Hydrogeology in the Service of Man, Mémoires of the 18th Congress of the International Association of Hydrogeologists, Cambridge, 1985. SOME CONSIDERATIONS ON GROUND WATER DATING USING ENVIRONMENTAL ISOTOPES. J.Ch. FONTES Laboratoire d"HYDROLOGIE et de 6E0CHIMIE ISOTOPIQUE Université de Paris-Sud, E-9I405 ORSAY Cedex. ABSTRACT : Some aspects of the use of tritium, carbon 14 and chlorine 36 for ground water dating are discussed with special reference to possible future development of these methods. Main principles of tritium modelling and various processes of estimation of ,4C initial activity are reviewed. Some examples of "*C1 variations in old and recent ground waters illustrate the considerable potentialities of this new technique. 1. INTRODUCTION: Time, or velocity are the least easy to estimate among hydrogeological variables or parameters. Requirements of Darcy's law applicability and especially the condition of steady state flow, put severe limitations on any transit time evaluation. This is especially the case in confined aquifers where successive inputs have varied according to long-term climatic fluctuations. This and further weaknesses of Darcy's law (extrapolation of local measurements of permeability, estimation or "guesstimation" of effective porosity and tortuosity, requirement of laminar flow) have induced considerable efforts to develop independent methods of time evaluation. Environmental isotopes provide many potential chronometers for ground waters studies (cf. table 1). However, because of either practical or theoretical limitations, suitable hydrochronometers are essentially : - tritium - carbon 14 - chlorine 36 whereas other method suffer problems of : sampling difficulties (krypton 85, argon 39), non conservative tracers (tritium-helium 3, silicon 32, uranium 234 - uranium 238, isotope 2 equilibration rate S04 ~-H20, helium) development of proper analytical techniques ( krypton 81 ). 2.TRIT1UM : Tritium has been for many years the most popular environmental isotope in ground water studies. This was certainly due to the simplicity of the interpretations in some favourable cases. 2.1. Sources : Tritium is produced by the action of neutrons on nitrogen atoms: ,4N + 'n = 3H + 12C. It decays into ^He with a half-life of 12.43 a. 2.1.1. Natural production : The production is induced by secondary neutrons from the cosmic radiations. A production - 118 - Isotopes Half-life (a) Origin Time range Qualities Limitations or process (a) 85Kr 10.8 Nuclear reactors since 1960 No interactions Sampling, counting 3H 12.43 Cosmic rays since 1952 Ideal behaviour Various sources Thermonuclear tests commonly suitable Reactors 3H-3He 12.43 Cosmic rays since 1952 Direct determination Analytical - Crustal Thermonuclear tests and possibly of turnover time production of ^He Reactors 100 32 Si « 100 Cosmic rays « 100 (?) Few, link between A0 (?) - Samples Nuclear tests 3H, 39Ar, 14C (?) 10m , counting time Crustal (?) 39Ar 269 Cosmic rays « 2000 BP No interactions, sample 10m3 Crustal comparison with '\ counting time '\ 5730 Cosmic rays, «3.104 In all ground Complicated chemical Thermonuclear tests *7.104 waters and isotopic systems Crustal *kr 2.!xt05 Cosmic rays 5x105(?) No interactions, Analytical ideal for very old waters 36c, 5 6 3.06x10 Cosmic rays 2x10 Hydrophilous, AQ (?), various Nuclear tests time range sources, acess to Crustal accelerator 234u_ 5 5 2.5x10 Decay of uranides 5x10 (?) Time range of very AQ (?) chemical 238u chain old waters interactions !80 Reaction Stable 105 (?) Additional tool Calibration (?), 2 (so4 " rate commonly suitable redox and pH -H20) dependent 4He Accumu- Stable « 106 Additional tool Not conservative mutation rate Tableau 1 : Theoretically available environmental isotopes for ground water dating. - 119 - rate of 0.5 ± 0.3 nuclei per cm2 of earth surface per second (Nir et al, 1966 in Gat, 1980) or 1.5 x !017Bq.a~' (Eisenbud et al, 1978) was estimated. 3 After formation, H is oxidized into H20 and is removed from the troposphere by precipitation. At steady state (production rate equals removal rate) the total amount of natural tritium in the atmosphere correspond to about 2.6 x 1018 Bq (Eisenbud et al, 1978). 2.1.2. Thermonuclear detonations : Since the first test (31st October 1952) large amounts of 3H were introduced into the stratosphere. Global inventory of tritium from weapon tests reached 1.15 x 1020 Bq in 1963. The moratorium on aerial thermonuclear tests in 1963 stopped almost completely (further aerial tests by China, France and India did not contribute significantly to the tritium budget), the supply of weapon bomb in the atmospheric reservoir. The radioactive decay will decrease this figure to the natural level approximately by the year 2030 (Eisenbud et al, 1979).. 