Application of isotopic methods to dating of very old groundwaters: Milk River aquifer, Alberta, Canada

Applied Geochemistry, Vol. 6, pp. 465-472, 1991

0883-2927/91 $3.00+ .00 Pergamon Press plc

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Application of isotopic methods to dating of very old groundwaters: Milk River aquifer, Alberta, Canada K. FR6HLICH Isotope Hydrology Section, IAEA, PO Box 100, A-1400 Vienna, Austria M . IVANOVICH Isotope Geoscience Section, AEA Technology, Harwell Laboratory, Oxon O X l l 0RA, U.K.

M. J. HENDRY National Hydrology Research Institute, i 1 Innovation Blvd, Saskatoon, Saskatchewan S7N 3H5, Canada

J. N. ANDREWS Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights, Reading RG6 2AB, U.K.

S. N. DAVIS Department of Hydrology and Water Resources, University of Arizona, Tucson, A Z 85721, U.S.A.

R. J. DRIMMIE Earth Science Department, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada J. FABRYKA-MARTIN Isotope Geochemistry Group (INC-7), MS-J514, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A, T . FLORKOWSKI Institute of Physics and Nuclear Techniques, Krakow, Poland P. FRITZ GSF-lnstitute for Hydrology, D-8042 Neuherberg, Germany B . LEHMANN a n d H . H . LOOSLI Physics Institute, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland

and E. NOLTE Faculty of Physics, Technical University of Munich, D-8046 Garching, Germany

(Received 12 March 1991 ; accepted in revisedform 20 March 1991)

Abstract--The aim of this joint project was to evaluate the usefulness of available geochemical and isotope techniques for dating very old groundwater. This paper represents a synthesis of an IAEA sponsored study for which purpose the Milk River aquifer groundwaters were sampled from 16 wells during 1985 and subsequent years for the following measurements: 2 H, 3 H, 13C , ISO, 14C , 39Ar, Sl Kr, 85 Kr, noble gases including 2221Rn, -36CI, 1291 and U isotopes. The Milk River aquifer was selected for this study because several preceding investigations had established that this groundwater system contains waters whose ages range from recent to > 1 Ma. The present study has established that the Milk River aquifer system is very complex both in terms of groundwater origin and in terms of the evolution of its chemical and isotopic contents. However, this apparent complexity proved to be an interesting challenge with respect to the use of different and complementary approaches to interpretation of the geochemical and isotope data in terms of groundwater residence time. Thus, the groundwater ages based on the hydrodynamic model should be considered as a lower limit of the average groundwater age in the Milk River aquifer (~0.25 Ma) while those based on the 36C1/C1 data uncorrected for any dilution by dead C1 should be considered as an upper 465

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K. Fr6hlich et al. limit (<2 Ma), The Milk River Aquifer International Project is an excellent illustration of the variety of insight and increased confidence possible in flow-system analysis when multiple, independent dating methods are combined with detailed hydrogeological studies. The conclusion of the present study is that the potential for dating of very old groundwaters by a variety of isotope techniques is very high, provided a combination of methods is applied (never a simple method by itself).

INTRODUCTION THE AIM of this paper is to evaluate the application of isotopic methods for dating of very old groundwaters. It represents a synthesis of an I A E A sponsored study designed to test various methods available for that purpose. The Milk River aquifer, Alberta, Canada was chosen for this study because it fits the definition of an idealized artesian system dipping gently from the recharge area and being confined by aquitards above and below (HENDRY et al., 1991; SCHWARTZ and MUEHLENBACHS, 1979). This feature, coupled with the fact that the groundwater in the Milk River aquifer becomes "old" (beyond 14C dating range) a short distance downflow from the dominant recharge area, makes this aquifer potentially useful for evaluating various dating techniques for old groundwaters. Sampling of the Milk River aquifer was undertaken on several occasions. During the I A E A sponsored campaign in 1985, samples from 16 wells were collected for the following measurements: major and minor ions (Alberta Agriculture and I A E A ) , Stable isotopes 2H, 13C and 180, and t4C ( I A E A , University of Waterloo and University of Arizona), 39Ar, 81Kr and 85Kr (University of Bern), noble gases including 222Rn (University of Bath), 36C1 (University of Arizona and Technical University of Munich), 129I (University of Arizona) and U isotopes (Harwell Laboratory). Additional samples for the analyses of selected isotopes and chemical constituents such as D O C in the recharge zone of the aquifer were collected as recently as 1988. Detailed presentations of the results obtained from all these isotopic and geochemical studies are given in the preceding eight papers in this issue. What follows is a summary and comparative discussion of the findings with particular emphasis on the question of residence time (age) of the groundwaters in the Milk River aquifer system.

