Comparison of age distributions estimated from environmental tracers by using binary-dilution and numerical models of fractured and folded karst: Shenandoah Valley of Virginia and West Virginia, USA Richard M. Yager & L. Niel Plummer & Leon J. Kauffman & Daniel H. Doctor & David L. Nelms & Peter Schlosser Abstract Measured concentrations of environmental tracers in spring discharge from a karst aquifer in the Shenandoah Valley, USA, were used to refine a numerical groundwater flow model. The karst aquifer is folded and faulted carbonate bedrock dominated by diffuse flow along fractures. The numerical model represented bedrock structure and discrete features (fault zones and springs). Concentrations of 3 H, 3 He, 4 He, and CFC-113 in spring discharge were interpreted as binary dilutions of young (0–8 years) water and old (tracer-free) water. Simulated mixtures of groundwater are derived from young water flowing along shallow paths, with the addition of old water flowing along deeper paths through the model domain that discharge to springs along fault zones. The simulated median age of young water discharged from springs (5.7 years) is slightly older than the median age estimated from 3 H/ 3 He data (4.4 years). The numerical model predicted a fraction of old water in spring discharge (0.07) that was half that determined by the binary-dilution model using the 3 H/ 3 He apparent age and 3 H and CFC-113 data (0.14). This difference suggests that faults and lineaments are more numerous or extensive than those mapped and included in the numerical model. Keywords Groundwater age . Fractured rocks . Karst . Numerical modelling . USA Introduction Groundwater in fractured-rock aquifers flows through networks of fractures that reflect the structural history of the bedrock. Fracture networks in horizontally layered sedimentary rock typically consist of orthogonal sets of fractures that are either aligned with or cut across the bedding. Groundwater flow in fractured sedimentary rock has been simulated successfully at the kilometer scale with equivalent porous-media (EPM) models using effective hydraulic properties that represent the primary flow paths through the networks (e.g. Yager 1996; Scanlon et al. 2003; Davis and Katz 2007). Anisotropy in sedimentary-rock aquifers that results from preferential flow along bedding can be incorporated in EPM models by appropriate representa- tion of the bedrock structure (e.g. Senior and Goode 1999; Yager et al. 2009; Yager and Ratcliffe 2010; Tiedeman et al. 2010). Received: 22 August 2012 / Accepted: 7 May 2013 Published online: 28 June 2013 * Springer-Verlag Berlin Heidelberg (outside the USA) 2013 Electronic supplementary material The online version of this article (doi:10.1007/s10040-013-0997-9) contains supplementary material, which is available to authorized users. R. M. Yager ()) US Geological Survey, 30 Brown Rd, Ithaca, NY 14850, USA e-mail: ryager@usgs.gov L. N. Plummer : D. H. Doctor US Geological Survey, 12201 Sunrise Valley Dr., Reston, VA 20192, USA L. N. Plummer e-mail: nplummer@usgs.gov D. H. Doctor e-mail: dhdoctor@usgs.gov L. J. Kauffman US Geological Survey, 810 Bear Tavern Rd, West Trenton, NJ 08628, USA e-mail: lkauff@usgs.gov D. L. Nelms US Geological Survey, 1730 East Parham Road, Richmond, VA 23228, USA e-mail: dlnelms@usgs.gov P. Schlosser Earth and Environmental Sciences, Lamont-Doherty Earth Observatory, 139 Comer, 61 Route 9W, Palisades, NY 10964, USA e-mail: schlosser@ldeo.columbia.edu Hydrogeology Journal (2013) 21: 1193–1217 DOI 10.1007/s10040-013-0997-9