Groundwater Bodies Research Articles

Identification and Mapping of Groundwater Dependent Ecosystems in the AZORES Volcanic Archipelago (Portugal)

Groundwater contributes to the maintenance of the functioning of ecosystems, through aspects related to hydrodynamics and chemical composition. Groundwater-dependent ecosystems (GDE) also offer a wide spectrum of ecosystem services to populations; therefore, their identification and mapping, which is the focus of the present paper, is of high value to environmental policies; for example, WFD envisages protecting both water bodies and GDE. An ecosystem dependence index was applied to proceed with this task in the Azores archipelago, being estimated by adding the values of three partial variables (spring density; wetlands/lakes; river baseflow) over a 10 by 10 m2 grid; with this methodology avoiding pitfalls due to lack of data. The results enabled the identification and mapping of five GDE, in Flores and São Miguel islands, supported by only three of the 28 groundwater bodies delimited in the Azores RBD. Those groundwater bodies are considered to have a good status according to the WFD requirements; thus, GDE, regardless of their typology, are not at risk of deterioration as a result of the interaction with groundwater. Nevertheless, other studies have shown that some GDE are in conflicting ecological areas and require specific management and protection measures, coupling land use and water resource planning.

Identification of sources and transformations of nitrate in Cr(VI)-impacted alluvial aquifers by a hydrogeochemical and δ15N-NO3- and δ18O-NO3 - isotopes approach.

A coupled methodology of nitrogen isotopes, hydrogeochemical characterization, multivariate statistical analysis, and SIAR Bayesian modeling has been employed to identify the sources of NO3- and N transformation processes in three alluvial aquifers (Schinos, Thiva, and Central Evia) located in central Greece where geogenic Cr(VI) co-occurs with agricultural activity and rural development. Hexavalent chromium concentrations exceed 50μg/L in many sampling stations of the studied groundwater bodies, while nitrate contamination is evident in all three study areas with concentrations well over 50mg/L. The mean δ15N-NO3- and δ18Ο-NO3- values are 6.67 ± 1.77‰ and 2.68 ± 1.77‰ in C. Evia, 8.72 ± 4.74‰ and 3.96 ± 4.57‰ in Schinos and 4.44 ± 1.71‰ and 2.91 ± 1.02‰ in Thiva, respectively. Domestic sewage and N-bearing fertilizers are contributing in various degrees to the observed nitrification which is the dominant transformation process of N in the studied aquifers. Multivariate statistics indicated that the main processes identified in the study areas are salinization, silicate dissolution, and groundwater contamination due to fertilizer use. It is suggested that ultramafic rock-related alluvial aquifers must be closely monitored in terms of nutrient inputs as an effective measure for controlling Cr(VI) release in groundwater.

Gravity fingering control on evaporation and deep drainage in a 3D porous medium

Water infiltration in soils is a fundamental hydrologic process that determines the water availability for plant roots and the rate of recharge to deep groundwater bodies. When both the initial water saturation of the soil and the water infiltration rate is low, the infiltration front becomes unstable, resulting in gravity fingers that act as preferential flow channels. Preferential flow may allow for the rapid bypass of water through the evaporation zone, thereby promoting deep drainage of water into the subsoil even in arid and semi-arid regions, where the potential evapotranspiration (PET) is high and the mean annual precipitation (MAP) is low. Here, we investigate experimentally the impact of gravity fingering on deep drainage coupled with surface evaporation in a 3D porous medium. We use a cylindrical vessel packed with dry glass beads, and reproduce seasonal climatic patterns consisting of relatively short rainfall events alternating with periods of intense evaporation. We vary the volume of rainfall events and the interval between rainfall events to replicate climate conditions. We record the mass of water in the system throughout the experiment, and determine the partition between evaporation and deep drainage before and after the onset of fingering. We find that in semi-arid conditions, the fraction of water that bypasses the evaporation zone increases fourfold after gravity fingering starts. This finding has potentially important implications for understanding infiltration dynamics in arid environments and ecosystems’ response in regions susceptible to desertification.

