Fondo Concursable I y II

tracer-based age is not necessarily equal to the transit time

of the water carrying the substance. Rather than referring to groundwater dating, it would probably be more correct to say we are interpreting temporal aspects of chemical and isotopic substances in groundwater.

For many applications in hydrology, we are interested in the travel time of water through an aquifer. The accuracy of the age determined depends in part on how perfectly the chemical and isotopic substances we are using as dating tools are transported in the aqueous phase. The concentrations of all tracers are, to some degree, affected by transport processes, and in the case of some tracers, such as chlorofluorocarbons, their concentrations can also be affected by chemical processes such as degradation and sorption during transit.

Various environmental isotopes and tracers are used to determine the age of groundwater. Carbon-14 is used to date ground waters older than 1000 years. Chloroflurocarbons (Freon) and tritium techniques are used to date groundwater that is less than 50 years old (Table 1).

The residence time of water underground has always been a topic of considerable speculation. But with the advent of radioisotopes, determination of the age of groundwater has become possible (1).

Carbon-14 (14_{C), commonly referred to as radiocarbon,}

is continuously being produced in the atmosphere. This results from cosmic ray bombardment of nitrogen nuclei. Atmospheric testing of thermonuclear weapons doubled the 14_{C concentration in the atmosphere. Carbon-14 is}

expressed in terms of percent of modern carbon (pmC).

14_{C is useful when dating groundwaters that are between}

1000 and 30,000 years old.

This isotope is present in groundwater as dissolved bicarbonate originating from the biologically active layers of the soil where CO2is generated by root respiration and

the decay of humus (2).

14_C

The14_{C generated in the atmosphere is carried down to the}

earth’s surface by precipitation and becomes incorporated into the biomass or transported into waterbodies such as lakes, oceans, and groundwater.14_{C undergoes radioactive}

decay (to14_{N), so that once isolated from the atmosphere,}

the amount of 14_{C decreases with time according to}

the equation

(14C)t= (14C)oe−kt

where (14_C)

t is the amount present at time t, (14C)o is

the amount present at t= 0, and k is the decay constant, Table 1. List of Environmental Tracers and Isotopes Used for Age Determination

Environmental Isotope/Tracer Age Range, Years Chloroflurocarbons (CFC-11, CFC-12, and CFC-113) 0 to 50 Tritium 0 to 50 Tritium/Helium-3 0 to 30 Carbon-14 1000 to 30,000

which is related to the half-life t1/2by the equation

t1/2= ln 2/k

To determine the time since water lost contact with the atmosphere, it is necessary to know (14_C)

o. This is

determined by tree rings for the most recent 7000 years; there is no accurate way to determine it prior to 7000 years ago, so it is generally assumed arbitrarily to have been constant. This gives rise to a timescale in ‘‘14_{C years,’’}

which may be different from astronomical years (3). There are some complications in the behavior of

14_{C during recharge, so that the ‘‘absolute’’ age of a}

groundwater cannot be determined reliably. However, if the14_{C concentration is measured at several points along a}

flow line within an aquifer, the differences in age between the points and hence the flow velocity can be determined. One complication is that dissolution of carbonate minerals or oxidation of organic matter within the aquifer may add ‘‘old’’ or ‘‘dead’’ (no detectable 14_{C) carbon to the water}

and give an erroneously old age. The contribution of carbon from these sources can sometimes be estimated from13_C/12_{C measurements and chemical arguments (4),}

so that corrections can be made. Another complication is mixing. A low 14_{C concentration may mean that we are}

looking at relatively ‘‘old’’ water, or it may mean that we are looking at a mixture of relatively ‘‘young’’ water and ‘‘dead’’ water. 14_{C measurements can be interpreted as}

ages only when mixing is insignificant.

Measurements of water samples taken from deep wells in deserts of the United Arab Republic and Saudi Arabia indicate ages of 20,000 to 30,000 years (5). This period is compatible with the Wisconsin Ice Age, when these desert areas last had high rainfall capable of recharging the underlying major aquifers.

