long will it last?

What is the source of contamination?

What is the risk of a contaminant?

What is the value of groundwater ages?

Darcy Equation

Q is Darcy velocity

K is intrinsic aquifer property

is hydraulic head

€

Q = Kdhdl

€

dhdl

€

v = Q / ρ

€

time =distance

v

Can we measure the necessary parameters?

v is actual microscopic velocity

is porosity

dl

dhK

distancetime =

Distance Can be measured between two groundwater wells.But what is the distance between a recharge point and a well?

Groundwater elevation in wells measured with great accuracyAt larger scales topography will suffice

K Cannot be measured in the fieldDifficult to measure in the laboratorySensitive to geographic and depth scaleSource of most uncertainty in hydrogeologic analysis

dl

dh

Material Hydraulic conductivity (m/s)

Clay 10-11 to 10-8

Silt, sandy silts, clayey sands, till

10-8 to 10-6

Silty sands, fine sands 10-7 to 10-5

Well-sorted sands, glacial outwash

10-5 to 10-3

Well-sorted gravel 10-4 to 10-3

• What about fractured rock?

• Water about karst?

10-2 to 10-11!

time Can be measured by tracers or other markers of timeCan be measured with variable accuracyCan be measured over a wide age range

Age-Dating Methods

• Natural radioactivity

• Climate change

• Inadvertent TracersTritiumChlorofluorocarbonsKrypton-85Stable isotopesDissolved contaminants

• Intentional TracersSulfur-hexafluorideNoble gasesDyes

SUPPLY

DEMAND• Agriculture• Urban• Recreation• Environmental

How much is there? Demand = Supply

• Natural recharge rates difficult to measure directly

• Age-dates of groundwater older than human occupation provide natural recharge rate

• Age-dates of youngest groundwater provide modern recharge rates=

=

=

AAge

distanceQ

A

Qv

v

distanceAge

aq

aq

porositytime

volumeaquiferQaq

=

=

What is the distance traveled by the groundwater?

• In basins with little elevation gain, distance approximately equals depth to groundwater well extraction level

• In basins with large elevation differences, recharge sources need to be determined

Tropical Arid

Large elevation change

Small elevation change

DistanceGroundwater TravelsIncreases

•Many choices of naturally-occurring isotopes for age-dating

•Which ones behave most like water?

Natural radioactivity

Isotopic age-dating methods• Unstable isotopes with relatively high decay constants• Either natural abundances or concentration spikes created by nuclear fallout

t

0

eN

N λ−=N = measured isotope abundanceN0 = abundance at time of rechargeλ = decay time constantt = time

N0 dependent on reactive and transport processes• Variation in source concentration• Dispersion/mixing/dilution• Phase changes

λ=

ln2T

2

1Half-Life =

VLV

L

R

R−=α

10001⎟⎟⎠

⎞⎜⎜⎝

⎛−=

STD

SA

RRδ

> 1 for hydrogen and oxygen isotopes

-200

-150

-100

-50

0

-25 -20 -15 -10 -5 0

δD

δ18O

SMOW

EvaporationRain-o

ut

GMWL

Mean Annual Precipitation

• Isotopic values controlled by temperature

LatitudeElevationInland distance

• Groundwater reflects mean annual precipitation values

Climate Change

Paleo-Recharge

Modern-Recharge

Climate Change

• Recharge during last glacial maximum (~10kyr ago) likely had lower isotopic values

• Groundwater values significantly lower than mean annual precipitation (except in karst)

• No plausible higher elevation recharge sources

• No plausible surface water recharge sources with low isotopic values

• Must make hydrologic sense

Water Table elevation – Sacramento Valley

Groundwater Oxygen-18 Values – Sacramento Valley

Potential Sources

• Rain/Snow Low elevation

High elevation• Rivers

• Agricultural irrigation Local sources Imported sources

• Urban landscaping

Age-dating groundwater older than human occupation

Radiocarbon

( )( )

ln(fmc)x8267Age

fmc orcarbonmodernfraction

CC

CC

C

std12

14meas

12

14

14 −=

=(14C/12C)std is an oxalic acid whose radiocarbon abundance is equal to the abundance of atmospheric CO2 in 1950

•Radiocarbon dating typifies challenges in age-dating methods

•Where carbon comprises significant amount of aquifer matrix, water-rock rxn dominates over radioactive decay

•Volcanoes are another source of dissolved carbon absent in 14C

Closed System Rxn: 14CO2 + H2O + M12CO3 H14CO3 + H12CO3 + M++

Open System Rxn: 14CO2 + H2O H214CO3 + H12CO3 H2

12CO3 + H14CO3 fast slow

Saturated Flow: H14CO3 + M12CO3 H12CO3 + M14CO3

10-8 10-10/cm2s

< 1yr

Possible Correction Method

• Establish all plausible initial 14C content of recharge

• Draw reaction lines (straight lines) toward 14C-absent source material

• Compute horizontal off-set of measured values from reaction lines

• Subtract off-set from one and compute age

4He 4He 4He

Dissolved 4He concentration increases

Natural uranium and thorium decay

Steady-state 4He flux from crust ~1e9 atoms/cm2-yr

• Rate dependent on

Regional uranium-thorium concentrations in crust Localized geologic faulting• Uncertainties factor of two or more• Good for only groundwater >1000 years old

