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esm_15_Reading_Abstracts - 36-Cl papers

esm_15_Reading_Abstracts - 36-Cl papers - WATER RESOURCES...

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Unformatted text preview: WATER RESOURCES RESEARCH. VOL. 38, NO. 8, 10.1029/2001WR000772, 2002 Estimating groundwater recharge in fractured rock from environmental 3H and “Cl, Clare Valley, South Australia P. G. Cook CSIRO Land and Water, Glen Osmond, South Australia. Australia N. 1. Robinson CSIRO Mathematical and Information Sciences, Glen Osmond, South Australia, Australia Received 13 July 2001; revised 29 January 2002; accepted 4 March 2002; published 10 August 2002. [1] Vertical profiles of31-1 and 36Cl concentrations are obtained from piczometer nests installed in fractured metasedimentary aquifers in the Clare Valley, South Australia. Because 3H is lost during cvapotranspiration with negligible fractionation, while 36C1 is retained within the soil, comparison of 3H and 36Cl concentrations allows estimation of the aquifer recharge rate. An analytical solution for the transport of 3 H and 36Cl through planar, parallel fractures is used to investigate the effect of variations in matrix porosity, tortuosity, fracture aperture, fracture spacing and aquifer recharge rate on tracer profiles and then to reproduce observed profiles within piezometcr nests. While the measured distributions of these tracers are not able to constrain most model parameters, they are able to tightly constrain the aquifer recharge rate. The broad nature of the 36Cl and 3 H peaks measured at our sites is simulated using a constant fracture spacing, lognormal distributions of fracture apertures, and mean recharge rates of 60—75 mm yr‘l. INDEX TERMS: 1829 Hydrology: Groundwater hydrology; 1832 Hydrology: Groundwater transport; KEYWORDS: fractured rocks, recharge, groundwater, isotopes, environmental tracers . Introduction [2] 3H and 3"C1 are environmental tracers whose concen- trations in rainfall were elevated during the 19503 and 19605 as a result of aboveground thermonuclear testing. Because these compounds are chemically stable, they have received widespread application as hydrological tracers in porous media. The depths of the highest concentrations of 3H and 3("CI in soil water profiles have been used to estimate rates of soil water movement leading to groundwater recharge [Smith 6101., 1970; Plrr'flrps et at, 1988]. In a few cases, multilevel piezometcrs have been used to determine 3 H and “Cl profiles in aquifers, and hence determine vertical rates of ground- water movement [Robertson cmd Cherry, 1989; Bentley et 01., 1982]. Also, because 3H is lost to evaporation, with little fractionating of the remaining liquid, comparison of the 31-1 mass in the subsurface with that deposited on the soil surface in rainfall, has been used to estimate the amount of evapo- ration, and thus aquifer recharge [Solomon and Cook, 1999]. [3] The interpretation of environmental tracer measure— ments from fractured rock aquifers, however, is complicated by diffusion between mobile water in the fractures and less mobile water in the rock matrix. For this reason, observa— tions on the transport of dissolved solutes cannot be readily used for determining water fluxes. Even in relatively simple systems, with identical, planar, parallel fractures, solute transport velocities cannot be used to estimate water veloc- ities without information on the fracture aperture, fracture spacing, dispersion coefficient within the fracture, matrix porosity and matrix diffusion coefficient of the solute. Furthermore, water velocities cannot be related to water Copyright 2002 by the American Geophysical Union. 0043-1397102/2001WR000772$09.00 fluxes without knowledge of fracture aperture and fracture spacing. 3H and “Cl concentrations in soil and groundwater obtained fi'om fractured rocks have been used to estimate minimum depths of infiltration of recent rainfall, and depths of circulation of groundwaters [Ruland et at, 1991]. How- ever, the depths of penetration of these tracers cannot readily be used to estimate vertical water velocities or aquifer recharge rates. To date, quantitative interpretation of environmental tracer measurements has been largely restricted to cases where equivalent porous media assump- tions can be made [Cook 2! at, 1996]. [4] In this paper we show how vertical profiles of 3H and 3’E’Cl in fractured rock aquifers can be used to estimate the aquifer recharge rate without making equivalent porous media assumptions, and without a knowledge of fracture and matrix transport properties. Because 3H is lost during evapotranspiration with negligible fractionation, while 3‘E’Cl is retained within the soil, and these tracers behave similarly in all other respects, comparison of 3H and 36CI concentra- tions allows estimation of the aquifer recharge rate. Aquifers within the Clare Valley, South Australia, are dominated by fractures that dip at angles close to 90°, allowing us to model the aquifer as a system of parallel vertical fi'actures. Ana- lytical simulations of 3H and 36C1 transport through parallel vertical fractures are compared with field data obtained from multilevel wells to provide estimates of aquifer recharge. 