HS8.2.1

After abundant rainfalls, the Mucille area (Ronchi dei Legionari, Northeastern Italy) is subject to frequent flooding. Although this area has always been exposed to such hazard, these inundations become problematic since 2001 as they more frequently affect housing and recreational areas, leading the population to believe that the swallow holes draining the area stopped functioning. The increased frequency of intense rainfall events led the municipal technicians to involve the Department of Mathematics and Geosciences of the University of Trieste to assess the situation. The Mucille karstic depression is fed by a spring area and drained by two swallow holes one of which is permanently active while the other operates only during floods. The Mucille springs represent the westernmost drain of the Classical Karst aquifer. During floods, as in-situ discharge measurements are impossible, only a hydrologic balance model may assess the inflow or outflow discharges. The extension of the flooded areas has been mapped. The obtained flooded surface together with high resolution DEM coverage allows to calculate the volume of surface water. Combined with water table levels recorded in an adjacent piezometer, this volume can be computed over time. Thus, the hydrologic balance (inflow minus outflow) can be estimated. This model has been applied to several flood events among which, two were the most important in terms of flooded areas: one in December 2017 and the other in November 2019. During the event of December 2017, the water level reached 7,5 m a.s.l. and the difference between the inflow and the outflow was 880 l/s. The day following the peak, the discharge difference decreased to 273 l/sand the 5 subsequent days the water balance was close to equilibrium. From the eighth day on, the outflow became predominant resulting in a negative budget between -233 and -78 l/s. The flood event of November 2019 reached the maximum inundated area at a water level of 7,8 m a.s.l. with a difference between the inflow and the outflow of 750 l/s . Two days after the peak a negative balance of -200 l/s was recorded and remained negative for the next 5 days. A period of intermittent precipitations increased again the inflow up to 600 l/s. Following a period of ten days with a negative balance the water level returned to the initial values of 5 m a.s.l. This study provides evidences fundamental for the design of measures to mitigate the risk. It estimates the discharge of the swallow holes, confirming their efficiency. Nonetheless it also emphasises the need to improve their draining capacity, especially considering the unsuspected high outflow of the springs at the onset of the flood.

How to cite: Zini, L., Turpaud, P., and Calligaris, C.: Water management in the Mucille area (NE Italy) through hydrologic balance estimation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12006, https://doi.org/10.5194/egusphere-egu21-12006, 2021.

European water policy requires to carry out nine tests for characterisation of groundwater bodies status, including the development of joint transboundary groundwater management principles. 

Gauja/Koiva and Salaca/Salatsi Rivers have a joint, Estonian/Latvian transboundary water cycle, including the groundwater recharge and discharge cycling. Despite the fact that groundwater is the only drinking water source in Gauja/Koiva and Salaca/Salatsi River basins and ensures the existence of many groundwater dependent ecosystems, the overall awareness of integrated cross-border management practice is still poorly understood and poorly linked in implementation of concrete groundwater protection actions. 

Taking into account the above-mentioned aspects, Latvian and Estonian groundwater and groundwater dependent ecosystems specialists from research, nature protection and groundwater resources management institutions have joined forces in the project funded by the Interreg Estonia-Latvia program: "Joint actions for more efficient management of common groundwater resources in Estonia and Latvia "(WaterAct).

The ongoing Est-Lat project “WaterAct” (2020–2022) of joint transboundary groundwater management project organised into the three activity blocks: (1) The capacity building of the joint groundwater transboundary management through exchange of knowledge and best management practices between project partners and key experts in other European Union countries; (2) Assessment of groundwater resources in transboundary River basins to improve groundwater management in accordance with valid international directives; (3) Dissemination and outreach activities to increase the overall awareness of ecosystems friendly groundwater management and protection of key actors working with groundwater assessments and locals. 

Firstly, the joint cross-boundary principles of identification and status assessment of shared groundwater bodies will be developed. Adaptation of existing knowledge, cross-boundary harmonization and development of needed methodologies will be used. 

Secondly, the joint cross-boundary assessment principles will be implemented into groundwater resources management. The status assessment of shared groundwater bodies will be carried out in close cooperation between project partners to create materials necessary for the development of last River Basin Management Plans (2022-2027), required by the Water Framework Directive and Groundwater Directive.

