Several recent studies have evidenced the relevance of machine-learning for soil salinity mapping using Sentinel-2 reflectance as input data and field soil salinity measurement (i.e., Electrical Conductivity-EC) as the target. As soil EC monitoring is costly and time consuming, most learning databases used for training/validation rely on a limited number of soil samples, which can affect the model consistency. Based on the low soil salinity variation at the Sentinel-2 pixel resolution, this study proposes to increase the learning database’s number of observations by assigning the EC value obtained on the sampled pixel to the eight neighboring pixels. The method allowed extending the original learning database made up of 97 field EC measurements (OD) to an enhanced learning database made up of 691 observations (ED). Two classification machine-learning models (i.e., Random Forest-RF and Support Vector Machine-SVM) were trained with both OD and ED to assess the efficiency of the proposed method by comparing the models’ outcomes with EC observations not used in the models´ training. The use of ED led to a significant increase in both models’ consistency with the overall accuracy of the RF (SVM) model increasing from 0.25 (0.26) when using the OD to 0.77 (0.55) when using ED. This corresponds to an improvement of approximately 208% and 111%, respectively. Besides the improved accuracy reached with the ED database, the results showed that the RF model provided better soil salinity estimations than the SVM model and that feature selection (i.e., Variance Inflation Factor-VIF and/or Genetic Algorithm-GA) increase both models´ reliability, with GA being the most efficient. This study highlights the potential of machine-learning and Sentinel-2 image combination for soil salinity monitoring in a data-scarce context, and shows the importance of both model and features selection for an optimum machine-learning set-up.

The Altiplano-Puna is a distinctive plateau and pristine environment to study geochemical, ecological, and cli-matic phenomena. It is the second highest plateau on Earth after Tibet and contains the largest endorheic system of South America. Nevertheless, water is a vital resource in the region, and surface water and groundwater are essential sources of drinking water for inhabitants. Environmental characterizations in the Altiplano-Puna plateau are fairly new and increasingly important due to climate change, the sustained decrease of surface water bodies (e.g. Lake Poop ' o), as well as anthropogenic and natural contaminants (e.g. arsenic) which can pollute the scarce water resources. Despite extreme and unique climatic, hydrologic, and geochemical conditions of the Altiplano-Puna plateau, some indigenous populations and biota have adapted. This Special Issue presents seven original research papers covering a wide range of topics in the Altiplano-Puna related to hydrogeochemical, climatic, and anthropogenic processes on the origin and mobility of economically important elements and contaminants, as well as environmental issues and bioindicators of ecological conditions in surface waters. Its aim is to present new and comprehensive analyses with interpretations in this extreme, yet pristine area which has been scarcely characterized from an environmental perspective, and also inspire future studies in the region.

Drinking water sources in the southeastern part of Lake Titicaca (Lower Katari Basin: LKB) and the southern part of Lake Poopo (Southern Poopo Basin: SPB) have high concentrations of arsenic (As), >10 mu g/L compared to the WHO and NB-512 guideline value. These regions belong to the Bolivian Altiplano and are characterized by a semiarid climate, slow hydrological flow, with geological formations of volcanic origin, in addition to brines and other mineral deposits. The present study is focused on comparing the geochemical processes of As in relation to the sources and mobilization in groundwater (GW) in LKB and SPB. Groundwater (GW), surface water (SW) and sediment samples were collected from both basins. The As (LKB: 0.8-288 mu g/L and SPB: 2.6-207 mu g/L), boron (B) (LKB: 96-2473 mu g/L and SPB: 507-4359 mu g/L), manganese (Mn) (LKB: 0.6-7259 mu g/L) and salinity (LKB: 125-11740 mu S/cm) were found to be higher than the WHO guideline values, which is a serious concern about the GW quality for human consumption. The dissolution and exchange of bases are the processes that govern the mineralization of GW. Load of solids and liquids of anthropogenic origin in surface water (LKB) represents an environmental problem for communities on river banks. The spatial distribution of As was attributed to the geology of both the basins and the heterogeneously distributed evaporites in the sediments. The highest As concentrations are found in alluvial sediments of the northern region of LKB and "PACK belt" (an approximately 25 km long belt stretching along the southern shores of the Lake Poopo, between the villages of Pampa Aullagas and Condo K) in SPB. Sequential extraction of sediment and mineral saturation indices indicate that iron (Fe) and aluminum (Al) oxides as well as hydroxides are the most predominant mineral phases as potential sorbents of As.