2. Field Setting 2.1. Site Description [5] The Clare Valley, located approximately 100 km north of Adelaide, South Australia, lies within the Northern Mount Lofiy Ranges, and forms part of the Adelaide Geosyncline (Figure 1). The geology consists of low—grade metamorphic, 11-1 WATER RESOURCES RESEARCH, VOL. 38, NO. 5, 10.1029/2001WR000399, 2002 Transport modeling applied to the interpretation of groundwater 36Cl age Jungho Park and Craig M. Bethke Department ot‘Geology, University oflllinois, Urbana, Illinois, USA Thomas Torgersen Department of Marine Science, University of Connecticut, Groton, Connecticut. USA Thomas M. Johnson Department of Geology, University oflllinois, Urbana, Illinois, USA Received 13 February 2001; revised 9 November 2001; accepted 9 November 2001; published 3 May 2002. [1] We use reactive transport modeling to consider how diffusion and hydrodynamic dispersion, cross—formational flow, and subsurface production affect the steady state distribution in flow regimes of tire radioactive isotope 36C1, and the relationship of the isotope distribution to groundwater residence time, or “age.” The isotope forms naturally in the atmosphere, dissolves in rainwater, and then decays in the subsurface with a half—life of ~30 1 ,000 years; hence it is important for age dating very old groundwater. In a simple flow regime composed of an aquifer confined above and below by aquitards, isotopic age may correspond rather well with a groundwater’s “piston flow” age. This correspondence is favored where the aquifer is thick, cross—formational flow is insignificant, salinity is low, and the difiirsion coefficient within the aquitards is small. The maximum dateable age, however, is somewhat smaller than expected from the isotopc’s half-life. Owing to the effect of “dead" chloride, dating based on isotope abundance (the 3 6C11C1 ratio) may be less accurate than that based on 36C1 concentration. Cross—formational flew can strongly affect the 36C1 distribution and abundance, preventing the rates and even direction of flow within an aquifer from being interpreted using the piston flow model. Where salinity is moderate or high, the isotope distribution is controlled by subsurface production, and dating on the basis of the decay of atmospheric 3 6C1 is not possible. In models of simple flow regimes the 3“Cl method fails to predict groundwater age accurately where groundwater chlorinity exceeds ~75—150 mg kg”. Reactive transport models hold considerable promise for improving interpretation of the rates and patterns of groundwater flow from radioisotope distributions. INDEX TERMS: 1035 Geochemistry: Geochronology; 1040 Geochemistry: Isotopic composition/chemistry; 1829 Hydrology: Groundwater hydrology; 1832 Hydrology: Groundwater transport; 3210 Mathematical Geophysics: Modeling; KEYWORDS: chlorine 36, groundwater age, groundwater modeling, diffusion 1. Introduction [2] One of the most fundamental tasks in the study of a ground— water flow regime is determining how the residence time (or age“) of the groundwater varies across the system. Most commonly, we use Darcy's law to calculate flow velocity, the knowledge of which lets us compute groundwater age. Darcy's law requires as input the distribution within the domain of hydraulic conductivity, hydraulic gradient, and porosity. None of these values can be known exactly; especially, appropriate values for hydraulic conductivity can be difficult to constrain. Hence characterizing a flow regime using Darcy’s law carries a certain level of inherent uncertainty. [3] Groundwater age can also be computed directly by observ- ing the distribution of an isotope that decays in the subsurface or is produced there at a predictable rate; the velocity field can then be calculated from variation in age with distance. A number of radiogenic and radioactive isotopes, most notably l4C, have been used for this purpose [e.g., Fro'hlich er of, 1991; Lehmann et at, 1993]. Residence time determined by isotopic methods can provide Copyright 2002 by the American Geophysical Union. 0043- l 397/02/200 l WR000399$09.00 an alternative means of characterizing a flow system or serve as a check on the results of applying Darcy's law. [4] In the study of very old groundwater, such as that found in regional flow regimes or deep in the Earth’s crust, isotopic methods can be especially important because reliable information about the distribution of hydraulic conductivity in such environments is commonly difficult to obtain. To be useful in dating very old groundwater, an isotope must decay or be produced quite slowly and be little affected by chemical reaction. The noble gas isotopes 4He and 40Ar, which are derived in the subsurface from the radioactive decay chains of U, Th, and K, meet these requirements, as does MCI, which is produced naturally in the atmosphere and decays in groundwater [Lehmamt er £11., 1993; Bethke er al., 1999b]. [5] The 3‘5C1 method is attractive for dating very old ground— water because