BIBLIOGRAPHY

1. Todd, D.K. (1995). Ground Water Hydrology. Wiley, Toronto, Canada, pp. 24–25.

2. Wigley, T.M.L. (1975). Carbon-14 dating of groundwater from closed and open systems. Water Resour. Res. 11: 324–328. 3. Drever, J.I. (1982). The Geochemistry of Natural Waters.

Prentice-Hall, Englewood Cliffs, NJ.

4. Wigley, T.M.L. (1976). Effect of mineral precipitation on isotopic composition and14_{C dating of ground water. Nature} 263: 219–221.

5. Thatcher, L. et al. (1961). Dating desert ground water. Science 134(3472): 105–106.

GROUNDWATER DATING WITH H–HE

CRAIGE. DIVINE JOHND. HUMPHREY Colorado School of Mines Golden, Colorado

PRODUCTION AND DECAY BEHAVIOR OF ATMOSPHERIC TRITIUM

Tritium (3_{H) is a relatively short-lived radioactive}

66 GROUNDWATER DATING WITH H–HE

atmosphere and enters the hydrologic cycle through meteoric precipitation. Natural atmospheric 3_{H results}

from the interaction of nitrogen-14 (14_{N) and cosmic-ray}

neutrons (n) by

14_{N+ n −−−→}3_H+12_{C (1)}

Tritium is then rapidly incorporated into water molecules. Tritium decays to helium-3 (3_{He) by beta decay:}

3_{H−−−→}3_{He+ β}− ₍₂₎

Tritium concentrations in water are typically reported as ‘‘tritium units’’ (TU), where 1 TU is equivalent to one

3_{H atom per 10}18 _{hydrogen atoms and 0.118 Bq kg}−1

(1 Bq= 1 disintegration per second). Decay of 1 TU yields approximately 0.402 pcm3_kg−1_of3_{He. The natural}

atmospheric production rate of 3_{H is approximately}

0.5± 0.3 atoms of 3_{H cm}−2_s−1_{(1). The background} 3_H

concentration in meteoric precipitation before 1951 is estimated in the range between 0.5 and 20 TU, and most measurements are less than 10 TU (2) and (3, Fig. 7.2). The rate of 3_{H decay and} 3_{He production follow these}

first-order rate laws:

3_H

t=3H0e−λt (3a) 3_He

t=3H0eλt (3b)

where3_H

tand3Hetare the number of tritium and helium-

3 atoms at time t, 3_H

0 is the initial number of tritium

atoms, and λ is the tritium decay constant (0.05626 yr−1). The tritium ‘‘half-life’’ (t1/2), or time it takes for half of

the starting tritium to decay to 3_{He, is best estimated}

at 12.32 yr (4500± 8 day) (4). Figure 1 graphically shows the decay and generation behavior of3_{H and}3_{He through}

time. The half-life is related to the decay constant by

λ= ln 2 t1/2 (4) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 5 10 15 20 25 30 35 40 45 50 55 60 Time, yr Nor maliz ed concentr ation 3_He 3_H 2 half-lives 3 half-lives 1 half-life

Figure 1. Tritium (3_{H) decay and}3_{He production through time.}

Values are normalized to the initial3_{H concentration (}3_H 0). 0 10 20 30 40 50 60 70 80 90 100 110 1950 1952 1954 1956 1958 1960 1962 1964 1966 1968 1970 1972 1974 Year T ritium released, 12₁₀ becquerels 1963 Soviet–American Test Ban Treaty

Figure 2. Anthropogenic 3_{H released from atmospheric deto-}

nation of thermonuclear weapons testing (developed from data summarized by Reference 3).

During 1951, it was demonstrated that significant energy is released during hydrogen fusion to provide the basis for developing thermonuclear superbombs. Atmospheric testing (detonation) of these weapons during the next decade resulted in a dramatic increase in atmospheric3_H

concentration, until implementation of a Soviet–American atmospheric test ban treaty in 1963 (Fig. 2). Atmospheric thermonuclear weapons testing released vast quantities of high-energy neutrons that interacted with14_{N and led}

to the production of 3_{H (Eq. 1). The incredible amount}

of 3_{H produced in the atmosphere resulted in a distinct}

precipitation input signal of anthropogenic3_{H (Fig. 3), and}

a notable peak in 1963.

