Vol.:(0123456789) 1 3 Carbonates and Evaporites (2020) 35:67 https://doi.org/10.1007/s13146-020-00600-w ORIGINAL ARTICLE Preliminary hydrogeological observations in Phong Nha–Ke Bang National Park and Tu Lan Karst Area, Quang Binh Province, Vietnam Howard Limbert 1  · Deborah Limbert 1  · Gheorghe M. L. Ponta 2  · Nam Nguyen Xuan 3  · Florentin Stoiciu 4  · Ana‑Maria Mocioiu 4 Accepted: 21 May 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Preliminary hydrogeological and geochemical observation were conducted in the Phong Nha—Ke Bang National Park and Tu Lan Karst Area, Quang Binh Province, Vietnam to evaluate and document the hydrogeologic conditions, karst surface features, water geochemistry and limestone composition. The study area provides an excellent benchmark for karst hydro- geological investigations in central Vietnam. The data generated from this project will guide future research and conservation eforts of this unique karst landscape. Five rock samples and 20 water samples were collected in 2016. Four of the fve rock samples are pure limestones, easy to dissolve which combine with a well-developed secondary porosity, and large amount of rain, forms a good environment for karst landscape and cave formation. There is a signifcant variation between 20 study sites in water chemistry baseline, specifc conductance and temperature. The water samples had slightly acidic-to-neutral pH values (6.15–7.91), moderate specifc conductance (SC) and total dissolved solids (TDS) values 65–418 μS/cm and 32–209 milligrams per liter (mg/L), respectively. Temperatures varied from 17.9 to 22.8 ºC. ORP ranged from 68 to 194 at Phong Nha Cave outlet. Total alkalinity as CaCO 3 fuctuates between 38 mg/L and 166 mg/L and total hardness ranged between 28 mg/L and 248 mg/L CaCO 3 . All samples exhibit slightly elevated chlorite, nitrate, and sulphate levels. Keywords Hang (cave) · Karst spring · Carbonate rocks · Hydrogeology · Stable isotopes · Tower karst Introduction Vietnam is the easternmost country of the Indochinese Pen- insula (Fig. 1) with an area of 331,210 km 2 of which 40% is mountains. Carbonate rocks comprise 18.12% of the surface of Vietnam or over 60,000 km 2 (Clements et al. 2006). In February 2016, Gheorghe Ponta with Geological Sur- vey of Alabama and Nguyen Xuan Nam with the Vietnam Institute of Geoscience and Mineral Resource performed a preliminary hydrogeologic study in Phong Nha—Ke Bang National Park and Tu Lan Karst Area, Quan Binh Prov- ince, in central Vietnam and collected 5 rock samples for microscopic/chemical analyses, and 20 water samples from springs, surface waters, sinking streams (ponors), and a rimstone pool (gour), to establish a water-quality baseline (Fig. 2). A follow-up trip took place in January 2019. * Gheorghe M. L. Ponta gponta@yahoo.com Howard Limbert howard@howardlimbert.plus.com Deborah Limbert howard@howardlimbert.plus.com Nam Nguyen Xuan nam_nguyen_xuan@hotmail.com Florentin Stoiciu fstoiciu@yahoo.com Ana-Maria Mocioiu ammocioiu@imnr.ro 1 British Cave Research Association (BCRA), The Old Methodist Chapel, Great Hucklow, Buxton, UK 2 Geological Survey of Alabama, 420 Hackberry Lane, Tuscaloosa, AL 35401, USA 3 Vietnam Institute of Geoscience and Mineral Resources, 67 Chien Thang Road, Thanh Xuan District, Hanoi, Vietnam 4 R&D National Institute for Nonferrous and Rare Metals, Biruintei Blvd, 102 Pantelimon, Ilfov, Romania