Arsenic (As) contamination of drinking water is a world-wide problem. The natural origin of As, its mobility and transport are of great interest in Bolivian Altiplano due to its presence in mineral ...

The challenge for many communities in Latin America is to find adequate solutions which are feasible given the local economic and technical conditions and which enable them to source water with arsenic concentrations below the WHO guideline value for drinking water (<10 μg/L) of arsenic (As) pollution, suitable for human consumption and the irrigation of crops. Three regions where geothermal fields are present were selected for study out of the several hundred locations in Latin America where the water environment is contaminated with As and where there is a critical water shortage problem. These are Cerro Prieto in Mexico, Momotombo in Nicaragua and Lake Poopó in Bolivia. The paper presents the results of research on the use of low-enthalpy geothermal energy sources and waste heat from geothermal power plants in membrane distillation (MD) processes, which is the only heat-powered membrane technology, in order to obtain potable water and/or water for crop irrigation. It was concluded that MD could be considered as a solution for obtaining water of good quality with a high retention of toxic solutes such as As as well as other different species found in groundwater. In addition, it is not only geothermal energy, but also the geothermal water itself that can be considered as a source of freshwater produced through the MD process, a process which is most suitable to be used in areas where cheap sources of heat are available.

Spatial patterns, cluster or dispersion trends are statistically different from random patterns of trace elements (TEs), which are essential to recognize, e.g., how they are distributed and change their behavior in different environmental processes and/or in the polluted/contaminated areas caused by urban and industrial pollutant located in upstream basins and/or by different natural geological conditions. The present study focused on a statistical approach to obtain the spatial variability of TEs (As, B and Sb) in shallow groundwater (GW) in a high-altitude arid region (Lower Katari Basin, Bolivian Altiplano), using multivariate analysis (PCA and HCA), geochemical modeling (PHREEQC, MINTEQ) and spatial analyses (Moran's I and LISA), considering the community supply wells. The results indicate that despite of the outliers there is a good autocorrelation in all cases, since Moran's I values are positive. The global spatial dependence analysis indicated a positive and statistically significant spatial autocorrelation (SA) for all cases and TEs are not randomly distributed at 99% confidence level. The results of hydrochemical modeling suggested the precipitation and stability of Fe (III) phases such as goethite. The re-adsorption of As and Sb on the mineral surface in the aquifer could be limiting the concentrations of both metalloids in southern regions. Spatial autocorrelation was positive (High-High) in northwestern (arsenic), southeastern (boron) and northeastern (antimony) region. The results reflected that the As and Sb are the main pollutants linked to the natural geological conditions, but B is a main pollutant due to the anthropogenic activities. Furthermore, >50% shallow groundwater exceeded the WHO limit and NB-512 guideline values for Sb (87%), B (56%) and As (50%); therefore the spatial distribution and concentrations of these TEs in GW raise a significant concern about drinking water quality in the study area.

Elevated concentrations of arsenic in water supplies represent a worldwide health concern. In at least 14 countries of South America, high levels have been detected relative to international standards and guidelines. Within these countries, the high plateau referred to as the "Altiplano-Puna", encompassing areas of Argentina, Bolivia, Chile, and Peril, exhibits high arsenic concentrations that could be affecting 3 million inhabitants. The origins of arsenic in the Altiplano-Puna plateau are diverse and are mainly natural in origin. Of the natural sources, the most important correspond to mineral deposits, brines, hot springs, and volcanic rocks, whereas anthropogenic sources are related to mining activities and the release of acid mine drainage (AMD). Arsenic is found in all water types of the Altiplano-Puna plateau over a wide range of concentrations (0.01 mg.L-1 < As in water > 10 mg.L-1) which in decreasing order correspond to: AMD, brines, saline waters, hot springs, rivers affected by AMD, rivers and lakes, and groundwater. Despite the few studies which report As speciation, this metalloid appears mostly in its oxidized form (As[V]) and its mobility is highly susceptible to the influence of dry and wet seasons. Once arsenic is released from its natural sources, it also precipitates in secondary minerals where it is generally stable in the form of saline precipitates and Fe oxides. In relation to human health, arsenic adaptation has been detected in some aboriginal communities of the Puna together with an efficient metabolism of this metalloid. Also, the inefficient methylation of inorganic As in women of the Altiplano might lead to adverse health effects such as cancer. Despite the health risks of living in this arsenic-rich environment with limited water resources, not all of the Altiplano-Puna is properly characterized and there exists a lack of information regarding the basic geochemistry of arsenic in the region. (C) 2019 Elsevier B.V. All rights reserved.

