age-dating of groundwater lecture at washington university, st. louis april 11, 2007
DESCRIPTION
Age-dating of Groundwater Lecture at Washington University, St. Louis April 11, 2007 Publication # UCRL-PRES-229859 By M. Lee Davisson Lawrence Livermore National Laboratory. What is the value of groundwater ages?. Helps answer: How much is there? How long will it last? - PowerPoint PPT PresentationTRANSCRIPT
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.