In this study hydrochemical, isotopic and multivariate statistical tools are combined with a recharge analysis and existing geophysical data to improve understanding of major factors controlling ...freshwater occurrence and the origins of high salinities in the multi-layered coastal aquifer system of the Great Maputo area in Mozambique. Access to freshwater in this semi-arid area is limited by an inefficient public supply network, scarce surface waters, long droughts and an increasing population growth. Groundwater has a large potential to enhance water security, but its exploitation is threatened by both coastal and inland salinization mechanisms that are poorly understood. A GIS approach is utilized to classify potential recharge zones based on hydrogeological properties and land use/cover, whereas potential recharge rates are estimated through a root zone water balance method. In combination with water stable isotope data results reveal that extreme rainfall events provide the most relevant contributions to recharge, and interception and evaporation play an important role in the low recharge areas. Hierarchical clustering of hydrochemical and isotopic data allows the classification of six water groups, varying from fresh to brackish/salt waters. Corresponding scatter plots and PHREEQC modelling show evaporation and mixing with seawater (up to 5%) as major processes affecting salinity in the area. The co-occurrence of high alkalinity and Cl concentrations, in combination with piezometric and geo-electrical data, suggests that: 1) inland brackish/salt groundwater is caused by mixing with seawater trapped within clay layers; and 2) brackish/salt surface waters result from seepage of brackish groundwater into rivers and wetlands, followed by evaporation, hence increasing salinity and δ18O values. Mixing with small fractions of trapped seawater as main salinity source, rather than halite dissolution, is further corroborated by Br/Cl ratios of brackish/salt water samples near the ocean ratio. Cation exchange upon salinization is mainly observed in the semi-confined aquifer, while freshening takes place in the phreatic aquifer, particularly in areas presenting high recharge rates.
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•Groundwater salinization in Great Maputo aquifer is caused by relic seawater.•Relic seawater is entrapped within clay layers of the aquifer system.•Seepage of salt groundwater and evaporation results in saline river and wetland.•Recharge dominated by intense rainfall is main source of freshwater.•Fresh groundwater exploitation within saline environment needs to be optimised.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
In this study multivariate and geostatistical methods are jointly applied to model the spatial and temporal distribution of arsenic (As) concentrations in shallow groundwater as a function of ...physicochemical, hydrogeological and land use parameters, as well as to assess the related uncertainty. The study site is located in the Mondego River alluvial body in Central Portugal, where maize, rice and some vegetable crops dominate. In a first analysis scatter plots are used, followed by the application of principal component analysis to two different data matrices, of 112 and 200 samples, with the aim of detecting associations between As levels and other quantitative parameters. In the following phase explanatory models of As are created through factorial regression based on correspondence analysis, integrating both quantitative and qualitative parameters. Finally, these are combined with indicator-geostatistical techniques to create maps indicating the predicted probability of As concentrations in groundwater exceeding the current global drinking water guideline of 10μg/l. These maps further allow assessing the uncertainty and representativeness of the monitoring network. A clear effect of the redox state on the presence of As is observed, and together with significant correlations with dissolved oxygen, nitrate, sulfate, iron, manganese and alkalinity, points towards the reductive dissolution of Fe (hydr)oxides as the essential mechanism of As release. The association of high As values with rice crop, known to promote reduced environments due to ponding, further corroborates this hypothesis. An additional source of As from fertilizers cannot be excluded, as the correlation with As is higher where rice is associated with vegetables, normally associated with higher fertilization rates. The best explanatory model of As occurrence integrates the parameters season, crop type, well and water depth, nitrate and Eh, though a model without the last two parameters also gives quite satisfactory results.