GEOLOGICAL, HYDROLOGICAL AND G E O C H E M I C A L BACKGROUND

The Milk River Formation consists of thin (3075 m thick) sandstone and is confined by marine shale above by the Pakowki Formation (typically 120 m thick) and below by the Colorado Group (ranging in thickness from 500 to 650 m). The aquifer underlies 15,000 km 2 of southern Alberta. Most of the area is covered by Pleistocene glacial drift (HENDRY et al., 1991). The aquifer crops out in southern Alberta and

more dominantly, on the flanks of Sweetgrass Hills, northern Montana, at the southern end of the study area. Groundwater flows generally toward the north, west and east, following the dip of the aquifer, The two flowpaths identified along the valleys of the low C1 concentrations in the eastern and western part of the study area (see Fig. 1 in HENDRY et al., 1991) reach a maximum length of - 8 0 km. The transmissivity decreases from the south to the north, west and east. This change is caused by a general thinning of the aquifer in these directions and a decrease in hydraulic conductivity toward the north due to an increase in the shale content. One of the unique features of the Milk River aquifer is the consistency in the distribution of major ions and environmental isotopes 180 and 2H. Concentrations of major ions such as CI, Na and HCO 3 + CO 3 are the lowest in the recharge area and increase progressively along the flow system. Dissolved CI and I concentrations range from <0.05 and 0.001 mmol/1, respectively, to >140 and 0.15 mmol/l at the distal part of the flow system. Concentrations of Na and HCO 3 + CO3 vary from - 1 5 mmol/l to >50 mmol/l and <12 mmol/1 to >25 mmol/1 along the flow system, respectively (HENORV et al., 1991). Although the concentrations of these species vary systematically, their patterns are complex. Thus, the distributions of C1, Na and HCO 3 + CO 3 are marked by a large north-trending bulge of groundwater with low concentrations coinciding with a zone of higher transmissivity. The patterns of Ca and Mg variations are similar with typical concentrations of <0.1 mmol/ 1 but at the northeastern margin of the aquifer, concentrations are >0.1 mmol/l. In contrast, SO 4 concentrations decrease from >5 mmol/l in the south to <0.1 mmol/1 throughout the remainder of the aquifer (HENORY et al., 1991). The 6180 and 62H values become higher toward the north. The 6180 and 62H data from samples collected from the recharge area plot on the global meteoric line, but deviate increasingly from it with distance downflow (HENDRY and SCHWARTZ, 1988). Four mechanisms have been suggested to explain the origin of the chemical and isotopic patterns: (1) the introduction of connate formation water through the Colorado shale and subsequent mixing with infiltrating meteoric water (SCHWARTZand MUEHLENBACHS, 1979; SCHWARTZ et al., 1981); (2) a finite source of meteoric recharge mixing with more saline water in the aquifer (DOMENICO and ROBBIr~S, 1985); (3) chemical and isotopic enrichment due to ion filtration (PHILLIPS et al., 1986); and (4) aquitard diffusion based on diffusion gradients between

