J. Benavente

Chemical evolution of groundwater in Triassic gypsum-bearing carbonate aquifers (Las Alpujarras, southern Spain)

A hydrochemical study employing modelling techniques, was carried out using samples taken at 65 points (springs and wells) in Triassic carbonate aquifers (Lujar-Gador Unit, Alpujarride Complex, Betic Cordillera). These aquifers are made up of limestones and dolomites with some gypsum scattered or interbedded. Though the area is semi-arid, recharge is relatively high because of their mountainous nature. The carbonate rocks contain dense microfissuration; the groundwater flow regime is predominantly diffuse. The karstic forms are in general poorly developed. Two main hydrochemical processes have been identified in these aquifers. One is incongruent dissolution of dolomite that determines the chemical composition of the less mineralised water. The other is dedolomitisation (dolomite dissolution together with calcite precipitation caused by dissolution of gypsum), which becomes predominant when the flow encounters interbedded gypsum. This reaction is also frequently associated with low temperature thermalism, and can play a part in more intense local karstification (cavities, sinkholes, high transmisivity in wells) observed in the sectors of these aquifers where gypsum is more abundant. A reaction path model has been used to simulate the geochemical processes through a hypothetical aquifer (with similar lithology to the Alpujarride carbonate aquifers). Successive stages of evolution through the carbonate sequence, represented by different saturation states with respect to calcite, dolomite gypsum and CO2, have been modelled and then compared with the field data.

Characterizing Abandoned Mining Dams by Geophysical (ERI) and Geochemical Methods: The Linares-La Carolina District (Southern Spain)

The mining exploitation of metallic sulphides, together with the activities associated to the mineral treatment and smelting, when maintained through centuries due to the wealth of the ores, generate important accumulations of wastes in structures of different kind of tailing dams and ponds, for instance. When no previous corrective steps are taken, as usually happens in old exploitations, this means a serious risk of environmental pollution, due to the mobilisation of heavy metals. The present study has been carried out in a mining district, actively exploited during the last two millennia, that was the first world’s producer of lead during some periods (Linares-La Carolina, southern Spain). In this district, the mining activity was associated to a philonian network of metallic sulphurs and ended by the 1980s of the past century. The ancient mining operations, mostly subterranean, have generated large accumulations of residues without any prior corrective action. Therefore, this work intends to characterise these mining dams and determine the influence of these mining wastes on the quality of surface and ground waters. With this goal, three structures that store the mining refuse of different mineralogical origin have been selected. First, a geochemical characterisation of the soil was performed in the area surrounding each of the structures. In all cases, high levels of trace elements (including Pb, Zn, Cu, Cd, Mn, As, Sb and Ba) were observed. A hydrochemical study revealed the mobilisation through the aqueous medium of certain contaminants from the leachate of these ancient accumulations; these contaminants will flow to the streams that drain the area or to the aquifers of the sector. The internal characterisation of these structures was performed with geophysical techniques, specifically electrical resistivity imaging (ERI). The six generated resistivity models have allowed the identification of the morphology of the structures, variations in the vertical and horizontal distribution of the deposited material, fracture zones, water content and reload–unload zones and the contact of the mining wastes with the substrate. Thus, the ERI study confirms the lack of impermeabilisation measures for the terrain in the spill zones in all three cases, which indicates a high risk of contamination of the soil and waters. The obtained images also permit the identification of the ideal positions to conduct future borehole controls.

Isotopic (13C) Signature of CO2 Sources in the Vadose Zone of a Mediterranean Karst (Nerja Cave Site, Southern Spain)

This study is based on in situ measurements of the soil and the vadose zone (<60 m) in a Mediterranean karst experimental site near Nerja Cave (a show cave in dolomite marbles in South Spain). CO2 concentrations in depth were registered in boreholes drilled in experimental site. The CO2 content generally increases with depth. Measurements indicate average vadose air CO2 concentrations of nearly 40,000 ppm, with a maximum of nearly 60,000 ppm. In this context, the cave itself appears to be a vadose subsystem above the groundwater level, with significantly lower CO2 concentrations (a few thousands of ppm maximum) due to its ventilation. The δ13C–CO2 data of the vadose air point to an origin of the gas mainly related to microbiological processes associated to the consumption of dissolved organic matter in the groundwater surface. This gas can diffuse or flow laterally, upward or downward through karst conduits. Interactions between air masses of surface origin (relatively dry, with variable temperature and low CO2 content) and typical vadose attributes (relatively high CO2 content, near-saturated humidity and 21 °C temperature) produce clear ascendant or descendant air fluxes inside the boreholes, especially those that cross significant karst voids.