PRINCIPLES OF THE3_H–3_{HE DATING METHOD}

The mid-1960s3_{H peak has been widely used as a direct}

time marker to determine groundwater ages. However, because of the relatively short half-life of 3_{H, this peak}

has become difficult to differentiate in many systems. This is particularly true in the Southern Hemisphere where the anthropogenic3_{H in precipitation was notably lower than}

in the Northern Hemisphere. However, measurement of both parent 3_{H and increases in its decay product,}3_He,

can dramatically increase the sensitivity of the method. The ‘‘in-grown’’3_{He (or}3_{He produced by}3_{H decay) can be}

equated to the initial3_H

0and3Htby 3_He

t=3H0(1− e−λt)=3Ht(eλt− 1) (5)

Assuming that other 3_{He sources are insignificant}

or can be accounted for and that there have been no systematic losses of tritiogenic3_{He, the relationships in}

Equation 5 can be rearranged, and the apparent age of the water (τ ) can then be calculated from (Fig. 4)

τ= t1/2 ln 2ln
1+ 3_He t 3_H t  (6)

The 3_H–3_{He method offers two significant advantages:}

GROUNDWATER DATING WITH H–HE 67 1 10 100 1000 10000

Jan-55 Jan-60 Jan-65 Jan-70 Jan-75 Jan-80 Jan-85 Jan-90 Jan-95 Jan-00

T ritium in precipitation, TU (b) 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Jan-55 Jan-60 Jan-65 Jan-70 Jan-75 Jan-80 Jan-85 Jan-90 Jan-95 Jan-00

T

ritium in precipitation,

TU

(a)

Figure 3. Tritium in precipitation (reported in tritium units, TU) measured at more than 50 locations in North America between 1955 and 2001 (top: arithmetic scale, bottom: logarithmic scale) (data obtained from Reference 5).

original3_{H input or identification of the mid-1960s peak,}

and (b) the method is generally less sensitive to analytical measurement errors (2).

The ‘‘clock’’ for the 3_H–3_{He method begins at the}

seasonal water table low, where 3_{He produced is no}

longer lost to the atmosphere and begins to accumulate in the groundwater. If the unsaturated zone is thick, the apparent3_H–3_{He age at the ‘‘bomb peak’’ signal may be}

significantly less than the actual time since the peak3_H

fallout. The difference in apparent ages can be used to estimate the travel time through the unsaturated zone. For example, Solomon et al. (6) determined an apparent

3_H–3_{He age of 16 years for groundwater samples near the} 3_{H peak signal in a study at Cape Cod, Massachusetts.}

However, the samples were collected 30 years after the actual peak 3_{H fallout in 1963. The difference in these}

age values resulted in an unsaturated zone travel time estimate of 14 years.

68 GROUNDWATER DATING WITH H–HE 0 10 20 30 40 50 60 70 0 5 10 15 20 25 30 3_He t/3Ht

Time since recharge

, yr

Figure 4. Calculated time since recharge from measured

3_He

t/3Htratio (Eq. 6).

There are additional sources of 3_{He in groundwater,}

and the measured 3_{He value must be corrected for}

these sources before applying Eq. 6. Tritium-generated

3_{He (}3_He

trit) is determined from total measured tritium

(3_He total) by

3_He

trit=3Hetotal−3Heatm−3Heexcess−3Hecrust

−3_He

mantle (7)

where 3_He

atm is the 3He concentration in solubility

equilibrium with the atmosphere, 3_He

excess is the 3He

resulting from excess air,3_He

crust is the3He produced in

the crust, and3_He

mantleis the3He produced in the mantle

(also called primordial3_{He). Atmospheric}3_{He dissolving}

in groundwater at the time of recharge is typically the most significant process resulting in excess 3_{He. This process}

can be accounted for by considering the following (3): • The atmospheric4_{He concentration is 5.24 parts per}

million (by volume), and the atmospheric 3_He/4_He

ratio is 1.3× 10−6.