High groundwater arsenic (As) across the globe has been one of the most well researched environmental concerns during the last two decades. Consequently, a large scientific knowledge-base has been developed on As distributions from local to global scales. However, differences in bulk sediment As concentrations cannot account for the As concentration variability in groundwater. Instead, in general, only aquifers in sedimentary basins adjacent to mountain chains (orogenic foreland basins) along continental convergent tectonic margins are found to be As-enriched. We illustrate this association by integrating observations from long-term studies of two of the largest orogenic systems (i.e., As sources) and the aquifers in their associated foreland basins (As sinks), which are located in opposite hemispheres and experience distinct differences in climate and land-use patterns. The Andean orogenic system of South America (AB), an active continental margin, is in principle a modern analogue of the Himalayan orogenic system associated with the Indus-Ganges-Brahmaputra river systems in South Asia (HB). In general, the differences in hydrogeochemistry between AB and HB groundwaters are conspicuous. Major-solute composition of the arid, oxic AB groundwater exhibits a mixed-ion hydrochemical facies dominated by Na-Ca-Cl-SO4-HCO3. Molar calculations and thermodynamic modeling show that although groundwater of AB is influenced by cation exchange, its hydrochemical evolution is predominated by feldspar dissolution and relationships with secondary clays. In contrast, humid, strongly reducing groundwater of HB is dominated by Ca-HCO3 facies, suggestive of calcite dissolution, along with some weathering of silicates (monosiallitization). This work demonstrates that although hydrogeochemical evolutionary trends may vary with local climate and lithology, the fundamental similarities in global tectonic settings can still lead to the elevated concentrations of groundwater As.

The southeastern part of the Titicaca Lake near the Cohana Bay in the Bolivian Altiplano, has environmental problems caused mainly by urban and industrial wastes upstream of the Katari Basin and by ...

Hydrochemical investigations of groundwater and surface water were carried out to better understand the spatial distribution of As, major ions and trace elements. The study was carried out to evaluate the sources of dissolved species and elucidate the processes that govern the evolution of natural water in the Lower Katari Basin. The study area is close to the Titicaca Lake (Cohana Bay) formed by sediments of the Quaternary system, deposited in the fluvio-glacial to fluvio-lacustrine environment and geologic formations of the Devonian and Neogene system of volcanic origin. The study area has several environmental problems mainly caused by contaminants such as heavy metals, nutrients, and bacteria. These problems are linked to the urban and industrial wastes, natural geologic conditions, and mining activities carried out upstream of the Katari Basin, where rivers discharge into the Cohana Bay. A total of 37 water samples were collected during wet season, 31 groundwater samples including drinking water wells and six surface water samples. The hierarchical cluster analysis and principal component analysis were applied to hydrochemical data. Results show high salinity in groundwater related to the evaporation causing serious problems for the groundwater quality and rendering it unsuitable for drinking. Dissolved As concentration ranges from 0.7 to 89.7 mu g/L; the principal source of As could be the alteration of volcanic rocks, more than 48% of the shallow groundwater samples exceeded the WHO guideline value for As and more than 22% for NO3-. Groundwater has neutral to slightly alkaline pH, and moderately oxidizing character. The groundwater chemistry reveals considerable variability, ranging from Na-SO4,Cl type through mixed Na-HCO3 type and Ca,Na-HCO3,Cl type. The distribution of trace elements shows a large range of concentrations. Speciation of As indicates that the predominant oxidation state is As (V). The geochemical modelling indicates that As could be associated with iron oxides and hydroxides which are probably the most important mineral phases for the As adsorption. The spatial distribution and the variation of dissolved As concentration in groundwater is governed by the variability in geological characteristics of the region that raises a significant concern about drinking water quality.

