Unit 09b : Advanced Hydrogeology Isotopic Processes
Isotopes Atoms of the same element that differ in terms of their mass are called isotopes. Hydrogen has three isotopes 1 H, 2 H, 3 H 1 H and 2 H (deuterium) are stable isotopes. 3 H (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.
Radioactive Decay Radioactive decay involves emission of an alpha particle ( 4 He), a beta particle ( 0 e), or gamma radiation produced when atoms in an excited state revert to their ground state Alpha Decay 232 Th > 228 Ra + 4 He Beta Decay 228 Ra > 228 Ac + 0 e Gamma Emission 236 U * > 236 U +
Isotopic Decay Radioactive decay is an irreversible reaction. Decay follows a first order rate law: dN/dt = -kN Solving for N gives: N(t) = N o exp(-kt) The time when N(t) = 1/2 N o is called the half-life. 1/2 = exp(-kt 1/2 ) ln(1/2) = - kt 1/2 -ln(2) = -kt 1/2 t 1/2 = ln(2) / k = / k
Radioisotopes Radioisopes are important for two reasons: –they are contaminants that represent a radiation hazard to living tissue. –They provide a means of age-dating groundwaters The most important radioisotopes used in age-dating waters are 3 H, 14 C, 32 Si and 36 Cl.
Half-Lives 3 H 12.3 y 32 Si 100 y 14 C 5300 y 36 Cl 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, 32 Si to about 500 years, radiocarbon to about 26,000 years and 36 Cl to around 1.5 My.
Isotopic Abundance The isotopes of H, O, C and S are ubiquitous in natural groundwaters and are useful in studying chemical processes. Oxygen 16 O O O Hydrogen 1 H H H * ~ Carbon 12 C C C * ~ Sulphur 32 S S S S 0.02 Relative abundance data show that one isotope dominates the rest in all cases. Radioisotopes ( 3 H and 14 C) have very low abundance
Delta Values Isotopic ratios are reported as positive or negative deviations from a standard: = R sample - R standard x 1000 R standard The units for are parts per thousand or permil. For example, the oxygen isotopic ratio 18 O/ 16 O : 18 O = ( 18 O/ 16 O ) sample - ( 18 O/ 16 O ) standard x 1000 ( 18 O/ 16 O ) standard A value of 18 O of -20% o means that a sample is depleted in 18 O by 2% relative to the standard.
Stable Isotopes in Water Water contains hydrogen and oxygen both of which have two or more stable isotopes. 2 H and 18 O 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.
Fractionation Water vapour in equilibrium with water is typically depleted in heavy isotopes by 80% o 2 H and 10% o 18 O so for water vapour in equilibrium with ocean water the isotopic ratios are 2 H = -80% o and 18 O = - 10% o The 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).
Latitude Effect The 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: 2 H = 8 18 O + 10% o
Meteoric Water Line MWL
Meteoric Water Line Key to interpretation of 2 H and 18 O Water falling on the line is assumed have originated from atmosphere. Deviations from the line are caused by other processes –Water/rock interactions (O only) –H 2 S exchange reactions (H only) –Open surface evaporation –Condensation
CO 2 -exchange Isotopic Reaction Paths MWL H 2 S-exchange Water / Rock interaction Evaporation Condensation Silicate Hydration
Age Dating There are two approaches: –Direct interpret concentration distribution of naturally occurring radioisotopes ( 3 H, 14 C, 36 Cl) –Indirect interpret changes of stable isotopes ( ) or organic tracers CFCs (chlorofluorocarbons) in recharge
Direct Method The residence time of a radioisotope in the system (or the age of the recharge water) is given by: t = t 1/2 ln (A o /A obs ) / ln(2) where A o is the presumed initial activity of the isotope and A obs is the observed activity.
Tritium 1 TU = one 3 H atom per atoms 1 H Typical natural tritium levels at ~ 20 TU Bomb tritium in N.America peaked at >2000 TU in 1963 Problem with 3 H for dating is that A o has to be estimated from historic records –30 TU water in year 2001 –could be 1952 water (500 TU) after 4 half lives 500/16 30 –could be 1976 water (75 TU) after 2 half lives 75/4 30 Main use is to identify pre-test (pre-1952) waters
Radiocarbon Measurements are reported as percent modern carbon-14 (pmc). So current source is 100 pmc. 14 C in groundwater comes from solution of CO 2(g) in the soil zone. Method assumes that carbonate in the saturated zone carries 14 C and that no additional C is added to the system. Age dating is valid if the only process removing 14 C is radioactive decay. This is not generally the case.
Processes affecting 14 C Dissolution 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.
Correction Procedures Procedures have been devised to correct 14 C activities for the various processes adding dead carbon and removing 14 C These procedures include: –empirical average estimation of dead carbon –accounting for calcite solution only as a source of dead carbon –using 13 C to estimate dead carbon addition. –complex geochemical modelling procedures accounting for all processes Uncertainties make 14 C dating at best a semi- quantitative tool.
Chlorine Cl 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 14 C. Source is atmospheric and believed to be constant (like natural 3 H and 14 C) Typical 36 Cl values for meteoric waters lie in the range 100 to 500 x for 36 Cl/ 34 Cl ratios. Atmospheric bomb testing elevated 36 Cl levels by 2 or 3 orders of magnitude so the bomb-pulse can be used in the same way as 3 H.
Indirect Method Method 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.
Climate Change Shallow groundwaters show -9 to -10% o 18 O Deeper waters give -14 to -17% o 18 O The 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.