Helium-4 Accumulation in Age-Dating

Castro et al., 2000

Carrizo Aquifer, TX

Age-dating groundwater since human occupationImpacts of engineered systems

Land UseHow groundwater recharge is affected

Arid Climate Wetter Climate

Agriculture Significantly enhances recharge; depletes and often contaminates groundwater

Modest changes in natural recharge rates; nutrient contaminants

Urbanization Significantly reduces natural recharge; petroleum and solvent contamination

Modest changes in natural recharge rates; petroleum and solvent contamination

Seawater intrusion Common in coastal environments

Common in coastal environments using groundwater

Surface water management

Changes where recharge occurs Reduces river recharge

Young groundwater age-datingChlorflourocarbons (CFCs) Krypton-85 (85Kr)

NO NATURAL SOURCES

Age = mol/Lin air = mol/Lin water x Hair-water

H = Henry’s Law partitioning coefficient

f (mean soil temperature)

CFCs Drawbacks• Reducing conditions• Point sources (e.g. landfills)• However:

CFC-113/CFC-111 ratios verify conservation

85Kr Drawbacks• Point sources (e.g. nuclear sites)• Not many labs measure it

Tritium (3H)

• Numerous studies since the 1960s

• Part of the water molecule

• Useful half-life (12.4 years)

• Atmosphere is sole source

• Point source contamination rare

• Atmospheric concentration has large variation

• 3H alone is excellent post-1950 age indicator

t

0

eN

N λ−=

3Hemeas = 3Hetrit + 3Heequil + 3Heexcess + 3Herad

4Hemeas = 4Heequil + 4Heexcess + 4Herad

22Nemeas = 22Neequil + 22Neexcess

Over determined system allows the calculation of 3Hetrit

ët

trit3

meas3

meas3

eHeH

H −=+

Noble Gas Mass Spectrometry

• Chemically suitable for potable supplies

• Conservative behavior

• Water soluble and measureable over large dynamic range

• Inexpensive

Common Tracers

Sulfur-hexafluoride

Noble gases (He, Xe)

Dyes (Rhodamine)

Artificial Tracers

0

20

40

60

80

1000 2000 4000 6000 8000 10000

AL recharge

North FP

South FP

Percent DOC Removal

Distance From Recharge Point (ft)

Anaheim Lake/Kramer Basin

Shallow Monitoring

AMD-9/1

OCWD-KB1AM-44

AM-7

KBS-4

AM-8

AM-10

AM-9

AM-14

0.0

2.0

4.0

6.0

8.0

10.0

0 100 200 300 400 500 600 700

AM-7

AM-8

SCWC-PLJ2

A-26

C/Co x100 (

136 X

e)

Days

North Flow Path

(a)

• High degree of accuracy

• Discriminate individual flow paths

• Track contaminant fate

• Evaluate health risks

Selected Reading

Craig, H., 1961, Isotopic variations in meteoric water. Science, 133, 1702-1703.

Dansgaard W., Stable isotopes in precipitation. Tellus XVI 4, 436-468, 1964.

Handbook of Environmental Isotope Geochemistry. Elsevier: New York, Fritz, P., Fontes, J.Ch. (eds.); 1980.

Heaton T.H.E. and Vogel J.C., 1981, "Excess air" in groundwater. J. Hydrol., 50, 210-216.

Ian D. Clark, Peter Fritz, 1997, Environmental Isotopes in Hydrogeology. CRC Press; 352 pgs

Ingraham, N.L., Taylor, B.E., Light stable isotope systematics of large-scale hydrologic regimes in California and Nevada, Water Resour. Res., 27, 77-90, 1991.

Mazor, E., 1991, Applied Chemical and Isotopic Groundwater Hydrology. Halsted Press: New York, 274 pgs.

Poreda, R.J., Cerling, T.E., Solomon, D.K., 1988, Tritium and helium-isotopes as hydrologic tracers in a shallow unconfined aquifer. J Hydrol. 103, 1-9.

Schlosser, P. Stute, M., Dorr, H., Sonntag, C., Munnich, O., 1988, Tritium/3He dating of shallow groundwater. Earth, Planet. Sci. Lett., 89, 353-362.

Schlosser, P. Stute, M., Sonntag, C., Munnich, O., 1989, Tritiogenic 3He in shallow groundwater. Earth, Planet. Sci. Lett., 94, 245-256.