Thirdly, the dissemination of project results will be carried out (1) by compilation of Guidelines of Groundwater Dependent Ecosystems for different levels and fields of decision making and experts, (2) by compilation of Spring Water Monitoring Guide for Volunteers and starting volunteer monitoring. For volunteer monitoring, a special web-based map application will be developed (allikad.info). 

The project of “Joint actions for more efficient management of common groundwater resources” (WaterAct, Est-Lat155) funded by ERDF Interreg Estonia-Latvia cooperation programme.

How to cite: Krauze, A., Terasmaa, J., Lode, E., Bikše, J., Türk, K., Tarros, S., Polikarpus, M., Marandi, A., Ojamäe, K., and Priede, A.: Joint actions for more efficient management of common groundwater resources in Estonia and Latvia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1979, https://doi.org/10.5194/egusphere-egu21-1979, 2021.

Most future scenarios for water resources are predicting water scarcity, with a decrease in the amount
of precipitation and limitation on groundwater recharge for the next five decades. In arid and semi-
arid areas, the water quality is a great problem and groundwater salinization is one of the principal
causes of degradation of water resources worldwide.
Menzel Habib aquifer is located in the northwest of Gabès region (southeastern Tunisia), included in
the arid Mediterranean bioclimatic area, with dry hot summers and relatively warm winters.
Groundwater geochemistry from the study area shows a Na-Cl and Ca-Mg-Cl-SO 4 dominant facies.
The high groundwater mineralization and its correlation between total dissolved solids and major ions
suggest a contribution of SO 4 , Cl, Na, Ca and Mg in groundwater salinization processes.
The salinization of groundwater is mainly associated with the Triassic evaporites, with the dissolution
of halite, anhydrite and gypsum, occurring in the area, and related to the tectonic context of the
region. Additionally, other geochemical processes occurred, such is the cation exchange mechanisms.
Changes in precipitation patterns and intensity, with water scarcity, low recharge and excessive
pumping have affected groundwater quantity and quality. Nowadays, the occurrence of climate
changes scenarios is a major drawback for water use for irrigation and drinking water supply in arid
and semi-arid regions, such as Menzel Habib aquifer.

How to cite: Dhaoui, O., Horta Ribeiro Antunes, I. M., Agoubi, B., and Kharroubi, A.: Management of groundwater salinization under a climate change scenario in an aridarea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7783, https://doi.org/10.5194/egusphere-egu21-7783, 2021.

Abu Dhabi is one of the arid regions in the world having less than 100 mm of rainfall per annum. The renewability of freshwater occurs only in the eastern part. The groundwater resources under desirable quality are very concise due to limited dilution/rainfall and higher rate of evaporation. Hence, in recent decades, desalinated water has been introduced for agriculture activities and surplus desalinated water is injected into the aquifer as artificial recharge. This study is conducted to understand the impacts in the aquifer system caused by the introduction of desalinated water for agriculture activities and for aquifer recharge structures. The simulation was carried out from 2000 to 2050 using reported rate of groundwater pumping and of desalinated water with 0.1 g/l, 0.5 g/l, 1 g/l, 1.5 g/l and 2 g/l degrees of salinity. A wide range of decline in the groundwater table is noticed in the western part of the aquifer due to less rainfall recharge. The results confirm that this region demands either reduction in agricultural activities or additional usage of desalinated water by which the pumping of groundwater can be reduced further. The improvement in the groundwater quality is noticed in the aquifer due to the addition of less saline desalinated water into the aquifer. This study confirms the long term suitability of existing aquifer recharge structure. Also, it expresses the need of further management practices in quantifying the desalinated water contribution for agriculture activities.

How to cite: Cherubini, C., Sadhasivam, S., Pastore, N., and Ghirotti, M.: Impacts of Desalinated Irrigation Water in the Abu Dhabi surficial aquifer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13095, https://doi.org/10.5194/egusphere-egu21-13095, 2021.

