www.VadoseZoneJournal.org | 653 2010, Vol. 9 Idenピficaピon of Underground Karst Features using Ground- Penetraピng Radar in Northern Yucatán, México Northern Yucatán is a dry tropical area where limestone karst terrain supports a subde- ciduous forest that is criƟcal for sustaining the local economy of Mayan people. The 5- to 10-m-deep vadose zone is characterized by shallow soils (<30 cm thick) with frequent rock outcrops overlying limestone bedrock, which contains the aquifer. This limestone has two important characterisƟcs: (i) lithologic proper Ɵes that change with depth and (ii) numer- ous dissoluƟon caviƟes ranging from small pores to caves, some of them filled with soil (soil pockets). Due to soil shallowness, di fferences in rock proper Ɵes are relevant because they restrict or favor root growth; in par Ɵcular, caves, crevices, and soil pockets allow pref- erenƟal movement of soil, water, and roots. These features may be important for water and nutrient availability, especially for tree species, but they have not been invesƟgated within an ecological context. We studied the capability of ground-penetraƟng radar (GPR) for idenƟfying various limestone layers—the laja (0.3–2.5 m), the sascab (2.5–5 m), and the coquina (5–9 m)—and dissoluƟon caviƟes. Research was conducted in a limestone quarry so that radar records could be compared with karst features revealed as blasƟng and rock extracƟon advanced. Radar records obtained using a 200-MHz antenna were generally of good interpretaƟve quality; observaƟon depths ranged from 3 to 5 m. In areas that lacked a soil mantle, the interface between the laja and the sascab was clearly idenƟfied by GPR at about 2 m. However, the sascab–coquina, and coquina–aquifer boundaries could not be idenƟfied. Areas of deep soils (>1 m), microrelief (mounds and plains), and large soil pockets within the limestone matrix were also idenƟfied. Main sources of GPR signal aƩen- uaƟon were suspected to be the higher clay and water contents of soil material contained in underlying caviƟes. As a noninvasive tool, GPR can help to determine proper Ɵes of the limestone and its dissoluƟon features that are criƟcal to vadose zone–forest interacƟons. AbbreviaƟons: CEC, caƟon exchange capacity; EC, electrical conducƟvity; GPR, ground-penetraƟng ra- dar; SP, soil pockets. Northern Yucatán is a dry tropical area where the vadose zone is composed of very shallow soils (<30 cm) underlain by limestone. his karst area is primarily made up of marine Miocene–Pliocene materials (Lopez-Ramos, 1975; Lugo et al., 1992; Servicio Geológico Mexicano, 2007). Diagenesis and weathering processes have shaped this land- scape which is characterized by lack of surface streams, no major topographic landforms, good drainage, dissolution features (e.g., crevices, caves, soil pockets), and frequent rock outcrops. he surface features of this karst landscape have been widely described (Finch, 1965; Wilson, 1980; Duch, 1988, 1991; Peniche, 1994; Bautista-Zúñiga et al., 2004); however, except for its caves, very little is known about its dissolution features and their importance for the functioning of the ecosystem. Limestone properties vary with depth (Querejeta et al., 2006, 2007). hree layers of limestone are distinguishable within the study area: the laja, a consolidated rock exposed or immediately underlying soils (Duch, 1988; Espinosa et al., 1998; Perry et al., 1989); the sascab, a subsurface nonindurated soter limestone with high porosity (Duch, 1988; Espinosa et al., 1998); and the coquina, a highly fossiliferous rock with high void percentage, found above the aquifer (Espinosa et al., 1998). hese limestone layers are composed mainly of calcite without signi icant amounts of sili- cate minerals (Bautista-Zúñiga et al., 2004; Leticariu, 2005). Dissolution features are scattered within the limestone matrix, acting as preferential low paths promoting the downward movement of water and soil materials, as well as deeper root growth. Subsurface bedrock containing cavities (from small pores to caves) are thought to represent an addi- tional source of water for plants. Cavities illed with soil material (soil pockets) may provide Ground-penetraƟng radar (GPR) was used for identifying underground limestone features in Yucatan, México. Limestone layers, soil pock- ets, and caves located up to 4 m depth were clearly idenƟfied by using a 200-MHz antenna. The main sources of GPR signal aƩenuaƟon were the higher clay and water contents of soil pockets and limestone bedrock. H. Estrada-Medina, Soil and Water Sciences Program, Univ. of California, Riverside, CA 92521. Departamento de Manejo y Conser- vación de Recursos Naturales Tropicales (PROTROPICO), Campus de Ciencias Bioló- gicas y Agropecuarias (CCBA), Universidad Autónoma de Yucatán (UADY), Km 15.5 car- retera Mérida-Xmatkuil. Mérida, Yucatán, México, C.P. 97315. W. TuƩle, USDA-NRCS, NaƟonal Soil Survey Center, 100 Centennial Mall North, Lincoln, NE 68508-3866. R.C. Graham, Soil and Water Sciences Program, Univ. of California, Riverside, CA 92521. M.F. Allen, Center for ConservaƟon Biology, Univ. of California, University Ave., Riverside, CA 92521. J.J. Jiménez-Osornio, Departamento de Manejo y Conservación de Recursos Natu- rales Tropicales (PROTROPICO), Campus de Ciencias Biológicas y Agropecuarias (CCBA), Universidad Autónoma de Yucatán (UADY), Km 15.5 carretera Mérida-Xmatkuil. Mérida, Yucatán, México, C.P. 97315. *Corresponding author (hector.estrada@uady.mx). Vadose Zone J. 9:653–661 doi:10.2136/vzj2009.0116 Received 21 Aug. 2009. Published online 3 Aug. 2010. © Soil Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmiƩed in any form or by any means, electronic or mechanical, including photo- copying, recording, or any informaƟon storage and retrieval system, without permission in wriƟng from the publisher. Original Research Héctor Estrada-Medina* Wes Tu─le Robert C. Graham Michael F. Allen Juan José Jiménez-Osornio