Isotope hydrology to provide insights into the behaviour of temporary wetlands as a basis for developing sustainable ecohydrological management strategies in Mediterranean regions

AbstractThe lack of hydrological considerations in the existing protection statutes for small temporary wetlands has recently been pointed out as the main cause of their rapidly increasing disappearance. In the present study, we aim to demonstrate the interest of using the tools of isotope hydrology to obtain a rapid and cost effective understanding of the hydrological connectivity of Mediterranean temporary wetlands in order to define the optimal environmental conditions needed to ensure their sustainability. By combining physical and isotope measurements, we have investigated the fragile balance of direct precipitation, subsurface water and groundwater influencing the hydroperiod of a typical Mediterranean temporary pond. The use of the water stable isotopes enabled us to demonstrate (i) the major role played by groundwater and subsurface water in the flooding phase of the temporary pond; (ii) the involvement of different water reservoirs in varying proportions over time to maintain the pond filled with water: precipitation, subsurface and groundwater by using d‐excess; (iii) the main role played by evaporation in starting the drying up phase; and (iv) the connection existing between a geographically isolated wetland and the regional groundwater body. These results highlight the urgent need to consider both surface and groundwater fluxes in specific protection statutes for wetlands. To this end, we recommend the use of the water stable isotopes to provide insights into the hydrological behaviour of the wetlands in order to dispose of additional cost‐effective arbitration arguments to help ensure their sustainability. The lack of hydrological considerations in existing protection statutes for small temporary wetlands has recently been pointed out as the main cause of their disappearance. Here, by combining physical and isotopes measurements, we first highlight the main role played by groundwater in the flooding phase of the temporary pond and the involvement of different water reservoirs in varying proportions over time to maintain the pond filled. These results demonstrate the connection existing between a geographically isolated wetland and the regional groundwater and highlight the urgent need to consider both surface and groundwater fluxes in specific protection statutes for wetlands.

Assessing groundwater salinity across Africa

In Africa groundwater is the principal source of drinking water (https://washdata.org/) and has significant potential to improve food production as a source of irrigation water. Although volumes of stored groundwater are high when compared to surface water, not all the available groundwater is suitable for irrigation. One of the principal factors that limit the development of groundwater is salinity, but quantitative evidence regarding the scale of salinization in Africa has been lacking. This paper presents an initial analysis of the extent of groundwater salinization in Africa, bringing together groundwater salinity data (n = 12,255) from across the continent. In our dataset c.80% of the samples have electrical conductivity values less than 2000 μS/cm. Samples with high conductivity values of more than 2000 μS/cm are restricted to limited geological and climatic environments. The data reveals salinity does not affect the water security status of most countries in Africa. However, salinity compounds water insecurity issues in arid low groundwater recharge environments. The paper discusses the role of anthropogenic processes such as river valley water resources development, extensive groundwater pumping, inter-basin groundwater transfer, and irrigation in altering the salinity of groundwater bodies. The paper further elucidates the origin of groundwater salinity by critically reviewing the natural and human-induced factors that control the salinization of the various groundwater bodies across Africa. Existing case studies reveal several causes of salinization, including i) human-induced salinization, ii) climate and hydrological change-induced paleo groundwater salinity, iii) rock dissolution, and iv), saltwater encroachment.

Configuration of quality control network of coastal groundwater bodies by model of hydrosectors specific case of Campo de Cartagena (Murcia, Spain)