of the isotope’s long half-life (Tug) of r~a30],000 ($4000) year [Bentley e1 (11., 1986a], which allows dating of samples with ages ranging from ~105 to 106 year. In addition, because of its high electronegativity, chlorine most commonly occurs in the subsurface as the free chloride ion. Hence the distribution of 36C1 should be little affected by chemical interaction with the host rock [Bentiev er of, 1986a; Seaman er (IL, 1996]. The 1-1 The distribution of meteoric 36(ll/CI in the United States: a comparison of models Stephen Moysey ‘ Stanley N. Davis - Marek Zreda - L. DeWayne Cecil Abstract The natural distribution of 36Cl/Cl in ground- water across the continental United States has recently been reported by Davis et al. (2003). In this pa er. the large-scale processes and atmospheric sources of "’Cl and chloride responsible for controlling the observed 3bCU'CI distribution are discussed. The dominant process that affects MCI/CI in meteoric groundwater at the continental scale is the fallout of stable chloride from the atmosphere, which is mainly derived from oceanic sources. Atmospheric circulation transports marine chloride to the continental interior. where distance from the coast. topography. and wind patterns define the chloride distribution. The only major deviation from this pattern is observed in northern Utah and southern Idaho where it is inferred that a continental source of chloride exists in the Bonneville Salt Flats, Utah. In contrast to previous studies. the atmospheric flux of 36Cl to the land surface was found to be approximately constant over the United States. without a strong corre— lation between local 36C] fallout and annual precipitation. However. the correlation between these variables was significantly improved (R 320.15 to R 2=0.55) when data from the southeastern USA. which presumably have lower than average atmospheric 3‘’Cl concentrations. were excluded. The total mean flux of 36C] over the continental United States and total global mean flux of 3f’Cl are Received: 2] October 2002 I‘ Accepted: 9 July 2003 Published online: 17 October 2003 © Springer-Verlag 2003 S. Moysey (E) - S. N. Davis - M. Zreda Department of Hydrology and Water Resources. University of Arizona. Tucson. AZ 3572i. USA e-rnail: [email protected] TEL: +l-650-7236l 17 Fax: “4150-7257344 L. D. Cecil US Geological Survey. 900 N. Skyline Drive. Suite C. Idaho Falls. ID 83402. USA Present address: S. Moysey. Department of Geophysics. Stanford University. Stanford. CA 94305. LISA Hydrogeology Journal (2003) 111615497 ‘ I calculated to be 30527.0 and 01624.5 atoms in" s‘. respectively. The 3t"CI/Cl distribution calculated by Bcntlcy ct a]. U986) underestimates the magnitude and variability observed for the measured MCI/Cl distribution across the continental United States. The model proposed by l-lainsworth (1994) provides the best overall fit to the observed 36Clr'Cl distribution in this study. A process— oriented model by Phillips (2000) generally overestimates 36C1/CI in most parts of the country and has several significant local departures from the empirical data. Résumé La distribution naturelle du rapport 3hCl/Cl dans les eaux souterraines des Etats—Unis a été recemrnent présentée par Davis et a1. (2003). Dans ce travail. les processus a grande échelle et les sources atmosphériques de 36C] et de chlorure responsables du controle de la distribution observée du rapport 3t‘Cl.’Cl sont discutés. Le processus dominant qui affecte le rapport 3r‘Clr‘Cl dans les eaux souterraines d'origine météoriquc a l‘e’chelle conti- nentale est l‘apport atmosphérique de chlorure stable. qui provient pour l‘essentiel de sources océaniqucs. La circulation atmosphérique transporte des chlorures marins vers l’intérieur des continents. oi: la distribution dc chlorure est définie par in distance a la cote. 1a topogra- phie et les regimes des vents. La scule exception majcure a ce schéma est observee dans le nord de I'Utab et le sud de l'ldaho oii l'on suppose qu'il existc one source continentale de chlorure dans les has-fonds sales dc Bonneville. Au contraire dc précédcntes eludes (Knics ct al. I994: Phillips 2000). on trouve que le flux atmosphe- rique de 3"Cl vers le sol est approximativcment constant sur l'ensemble des Etats—Unis. sans forte correlation entrc la retombée locale de 3f‘Cl et les précipitations annuclles. Cependant. la correlation entre ces variables dcvicnt significative (R 2=0.15 a 0.55) Iorsqu'on supprime les données du sud-est des Etats—Unis. donl on pcnse qu‘cllcs présentent des concentrations en 3hCI atnrosphérique inférieures a la moyennc. Le flux total moyen dc MCI sur les Etats—Unis continentaux et le flux moyen global de 3t"Cl sont respectivement évalue's a 30.5 i 7.0 el |9.6 i 4.5 atontesm’zs“. La distribution du rapport 3r‘Clr‘Cl calcu- lée par Bentley et a1. (1986) sous-estime l‘ordre de grandeur et la variabilité observes pour la distribution mesurée du rapport 3“CU’CI sur les Etats—Unis continen- taux. Le modele propose par Hainswortb (1994) l’ournil to DO] I 0. I 007/5; I 0040—003-0287-2 ...
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