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saline, isotopically enriched water in the Colorado waters to - 2 0 km distance from the recharge zone shale and fresher, isotopically depleted water in the indicate minimum flowrate of - 1 - 1 . 5 m/a in this part aquifer (HENDRY and SCrtWARTZ, 1988). of the aquifer. The data in Cluster B fall close to the The last of the above models was tested using a modern meteoric water line for the region. However, mass transport model (HENORY and SCHWARTZ, these groundwaters contain no measurable 14C and 1988). The period during which the flow system has thus could have recharged before the last glaciation. operated in the manner observed at present, i.e. Assuming constant flowrates beyond the Milk River active inflow of meteoric water in the system orig- of the order of 0.1 m/a, DRU,IMm et al. (1991) suggest inally filled with formation water, was estimated tO be groundwater ages >50 ka B.P. and possibly as much 1-2 Ma. Although the model parameters are subject as 600 ka B.P. along path 1. Cluster C data represent to large uncertainties, the calculations showed that groundwaters in the most distant section of the Milk the observed chemical patterns in the aquifer can be River aquifer. They deviate from the meteoric water explained by diffusion from the aquitard. The best line and may indicate mixing with formation waters. model fit to both CI and lSo data yielded an effective This may occur through diffusive transport of isotopidiffusion coefficient D ' for these species through the cally enriched groundwaters from the confining aquitard (the most critical parameter in the model) of shales resulting in alteration of the warm climate 6 × 10 -12 mZ/s. The flowrate was assumed to be meteoric water isotopic signatures. constant along the flowpaths. For path 1 (see Fig. 1 in Isotope analyses of aqueous SO 4 indicate bacterial HENDRY et al., 1991), the flowrate was 0.3 m/a yield- reduction of SO4. Sulphate concentrations are high in ing a groundwater age at 80 km downflow from the groundwaters of the recharge area but SO4 virtually outcrop of 0.25 Ma. The flowrate along path 2 was disappears in the groundwaters of the central part of estimated to be half of the flowrate obtained for path the aquifer. Similarly, bacterial action producing or 1, requiring 0.51 Ma for groundwater to travel 80 km. oxidizing CH 4 can affect the distribution of C isoThese estimates of the maximum groundwater resi- topes. Methane is present in most sampled wells and dence times in the aquifer bracket earlier estimates of the plot of 13C against 62H (see Fig. 4 in DRIMMIEet 0.3 Ma based on hydraulic modelling (SWANICK, al., 1991) suggests that all CH 4 is of biogenic origin. 1982). Unfortunately, the bacterial processes and the consequent change in at least the DIC pool makes the interpretation of 14C data in terms of groundwater ISOTOPIC STUDIES IN MILK RIVER AQUIFER residence time very problematic. Radiocarbon was measurable only in the groundRadiocarbon and stable isotopes in groundwater and waters sampled from the wells located within 20 km of the recharge zone. Comparison of 14C data from dissolved constituents two wells near the recharge area of the aquifer DRIMMIEetal. (1991) summarize the distribution of obtained from DIC, D O C - H M W and D O C - L M W 2H, 3H and 180 in groundwaters, 2H and I3C in CH 4 , fractions has revealed that they are very similar (see 1~O and 345 in SO 4, i3C and 14C in dissolved inorganic Table 3 in DRIMMIE et al., 1991). Although the DIC 14C pmc values are much lower than those for DOC C (DIC) and dissolved organic C (DOC). Although the light stable isotopes cannot determine the absol- in both wells, the age differences between the two ute age of the Milk River aquifer groundwaters, they wells are quite similar, averaging - 1 4 ka. The lower DIC values lead to the conclusion that the initial DIC can assist in identifying chemical reactions in the J4C content when the water entered the aquifer was aquifer that may contribute to the DIC pool and thus affect 14C concentrations. This information, in turn, as low as 30 pmc. From these data the flowrate of - 1 m/a has been estimated for the distance of 12 km is required to interpret lac data in either DIC or between the two wells. This flowrate agrees surprisDOC, or both, in terms of groundwater age, At least three distinct zones can be recognized in ingly well with the stable isotope data but is an order the 6180 against 62H plot: Cluster A with 6180 values of magnitude higher than the flowrates obtained of approx. - 19%o, Cluster B with 6180 of - 16%o, and from hydraulic considerations (SwANlCK, 1982; Cluster C with 6180 of -8%0 (see Fig. l in DRI~MIE et HENDRV and SCHWARTZ, 1988) or based on age determinations from 36C1 (PHILLIPS et al., 1986; NOLTE et al., 1991). Cluster A represents groundwaters near the recharge area in the southern part of the aquifer al., 1991) and from U isotopes (IVANOVICH et al., 1991) for the northern part of the aquifer. Similarly, (some samples contained measurable levels of bomb extrapolation of 14C groundwater ages obtained in 3H). The isotopic signatures are not followed by even more negative 6180 and 62H values downflow as the southern area of the aquifer near the recharge might be expected were Pleistocene recharge pres- along the flowpath north of the Milk River yields low ages of only 70-100 ka for the groundwaters in the ent. This absence of a glacia[ signature suggests that northern-most locations. Clearly, these results canthese groundwaters were recharged <15 ka B.P. and that the Pleistocene period of that region is character- not be reconciled easily with other isotopic and hydraulic evidence for older groundwater ages. The ized by insignificant recharge during long periods. 14C is absent in any significant amounts in groundThe uniform di2H and 6180 values found in groundAG 6:4-E