► We studied arsenic levels in shallow alluvial groundwater under agricultural land. ► Multivariate geostatistics is used to model spatio-temporal arsenic occurrence. ► Results indicate the dominant role of reductive dissolution of Fe (hydr)oxides. ► Redox state is linked to rice crops, fertilizers can provide additional As source. ► Best regression model for As uses season, crop, well and water depth, Eh and NO3−.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The present study has analyzed the origin of recharge and groundwater salinity in the Haouz alluvial plain, located in Central Morocco. Stable isotopic data (¹⁸O,²H) indicate that the recharge of the ...alluvial aquifer is dominated by high-altitude meteoric water originating from the High-Atlas Mountains that border the plain to the south. Tritium globally indicates the presence of recent groundwater, as well as mixing between old and recent groundwater. Alluvial groundwater salinity varies from east to west. This variability is mainly induced by enrichment in chloride and sulfate salt contents. Strontium content reveals that sulfate could be related, respectively, to the influence of the Senonian carbonate deposits containing gypsum. Dissolution of halite is revealed by the plot of¹⁸O vs Cl, where the most saline samples do not show any changes in¹⁸O. Br/Cl ratio varies in a wide range, probably due to an orographic effect. The nitrate distribution was analyzed using indicator-geostatistical techniques resulting in probability maps. Nitrate contamination is shown to occur particularly in the eastern part of the Haouz plain. In this paper, some aspects of groundwater resources management are addressed. Different management actions are identified with regard to the current water resources status and to the land and water uses. In this framework controlling groundwater abstraction, optimizing irrigation and enhancing mountain-front recharge constitute the major key management actions that should be carried out.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
In this study deterministic, multivariate and stochastic methods are applied to a combined temporal and spatial monitoring data set, in order to assess nitrate and pesticide levels and contamination ...risk in shallow groundwater. The case study involves an area in the Mondego River alluvial body in central Portugal, where agriculture is the main land use, with predominantly maize, rice and some vegetable crops supported by river water irrigation. Factorial correspondence analysis (FCA), reducing the original data matrix to a small number of independent orthogonal factors, is applied to detect associations between nitrate levels, land use (crop type), lithology and groundwater depth. Indicator-geostatistical techniques are used to create maps indicating the probability of nitrate concentrations in groundwater exceeding predetermined threshold values, including the drinking water standard (98/83/EC) and vulnerable zone designation criterion (91/676/EEC) of 50
mg/l NO
3
−. For pesticides the leaching potential is determined by calculating the Groundwater Ubiquity Score (GUS), based on the sorption coefficient and soil half-life for each pesticide compound. Results for nitrate show an overall very low risk of exceeding 50 or 25
mg/l, whereas the risk of exceeding 9.5
mg/l (third data quartile) is particularly high in areas where FCA shows correlation of nitrate contamination with vegetable and maize crops, aerobic conditions, lower groundwater levels and to some extent, coarser grained sediments. On the contrary, nitrate levels under rice are lowest and correlated to a reduced environment, finer-grained sediments and a higher water table. Denitrification is found to be an important attenuation process, as well as dilution by surface water irrigation and precipitation. Crop type and irrigation source are seen to have a large influence on the nitrate contamination potential of groundwater. Total concentrations of the analysed pesticide compounds above the regulatory limit of 0.5
μg/l are observed in 32% of the analysed water samples, with a maximum value of 16.09
μg/l. The probability maps provide a particularly interesting example of how multiple-well monitoring results over a certain period can be condensed into single maps and used by water engineers, managers and policy-makers.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The following paper presents a discussion of spatial and temporal distribution of groundwater discharge estimates at a regional scale for two coastal aquifer systems in the Algarve region. A finite ...element distributed parameter groundwater flow model is applied to analyze the effect of uncertainties regarding definition of model boundary conditions and seasonality on estimated values. Finally, estimates of sustainable yields are provided based on predefined sustainability criteria and the importance of well location is analyzed. Steady-state simulations indicate a range of average annual coastal discharge between 6.5 × 10⁶and 15 × 10⁶ m³/year from the Albufeira-Ribeira de Quarteira aquifer, and 4.5 × 10⁶and 11.3 × 10⁶ m³/year from the Quarteira aquifer, as well as significant spatial variation. Transient simulations show that seasonal variability inherent to these systems can lead to inversions of hydraulic head gradient during short periods. Model results indicate that coastal discharge rates are between 1.5 and 2 times higher during the peak winter months than during the minimum in the summer, and as such seasonality has a larger impact on discharge rates than BC conceptualization. Up to 3.31 × 10⁶ m³/year could be abstracted from existing well fields without causing seawater intrusion problems. Historical levels of abstraction are not within the selected sustainability criteria. However, by placing abstraction further from the coast sustainable yield increases. This work is part of ongoing research that aims to identify and characterize groundwater flow from the coastal strip towards the continental platform, taking into account structural geology, marine geology and the effects of the hydrological/hydrogeologic conditions on the associated ecosystems.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