Groundwater is vulnerable to contamination from natural and anthropogenic activities. The agricultural and human activities associated with hydrological characteristics influence the quality of groundwater. The City of Tete is in the Nharthanda Valley (Zambezi River, Central Mozambique). The city faces a set of serious structural issues of access to water such as a precarious public water supply system, including a lack of network management, water rationing, and a poor sewerage system. Groundwater is collected from the aquifer for the public water supply system of the old city of Tete and a for a traditional agro-livestock farm, which is irrigated by artesian wells. Groundwater abstraction has increased in the last few decades, and it was identified as a risk for groundwater quality and quantity. Groundwater physic-chemical and microbiological parameters obtained from fifteen boreholes and eleven wells have been determined to assess water quality. The presence of potential contaminant activities throughout the Nhartanda Valley and adjacent areas associated with contamination of the Zambezi River contribute to the degradation of water quality. The high vulnerability index for most chemical and microbiological elements indicates that groundwater is easily reached by bacteria and viruses and other potentially toxic substances. Most of the water parameters, from wells and boreholes, exceed the water referenced values allowed for human consumption and agricultural use. The protection of the Nhartanda Valley aquifer system is necessary and urgent. The identification of the most vulnerable areas provides important information for groundwater management, such as the indication of protection measures in aquifer systems.

How to cite: Antunes, I. M. H. and Bande, A.: Management of groundwater sustainability and contamination - a Mozambique case study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-910, https://doi.org/10.5194/egusphere-egu21-910, 2021.

Many drinking water systems worldwide are based on river bank filtration. From a quantitative point of view river bank filtration systems are highly reliable because of the high permeability of alluvial aquifers linked to high production rates. However, there might be an increased risk of contamination because of the short residence time between the river and the production well, especially during flood events.

Flood events change the river-aquifer hydraulic interactions and may increase infiltration rates (e.g., due to an increased hydraulic head, larger river infiltration widths, or erosion of a siltation layer). This leads to changes in groundwater flow paths and production wells might abstract water with a shorter residence time and lower quality. Groundwater quality may degrade during flood events due to the presence of undesirable chemicals (e.g., wastes water treatment plant overflow) and the occurrence of faecal indicator bacteria such as E.Coli.

Groundwater modelling can assist in developing strategies to protect river bank filtration from such undesired contamination by predicting optimal operation conditions. The key impediment of this approach is significant uncertainties in subsurface properties and the associated uncertainties of the groundwater flow paths. To reduce uncertainties in model predictions, anthropogenic tracers including the MRI contrast agent gadolinium and artificial sweeteners were used in this study. They revealed sources and flow patterns, and have been used to derive mixing ratios representing different temporal and spatial scales. Including anthropogenic tracers into the objective function of the calibration process also lead to more accurate estimation of groundwater flow paths. This was critical to predict the best water works operation strategy during flood events.

How to cite: Marazuela, M. A., Herrera, P., Erlmeier, K., Brünjes, R., Brunner, P., and Hofmann, T.: Reducing the risk for contamination of river bank filtration systems using inverse modelling and anthopogenic traces, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2388, https://doi.org/10.5194/egusphere-egu21-2388, 2021.

One of the major challenges for the sustainable development of the federal capital territory of Abuja (Nigeria) is related to the access to safe fresh water resources. This area lies within the drought prone parts of the Sahel region. As in many regions of the world there has been growing competing demands for fresh water as a result of population growth and groundwater quality degradation. In this context, the paucity of data and in depth knowledge of aquifer features and groundwater flow makes groundwater management even more complex, with a severe impact on access to safe water resources for the local populations. To address this challenge, the purpose of the presented research is to generate information on aquifer settings and its vulnerability and on the qualitative and quantitative assessment of the available groundwater resources. Remote sensing and GIS were applied to improve the available information on groundwater resources of Abuja. Fundamental information such as recharge rate, availability and vulnerability of groundwater to pollution was determined. Aquifer vulnerability zones were delineated using the DRASTIC model by integrating layers of depth to groundwater, aquifer recharge, aquifer media, soil type, topography, impact of vadose zone and hydraulic conductivity.  The study area covers about 8000km². The elevation ranges from 62 to 843m a.s.l. with the highest elevations at the North Eastern parts and the lowest elevations at the South Western parts of the study area. There are three soil types in the area, the silty clay, silt loam and clay with clay being the predominant soil type. The five major rock types in the area include migmatite gneiss, schist and metasediment, sandstone and river alluvium, granite and quartzite. The aquifer type is phreatic and the depth to groundwater ranges from 2.8 to 21.9 m. The high recharge areas occurred mostly in highly fractured areas covered with metasedimentary rocks, migmatite gneiss and sandstones. The groundwater vulnerability zones in the study area were grouped into four classes: High, moderate, low and very low. The highly vulnerable zones are the North Eastern parts of the study area covering most parts of Bwari and parts of the municipal council areas and also the Southern parts of the study area covering parts of Kuje and Abaji. They constitute the highly fractured areas covered with silt loam soil type. The very low vulnerable zones are the North Western and Central parts covering mostly Gwgwalada and Kwali areas.  This study demonstrates that GIS and remote sensing techniques are efficient and cost effective tool for delineation of groundwater vulnerability zones. The information obtained will be used as a basis for a geochemical characterization of groundwater quality in the region with the overall goal of supporting new groundwater management plans in the region.