• The temperature-dependent aqueous solubility of helium (at 10◦C, the value is 4.75× 10−8cm3

STP/cm3_H 2O).

• 4_{He is more soluble than} 3_{He in water and has a}

fractionation factor (αwater−air) of 0.983.

Although the 3_H–3_{He method is fairly insensitive to}

recharge temperatures, it is very sensitive to excess air, particularly for young groundwater (7). In some systems,

3_{He from excess trapped air caused by a transient}

wetting front may be significant and can be identified by supersaturation of other gases, such as Ne and N2.

For most systems, subsurface generated3_{He (}3_He crustand 3_He

mantle) is insignificant; however, correction methods

for these cases are discussed by Schlosser (8,9), Solomon et al. (2), and Cook and Solomon (7). Other potential errors in the 3_H–3_{He method may be caused by diffusive loss}

and dispersion/mixing effects. 3_{He diffusive loss to the}

atmosphere was greatest during the mid-1960 peak (due to the high concentration gradient), so this method may incorrectly calculate younger ages for groundwater below the mid-1960 peak (2). Because helium has a relatively high Henry’s law constant (105.2 [dimensionless] at 25◦C) (10), care must be taken during sampling to prevent

3_{He loss due to gas stripping.}

ANALYTICAL MEASUREMENT METHODS

Tritium is typically measured by either low-level dis- integration counting or by the more sensitive 3_{He in}

growth method using mass spectrometry. Depending on the specific instruments and analytical procedures, 3_H

detection limits of∼0.05–0.8 TU and analytical precision of ±2.5–5% can be achieved (9). For 3_{He measurement,}

water samples are commonly collected in pinched-off cop- per tubes or gas-filled diffusion samplers. The helium is then isolated from all other dissolved gases and analyzed by a helium isotope mass spectrometer to resolve the

3_He/4_{He isotopic ratio. Achievable measurement precision}

for the3_He/4_{He ratio is approximately 0.2% (9). Although}

approximately 40 years have passed since the mid-1960

3_{H ‘‘bomb peak,’’ these analytical capabilities should per-}

mit identifying the peak signal in favorable hydrogeologic settings for several more decades (8). In most cases, ana- lytical uncertainties result in calculated age uncertainties of less than approximately 10% (7).

CASE STUDY: COMPARISON OF THE3_H–3_{HE METHOD}

TO THE CFC AND85_{Kr DATING METHODS}

Ekwurzel et al. (11) compared calculated groundwater ages for three different dating methods, including the

3_H–3_{He method, in a study on the Delmarva Peninsula}

(approximately 15,690 km2_{) located on the east coast of the}

United States. The Delmarva Peninsula is characterized by low topographical relief and little urban development. The hydrogeology of the area is relatively simple, consisting of low hydraulic gradients, highly permeable surficial materials, and shallow water tables. Discrete groundwater samples were collected from approximately 30 wells throughout the Delmarva Peninsula (most wells were screened across less than 1 m of the aquifer). Additionally, age dating methods were compared for a single flow system on the peninsula at Locust Grove.

Most age values calculated from the three methods agreed within approximately 2 years, indicating conserva- tive behavior of the various tracers and reliability of the dating methods. A few wells that produced large appar- ent age discrepancies were located in areas of significant mixing. Dissolved N2 concentrations from several wells

with large tracer age variations (>10 years) indicated that gas stripping may have occurred in these samples. A one-dimensional advection–dispersion model, applied to estimate the effect of dispersion on the age corre- lation, determined that hydrodynamic dispersion effects were negligible. As stated earlier, fairly close agreement was observed among the different dating methods for

In document PROGRAMA MINERO DE SOLIDARIDAD CON EL PUEBLO (página 76-82)