The design of groundwater quality control networks must be based, fundamentally, on an index of optimal average density of sampling points per surface of groundwater body (hereinafter, MSBT), estimated at 1 point per 50 km2 , and defining some hydrosectors with homologated conditions as hexagonal cells, in each of which a selection of equivalent control stations with analytically harmonized series is made in order to be able to extend the historical series of quality data of the extractions of water sampling, to improve periodicity and monitoring through possible concatenations and / or overlaps between said simultaneous co-series obtained (between 2 or more series), and define associations of sectoral stations that maintain comparable concentrations of anthropic components, at least for a sampling period within the same hydrogeologically homogeneous sector. These concepts are accompanied by the introduction of a precise formulation through the adoption of new statistical indices that would be applied to the different masses of groundwater but, mainly, to one of the coastal MSBTs that presents one of the greatest management complexities, among other external causes, to its complicated intrinsic hydrogeological structure (heterogeneity and anisotropy), such as the Campo de Cartagena MSBT. The objective is to establish a statistical modeling framework that helps us improve the evaluation of the status and chemical quality of groundwater bodies, in general, but, this time, not only on the unique interpretation of the isolated data series of each of the control stations definable in time, but rather by evaluating the correlation between these co-series assigned to the same hydrosector with homologous conditions, as well as in the interrelation between the different hydrosectors belonging to the same aquifer, and for each of the aquifers within the same complex structure groundwater body.

The Electrochemical Detection of 4-chloro-2-methylphenoxyacetic Acid (MCPA) Using a Simple Activated Glassy Carbon Electrode

4-Chloro-2-methylphenoxyacetic acid (MCPA) is one of the pesticides most widely used to control broadleaf weeds in arable and horticultural crops and it leaches readily into groundwater bodies causing pollution. In this study, a sensor was fabricated by the simple activation of a glassy carbon electrode (GCE) in an aqueous phosphate solution by cycling the GCE between –2.0 and 2.4 V vs SCE. The activated GCE exhibited very good detection of MCPA, with a linear concentration range extending from 1 to 850 μM (cyclic voltammetry) and a limit of detection (LOD) of 0.008 μM, which was obtained using differential pulse voltammetry. A more thermodynamically favoured oxidation of MCPA was observed at the activated GCE, with an approximate shift of 110 mV in the peak potential to lower potentials. Very good reproducibility and stability were achieved, with the sensor giving similar peak currents over a 30-day immersion period. Good selectivity was achieved in the presence of nitrates, nitrites, sulfates and structurally related compounds such as nitro-phenols. The promising performance of the activated GCE in the sensing of MCPA was attributed to the generation of oxygenated functional groups and an increased surface area arising from the local dissolution of the GCE during activation.

Formation of Sediment-Hosted Opal-AG at Lightning Ridge (New South Wales, Australia): Refining the Deep Weathering Model

Formation of sedimentary opal-AG in opal fields of eastern Australia has been linked to intensive weathering of their Cretaceous host rocks—the “deep weathering” model. Here we examine possible links between weathering history as recorded in mining exposures and textural observations in thousands of opal nodules from Lightning Ridge (New South Wales, Australia) to further constrain the timing and depositional environment of opal-AG. Satellite imaging identifies river channels—now marked by pedogenic silcrete—associated with an inland river delta as the intermittent source of water that drove localized deep weathering and silicification in reactive volcanogenic sediments. Variably mineralized weathering profiles show evidence for wet/dry cycling that controlled redox and pH fluctuations critical in the conversion of detrital feldspar to kaolinite, release of silica, formation of opaline silica spheres, and opal mineralization during the initial intense phase of weathering. Subsequent less intense weathering under consistently oxidizing conditions modified the weathering profiles but produced little further opal. Textures in oriented opal nodules indicate that cavities filled rapidly under changing Eh-pH. Opal nodules formed when opaline silica spheres, nucleated and grown in perched groundwater bodies, accumulated and drained into cracks and dissolution cavities in underlying claystone. Drier conditions promoted sol-gel ripening processes that produced the solid opal. Patches and bars of precious opal-AG within common opal nodules suggest that it formed through oxidation, diffusion, and leaching in common opaline Si gel during periods of low water flux and was rate limited by the solidification of the Si gel. Ostwald ripening increased silica sphere size to produce the characteristic “play of color.” Opal formation largely ceased once acidification potential was lost or all feldspar had been converted to kaolinite.