A simple methodology based on multivariate analysis is developed to create a groundwater quality index (GWQI) and a composition index (GWCI), with the aim of monitoring the joint influence of ...agriculture on several key parameters of groundwater chemistry and potability. The methodology is based on the definition of two standard water samples of high and low quality that, together with the actual data, are run through a statistical algorithm known as correspondence factor analysis. The applicability of the constructed indices as an assessment and communication tool is evaluated in two case studies in the south of Portugal. Index maps are created, providing a comprehensive picture of the contamination problem and easily interpretable for people outside the scientific domain. Such maps could be particularly useful in agro-environmental policies such as Nitrates Directive 91/676/EEC. Drawn up in order to reduce the environmental load of nutrients from agriculture, its implementation continues to be very problematic in most EU member states. In the case studies, the GWQI maps reveal that groundwater quality in the upper aquifers is extremely low, with an almost complete absence of potable water. However, the impact of agricultural activity on the groundwater composition shows a large spatial variability, which is accurately depicted by the GWCI maps and mainly related to crop type and aquifer lithology. In addition, contamination-freshening sequences occur and are displaced by groundwater flow, partly affected by faults that form either preferential flow paths or flow barriers.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A comparative study on climate change and its impacts on coastal aquifers is performed for three Mediterranean areas. Common climate scenarios are developed for these areas using the ENSEMBLES ...projections that consider the A1b scenario. Temperature and precipitation data of three climate models are bias corrected with two different methods for a historic reference period, after which scenarios are created for 2020–2050 and 2069–2099 and used to calculate aquifer recharge for these periods based on two soil water budget methods. These multiple combinations of models and methods allow incorporating a level of uncertainty into the results. Groundwater flow models are developed for the three sites and then used to integrate future scenarios for three different parameters: (1) recharge, (2) crop water demand, and (3) sea level rise. Short-term predictions are marked by large ranges of predicted changes in recharge, only showing a consistent decrease at the Spanish site (mean 23 %), particularly due to a reduction in autumn rainfall. The latter is also expected to occur at the Portuguese site, resulting in a longer dry period. More frequent droughts are predicted at the Portuguese and Moroccan sites, but cannot be proven for the Spanish site. Toward the end of the century, results indicate a significant decrease (mean >25 %) in recharge in all areas, though most pronounced at the Portuguese site in absolute terms (mean 134 mm/year) and the Moroccan site in relative terms (mean 47 %). The models further predict a steady increase in crop water demand, causing 15–20 % additional evapotranspiration until 2100. Scenario modeling of groundwater flow shows its response to the predicted decreases in recharge and increases in pumping rates, with strongly reduced outflow into the coastal wetlands, whereas changes due to sea level rise are negligible.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The applicability of two vulnerability assessment methods in evaluating the impact of agricultural activities on groundwater quality, is tested in two areas in the south of Portugal with modest ...results. Intensive citri- and horticulture require large amounts of fertiliser and water supplied by irrigation, which induces groundwater salinisation and contamination by nitrates. The degree of contamination varies highly within and between the study areas and is related to hydrogeological factors as well as intensity of agricultural practices. Vulnerability mapping is performed with the intrinsic DRASTIC method and the specific Susceptibility Index (SI), which is an adaptation of DRASTIC. These methods can constitute useful groundwater management tools, for instance when designating new Nitrate Vulnerable Zones as defined in the European Directive 91/676/EEC. However, in the case of DRASTIC, little correspondence exists between the most vulnerable and the most contaminated areas. This is mainly a result of underestimating the dilution capacity and overemphasising the attenuating potential of the unsaturated zone and aquifer, as both chloride and nitrate prove to be very stable