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waters north of the Milk River so that detailed extrapolation over the entire flow system may not be valid. It is therefore concluded that the groundwater movement in the southern part of the aquifer is rather fast ( - 1 m/a) up to the Milk River. The results of ROBERTSON (1988) show that the river acts as a point of discharge rather than recharge. In consequenee, lower flowrates ( - 0 . 1 m/a) may exist north of the river. Further, 36C1 and C1 data from the zone near the end of the flowpaths suggest further decrease of flowrates in the northern part of the Milk River aquifer due to distributed discharge along the flowpath (see discussion on 36C1 data below). This observation is consistent with the observed decrease of the aquifer transmissivity downflow (HENDRY and SCHWARTZ, 1988).

Dissolved gases and Kr isotopes

ANDREWS et al. (1991a) discuss in detail the geochemical relations among the noble gases (He, Ne, Ar, Kr and Xe), the 36Ar/a°Ar isotope ratio, N~ and CH 4 contents of Milk River aquifer groundwaters. In particular, inferred recharge temperatures based on noble gas and stable isotope data together with the observed correlation between He and CI contents of the groundwaters shed further light on the origin and age of the groundwaters in this aquifer. The stable isotope composition of the Milk River aquifer groundwaters suggests that the most recent recharge took place under cooler climatic conditions than the earlier recharge episodes (see above). Indeed, the inferred recharge temperature difference from the noble gas data of - 4 - 5 ° C is consistent with the stable isotope data considerations. The concentration of 4He in the Milk River aquifer groundwater varies with depth consistent with the radiogenic 4He accumulation in the aquifer and the adjacent shales. For groundwaters which did not degas due to the CH 4 presence, a trend of increasing He content with increasing Cl content is observed (ANDREWS et al., 1991a). Increasing CI content is regarded as an index of groundwater ageing. Thus, increase of 4He content is consistent with increasing groundwater age. The relation between 4He and C1 contents must be limited at high C1 concentrations as the equilibrium between dissolved 4He and 4He stored in the rock matrix is approached. Some of the measured concentrations follow this trend. However, the data from several locations fall below the expected trend. These data represent de-gassed groundwaters due to the presence of CH4. Therefore, no reliable groundwater age could be inferred from the available 4He data. LEHMANN et al. (1991) measured 81Kr and 85Kr in well 9, a well intermediate between the outcrop and the northern end of the aquifer. The activity of 85Kr is only 0.3% of the modern atmospheric activity indicating absence of any admixture with younger

groundwaters at that location. The measurement of 81Kr yields an upper age limit of - 1 4 0 ka in excellent agreement with the hydraulic models, 36C1 and U isotope dating techniques applied to the groundwaters in that part of the aquifer (see below).