Mapping groundwater-surface water interactions on a national scale for the stream network in Denmark

Study regionDenmark. Study focusGroundwater-surface water interaction (GSI) occurs when groundwater discharges into surface water, or surface water seeps into the groundwater. Understanding GSI aids in understanding the quality and quantity of water and the ecosystem's health, assisting water resource management and the protection of land and aquatic environment. This paper presents a classification mapping routine to classify the Danish stream network into an established GSI typology framework. We mapped three classes for GSI contact types: connected clayey, connected sandy, and disconnected, and three classes for riparian flow paths: diffuse and/or overland, direct, and artificial drainage. New hydrological insights for the regionResults revealed that about 85% of Denmark's streams are connected to groundwater with approximately equal portions in sandy and clayey subsoil areas. Also, about 87% of these streams exchange water with groundwater, either as the only flow path or combined with other flow paths. Furthermore, 41% and 19% of stream sections receive riparian flow water through artificial drainage and diffuse/overland flow paths as the dominant flow path or combined. These numbers correspond to Denmark's land use, which is approximately 60% agricultural, restricting runoff potential, and half of which is artificially drained. We anticipate that the maps will assist policymakers in controlling water and pollutant transfer from the uplands to streams to attain good environmental status for surface and groundwater bodies.

Interpretation of the anomalous groundwater chemistry and 234U/238U activity ratio disequilibrium in the northern part of the baltic region

The anomalous isotope-hydrogeochemistry phenomena in the groundwater of Estonian Cambrian-Vendian (Ediacaran) and Ordovician-Cambrian aquifer systems were formed in the Late-Middle Pleistocene. In the periglacial environment, in northern and northwestern Estonia, these aquifer systems with fracture porose crystalline basement are connected to hydraulically joint unit characterized by high radioactivity groundwater. A significant alteration of groundwater occurred by series of isotope and chemistry facies fractionation. In this study, uranium isotopes activity ratio (234U/238U), 4He content, isotope-hydrogeochemistry and adjusted 14C ages are coupled for a new prospect of the estimation of northern Baltic Basin groundwater evolution. Analyzing radiocarbon and 4He groundwater residence time results and uranium isotope activity ratio distribution suggests a prolonged periglacial environment in which groundwater evolved. Stable isotope ratios of δ18O and δ2H correlation and hydrochemical composition changes support the cryogenic origin of groundwater. Pleistocene glaciations cyclically affect groundwater in multiple ways: permafrost isotope-geochemistry partitioning; periodically changing reversed flow directions of recharge and discharge areas; oscillations of the sea, river system, and periglacial lakes level, surface and sub-permafrost water mixing via taliks and fractured basement rocks. These processes lead to forming the sequence of isotope-hydrogeochemistry types and specific zoning; in general, two separate groundwater fractions – brackish in the lower part and freshened above. An extensive groundwater exploitation on the northern coast sites influenced a sharp dysfunction in the groundwater body, destabilizing the natural equilibrium state formed in the Holocene and Pleistocene.

Assessment of Risk and Social Impact on Groundwater Pollution by Nitrates. Implementation in the Gallocanta Groundwater Body (NE Spain)

Groundwater is an essential resource for humans concerning freshwater supply; therefore, preserving and protecting its quality is necessary. Risk assessment, based on hazard, intrinsic vulnerability information and mapping, may be considered as a key aspect of sustainable groundwater management. An approach has been made by combining the Nitrogen Input Hazard Index and the hydrogeological parameters considered in a modified DRASTIC method. A three-level classification has been used to determine the degree of risk, and the thresholds have been established following measurable criteria related to the potential nitrate concentration in groundwater. The second part of the study focused on estimating the socioeconomic impact of groundwater pollution by relating the degree of risk and social vulnerability to groundwater pollution. The method has been tested in the Gallocanta Groundwater Body (Spain). As a result, a risk map and an impact map are provided. The risk map shows that 67% of the study area can be classified as moderate and high-risk areas, corresponding to high hazard sources located in moderate and high vulnerability zones, whereas the impact of groundwater pollution is classified as moderate in the whole groundwater body. The proposed analysis allows comparison between aquifers in different areas and the results required by water authorities to implement control and mitigation measures.