How to cite: Etuk, M., Ogbonnaya, I., Viaroli, S., Petrini, R., and Re, V.: Vulnerability Assessment of Shallow Aquifers in Abuja using GIS and Hydrogeological Parameters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8712, https://doi.org/10.5194/egusphere-egu21-8712, 2021.

Groundwater is a key resource to fulfil human drinking needs worldwide. Therefore, guaranteeing a safe and constant supply of drinking water to the public has been an important focus at European level. Recently, the EU approach to drinking water monitoring radically changed, moving from the simple water quality monitoring, toward a more comprehensive risk assessment, involving the whole supply chain from collection to distribution. Particularly, EU Directives 2015/1787 and 2020/2184 endorsed the Water Safety Plan (WSP) system which requires a detailed assessment of every possible dangerous event.

Groundwater extraction constitutes the first step of the supply chain, and therefore the most vital. In this work, an approach to assess groundwater wells vulnerability in the scope of WSP is proposed, considering natural and anthropogenic hazards, through a hydrogeological, hydrochemical and hydrodynamical characterization. The study area is the Lake Iseo morainic amphitheatre (ca. 180 km²) in the Brescia province, Northern Italy. Particularly, 17 wells have been analyzed, serving 4 municipalities.

Two main dangerous events have been considered as possible hazard for the collected groundwater: a) anthropogenic impact from the surface, related to the land use, and b) natural contamination by reduced species consequent to the degradation of natural organic matter.

Groundwater extraction vulnerability to these two dangerous events has been assessed, considering several hydrogeological aspects: a) the kind of the exploited aquifer (shallow, confined, semiconfined), b) groundwater depth for the shallow aquifers, c) permeability of the vadose zone for the shallow aquifers and d) red-ox conditions of the collected groundwater.

To assess these parameters, lithostratigraphic, chemical and piezometric data were analyzed, reaching a deep understanding of the system by characterizing the different exploited groundwater bodies from a hydrogeological, hydrochemical and hydrodynamic point of view.

Hydrogeological sections were elaborated, covering the whole amphitheater, 7 in the N-S direction and 7 in the W-E direction. The interpretation of these sections allowed to identify the distribution of the main aquifer bodies and the relationships between the various hydrogeological units. To evaluate the red-ox conditions and perform groundwater quality characterization, chemical data were analyzed, including major ions and red-ox sensitive species, through boxplot and statistical analysis. Furthermore, piezometric levels were analyzed to identify groundwater depth, flow directions and watersheds. Of the 17 wells, one resulted to be confined with reducing conditions. Among the remaining, 7 are semiconfined while 9 are shallow, with oxidizing conditions in both cases. Concerning groundwater depth, 13 present values above 40 m, 2 between 20 m and 40 m, and 1 below 20 m. As regards the vadose zone permeability, 9 present high permeability, 7 medium. Totally, in terms of vulnerability to anthropic impacts, one well has low vulnerability, 9 medium and 6 high, while in terms of vulnerability to natural contamination one well has high vulnerability and the remaining low.

This approach allowed a deep understanding of the system and constitutes a reproducible methodology to assess groundwater wells vulnerability to natural and anthropogenic contaminations.

Funding: this work was supported and carried out in cooperation with Acque Bresciane, water supplier.

How to cite: Zanotti, C., Rotiroti, M., Fumagalli, L., Caschetto, M., Sartirana, D., and Bonomi, T.: Hydrogeological and hydrochemical characterization to assess wells vulnerability in the scope of Water Safety Plans, a case study in Northern Italy., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2106, https://doi.org/10.5194/egusphere-egu21-2106, 2021.

