2IsotopesAtoms of the same element that differ in terms of their mass are called isotopes.Hydrogen has three isotopes 1H, 2H, 3H1H and 2H (deuterium) are stable isotopes.3H (tritium) is radiogenic and decays (with a half life of about 12.3 years.Relative isotopic concentrations change in processes where mass fractionation is important such as evaporation, condensation and water-rock interactions.
3Radioactive DecayRadioactive decay involves emission of an alpha particle (4He), a beta particle (0e), or gamma radiation produced when atoms in an excited state revert to their ground stateAlpha Decay 232Th > 228Ra + 4HeBeta Decay 228Ra > 228Ac + 0eGamma Emission 236U* > 236U + g
4Isotopic Decay Radioactive decay is an irreversible reaction. Decay follows a first order rate law:dN/dt = -kNSolving for N gives:N(t) = Noexp(-kt)The time when N(t) = 1/2 No is called the half-life.1/2 = exp(-kt1/2)ln(1/2) = - kt1/2-ln(2) = -kt1/2t1/2 = ln(2) / k = / k
5Radioisotopes Radioisopes are important for two reasons: they are contaminants that represent a radiation hazard to living tissue.They provide a means of age-dating groundwatersThe most important radioisotopes used in age-dating waters are 3H, 14C, 32Si and 36Cl.
6Half-Lives 3H 12.3 y 32Si 100 y 14C 5300 y 36Cl 310,000 y To obtain “ages”, concentrations of isotopes must be measured in mass spectrometers. The analysis becomes difficult after about 5 x the half life. So tritium is good to about 60 years, 32Si to about 500 years, radiocarbon to about 26,000 years and 36Cl to around 1.5 My.
7Isotopic AbundanceThe isotopes of H, O, C and S are ubiquitous in natural groundwaters and are useful in studying chemical processes.Hydrogen1H2H3H* ~10-16Oxygen16O17O18OCarbon12C13C14C* ~10-10Sulphur32S33S34S36SRelative abundance data show that one isotope dominates the rest in all cases.Radioisotopes (3H and 14C) have very low abundance
8Delta ValuesIsotopic ratios are reported as positive or negative deviations from a standard:d = Rsample- Rstandard x 1000RstandardThe units for d are parts per thousand or permil.For example, the oxygen isotopic ratio 18O/16O :d18O = (18O/16O )sample- (18O/16O )standard x 1000(18O/16O )standardA value of d18O of -20%o means that a sample is depleted in 18O by 2% relative to the standard.
9Stable Isotopes in Water Water contains hydrogen and oxygen both of which have two or more stable isotopes.d2H and d18O compositions are usually measured with respect to the Standard Mean Ocean Water (SMOW) standard.SMOW is a particularly appropriate standard for groundwater studies because precipitation that enters the groundwater system originates from evaporation of ocean water.
10FractionationWater vapour in equilibrium with water is typically depleted in heavy isotopes by 80%o d2H and 10%o d18O so for water vapour in equilibrium with ocean water the isotopic ratios are d2H = -80%o and d18O = -10%oThe depletion of heavy isotopes in water as a result of evaporation is an example of a fractionation process.Water vapour in air masses is typically not in equilibrium with ocean water. Near the equator vapours are slightly depleted and the depletion increases with latitude (mainly because of temperature differences).
11Latitude EffectThe “latitude effect” is related to progressive temperature-controlled removal of heavy isotopes from the vapour during precipitation.At lower temperatures, the fractionation is more pronounced but all rainwaters are systematically depleted relative to SMOW.When isotopic compositions of rainwaters from around the world are plotted, they lie along a straight line known as the meteoric water line:d2H = 8d18O + 10%o
13Meteoric Water Line Key to interpretation of d2H and d18O Water falling on the line is assumed have originated from atmosphere.Deviations from the line are caused by other processesWater/rock interactions (O only)H2S exchange reactions (H only)Open surface evaporationCondensation
14Isotopic Reaction Paths H2S-exchangeSilicate HydrationEvaporationCO2-exchangeWater / Rock interactionCondensationMWL
15Age Dating There are two approaches: Direct Indirect interpret concentration distribution of naturally occurring radioisotopes (3H, 14C, 36Cl)Indirectinterpret changes of stable isotopes (d18O) or organic tracers CFC’s (chlorofluorocarbons) in recharge
16t = t1/2 ln (Ao/Aobs) / ln(2) Direct MethodThe residence time of a radioisotope in the system (or the age of the recharge water) is given by:t = t1/2 ln (Ao/Aobs) / ln(2)where Ao is the presumed initial activity of the isotope and Aobs is the observed activity.
17Tritium 1 TU = one 3H atom per 1018 atoms 1H Typical natural tritium levels at ~ 20 TUBomb tritium in N.America peaked at >2000 TU in 1963Problem with 3H for dating is that Ao has to be estimated from historic records30 TU water in year 2001could be 1952 water (500 TU) after 4 half lives 500/16 30could be 1976 water (75 TU) after 2 half lives 75/4 30Main use is to identify pre-test (pre-1952) waters
18RadiocarbonMeasurements are reported as percent modern carbon-14 (pmc). So current source is 100 pmc.14C in groundwater comes from solution of CO2(g) in the soil zone.Method assumes that carbonate in the saturated zone carries 14C and that no additional C is added to the system.Age dating is valid if the only process removing 14C is radioactive decay.This is not generally the case.
19Processes affecting 14CDissolution of carbonate minerals may add “dead carbon” to the system.Oxidation of organic carbon, sulphate reduction and methanogenesis may add “dead carbon”.Precipitation of carbonates may remove radiocarbon.Isotopic exchange with carbonates may remove radiocarbon.
20Correction Procedures Procedures have been devised to correct 14C activities for the various processes adding “dead carbon” and removing 14CThese procedures include:empirical “average” estimation of “dead carbon”accounting for calcite solution only as a source of “dead carbon”using 13C to estimate “dead carbon” addition.complex geochemical modelling procedures accounting for all processesUncertainties make 14C dating at best a semi-quantitative tool.
21Chlorine-3636Cl is a potentially useful tool for dating waters up to 2 million years old.Few natural processes add “dead chlorine” so problems are much reduced by comparison with 14C.Source is atmospheric and believed to be constant (like natural 3H and 14C)Typical 36Cl values for meteoric waters lie in the range 100 to 500 x for 36Cl/34Cl ratios.Atmospheric bomb testing elevated 36Cl levels by 2 or 3 orders of magnitude so the bomb-pulse can be used in the same way as 3H.
22Indirect MethodMethod relies on interpreting systematic changes in stable isotopes along groundwater flow paths.Unlike radioisotopes, the time marker is provided by an interpreted event that changed the tracer in a systematic way.Shifts in stable isotope ratios are apparent in groundwater profiles.
23Climate Change Shallow groundwaters show -9 to -10%o d18O Deeper waters give -14 to -17%o d18OThe shift is interpreted as gradual climatic warming following deglaciation and has been “calibrated” using radiocarbon.The more depleted water is indicative of older infiltration generated at a time when the climate was colder.Method provides a useful time scale in low K materials where groundwater moves slowly.