Procena rizika sadržaja pesticida u zemljištu kao funkcija osobina zemljišta, pesticida i padavina (kiše), upotrebom numeričkog modela HYDRUS 1D

It has been observed through experiments that many chemicals especially pesticides found in streams and groundwater bodies are transported through surface runoff and/or preferential flow processes. Researchers have also proved that the timing and nature of rainfall since the last pesticide application are often described as significant controls on pesticide transport through flow processes. Rainfall variability is now considered as the climate drivers on contaminant transport. The amount of pesticides transported through surface runoff and/or preferential flow pathways to streams and groundwater respectively has been shown to be strongly correlated with the concentration of that chemical in a thin near-surface layer of the soil immediately prior to such an event. The thin near-surface layer of a soil is regarded as the source where all the dynamics of solute transport starts. The thin near-surface layer is the store of pesticide from which they are leached by both fast episodic as well as slow steady processes. In this research, we seek to better understand the idea of driver-source-trigger concept of pesticide solute transport in the subsurface. The driver (rainfall variability) triggers (surface runoff and/or preferential flow) from the source (a thin near-surface layer) for pesticides transport to stream and/or groundwater. The methodology for this study is by the use of HYDRUS 1D Numerical Model.

Analysis of factors affecting water injection effect of single sand body and research on control measures

According to the isotopic data annular monitoring, test results, well group control degree and other data, the water absorption status trend map of single sand body is manually drawn by using the oil well sealing method and the equal interval data interpolation method to determine the morphological distribution and dynamic connectivity of groundwater bodies. The relationship between the single sand body is confirmed, and the specific measures for coordinating and improving the single sand body are proposed.

Assessing Natural Background Levels for Chlorides in Groundwater Bodies Affected by Seawater Intrusion Processes. Application to Coastal Aquifers in the Mediterranean Coast of Spain

Assessing Natural Background Levels for Chlorides in Groundwater Bodies Affected by Seawater Intrusion Processes. Application to Coastal Aquifers in the Mediterranean Coast of Spain

Quantifying Groundwater Resources for Municipal Water Use in a Data-Scarce Region

Groundwater is a major source of drinking water worldwide, often considered more reliable than surface water and more accessible. Nowadays, there is wide recognition by the scientific community that groundwater resources are under threat from overexploitation and pollution. Furthermore, frequent and prolonged drought periods due to climate change can seriously affect groundwater recharge. For an appropriate and sustainable management of water systems supplied by springs and/or groundwater withdrawn from aquifers through drilling wells or drainage galleries, the need arises to properly quantify groundwater resources availability, mainly at the monthly scale, as groundwater recharge is influenced by seasonality, especially in the Mediterranean areas. Such evaluation is particularly important for ungauged groundwater bodies. This is the case of the aquifer supplying the Santissima Aqueduct, the oldest water supply infrastructure of the city of Messina in Sicily (Southern Italy), whose groundwater flows are measured only occasionally through spring water sampling at the water abstraction plants. Moreover, these plants are barely maintained because they are difficult to reach. In this study, groundwater recharge assessment for the Santissima Aqueduct is carried out through a GIS-based inverse hydrogeological balance methodology. Although this approach was originally designed to assess aquifer recharge at the annual scale, wherever a model conceptualization of the groundwater system was hindered by the lack of data, in the present study some changes are proposed to adjust the model to the monthly scale. In particular, the procedure for evapotranspiration assessment is based on the Global Aridity Index within the Budyko framework. The application of the proposed methodology shows satisfactory results, suggesting that it can be successfully applied for groundwater resources estimation in a context where monthly information is relevant for water resources planning and management.