Chloride a n d

36C1m e a s u r e m e n t s

NOLTE et al. (1991) report a set of 36C1/C1 ratio measurements in Milk River aquifer groundwaters. Their interpretation of the CI isotopic data is based on two approaches: (1) the evolution of 36C1 and CI concentrations along the two flowpaths is described by simple exponential decrease of 36C1 and linear increase of C1 with distance from the recharge area; this approach assumes constant flowrate whose value is a function of the amount of in situ 36C1production; (2) the evolution of 36C1 and CI concentrations along the two flowpaths is described by 36C1 decay and by diffusive exchange of both 36C1 and CI between the aquifer and the underlying Colorado shale aquitard. The groundwater flowrate is then inferred from a diffusion model in which the flowrate is assumed constant. The 36C1 input is much greater than can be obtained from atmospheric fallout even after concentration by much greater evapotranspiration than the 60% suggested by SWANICK(1982). ANDREWS et al. (1991b) have suggested that the input may be duc to solution of salt from the shallow soil zone at recharge where cosmic-ray irradiation would increase 36C1 production. However, the residence time of salt in the soil zone may not be sufficient for this mechanism to approach equilibrium. Alternatively, FABRYKAMARTIN et al. (1991) propose that the hundred-fold increase between halide concentrations in precipitation and recharge waters can be attributed to the combined effects of evapotranspiration, dry fallout, and a limited period of recharge. Further investigations of 36C1 in the soil zone and the mechanism for generating the 36C1 input is required. A satisfactory explanation of the input is a prerequisite to validate the derived 36C1 ages and would be positive evidence for exclusion of bomb-pulse 36C1by vertical leakage. Considerations of in situ production of 36C1 in the first of the two models have yielded groundwater flowrates 75% lower than when zero in situ production is assumed. The flowrates inferred from the diffusion model are only slightly higher than the corresponding values obtained by the first model. The above results lead to the conclusion that radioactive decay of 36C1, rather than diffusive losses, controls 36C1 concentration in groundwater during advective transport in the Milk River aquifer (NOLTE et al., 1991). Thus, groundwater ages based on 36Cl/Cl data for the Milk River aquifer range up to 2 Ma if uncorrected for any dilution by subsurface sources of dead Cl (PHILLIPSe t aL, 1986). If the assumption is made that the subsurface contributions

Application of isotopic methods to dating very old groundwaters of C1 range from low values in the fresh waters in the south to high values (up to 90% of total C1) in the much older saline waters in the northern parts of the aquifer, then maximum ages are reduced to <1 Ma, making the 36Cl-based ages more consisterft with those based on other isotope techniques considered here (FABRYKA-MARTINetal., 1991).

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water containing lower halogen concentrations if the thickness of the shale "lens" does not fall under a certain value.