contaminants. By including a parameter for land use, SI manages to produce more reliable results, although in many areas the vulnerability is overestimated.Original Abstract: L'application de deux methodes de calcul de la vulnerabilite permettant d'evaluer l'impact des activites agricoles sur la qualite des eaux souterraines, est testee dans deux zones du Sud du Portugal, avec des resultats modestes. La citriculture et l'horticulture intensives necessitent de grandes quantites e fertilisants et d'eau souterraine pour l'irrigation, ce qui induit la salinisation et la contamination des eaux souterraines par les nitrates. Le degre de contamination varie grandement a l'interieur et entre les zones d'etudes, en fonction des facteurs hydrogeologiques et de l'intensite des pratiques agricoles. La cartographie de la vulnerabilite est mise en oeuvre via la methodologie DRASTIC et l'Index de Susceptibilite (SI) specifique, qui est une adaptation de la methode DRASTIC. Ces methodes euvent constituer des outils de management des eaux souterraines, par exemple lors de la designation de nouvelles zones de vulnerabilite aux Nitrates selon la n Directive Europeenne 91/676/EEC. Par ailleurs dans le cas de DRASTIC, de petites correspondances existent entre les zones les plus vulnerables et les plus contaminees. Ceci est principalement le resultat d'une sous-estimation de la capacite de dilution et de la sur-accentuation du potentiel d'attenuation de la zone non-saturee de l'aquifere, car et le chlore et les nitrates sont des contaminants tres stables. En incluant un parametre d'utilisation des sols, SI produit des resultats plus realistes, bien que dans de nombreuses zones la vulnerabilite soit surestimee.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Communities located in the interface between marine/brackish and freshwater habitats are likely to be early responders to climatic changes as they are exposed to both saline and freshwater ...conditions, and thus are expected to be sensitive to any change in their environmental conditions. Climatic effects are predicted to reduce the availability of groundwater, altering the hydrological balance on estuarine-aquifer interfaces. Here, we aimed to characterise the estuarine faunal community along a gradient dependent on groundwater input, under a predicted climatic scenario of reduction in groundwater discharge into the estuary. Sediment macrofauna was sampled along a salinity gradient following both the wet and dry seasons in 2009. Results indicated that species abundance varied significantly with the salinity gradient created by the groundwater discharge into the estuarine habitat and with sampling time. The isopode
Cyathura carinata (Krøyer, 1847) and the polychaetes
Heteromastus filiformis (Claparède, 1864) and
Hediste diversicolor O.F. Muller, 1776 were associated with the more saline locations, while oligochaeta and Spionidae were more abundant in areas of lower salinity. The polychaete
Alkmaria romijni Horst, 1919 was the dominant species and ubiquitous throughout sampling stations. This study provides evidence for estuarine fauna to be considered as a potentially valuable indicator of variation in the input of groundwater into marine-freshwater interface habitats, expected from climatic pressures on aquifer levels, condition and recharge rates. For instance, a reduction in the abundance of some polychaete species, found here to be more abundant in freshwater conditions, and increasing Oligochaeta found here on higher salinities, can potentially be early warnings of a reduction in the input of groundwater into estuaries. Estuarine benthic species are often the main prey for commercially important fish predators such as in our case study, making it important to monitor the aquatic habitat interfaces taking into consideration the estuarine macrobenthos and groundwater availability in the system.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Major issues regarding the efficiency of moni toring programs for nitrate contaminated groundwater are analyzed in this paper: (i) representativeness of monitoring networks; (ii) correct ...interpretation of the monitoring data and resulting time series and trends; and (iii) differentiation among the different sources of nitrates in groundwater. Following an overview of the nitrate contamination problem and possible solutions, as well as some of the difficulties found, a relatively straightforward method for assessing monitoring network representativity is presented, namely interpolation standard error assessment. It is shown how nitrate-concentration time series resulting from periodic observations can be corrected with a conservative tracer, in order to avoid misinterpretation and confirm or correct apparent trends. Finally, coupled 15N and 18O isotope signatures of nitrate (NO3 –) in groundwater are used to differentiate among nitrogen (N) sources, to ensure correct targeting of restoration measures. The case study regards a Nitrate Vulnerable Zone in the south of Portugal, designated in compliance with the European Nitrates Directive, where coastal discharge of nutrient-rich groundwater threatens the good qualitative and ecological status of the Ria Formosa coastal lagoon. Results show that mineral fertilizer is the main source of N in groundwater, and that increases in N load can be masked by dilution phenomena.
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IJS, KILJ, NUK, PNG, UL, UM
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