Transcript What is the source of contamination?

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?
What is the source of contamination?
What is the risk of a contaminant?
Darcy Equation
Q is Darcy velocity
K is intrinsic aquifer property
dh
dl
is hydraulic head

v is actual microscopic velocity
 is porosity

dh
QK
dl
v  Q/
distance
time
v
Can we measure the necessary parameters?
Distance
Can be measured between two groundwater wells.
But what is the distance between a recharge point and a well?
dh
dl
Groundwater elevation in wells measured with great accuracy
At larger scales topography will suffice
K
Cannot be measured in the field
Difficult to measure in the laboratory
Sensitive to geographic and depth scale
Source of most uncertainty in hydrogeologic analysis
Material
Hydraulic conductivity (m/s)
Clay
10-11
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
Well-sorted gravel
10-4 to 10-3
to
10-8
distance
time 
dh
K
dl
• What about fractured rock?
to
10-3
• Water about karst?
10-2 to 10-11!
time
Can be measured by tracers or other markers of time
Can be measured with variable accuracy
Can be measured over a wide age range
Age-Dating Methods
• Natural radioactivity
• Climate change
• Inadvertent Tracers
Tritium
Chlorofluorocarbons
Krypton-85
Stable isotopes
Dissolved contaminants
• Intentional Tracers
Sulfur-hexafluoride
Noble gases
Dyes
DEMAND
• Agriculture
• Urban
• Recreation
• Environmental
SUPPLY
How much is there?
distance
Age 
v
Qaq
v

A
distance
Qaq 
A
Age
Qaq 
aquifer volume
time
  porosity
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
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
Large elevation change
Distance
Groundwater
Travels
Increases
Small elevation change
Tropical
Arid
Natural radioactivity
•Many choices of
naturally-occurring
isotopes for age-dating
•Which ones behave
most like water?
Isotopic age-dating methods
• Unstable isotopes with relatively high decay constants
• Either natural abundances or concentration spikes created by nuclear fallout
N
 t
e
N0
N = measured isotope abundance
N0 = abundance at time of recharge
= decay time constant
t = time
N0 dependent on reactive and transport processes
• Variation in source concentration
• Dispersion/mixing/dilution
• Phase changes
Half-Life =
ln2
T1 

2
Climate Change
RL
  L V
RV
> 1 for hydrogen and oxygen isotopes
 RSA

  
 11000
 RSTD 
0
SMOW
-50
D
• Isotopic values controlled by
temperature
Latitude
Elevation
Inland distance
• Groundwater reflects mean annual
precipitation values
Mean
Annual
Precipitation
-100
-150
-200
-25
GMWL
-20
-15
-10
 O
18
-5
0
Climate Change
PaleoRecharge
• 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
ModernRecharge
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
 C C
 C C
14
12
meas
14
12
 fraction modern carbon or fmc
std
Age14C  8267 x ln(fmc)
• 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
(14C/12C)
is an oxalic acid
whose radiocarbon abundance is
equal to the abundance of
atmospheric CO2 in 1950
std
Closed System Rxn: 14CO2 + H2O + M12CO3
H14CO3 + H12CO3 + M++
< 1yr
Open System Rxn: 14CO2 + H2O
H214CO3 + H12CO3
fast
Saturated Flow:
H14CO3 + M12CO3
H212CO3 + H14CO3
slow
H12CO3 + M14CO3
10-8 10-10/cm2s
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
Helium-4 Accumulation in Age-Dating
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
Dissolved 4He concentration increases
4He
4He
Natural uranium and thorium decay
4He
Carrizo Aquifer, TX
Castro et al., 2000
Age-dating groundwater since human occupation
Impacts of engineered systems
How groundwater recharge is affected
Land Use
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-dating
Krypton-85 (85Kr)
Chlorflourocarbons (CFCs)
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
N
 t
e
N0
3
H meas
 λt
e
3
3
H meas  Hetrit
3He
meas
= 3Hetrit + 3Heequil + 3Heexcess + 3Herad
4He
meas
= 4Heequil + 4Heexcess + 4Herad
22Ne
meas
Noble Gas
Mass Spectrometry
= 22Neequil + 22Neexcess
Over determined system allows the calculation of 3Hetrit
Artificial Tracers
• 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)
10.0
C/Co x100 (
136
Xe)
North Flow Path
AM-7
AM-8
SCWC-PLJ2
A-26
8.0
6.0
4.0
(a)
2.0
0.0
0
100
200
300
400
500
600
700
Days
• Discriminate individual flow paths
• Track contaminant fate
• Evaluate health risks
AL recharge
0
Percent DOC Removal
• High degree of accuracy
Anaheim Lake/Kramer Basin
North FP
Shallow
Monitoring
20
South FP
AMD-9/1
40
OCWD-KB1
AM-44
AM-8
AM-7
60
AM-9
80
KBS-4
100
0
AM-10
2000
4000
AM-14
6000
8000 10000
Distance From Recharge Point (ft)
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.