Uranium isotopes in the Milk River aquifer groundwaters

In general, the U content in Milk River aquifer groundwaters decreases with distance from the recharge, area in the south (IVANOVlCHet aL, 1991). FABRYKA-MARTINet al. (1991) present C1, Br and I The groundwaters can be divided into a small oxic data for the Milk River aquifer. The three halides are group (U content - 1 0 -5 mmol/l) and more predomihighly correlated. Concentrations are low in the nant anoxic group (U content - 1 0 -7 mmol/l). All freshwater zones of the aquifer but increase by as groundwaters are undersaturated with respect to much as two orders of magnitude along the margins. common U minerals. The dominant U complexes are The C1/I and C1/Br ratios are well below those of uranyl carbonates. The 234U/238U activity ratios inseawater and are consistent with the origin derived crease from 2 to 11 with distance downftow of the from the diagenesis of organic matter in the sedi- recharge area. The highest 234Ul238 U activity ratio of ments. However, moving downflow, the halide ratios 11.8 defines the redox front, whereupon a steady reach nearly constant values suggesting the domi- decrease downflow to 4 is observed. This decrease is nance of a common subsurface source for these ions. interpreted by IVANOVlCHetal. (1991) to be due to the FABRYKA-MARTIN et al. (1991) suggest that halide decay of 234Uexcess with groundwater age and due to ratios rule out unaltered seawater or leakage and/or U loss by sorption/precipitation processes from soldiffusion from the underlying Bow Island sandstone ution to solid surfaces. A phenomenological model as a major influence on the groundwater chemistry in (FR6HLICH and GELLERMANN, 1987) for the evolution the Milk River aquifer. They favour the hypothesis of U isotopes groundwater composition downflow that the primary source of the halides is altered was used. It is based on a transport (dispersion) connate seawater diffusing from low-permeability equation incorporating radioactive decay assuming units within the Milk River Formation. This is at that sorption processes are of first-order kinetics and variance with the hypothesis offered by HENDRY and yields a groundwater flowrate range of 0.2-0.6 m/a SCHWARTZ (1988) in which the halides and other along path 1 (see Fig. 1 in HENDRY et al., 1991) in dissolved constituents diffuse primarily from the good agreement with the flowrates of 0.3 m/a obunderlying Colorado shales. This fifth conceptual tained from hydraulic considerations. When applied model (see description of the other four models along path 2 (see Fig. 1 in HENDRV et al., 1991) the earlier in this paper) does not require mixing with an model yields a flowrate range of 0.1-0.4 m/a in very external water body or Complex mechanisms such as good agreement with the hydraulic model flowrate of ion filtration (see PHILLIPS et al., 1990; HENDRY and 0.2 m/a. Further, a simplified model of downflow decay of 234U excess in solution under steady-state SCHWARTZ, 1990). Arguments consistent with this fifth hypothesis are conditions yields a flowrate range along path 1 from derived from the observed deviations from halide 0. l to 0.2 m/a consistent both with the more sophistimixing curves (see diagram in Fig. 6, FABRYKA- cated FR6HLICH and GELLERMAN (1987) model and MARTIN et al., 1991). These deviations are inter- with the hydraulic models. preted in terms of differences in chemical composition of the saline water in the smaller pores of the less permeable sediments of the Milk River FormaDISCUSSION tion. Further, FABRYKA-MARTINet al. (1991) estimate from the C1 contents of the Milk River Formation As indicated earlier, four mechanisms have been drill-cuttings leachate (HENDRY and SCHWARTZ, suggested to explain the origin of the chemical and 1988) that the Cl concentrations in the fluids of isotopic patterns observed in the Milk River aquifer smaller pores are several times higher than the C1 groundwaters. Each suggests different sources of the concentrations in the groundwaters derived from the saline component and is supported by diverse parts of larger-sized pores. The fact that so much leachable the existing data base. The way out of this apparent salt is left in the formation is considered to be an dilemma is indicated by FABRYKA-MARTIN et al. (1991) who state that "other saline sources proposed indication that sedimentary rocks within the formation are still the most likely major source of C1 in the by other authors cannot be ruled out altogether and aquifer groundwater. FABRYKA-MARTINet al. (1991) are probably contributing to the groundwater geohave calculated diffusion rates for shales in the chemistry to some extent, but the model in which the aquifer indicating that. appreciable solute concen- solute source resides mainly within the Milk River trations are still available for diffusion to the ground- Formation is the simplest one and is able to account Geochemistry o f halogens

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for most of the observed data". More generally, this ambivalence in interpreting measured data by conceptual models appears to be inherent in regional groundwater studies such as the Milk River aquifer study. It seems to be a consequence not only of the limited data base available (new data often open the possibilities of additional interpretations) but also of inherent complexities of regional groundwater systems. Thus, parallel use of different approaches in interpreting the available data is a positive advantage in the Milk River aquifer study. Therefore, the groundwater ages based on the hydrodynamic model should be considered as a lower limit of the average groundwater ages in the Milk River aquifer while those based on the 36C1/C1 data should be considered as an upper limit. One of the basic requirements for isotopic dating of groundwaters is that no unaccounted-for radiogenic sources exist in the aquifer. For this reason, the underground production of various radionuclides of relevance in both the groundwater and the sandstone matrix have been considered in the Milk River aquifer. In sandstones generally, the underground production of 3H, SSKr and 14C is below detection limits. Thus, in wells 5 and 8 located close to the recharge area and in well 9 which is situated - 2 0 km downflow, the absence of SSKr shows that there is no admixture with young groundwaters. The 3TAr and 39Ar are produced within the sandstone. The latter is measurable in wells 8 and 9 (about 20% and 7% modern, respectively) where the hydrological age shows that cosmogenic input should have decayed. Release efficiencies for these radionuclides are estimated to be less than a few per cent and are similar to the release efficiency for 222Rn. The 39Ar/37Ar ratio is rather similar to the calculated ratio for in situ production within the sandstone and is very different from the ratio for production within the underlying Colorado shale. Therefore, it is plausible that both groundwaters have dissolved these isotopes from sandstone in their in situ production ratio and that the efficiency of release at these locations is different, Further, this result appears to be an argument in favour of the model of FABRYKA-MARTINet al. (1991) which assumes an internal source for the halide concentrations observed in Milk River aquifer groundwater. The 81Kr activity present in well 9 (82 _+ 18% modern) may be the residual cosmogenic input. New estimates of the mass 81 fission yield suggest that in situ production is unlikely to limit the use of 81Kr for groundwater dating. The 129I produced in situ by fission is probably released with high efficiency and is then isotopically equilibrated with dissolved I. The fractional release (ratio of 129I atoms measured in solution to that expected for total solution of the in situ production) is of the order of several per cent and shows a general trend of increase with increasing groundwater age. This seems to be an argument against the ion ill-

