Abstract
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
Highlights
About 7 %–12 % of the world’s continental area is covered by karst (e.g., Hartmann et al, 2014) and up to one-quarter of the earth’s population at least partially depends on drinking water from karst systems (e.g., Ford and Williams, 2007)
The elevated phase values at high (> 1 kHz) frequencies, which can be observed for all six samples, are typical electromagnetic coupling effects (Pelton et al, 1978) but do not affect our time-domain induced polarization (TDIP) measurements, due to the long initial delay before the sampling of the voltage decay starts
The main limitations regarding the applicability of seismic refraction tomography (SRT) measurement on the lake floor, which we identified in this study, are related to the specific surface conditions: on the one hand, the generation of seismic pulses was excessively labor intensive, as the steel plate bogged down into the soft lake bottom and had to be dug out after every single hit
Summary
About 7 %–12 % of the world’s continental area is covered by karst (e.g., Hartmann et al, 2014) and up to one-quarter of the earth’s population at least partially depends on drinking water from karst systems (e.g., Ford and Williams, 2007). Even though continued population growth and industrialization put pressure on these important resources in terms of both water quantity and quality, the response of karst systems to expected future climate change is still not well understood (Hartmann et al, 2014). E.g., from drillings, is not dense enough or not available at all, geophysical methods can be used to provide quasi-continuous indirect information on the subsurface geology in karst areas (Bechtel et al, 2007). Another possibility to understand climatic effects on karst water systems relies on the analysis of paleoenvironmental records (e.g., Medina-Elizalde and Rohling, 2012; VázquezMolina et al, 2016). To identify suitable drilling locations providing continuous paleoenvironmental records at a high temporal resolution, knowledge about sediment thickness and composition, depth to bedrock, and possible heterogeneities within the lake sediments is needed (Last and Smol, 2002)
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