Hierarchical clustering and compositional data analysis for interpreting groundwater hydrogeochemistry: The application to Campi Flegrei volcanic aquifer (south Italy)

Comprehensive hydrogeochemical studies have been conducted in the Campi Flegrei volcanic aquifer since late 20th century due to the volcanic unrest. In the last decade, groundwater samples were grouped based on the dominant anion species (i.e. bicarbonate, sulfate and chloride) to explain the general hydrogeochemical processes. In this article, 44 groundwater samples are collected from Campi Flegrei aquifer to geochemically and spatially capture the main characteristics of the groundwater body. The hierarchical clustering algorithm is then performed on proportion of bicarbonate, sulfate and chloride, and the optimum number of clusters are determined regarding the results of deep hydrogeochemical investigations published in the past. The collected samples are categorized in the following groups: (1) bicarbonate-rich groundwater; (2) chlorine-rich groundwater; (3) sulfate-rich groundwater; and (4) mixed groundwater. The first group (As = 158.2 ± 169 μg/l, electric conductivity = 1,732.1 ± 1,086 μS/cm and temperature = 25.6 ± 8 °C) is mainly derived from poor arsenic meteoric water, but there is significant thermal/seawater contribution in the second one (As = 1,457.8 ± 2,210 μg/l, electric conductivity = 20,118.3 ± 11,139 μS/cm and temperature = 37.1 ± 20 °C). Interaction of the bicarbonate-rich groundwater and hydrothermal vapors gives rise to the sulfate-rich groundwater (As = 847.2 ± 679 μg/l, electric conductivity = 3,940.0 ± 540 μS/cm and temperature = 82.8 ± 3 °C) around Solfatara volcano. The mixed groundwater (As = 451.4 ± 388 μg/l, electric conductivity = 4,482.9 ± 4,027 μS/cm and temperature = 37.1 ± 16 °C) is observed where the three main groundwater groups undergo a mixing process, depending on the hydrogeology of the volcanic aquifer. Contrary to the bicarbonate- and sulfate-rich groundwater, the chlorine-rich and mixed groundwater generally occurs at low piezometric levels (approximately <1 m above sea level) near the coastline. The hierarchical cluster analysis provides more information about the volcanic aquifer, particularly when compositional data analysis is applied to study hydrogeochemistry of the homogeneous groundwater groups and to uncover the relationships between variables. Addressing compositional nature of data is recommended in the future studies for developing new tools that help deeper understanding of groundwater evolution in volcanic aquifers and identifying promising precursors of volcanic eruption.

Projects which push the limits of efficient performance concepts of high-pressure grinding rollers

The ore pretreatment process evolution is briefly addressed, with some milestones and pioneer projects described. Two advanced projects of concentration mills based on the technology of the pioneer-level high-pressure roll milling are discussed. The operating Canadian Copper Mountain Mill is interesting for using the technology which eliminates bottlenecks of the classical semi autogenous milling circuit, with the resultant increase in the mill capacity more than two times as against the initial project value. This result has been achieved for the first time in the world owing to total rejection of the ore pretreatment circuit and thanks to complete transition to press roller milling. Design and engineering of high-pressure roll mill HRC3000, as well as auxiliary milling and flotation equipment is in progress. The high-capacity Iron Bridge Magnetite Mill under construction in Australia can be assumed as the breakthrough project owing to the world&rsquo;s first time trial of Enduron high-pressure grinding rolls by Weir Minerals in milling of magnetite quartzite and in dry production of fine products suitable for further processing without ball milling. The power saving in dry milling using high-pressure rolls will make 30 % at the least as compared with the present-day ore pretreatment by wet milling. Ball mills are withdrawn from the processing circuit, water consumption drops, carbon emissions are decreased, and the tailing storage becomes simpler and safer. In dry tailing storage, the capital investment is cut down as the operation of thickening of wet tailings is rejected, dam construction for tailings ponds is eliminated, and the environmental impact related with contamination of surface and ground water bodies, which is intrinsic to operation of conventional tailings storage facilities, is mitigated.