tration hypothesis, because, at least, it shows that the increase of 129I with groundwater age is controlled by release of in situ produced 129I rather than by enrichment of cosmogenic 129I. In general, the in situ production of 36C1 in aquifer matrices has been shown to be significant. However, in the case of the Milk River aquifer the high cosmogenie input of 36C1 limits the effects of in situ production on groundwater 36C1 content. Thus, groundwater ages calculated with in situ production of 36Cl are lower by --30% in the case of path 2 to 50% in the case of path 1 than the ages calculated for zero in situ production. This difference does not appear to be particularly large when the total range of uncertainty in the groundwater age estimates between different models and isotopic methods is taken into account. However, after the groundwater chlorinity reaches - 5 0 0 ppm, in situ production dominates the 36C1 content and no reliable groundwater ages can be determined. In the case of the confined part of the Milk River aquifer, the estimated in situ neutron flux would produce an equilibrium 36C1/C1 ratio of 11 x 10-15 in the aquifer matrix C1. Because of their somewhat lower U and Th contents, the unconfined sandstone and the confining shales would have low characteristic 36C1/C1 ratios ranging from 5 to 6 x 10 -15. The measured groundwater 36C1/C1 ratios from northernmost, western and eastern margins of the study area vary between - 4 and 10 x 10-15, a range coinciding with the ratios calculated for the confined part of the Milk River aquifer and confining shale matrix C1. Thus, it is difficult to deduce whether the source of the C1 resides within or without the Milk River aquifer. However, a fairly good agreement between the in situ 36C1/C1 ratio and the ratio measured in the very old groundwaters of the aquifer seems to favour the diffusion model over the ion filtration model. Unfortunately, the fact that the 37Ar/39Ar ratio in the groundwater matches that calculated for the sandstone (and not that for the confining shales) cannot be used as evidence that the source of salinity in the groundwater resides in the sandstone (and not in the shales) because the diffusion time from the shales is presumably much longer than the 37Ar half-life (35 d) i.e. any distinctive A r isotopic signature in shale waters (salts) would be lost due to decay soon after leaving the shale.

CONCLUSIONS The main aim of this joint project was to evaluate the usefulness of available geochemical and isotope techniques for dating very old groundwater. The Milk River aquifer was selected for this study because several preceding investigations had established that this groundwater system contains waters whose ages range from recent recharge to - 1 Ma. The present study has established that the Milk River aquifer

Application of isotopic methods to dating very old groundwaters system is very complex both in terms of groundwater origin and in terms of the evolution of its chemical and isotopic contents. However, this apparent complexity proved to be an interesting challenge with respect to the use of different and complementary approaches to interpreting the geochemical and isotope data in terms of groundwater residence time. A comprehensive examination of the hydrochemistry, in particular the study of halogen distribution, proved indispensable for identifying major groundwater flowpaths through the extended aquifer system. Information obtained from the previous hydraulic studies provided an independent estimate of the groundwater flowrate and hence its age and indicated that a decrease of the flowrate with increasing distance downflow from the recharge area should be considered. The stable isotopes 2H and ISO in groundwaters have been essential in providing insight into their origin and in corroborating, at least qualitatively, the age estimates derived from other dating methods. The stable isotopes 13C and 180 in both DIC and D O C have assisted in identifying chemical reactions taking place in the aquifer that may affect ~4C concentrations in those two phases. Unfortunately, 14C is of limited use in the Milk River aquifer because its concentration in the aquifer waters decreases to zero after a distance of only 20 km downflow of the recharge zone. Thus, for most of the flowpath the groundwater age is far beyond the 14C dating range. The study of dissolved gases, supplementary to 2H and 180, has provided the information on the groundwater recharge temperatures. The 4He content has been found to increase along the flowpath but no age could be inferred from the data. In contrast, 81Kr measurement in one well has yielded an estimate of groundwater age in good agreement with other dating methods. Thus, sJ Kr appears to be a promising tool for dating very old groundwaters in spite of the complexities involved in the measurement. Argon isotope measurements could not be interpreted in terms of groundwater age but they have been used to estimate 39Ar underground production in good agreement with theoretical estimates and to provide corroborative evidence in the origin of dissolved solids in the aquifer groundwaters. This study has shown clearly that the application of a 36Cl dating method requires an extensive study of the CI groundwater content. The high cosmogenic 36C1 input at the recharge of the Milk River aquifer makes radioactive decay the dominating process controlling the decrease of the 36C1 content in the groundwaters with distance downflow. This may be the main reason why the groundwater age estimates derived from the 36C1 data using a variety of conceptual models appears to be relatively invariant. Consideration of the 129I data available for the Milk River aquifer groundwater revealed an increase in concentration along the flowpaths. This increase

