Integrated land and water-borne geophysical surveys shed light on the sudden drying of large karst lakes in southern Mexico
Authors: Bücker Matthias , Flores Orozco Adrián , Gallistl Jakob , Steiner Matthias , Aigner Lukas , Hoppenbrock Johannes , Glebe Ruth , Morales Barrera Wendy , Pita de la Paz Carlos , García García César Emilio , Razo Pérez José Alberto , Buckel Johannes , Hördt Andreas , Schwalb Antje , Pérez Liseth 
Published: February 2021

Abstract. Karst water resources play an important role in drinking water supply but are
highly vulnerable to even slight changes in climate. Thus, solid and
spatially dense geological information is needed to model the response of
karst hydrological systems to such changes. Additionally, environmental
information archived in lake sediments can be used to understand past climate
effects on karst water systems. In the present study, we carry out a
multi-methodological geophysical survey to investigate the geological
situation and sedimentary infill of two karst lakes (Metzabok and Tzibaná)
of the Lacandon Forest in Chiapas, southern Mexico. Both lakes present large
seasonal lake-level fluctuations and experienced an unusually sudden and
strong lake-level decline in the first half of 2019, leaving Lake Metzabok
(maximum depth ∼25 m) completely dry and Lake Tzibaná (depth
∼70 m) with a water level decreased by
approx. 15 m. Before this event, during a lake-level high stand in
March 2018, we collected water-borne seismic data with a sub-bottom profiler
(SBP) and transient electromagnetic (TEM) data with a newly developed floating
single-loop configuration. In October 2019, after the sudden drainage event,
we took advantage of this unique situation and carried out complementary
measurements directly on the exposed lake floor of Lakes Metzabok and
Tzibaná. During this second campaign, we collected time-domain induced
polarization (TDIP) and seismic refraction tomography (SRT) data. By
integrating the multi-methodological data set, we (1) identify 5–6 m
thick, likely undisturbed sediment sequences on the bottom of both lakes,
which are suitable for future paleoenvironmental drilling campaigns, (2)
develop a comprehensive geological model implying a strong interconnectivity
between surface water and karst aquifer, and (3) evaluate the potential of the
applied geophysical approach for the reconnaissance of the geological
situation of karst lakes. This methodological evaluation reveals that under
the given circumstances, (i) SBP and TDIP phase images consistently resolve
the thickness of the fine-grained lacustrine sediments covering the lake
floor, (ii) TEM and TDIP resistivity images consistently detect the upper
limit of the limestone bedrock and the geometry of fluvial deposits of a river
delta, and (iii) TDIP and SRT images suggest the existence of a layer that
separates the lacustrine sediments from the limestone bedrock and consists of
collapse debris mixed with lacustrine sediments. Our results show that the
combination of seismic methods, which are most widely used for lake-bottom
reconnaissance, with resistivity-based methods such as TEM and TDIP can
significantly improve the interpretation by resolving geological units or
bedrock heterogeneities, which are not visible from seismic data. Only the use
of complementary methods provides sufficient information to develop
comprehensive geological models of such complex karst environments