Use of environmental or artificial tracers has been an effective approach to characterize groundwater flow and solute transport, tracking pollutant migration and determine travel time. However, availability of a distinctive number of tracers, variability in interaction with the aquifer matrix, and analytical detection limits are namely few of the significant concerns to be addressed and which led us to focus on employing novel DNA tracers.

Besides the quality of being unique, improbably prevalent in nature and environmentally friendly, DNA tracers can be synthesized virtually in infinite numbers of distinct sequences, rendering them a potential candidate for multi-tracer applications for subsurface and groundwater flow characterization. Studies have already demonstrated the potential of DNA tracing in groundwater studies but a blueprint for methodical application and analysis is required.

In this study, we investigate the applicability of DNA tracers in determining hydraulic parameters of a natural aquifer, such as, hydraulic conductivity, effective porosity, dispersivity, and travel time, the most significant characters of a matrix, influencing solute or pollutant transport. In addition, we aim to leverage the applicability of the tracers in terms of minimizing the uncertainty in estimating the parameters.

In order to capitalize on these advantages of DNA tracers with the aim of addressing the aforementioned objectives, this research focuses on employing multiple dsDNA (ds=double stranded) tracers in a 1.3 m long three-dimensional sand-filled aquifer tank. Under forced-gradient water flow conditions, distinctly sequenced, monodispersed dsDNA tracers are instantaneously injected through injection wells, taking into account different scenarios. The scenarios consider different configurations of injection and sampling strategies. Samples collected periodically were subjected to quantitative polymerase chain reaction (qPCR) for DNA concentration estimation. All the silica-encapsulated DNA particles were comparable in size and surface properties.

Individual breakthrough curves from each of the scenarios are carefully analysed for determining water flow and hydraulic properties. In addition, the experiments producing multiple breakthrough curves are cumulatively analysed for obtaining a minimal uncertainty for the parameter estimations.

How to cite: Chakraborty, S., Arachchilage, C., Elhaj, R. H. M., Foppen, J. W., Bogaard, T., and Schijven, J.: Multiple DNA-tracer transport approach for determining aquifer matrix properties in a laboratory 3D aquifer sand tank: a methodical perspective, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16366, https://doi.org/10.5194/egusphere-egu21-16366, 2021.

Crystalline rocks aquifers are usually represented with a low porosity and hydraulic conductivity giving low well yields. Over the world, more than 880 millions people live on crystalline basement rocks. Thus, abilities to spot sufficient groundwater resource in these systems are crucial. Nevertheless, assessment of the sustainable reservoirs in crystalline basement aquifers is challenging. The well-admitted conceptual model presents a stratiform-weathered profile above a fractured zone showing a decreasing fracture density with depth. The interconnection between these two compartments defines the hydraulic parameters: the weathered profile is capacitive while the fractured zone is transmissive.

French Guiana is mostly composed of Paleoproterozoic rocks belonging to the Guiana Shield. It was formed during protracted periods of intense suprasubduction related magmatism, metamorphism and deformation, culminating with the Transamazonian orogeny, bracketed between 2.3 and 1.9 Ga. This peculiar geological history creates a large diversity of geological units from undeformed granitic units to ultramylonitized shears-zone related meta-volcano-sedimentary units and through brittle to ductile deformed units. Furthermore, over almost 200 Ma, the French Guiana recorded a deep weathering phase leading to heterogeneous and complex profiles up to 80-100 m deep. In such a context, hydrogeological exploration is thus puzzling, especially as French Guiana is covered by the Amazonian Forest, reducing direct observations.    

We use a multi-disciplinary method from remote sensing to field observations through geophysical tomography to propose conceptual models of groundwater circulation helping us to localise precisely (meter scale) exploration borewells. After 15 years of hydrogeological surveys, the BRGM has studied plural units: (i) classical isotropic unit (Mahury Massif (MM)) and Granitic unit (Mana), (ii) ductile to brittle deformed units separated by strike-slip fault (Rosebel-Bonidoro unit (RBU) and Armina Unit (AU)), (iii) ultramilonitized unit (Paramaca Unit (PU)). A large heterogeneity of hydrogeological conceptual models for each context arise from our results. Notwithstanding this diversity and thanks to these conceptualizations, we were able to propose successfully useable sustainable resources, confirming the robustness of the method.     