A Field Analog of CO2-Closed Conditions in a Karstified Carbonate Aquifer (Nerja Cave Experimental Site, South Spain)

We present new data that illustrate the hydrochemical evolution of groundwater along a flow line in the Triassic marbles around the Nerja Cave, South Spain. Water dissolves calcite and dolomite, and then CaSO4. The environment is locally rich in CO2 (up to near 60,000 ppmv) and consequently the water increases significantly its content in Ca2+, Mg2+, HCO3 − and SO4 2− along the flow, with EC values between 500 and 900 μS/cm. The pH values are typically in the 7–8 range, and the equilibrium PCO2 of the water varies between 10−1.5 and 10−2.5 atm. In the considered flow line there is a relatively deep borehole (S2: 380 m; 280 m saturated) that shows pH values around 10 and equilibrium PCO2 of 10−6 atm, with EC values generally in the 150–200 μS/cm range. Most of its solutes derive from rainwater concentration, together with the dissolution of carbonate minerals in a system closed to CO2. For this reason we consider S2 to be a field analog of such conditions. The nearly stagnant water of this well also shows evidence of sulphate reduction. Unlike its solute contents, isotopically (δ2H and δ18O) the water of S2 does not show any modification with respect to the other points along the flow line.

Electrical resistivity imaging survey to detect uncharted mine galleries in the mining district of Linares, Jaén, Spain

The scarcity of information about the existence of old mining shafts and galleries in urban areas is an important issue for future urban development. Electrical resistivity tomography is a non-destructive geophysical technique that can detect and characterize such subsurface cavities based on differences in the behaviour of electrical current in the void and in the embedding rock. Here we present a study in which this technique was used to determine the location of old engineered structures around the city of Linares, southern Spain, and to relate these structures to the abandoned deep mines present in the area. Eight electrical resistivity imaging profiles were performed, with a total of 22 808 measurements. Correlations between geoelectrical anomalies allow detection of the depth and the direction of several galleries, as well as the voids that result from mining extraction. Given the depth at which these structures are located (in some cases less than 5 m), they pose an important risk for future construction projects in areas of urban expansion. This technique is shown to be a useful tool for locating areas that pose important urban risks and, by extension, for the decision-making process in territorial planning, especially in areas with a history of deep mining.

Highlighting the importance of transitional ventilation regimes in the management of Mediterranean show caves (Nerja-Pintada system, southern Spain)

This study shows the utilization of the air CO2 exhaled by a very high number of visitors in the Nerja Cave as both a tracer and an additional tool to precisely evaluate the air circulation through the entire karst system, which includes non-touristic passages, originally free of anthropogenic CO2. The analysis of the temporal - spatial evolution of the CO2 content and other monitoring data measured from January 2015 to December 2016 in the Nerja-Pintada system, including air microbiological controls, has allowed us to define a new general ventilation model, of great interest for the conservation of the subterranean environment. During the annual cycle four different ventilation regimes and two ventilation modes (UAF-mode and DAF-mode) exist which determine the significance of the anthropogenic impact within the caves. During the winter regime, the strong ventilation regime and the airflow directions from the lowest to the highest entrance (UAF-mode) contribute to the rapid elimination of anthropogenic CO2, and this affects the whole karstic system. During the summer regime the DAF-mode ventilation (with airflows from the highest to the lowest entrances) is activated. Although the number of visitors is maximum and the natural ventilation of the karstic system is the lowest of the annual cycle, the anthropogenic impact only affects the Tourist Galleries. The transitional ventilation regimes -spring and autumn- are the most complex of the annual cycle, with changing air-flow directions (from UAF-mode to DAF-mode and vice versa) at diurnal and poly diurnal scale, which conditions the range of the anthropogenic impact in each sector of the karst system. The activation of the DAF-mode has been observed when the temperature difference between the external and air cave is higher than 5°C.

Identification of Vadose Karst Voids and Ventilation Patterns Coupling Hydrochemical and Geophysical Methods (Maro Spring, Near Nerja, Southern Spain)

The Maro Spring, near Nerja, southern Spain, drains a Triassic carbonate aquifer. It presents a typical karstic behaviour. We have compiled more than 220 pH values of water from this spring and a nearby well. The spatial variation of this variable suggests a CO2 degassing effect as far as the groundwater flow gets near the spring. The average pH seasonal variation indicates a maximum during autumn and a minimum in spring. This can be explained by water degassing related to the start of the period of convective ventilation in vadose voids, as happens in the important Nerja Cave located nearby. There also seems to be an effect of downward gas diffusion when the soil respiration is at its peak and the ventilation of the vadose voids is nearly inhibited. These circumstances suggest that Maro Spring water can be spatially linked with unknown cavities. The second phase of this study is the identification of some of these voids by the way of combination of two gravity profiles of 200 m, one designed as a test of the method and the other for prospecting the voids. The link between hydrochemical and geophysical methods is the main aim of this study as such a transdisciplinary approach is not frequently used in hydrogeology.

Object-Oriented Modelling as a Decision-Making Tool in Agriculturally Overexploited Karstic Aquifers

Object-Oriented Bayesian Networks (OOBNs) utilise the power of Object-Oriented Programming (OOP) and offer a novel approach to the problems of integrated water management. This paper describes the building of an OOBN Decision Support System (DSS) that allows complex domains to be described in terms of inter-related objects. Thus, the DSS structure is able to represent an accurate reflection of a complex real-world water system made for an aquifer that has been used as an example of a successful application. In this research, conventional Bayesian Networks (BNs) are used to describe the probabilistic relationships between variables (objects) within each network. A network is a group of objects that can be described as a class. Different classes can possess similar sets of objects and be linked through other networks having common variables. Classes inherit commonly used states and behavior from other classes in a hierarchical way. This model of networks represents a participatory DSS for helping water managers.