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has been interpreted to be due to the transfer of in situ produced 1291 from the rock matrix to the liquid phase. The study of in situ production of cosmogenic radionuclides has clearly shown that any dating of groundwater with 36C1 and 1291 has to examine carefully the possible contribution from this source to the groundwaters. In the case of the Milk River aquifer, calculated estimates of production rates have been found to be generally in good agreement with the measurements. This agreement confirms that improvements have been made in the calculations of the production rates e.g. improvements in the used nuclear reaction cross-sections and inclusion of the contributions from the higher energies of the neutron flux spectrum. The U isotope dating method appears to be suitable for the Milk River aquifer yielding groundwater ages in good agreement with the hydraulic and 36C1 methods. Thus, when U isotope residence time in solution is of the order of the 234U half-life (2.48 x 105 a), the U isotope dating method is useful for estimating the groundwater age in the range of 104-5 x 105 a. Finally, this project has demonstrated that different approaches have been necessary to understand the geochemical and isotopic patterns observed in the measured data and to interpret them in terms of the origin, evolution and age of groundwater in the Milk River aquifer. In the case of 36C1 and U isotopes, the most useful of the isotope dating methods, different dating models were found to be equally applicable. This apparent ambivalence in interpretation of the measured data by conceptual models appears to be inherent in regional groundwater studies such as this. It seems to be a consequence not only of the limited data base available (new data often open additional possibilities of interpretation) but also of inherent complexities of regional groundwater systems. Thus, parallel and complementary use of different approaches in interpretation of the available data base is a positive advantage. In the present case, the groundwater ages based on the hydrodynamic model should be considered as a lower limit of the average groundwater age in the Milk River aquifer while those based on the 36C1/C1 data uncorrected for any dilution by dead C1, should be considered as an upper limit. In conclusion, the results of the present study demonstrate the high potential of isotope techniques for dating very old groundwaters provided a combination of methods is applied (never a single method by itself). The confidence in more conventional hydrodynamic models for estimating average groundwater flowrates and distinguishing flow paths or mixing zones is often misplaced in instances where the flow system is several hundred thousand years old. In cases such as the Milk River aquifer, the likelihood of alteration to groundwater flow due to climatic change (particularly glaciation) is often neg-

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lected a n d the use of p r e s e n t - d a y hydraulic h e a d s (or e v e n r e c o n s t r u c t e d o n e s ) for estimating g r o u n d w a t e r average age over geological timescales m a y b e inappropriate. T h u s , isotopic a n d g e o c h e m i c a l d a t a m a y r e p r e s e n t a m o r e realistic r e c o r d of average flow conditions t h a n do the hydraulic data. T h e Milk R i v e r A q u i f e r I n t e r n a t i o n a l Project is a n excellent illustration of the variety of insight a n d increased degree of confidence possible in flow-system analysis w h e n multiple, i n d e p e n d e n t dating m e t h o d s are c o m b i n e d with detailed hydrogeological studies. Acknowledgement--We thank Brenda Boardman, Harwell Laboratory for typing numerous versions of this paper. Editorial handling: M. Ivanovich and B. Hitchon.

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