Se ha realizado un análisis hidroquimico de 18 pozos ubicados en la zona periurbana oeste de la ciudad de Cochabamba. También se ha efectuado una evaluación de la viabilidad técnica de diferentes procesos de remoción de arsénico en función de los parámetros físicos y químicos de calidad de las aguas provenientes de los pozos más contaminados. Se han logrado los siguientes resultados importantes: La tendencia de evolución geoquímica hacia el tipo de agua Na-Ca-HCO3 puede ser causada por la mineralización y/o disolución de minerales tipo halita [NaCl], dolomita [CaMg(CO3)2], calcita [CaCO3] y/o magnesita [MgCO3]. El As disuelto presenta concentraciones variables en 3 órdenes de magnitud, desde 3 hasta 581,7 µg/L, lo que supone un alto riesgo a la salud por exposición. La variabilidad espacio-temporal en las concentraciones de arsénico se explica por la variabilidad litológica del lugar, donde las diferentes capas de arcilla, limo y/o arena contendrían composiciones heterogéneas y variables de As. La correlación positiva moderada entre el arsénico y el hierro presentes sugiere la disolución y/o mineralización de fases amorfas de oxi-hidróxido de hierro asociadas con el arsénico haciendo posible su movilización del As hacia el acuífero subterráneo. Las características físico-químicas de casi todos los pozos altamente contaminados, favorecen la viabilidad técnica de la mayoría los procesos comparados: coagulación con Fe(III) y Al(III), adsorción sobre alúmina activada, intercambio iónico, filtración en lechos de arena recubierta con óxido de hierro (IHE-ADART), Remoción de Arsénico Asistida por Oxidación Solar, RAOS, nano-filtración y osmosis reversa. El pozo con mayor contaminación y mayor salinidad presenta condiciones más desventajosas para su tratabilidad, siendo los procesos de membrana los más viables técnicamente; sin embargo, los procesos IHE-ADART, adsorción sobre alumina activada y RAOS también son potencialmente viables para este caso. En cada una de las OTBs afectadas, será necesario analizar comparativamente la viabilidad económica de la implantación de los procesos en función de sus costos de inversión, operación y mantenimiento, considerando una capacidad adecuada para la población servida.

Groundwaters from shallow aquifers and surface water from rivers of the southern part of Poopó Lake basin within the Bolivian Altiplano have significant quality problems such as high salinity and high concentrations of arsenic (As). The extent of As contamination is observed in the studied groundwater over large parts of the study area. Surface-waters are generally alkaline (pH 8.2–8.7) and oxidizing with dissolved oxygen (DO) concentrations in a range of 2.5–6.6mg/L The water chemistry is predominantly of Na–Cl–HCO3–type, with concentrations of dissolved As in the range of 8.6–117µg/L with As(V) as the main aqueous species. The concentration of Li varies in the range of 1.1–4.4mg/L, while other trace elements occur in low concentrations.

Numerous hot springs and fumaroles occur along the Andes Mountains, in the Bolivian Altiplano, where people use thermal springs for recreational purposes as pools, baths and also for consumption as drinking water and irrigation once it is mixed with natural surface waters; most of these thermal springs emerge from earth surface and flow naturally into the rivers streams which drain further into the Poopo Lake. Physicochemical characteristics of the thermal water samples showed pH from 6.3 to 8.3 with an average of 7.0, redox potential from +106 to +204 mV with an average of +172 mV, temperatures from 40 to 75 degrees C with an average of 56 degrees C and high electrical conductivity ranging from 1.8 to 75 mS/cm and averaged 13 mS/cm. Predominant major ions are Na+ and Cl- and the principal water types are 37.5% Na -Cl type and 37.5% Na-Cl-HCO3 type. Arsenic concentrations ranged from 7.8 to 65.3 mu g/L and arsenic speciation indicate the predominance of As(III) species. Sediments collected from the outlets of thermal waters show high iron content, and ferric oxides and hydroxides are assumed to be principal mineral phases for arsenic attenuation by adsorption/co-precipitation processes. Arsenic concentrations in cold water samples from shallow aquifers are higher than those in thermal springs (range < 5.6-233.2 mu g/L), it is likely that thermal water discharge is not the main source of high arsenic content in the shallow aquifer as they are very immature and may only have a small component corresponding to the deep geothermal reservoir. As people use both thermal waters and cold waters for consumption, there is a high risk for arsenic exposure in the area. (C) 2015 Elsevier Ltd. All rights reserved.

