U-Sr isotopic speedometer: Fluid flow and chemical weatheringrates inaquifers

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Both chemical weathering rates and fluid flow are difficultto measure in natural systems. However, these parameters are critical forunderstanding the hydrochemical evolution of aquifers, predicting thefate and transport of contaminants, and for water resources/water qualityconsiderations. 87Sr/86Sr and (234U/238U) activity ratios are sensitiveindicators of water-rock interaction, and thus provide a means ofquantifying both flow and reactivity. The 87Sr/86Sr values in groundwaters are controlled by the ratio of the dissolution rate to the flowrate. Similarly, the (234U/238U) ratio of natural ground waters is abalance between the flow rate and the dissolution of solids, andalpha-recoil loss of 234U from the solids. By coupling ... continued below

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Maher, Kate; DePaolo, Donald J. & Christensen, John N. December 27, 2005.

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Both chemical weathering rates and fluid flow are difficultto measure in natural systems. However, these parameters are critical forunderstanding the hydrochemical evolution of aquifers, predicting thefate and transport of contaminants, and for water resources/water qualityconsiderations. 87Sr/86Sr and (234U/238U) activity ratios are sensitiveindicators of water-rock interaction, and thus provide a means ofquantifying both flow and reactivity. The 87Sr/86Sr values in groundwaters are controlled by the ratio of the dissolution rate to the flowrate. Similarly, the (234U/238U) ratio of natural ground waters is abalance between the flow rate and the dissolution of solids, andalpha-recoil loss of 234U from the solids. By coupling these two isotopesystems it is possible to constrain both the long-term (ca. 100's to1000's of years) flow rate and bulk dissolution rate along the flow path.Previous estimates of the ratio of the dissolution rate to theinfiltration flux from Sr isotopes (87Sr/86Sr) are combined with a modelfor (234U/238U) to constrain the infiltration flux and dissolution ratefor a 70-m deep vadose zone core from Hanford, Washington. The coupledmodel for both (234U/238U) ratios and the 87Sr/86Sr data suggests aninfiltration flux of 5+-2 mm/yr, and bulk silicate dissolution ratesbetween 10-15.7 and 10-16.5 mol/m2/s. The process of alpha-recoilenrichment, while primarily responsible for the observed variation in(234U/238U) of natural systems, is difficult to quantify. However, therate of this process in natural systems affects the interpretation ofmost U-series data. Models for quantifying the alpha-recoil loss fractionbased on geometric predictions, surface area constraints, and chemicalmethods are also presented. The agreement between the chemical andtheoretical methods, such as direct measurement of (234U/238U) of thesmall grain size fraction and geometric calculations for that sizefraction, is quite good.

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  • Journal Name: Geochimica et Cosmochimica Acta; Journal Volume: 70; Journal Issue: 17; Related Information: Journal Publication Date: 2006

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  • Report No.: LBNL--60859
  • Grant Number: DE-AC02-05CH11231
  • Office of Scientific & Technical Information Report Number: 923345
  • Archival Resource Key: ark:/67531/metadc897537

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  • December 27, 2005

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  • Sept. 27, 2016, 1:39 a.m.

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  • Oct. 31, 2016, 3:59 p.m.

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Maher, Kate; DePaolo, Donald J. & Christensen, John N. U-Sr isotopic speedometer: Fluid flow and chemical weatheringrates inaquifers, article, December 27, 2005; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc897537/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.