Innovative Geophysics for Hydrologic Parameter Aquisition

The proposed project is part of an international cooperation submitted in the D-A-CH program entitled "Geometry, genesis and dynamic of the Yucatan karstic system". The project involves three teams: the University of Neuchâtel which will coordinate the project, the Geological Survey of Austria which has extensive experience in geophysical exploration in the region, and the Centro del Agua in Mexico which has a strong finite element modelling expertise. The aim of the project is to better understand the formation and behaviour of large and complex karstic systems. This implies the development of new measurement and field methods, as well as new modelling techniques allowing a detailed analysis of field observations and hypothesis testing. The field site for the project will be the Yucatan peninsula in Mexico, since it is one of the largest karstic systems in the world and because a large amount of data is already available even if, as outlined in the proposal, some key information are still missing. The research involves two main scientific questions: How has such a massive karstic system formed? How is it functioning today? These questions are not yet answered satisfactorily. The formation of the system is thought to be mainly the result of calcite dissolution enhanced by mixing of salt and fresh-water, but some authors argue that vertical heat transport in the peninsula enhances the mixing and this thermo-haline convection could be the main driving force for the formation of the vertical structures named cenote. Other authors disagree and reject the idea that heat is an important factor in the process. In terms of the current functioning of the aquifer, its detailed dynamic is not yet well understood and modelled because it involves complex interactions between freshwater, seawater, and tidal effects under a very small head gradient. All the current models of the system could not account for all those processes and they provide therefore only a general description of the overall behaviour. To go a step further, in this project it is proposed to focus on the two main aspects. It is firstly aimed at understanding the evolution of the permeability and the genesis of the karst system of the Yucatan peninsula over geological time scales. It includes in particular the understanding of the long term effect of the interaction between fresh water and salt water on the dissolution as well as the role of the thermal regime. It is then aimed at understanding the properties of the regional-scale preferential flow paths. More precisely it involves understanding the geometry of the current conduit systems and how it influences the regional flow and solute transport. The geometry is a result of the speleogenesis processes and therefore the two questions are intimately related. Trying to answer those two questions necessitates on the one hand the development of new groundwater flow and transport models and the acquisition of additional data to reduce the current lack of hydrological monitoring data on the other hand. This requires the development of new measurement techniques adapted to the special measurement environment of a karst groundwater regime for monitoring groundwater fluxes as well as electrical conductivities for high resolution halocline and mixing zone observation, quick capturing of conduit geometry under water and the setup of an extensive long term monitoring network for pressures and temperatures complemented by a multi-parameter probe. The task of the Geological Survey of Austria is the development and testing of those methods in Austria as well as coordination of the field work covering preparation, deployment of the monitoring setups and surveys. Further, the newly acquired data will be processed and integrated with the existing geophysical data base already developed in course of several field campaigns in the region conducted by the Geological Survey of Austria.

The research cooperation Xibalba focused on the karst ground water system in and around Tulum/Mexico, a small, but rapidly growing town at the Caribbean coast of the Yucatan peninsula, famous for Cenotes filled with crystal-clear water and the ancient Mayan ruins directly at the seaside. The study area comprises the town itself, part of the nearby biosphere reserve of Sian Ka An, a species rich world heritage for nature, and part of the second largest barrier reef of the world. All parts are under stress due to rapid urban expansion and water pollution. In 2007, 2008, and 2015 helicopter surveys with a special probe (AEM) have been conducted for imaging the hidden conduit network. The data showed a vast, complex network indicating high connectivity in the underground. So, pollution potentially can spread quickly everywhere. The question to answer was in short: How does this conduit network control the flow of the groundwater? This can be estimated by hydrological modelling. In the joint project Xibalba with the Geological Survey of Austria, the University of Neuchatel, Switzerland, and the NGO Amigos de Sian Kaan, Mexico, and local cave diving explorers as partners, standard and innovative measurements of ground water parameters, on the surface as well as directly in the conduits, should deliver an input data set for a stochastic groundwater modelling approach, developed by the Swiss partner. Data acquisition was done with GPS and water level measurements, chemical water analysis, borehole measurements, electrical underground tomography, tides observation, innovative laser-flow measurement and 3D-laserscanning of the shape of groundwater karst tunnels. It was found, that the AEM-method can deliver far more information about karst conduit structures embedded in saltwater as assumed so far. Advanced data processing showed structures beneath high conductive brackish lagoons as well as in the reef. For scanning the shape of submerged groundwater tunnels, a new laser optical device has been developed operable by cave divers. A new developed flux measurement system showed complex, turbulent velocity fields. The flexible hydrological modelling technique incorporates structural and hydrological parameters and simulates the ground water flow. It enables first estimates about travel times and contamination plume spreading in the groundwater. Basic result was the successful combination of innovative data acquisition methods with a new numerical modelling approach allowing incorporation of quantitative as well as structural information for understanding an important type of groundwater regime. Beyond pure science, the results provide information for the community of Tulum about the state and development of their only freshwater resource. This information would serve for raising awareness about the sensitivity of the karst water, and as fact base for decision makers and authorities controlling the process of urban and touristic development in the region.