Environmental settings in the southern area of Lake Poopó in the Bolivian highlands, the Altiplano, have generated elevated amounts of arsenic (As) in the water. The area is characterised by a semiarid climate, slow hydrological flow and geologic formations of predominantly volcanic origin. The present study aimed at mapping the extent of the water contamination in the area and to investigate the geogenic sources and processes involved in the release of As to the groundwater. Ground- and surface-water samples were collected from 24 different sites, including drinking water wells and rivers, in the southern Poopó basin in two different field campaigns during the dry and rainy seasons. The results revealed variable levels of As in shallow drinking water wells and average concentration exceeding the WHO guidelines value. Arsenic concentrations range from below 5.2 μg/L (the detection level) to 207 μg/L and averages 72 μg/L. Additionally, high boron (B) concentrations (average 1902 μg/L), and high salinity are further serious concerns for deteriorating the groundwater quality and rendering it unsuitable for drinking. Groundwater is predominantly of the Na-Cl-HCO3 type or the Ca-Na-HCO3 type with neutral or slightly alkaline pH and oxidising character. While farmers are seriously concerned about the water scarcity, and on a few occasions about salinity, there are no concerns about As and B present at levels exceeding the WHO guidelines, and causing negative long term effects on human health. Sediment samples from two soil profiles and a river bed along with fourteen rock samples were also collected and analysed. Sequential extractions of the sediments together with the calculation of the mineral saturation indices indicate that iron oxides and hydroxides are the important secondary minerals phases which are important adsorbents for As. High pH values, and the competition of As with HCO3 and dissolved silica for the adsorption sites probably seems to be an important process for the mobilisation of As in the shallow groundwaters of the region. Continuous monitoring and expansion of monitoring systems are necessary prerequisites for better understanding of the pattern of As mobilisation in the Southern Poopó Basin.

Natural arsenic occurrence and its removal from drinking water using a tubular photo-reactor enhanced with a solar concentrator in Cochabamba, Bolivia

This study deals with the chemical quality of water samples taken from manually constructed wells with depths between two to nine meters. Almost all well-waters are used for consumption as drinking and irrigation water, especially during dry season. The wells are located around the Poopo Lake, situated in the central part of the Bolivian Altiplano (BA). The wells are mostly shallow and ontaminated by arsenic (As) and other trace metals from natural and anthropogenic sources. The north east side of the lake is a semiarid area where strong mining activities are carried out since last century. The west south side of the lake is an arid area where agricultural and cattle activities are carried out. Due the mining and geothermal sources, rivers, soils and some wells in the semiarid area are polluted by trace metals. Few rivers in the arid area are seasonally used for irrigation and become scarce or disappear before reaching the lake and many wells become dry as well. Detailed hydrochemical analyses of the well waters around the Poopo Lake reveal elevated As concentrations in almost all wells in the region.