2.1.3. Nuclear power reactors : This supply is becoming the major source of 3H. Tritium is produced by ternary fission and by activation of some light elements (B, Li) which are used in reactors, it has been estimated that 3H from nuclear power reactors will become prédominent over bomb tritium in the atmosphere in 1985 (Eisenbud et al, 1978). Losses from tritium industry (watches, signals..) contribute also significantly to the increase in atmospheric 3H. 2.2. Units : Radioactivity units are not frequently used in environmental tritium studies. The very small amounts of 3H present in natural waters led to the definition of a tiny reference : the "Tritium Unit" (TU). Its corresponds to an atomic ratio of one atom of 3H for 1018 atoms of ^H. Strictly speaking the Tritium Unit should be referred to as the Tritium Ratio (TR). The corresponding radioactivity in water is0.118 Bq.kg"' (or 3.19 pCi.kg~',or 7.2dpm.kg~Mn the previously used units). 2.3. Distribution patterns : Both natural and artificial tritium are inequally distributed at earth's surface. 2.3.1. Cosmic tritium : Natural tritium is produced in major quantities at high latitudes. This is due to the minor magnetic deviation of charged cosmic particles (alpha and protons) which produce secondary neutrons by interaction with the first air molecules in the high atmosphere. Natural fall-out in precipitation is evaluated between 25 to 20 TU at high latitudes and A TU in equatorial regions (see 6at, 1980). 2.3.2. Bomb tritium .- 2.3.2.1. Global variations : The thermal convection which occurs during aerial tests brings a large fraction or radioactive nuclides and debris up to the stratosphere. There, the laminar structure of air masses and the monodirectional winds induce a general motion of rotation at the latitude of injection. The largest aerial tests were performed at mean latitudes in the Northern hemisphere. This explains the heterogeneity of 3H distribution in latitudes and the rather low tritium activity of rains in the austral hemisphere (Fig. 1 ). - 120 - Northern 2000 j- HemisDhere [TU] [ 1500!- !- t- 1000^ a tf 500^ U/'^v' ww V VVsyHl r 200^ Tropics au] - 150^ 100- S i •« 50h •A JWsAib ^ Southern 100 Hemisphere [TU] r 751- I h f 50r iii r s 25r- ! it M Nmutiw ^ 1955 60 65 70 1975 Fig. 1. Time and space distribution of bomb tritium. From Groeneveld ( 1977) in Gat ( 1980). - 121 - 2.3.2.2. Seasonal variations : The mechanisms of 3H fall-out from the stratosphere into the troposhere, where its residence time is short (21-40 days according to Eisenbud et al, 1978 ; 5 to 20 days according to Gat, 1980) is not completely understood. Several processes are possible (diffusion through the tropopause, sowing of convective clouds which penetrate into the stratosphere), but the main transfer probably occurs through tropopause discontinuities or extrusions of the tropopause. These discontinuity zones appear seasonnaly when polar and equatorial tropopauses begin their opposite migration toward pole and equator respectively, at the end of winter. The result is a "spring peak" which is approximately 3 times the average annual weighted activity. Then tropopauses move back at the end of fall and give rise to a "winter valley" with half of the annual average activity. 2.4 Sinks : Main sinks for tritium are residence time and related radioactive decay in soils, in aquifers and mainly in the "mixed layers" of ocean waters which corresponds to a thickness of about 100 m of surface waters. The relatively low renewal of these waters involves old (tritium free) bottom waters as well as supplies from the continents. The result is a very low tritium content of ocean surface waters. Through molecular exchange at the interface and also by mixing with oceanic vapour, the tritium content of the atmospheric moisture decreases as long as the depressions travel over ocean surfaces. Precipitations over oceanic islands and coastal regions are thus depleted in tritium as compared to continental rains. During its transit over the continents the initial marine vapour mixes gradually with increasing amounts of vapour generated by evaporation from soils and lakes and by évapotranspiration. This continental vapour corresponds to rapidly recycled precipitations which thus contain tritium. A continental effect occurs which relates the increase in 3H of meteoric water to the distance to the coastline along pathes of atmospheric moisture masses ( Fig.2). 2.5. Reference stations : Despite of the numerous, above discussed, causes of variations tritium distribution in precipitation is not erratic at the scale of the hemisphere. Weighted monthy averages are closely correlated (Fig.3). Therefore, any partial record at a given location may be reconstructed with a reasonable confidence by comparison with a reference stations where a long record is available. Available reference station and related results are found in the AIEA-WMOnetwork (AIEA, 1969, 1970, 1971, 1973, 1975, 1979). A special reference station is Ottawa because it provides the only records starting from the beginning of the fifties i.e. before the first thermonuclear tests. 2.6. The use of tritium for ground water studies : 2.6. /. Qualitative : Tritium is often used as a guide to evaluate ground water residence times. Last pre-bomb precipitation occured in 1951-52 with a 3H content of 4 to 20-25 TU.