A meta-modeling approach for hydrological forecasting under uncertainty: Application to groundwater nitrate response to climate change

We suggest a quantitative predictive approach based on meta-modeling techniques for long-term forecasting in hydrology. We apply our scalable methodological framework to a representative groundwater body in Emilia-Romagna region of Italy to evaluate the potential impact of climate change on groundwater nitrate concentration. The approach allows to (i) handle the uncertainty associated with the projection of climate variables, (ii) preserve an accurate description of flow and transport processes, and (iii) drastically accelerate computationally intensive simulations, at a time. A long-term climate scenario (i.e. referred to the 30-years 2061–2090) is developed for the study area by considering changes in the natural recharge to groundwater, the baseflow of the main rivers, and the rate of nitrate leaching. Global sensitivity analysis is performed to assess the relative influence of these factors on the quantity of interest (QoI), i.e. the long-term prediction of groundwater nitrate concentration. Our analysis enables also us to compute the spatial statistics and variability of the moments of the QoI over the study area and to perform Monte Carlo simulations to fully characterize the probabilistic behavior of the QoI at selected sensitive locations.

Application of Geostatistical Tools to the Geochemical Characterization of the Peloritani Mts (Sicily, Italy) Aquifers

Sources of groundwater contaminants in inhabited areas, located in complex geo-tectonic contexts, are often deeply interlocked, thus, making the discrimination between anthropic and natural origins difficult. In this study, we investigate the Peloritani Mountain aquifers (Sicily, Italy), using the combination of probability plots with concentration contour maps to retrieve an overall view of the groundwater geo-chemistry with a special focus on the flux of heavy metals. In particular, we present a methodology for integrating spatial data with very different levels of precision, acquired before and during the “geomatic era”. Our results depict a complex geochemical layout driven by a geo-puzzle of rocks with very different lithological natures, hydraulically connected by a dense tectonic network that is also responsible for the mixing of deep hydrothermal fluids with the meteoric recharge. Moreover, a double source, geogenic or anthropogenic, was individuated for many chemicals delivered to groundwater bodies. The concentration contour maps, based on the different data groups identified by the probability plots, fit the coherency and congruency criteria with the distribution of both rock matrices and anthropogenic sources for chemicals, indicating the success of our geostatistical approach.

Predictive modelling benchmark of nitrate Vulnerable Zones at a regional scale based on Machine learning and remote sensing

Nitrate leaching losses from arable lands into groundwater were a main driver in designating Nitrate Vulnerable Zones (NVZs) according to the Nitrates Directive, with a view to enhancing their water quality. Despite this, developing common strategies for effective water quality control in these areas remains a challenge in the European Union. This paper evaluates the performance of the Random Forest (RF) machine learning algorithm combined with Feature Selection (FS) techniques in predicting nitrate pollution in NVZs groundwater bodies in different periods and using updated environmental features in Andalusia, Spain. A set of forty-four features extrinsic to groundwater bodies were used as environmental predictors, with an aim to make this methodology exportable to other regions. Phenological features obtained through remote-sensing techniques were included to measure the dynamics of agricultural activity. In addition, other dynamic features derived from weather and livestock effluents were included to analyse seasonal and interannual changes in nitrate pollution. Three feature stacks and two nitrate databases were used in the predictive modelling: Period 1 (2009), with 321 nitrate samples for training; Period 2 (2010), with 282 nitrate samples for validation and initial spatial prediction; and Period 3 (2017), to assess the changes in the probability of groundwater nitrate content exceeding 50 mg/L. Random Forest as a wrapper with four sequential search methods was considered: sequential backward selection (SBS), sequential forward selection (SFS), sequential forward floating selection (SFFS) and sequential backward floating selection (SBFS). From among all the Feature Selection methods applied, Random Forest with SFS had the best performance (overall accuracy = 0.891 and six predictor features) and linked the highest probability of nitrate pollution with three dynamic features: the Normalized Difference Vegetation Index (NDVI) base level, NDVI value for the end of the growing season and accumulated manure production of livestock farms; and three static features: slope, sediment depositional areas and valley depth.