Water management in semiarid and arid catchments such as the Poopó Lake Basin requires improved understanding of the complex behavior of the various contaminants, which affect the drinking water quality and considered as crucial for sustainable development of the region. Mechanisms of arsenic (As) release in the surface and groundwater were studied. Hydrochemical data for surface water (4 samples) and groundwater (28 samples) were collected in a small watershed in the Poopó catchment at the highland of the Bolivian Andes (Altiplano). All of them show high electrical conductivity values and moderately oxidizing conditions. The surface water contains high concentration of sulfate and the trace elements As, Zn and Pb in the zone affected by acid mine drainage. There is a large variability of the concentration of As and of the trace elements in the groundwater in the five different regions within the Poopó catchment. The metal concentrations sensitive to changes of redox state and results of speciation modeling suggest that As (V) is a predominant aqueous species, which conforms to the prevailing oxidizing conditions in the shallow groundwater environment. Two generalized trends for As distribution were identified in groundwater: (a) high concentrations are found in the arid zone (100–250 μg/L) in the southern (region III) and in the northwestern (region V) regions, and (b) low concentrations (<50 μg/L) are found in the remaining part of the basin (region I, II and IV). However, the spatial distribution within these regions needs to be investigated further. A conclusion from the present study is that there are multiple sources of As as well as other trace elements (such as Cd, Mn and Zn) in the Poopó Lake Basin. Among the sources and the processes which led to the mobility of As and other trace metals in the region are: (a) weathering of sulfide minerals, (b) oxidation of pyrite and/or arsenopyrite in mineralized areas and (c) desorption from hydrous ferric oxide (HFO) surfaces. In non-mining areas, volcanic ash is suggested to be a significant source of As.

Se han construido y caracterizado cuatro foto-reactores tubulares de sección semi-circular, y se aplicaron al tratamiento de aguas subterráneas contaminadas con As(V), utilizando las técnica de la Remoción de Arsénico por Oxidación Solar (RAOS). Los concentradores solares fueron construidos reciclando materiales desechados: tubos de vidrio proveniente de lámparas de Ne, tubos de desagüe sanitario de 6” (PVC) y láminas de hojalata comercial, recubiertos por láminas de aluminio. Los diámetros de los foto-reactores fueron de 15 cm, 31,7 cm, 47,6 cm y 70,7 cm. Poseen una capacidad de radiación equivalente a 2,47, 4,73, 6,88 y 10,02 soles, respectivamente. Pruebas simultáneas sin agitación en los cuatro foto-reactores, mostraron que la remoción de As en el foto-reactor de mayor diámetro es más rápida, con remociones mayores al 80% en todos los casos. Los tiempos de crecimiento de los flóculos hasta tamaños mayores a 0,45 m (tamaño del microfiltro) fueron de 2-3, 3-4, 4-5 y 6-8 min para los foto-reactores de 71, 48, 32 y 15 cm de diámetro, respectivamente, para intensidades de radiación UVA integral incidente (290-390 nm) entre 56,8 y 59,5 Wm-2. Pruebas de irradiación seguidas de agitación controlada a 18-22 s-1 de gradiente de velocidad, mostraron que el foto-reactor de diámetro mayor acelera el proceso de formación de flóculos fácilmente sedimentables (0,5 – 1,0 mm de tamaño e índices de Willcomb de 6-8). Los tiempos de irradiación para los foto-reactores de 15 cm de diámetro (intensidad de radiación UVA efectiva de 141 Wm-2), 32 cm de diámetro (274 Wm-2), 48 cm de diámetro (409 Wm-2) y 71 cm de diámetro (569 Wm-2), fueron 15, 7,5, 5 y 3 min, respectivamente. Los tiempos de aparición del flóculo durante la agitación fueron inmediatos en todos los casos. La relación empírica entre las velocidades de crecimiento de flóculos hasta 0,5 mm y las intensidades de radiación UVA efectiva es lineal (r = 0,990), en el intervalo experimental estudiado. Esto significa que la generación de radicales libres en la solución, propiciada por la cantidad de fotones de diapasón UVA, es todavía la etapa limitante del proceso global de formación de precipitados de Fe(OH)3. Cálculos de capacidad de tratamiento, en régimen continuo (considerando tiempos de residencia hidráulica iguales a los tiempos de irradiación), demuestran la mayor capacidad del foto-reactor de 71 cm de diámetro, logrando un flujo diario de 190 Lm-2 para una operación de 5 h por día. Desde el punto de vista económico y de su construcción, este foto-reactor es más práctico que los reactores de menor diámetro, por la menor cantidad de accesorios y materiales involucrados.