Comparing conservative and nonconservative tracers in karst and using them to estimate flow path geometry Andrew J. Luhmann a,⇑ , Matthew D. Covington b , Scott C. Alexander a , Su Yi Chai a , Benjamin F. Schwartz c , Joel T. Groten a , E. Calvin Alexander Jr. a a Department of Earth Sciences, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, USA b Karst Research Institute, Titov Trg 2, 6230 Postojna, Slovenia c Department of Biology, Texas State University, 601 University Drive, San Marcos, TX 78666, USA article info Article history: Received 7 April 2011 Received in revised form 21 April 2012 Accepted 25 April 2012 Available online 5 May 2012 This manuscript was handled by Philippe Baveye, Editor-in-Chief, with the assistance of Magdeline Laba, Associate Editor Keywords: Tracer breakthrough curve Hydrograph Heat transport Spring Karst summary A controlled recharge event was conducted with multiple tracers in a karst aquifer in southeastern Min- nesota. A pool adjacent to a sinkhole was filled with approximately 13,000 L of water. After tracers were added and thoroughly mixed, the pool was emptied into the sinkhole. Data were collected at Freiheit Spring approximately 95 m north of the sinkhole to monitor spring responses. Flow peaked first at the spring, and suspended sediment peaked next. Then nearly identical uranine, chloride, and dD peaks occurred. Temperature was the last of the tracers to peak. The initial increase in flow at the spring recorded the time at which the water reached a submerged conduit, sending a pres- sure pulse to the spring at approximately the speed of sound in open water. The initial increase in ura- nine, chloride, and dD at the spring recorded the arrival of the recharge water. The initial change in temperature and its peak occurred later than the same features in the uranine, chloride, and dD break- through curves. As water flowed along this flow path, water exchanged heat with the aquifer, producing a lagged, damped thermal peak at the spring. The combination of hydraulic response and conservative and nonconservative tracers illustrates unique pressure, advective, and nonconservative processes. Geometrical properties of the flow system may be estimated using these tracers. By summing discharge between the time of the initial increase in stage produced by a pressure pulse in a fully phreatic flow path and the time of the chloride peak, the conduit volume is estimated as 47 ± 10% m 3 . Heat transport sim- ulations were used to reproduce the modified thermal signal, and simulations with planar flow paths and hydraulic diameters of 7 and 8 cm produced the best fits to the observed temperature breakthrough curve. These volume and hydraulic diameter estimates together predict a bedding plane flow path that is 3.5 cm high by 9 m wide or 4 cm high by 8 m wide. The different tracers provide complementary infor- mation, and the combination of parameters provides useful constraints on flow path geometry. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Tracers are often used in aquifer studies to provide basic infor- mation about a flow system. However, few studies have simulta- neously injected multiple tracers at a single site to identify similarities or differences in tracer responses at a spring or to ex- ploit complementary information that each tracer provides. Atkin- son et al. (1973) conducted a multiple tracer test using malachite green dyed Lycopodium clavatum spores, blue polyethylene dust, lithium acetate, and pyranine. All tracers except pyranine were in- jected at a stream sink; the pyranine was injected more than 150 m upstream. They did not recover either the polyethylene dust or the lithium acetate. The concentration curves of the Lycopodium spores and the pyranine dye were similar, with the initial arrival of the spores occurring just before the dye. Goldscheider et al. (2003) in- jected sodium-napthionate and clubmoss spores of L. clavatum into one well and eosin and fluorescent microspheres into another well to compare soluble and particle tracers. However, they only recov- ered 0.81% of the sodium-napthionate and negligible amounts of the other tracers. Geyer et al. (2007) injected uranine, sulforhoda- mine G, and tinopal CBS-X simultaneously into a sinkhole. Uranine peaked first at the spring and was followed by sulforhodamine G and tinopal CBS-X peaks 25 and 45 min later, respectively. Geyer et al. (2007) used the different breakthrough curves of the conser- vative and reactive tracers to estimate reactive transport and con- duit parameters and concluded that a combination of conservative 0022-1694/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2012.04.044 ⇑ Corresponding author. Tel.: +1 612 709 9783; fax: +1 612 625 3819. E-mail addresses: luhm0031@umn.edu (A.J. Luhmann), speleophysics@gmail.- com (M.D. Covington), alexa017@umn.edu (S.C. Alexander), chaix029@umn.edu (S.Y. Chai), bs37@txstate.edu (B.F. Schwartz), grote051@umn.edu (J.T. Groten), alexa001@umn.edu (E.C. Alexander Jr.). Journal of Hydrology 448–449 (2012) 201–211 Contents lists available at SciVerse ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol