Transport and retention from single to multiple fractures in crystalline rock at Äspö (Sweden): 1. Evaluation of tracer test results and sensitivity analysis

Abstract

[1] We evaluate the breakthrough curves obtained within a comprehensive experimental program for investigating the retention properties of crystalline rock, referred to as Tracer Retention Understanding Experiments (TRUE). The tracer tests were conducted at the Äspö Hard Rock Laboratory (Sweden) in two phases jointly referred to as TRUE Block Scale (TBS); the TBS tests comprise a total of 17 breakthrough curves with nonsorbing and a range of sorbing tracers. The Euclidian length scales are between 10 and 30 m, compared to 5 m for the earlier tests TRUE-1. The unlimited diffusion model is consistent with measured breakthrough curves and is adopted here for evaluation. The model has four independent parameters, two of which are related to advection and dispersion, one which is related to diffusion-sorption, and one which is related to surface sorption; the individual retention parameters or properties cannot be inferred from breakthrough curves alone and require additional constraints. The mean water residence times for the TBS tests are in the range 15-250 h, whereas the coefficient of variation of the water residence times is in the range 0.4-0.6. A consistent trend is found in the calibrated retention parameters with the sorption affinities of the tracers involved. Using Bode sensitivity functions, it is shown that sensitivity increases for the retention parameter with increasing sorption affinity; for nonsorbing tracers, diffusion and hydrodynamic dispersion are shown to "compete," exhibiting similar effects; hence, their estimates are uncertain. The analysis presented here exposes a few fundamental limitations and sensitivities when evaluating diffusioncontrolled retention in the subsurface; it is general and applicable to any site with comparable tracer test data. In part 2, it will be shown how discrete fracture network simulations based on the hydrostructural information available can be used for further constraining individual retention parameters, in particular, the active specific surface area (s f ) and the rock matrix porosity ().

Hydrological parameter estimations from a conservative tracer test with variable‐density effects at the Boise Hydrogeophysical Research Site

“…Advancements in the above areas are supported by numerical, laboratory, and field experiments [e.g., Rubin , 1995; Yeh and Liu , 2000; Cardiff et al , 2009; Pollock and Cirpka , 2010; Sudicky et al , 2010]. In particular, field experiments in various hydrological and geological contexts, at various scales, and with various survey strategies, are essential to better understand flow and transport processes, validate and improve modeling approaches, and guide novel developments [e.g., Sudicky , 1986; Ptak and Schmid , 1996; Illman et al , 2009; Cvetkovic et al , 2010; Brauchler et al , 2011]. Such field experiments commonly include variations on tracer tests, multiwell pumping or injection tests, and single‐well slug or flowmeter tests or direct‐push profiles [e.g., Hyndman and Gorelick , 1996; Brauchler et al , 2003; Doherty , 2003; Zhu and Yeh , 2005; Cardiff et al , 2011].…”

Tracer attenuation in groundwater

“…First, crystalline rock is a widespread geological formation globally, and is currently a most likely first host for high‐level nuclear waste disposal [ Milnes et al , 2008]. Second, as a sparsely fractured formation, dispersion in crystalline rock is relatively complex and challenging for quantification [ Neuman , 2005; Cvetkovic et al , 2007, 2010]. Finally, crystalline rock has historically been a focus of broad studies where many aspects of flow and transport have been investigated, and relatively comprehensive characterization data are available [ Shapiro , 2001; Cvetkovic et al , 2010].…”

“…The unconditional travel time density (i.e., the expected tracer discharge for pulse of unit mass, or the transfer function) h is written as [e.g., Cvetkovic et al , 2010; Cvetkovic and Frampton , 2010]. where [1/ T 1/2 ] with being the matrix porosity, D e [ L 2 / T ] effective diffusivity, the retardation coefficient for the matrix, [ M / L 3 ] the rock density, and s f [1/ L ] the active specific surface area [ Cvetkovic et al , 1999].…”

“…In Figure 5, we show recovery data from a comprehensive series of field tracer tests (TRUE) [ Cvetkovic et al , 2007, 2010; Cvetkovic and Frampton , 2010] with multiple tracers along multiple flow paths. At a given time , different tracers with different sorption affinities and loss rates, injected into an aquifer under similar conditions, would yield different recoveries; the data symbols in Figure 5 indicate these differences.…”

“…Comparison between model and data for tracer recovery [ Cvetkovic et al , 2007, 2010; Cvetkovic and Frampton , 2010] that is suggested as an experimental proxy for attenuation. The key retention parameter group (defined above ) is 1 1/h, consistent with the upper bound identified by Cvetkovic [2010b].…”