The MM and Mana are classical isotropic units displaying a deep weathered profile. The confined aquifer is located into the fractured layer with yield reaching 15 m³.h^-1. Crosscutting dolerite dyke is attested to be an interesting hydrogeological target with yield near 20 m³.h^-1. The highest yields in French Guiana for crystalline basement rocks (30 m³.h^-1) are found in confined aquifer in PU context. This record could be due to the ultramylonitic deformation giving a high permeable unit. Three different places were studied for the AU (Sparouine, Roura, Beauséjour). As for the PU, aquifers are all confined. Yields are systematically low (around 2-5 m³.h^-1). The RBU is an interesting and contrasting unit because it does not show developed weathered profile. It seems that an unconfined aquifer must probably recharge surroundings units (i.e. PU and AU).

This work highlights the high potential of ductile to ultramylonitic shear zones for groundwater resource. Taking together, these conceptual models highlight that, in French Guiana and probably in entire Guiana Shield, Transamazonian tectonometamorphic structures as well as early Jurassic extensive faults correspond to sustainable useable groundwater resources.

How to cite: Selles, A., Aertgeerts, G., Brisset, N., and Lhotelin, M.: Heterogeneity of hydrogeological conceptual models in crystalline basement aquifers under equatorial climate: case study of French Guiana, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2517, https://doi.org/10.5194/egusphere-egu21-2517, 2021.

Distributed integrated hydrological models (IHMs) are the most effective tools for estimating groundwater recharge in arid and semi-arid areas characterized by thick unsaturated zone. It is also important to capture spatio-temporal aquifer dynamics by using real-time or near-real-time data, for sustainable water resources management. However, such data is often unavailable in developing countries where monitoring networks are scarce. In recent years, remote sensing has played an important role in providing spatio-temporal information for evaluation and management of water resources. Nevertheless, application of remote sensing in groundwater studies is still limited and has mainly focused on assessment of groundwater recharge and groundwater storage as well as to provide boundary conditions and driving forces for both standalone groundwater models and IHMs. This study entails application of remote sensing data in developing the distributed integrated hydrological model for Stampriet transboundary multi-layered aquifer system shared between Namibia, Botswana and South Africa. A numerical model has been set – up using MODFLOW 6 coupled with the Unsaturated Zone Flow (UZF) Package where Climate Hazards Infrared Precipitation with stations (CHIRPS) rainfall data and Global Land Evaporation Amsterdam Model (GLEAM) potential evapotranspiration data were implemented as the model driving forces. Other input data used include digital elevation model, and land-use/landcover and also soil datasets to define unsaturated zone parameters. The model has been calibrated with groundwater level measurements as the state variables in transient conditions at daily time step for a period of 16 years. The model-simulated unsaturated zone and groundwater storage was compared to GRACE-derived sub-surface storage anomaly, further also used to constrain the model. The calibrated model provides spatio-temporal water flux dynamics as well as water balances and hence an understanding of the groundwater-resource dynamics and replenishment. This information is shown useful for proper management of the transboundary water resource as well as for policy making.

How to cite: Kinoti, I., Leblanc, M., Olioso, A., Lubczynski, M., and Poulain, A.: Remote sensing for assessment of groundwater resources, A case study of Stampriet Transboundary Aquifer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7444, https://doi.org/10.5194/egusphere-egu21-7444, 2021.

Imaging the extent of salt water intrusion in the Luy river coastal aquifer (Binh Thuan) using electrical resistivity tomography (ERT)

Diep Cong-Thi^1,3, Linh Pham Dieu^1,3, Robin Thibaut¹, Marieke Paepen¹, Hieu Huu Ho³,

Frédéric Nguyen², Thomas Hermans¹

¹Department of Geology,Ghent University, 9000-Gent, Belgium

² Department of Urban and Environmental Engineering,Liege University and Department of Civil Engineering, KU Leuven, B- 4000 Liège and 3000 Leuven, Belgium

³ Department of Marine Geology, Vietnam Institute of Geosciences and Mineral Resources (VIGMR), 100000 Hanoi, Vietnam

  ABSTRACT

Seawater intrusion has been one of the most concerning issues of the Vietnam South Central provinces in recent years, especially in the Binh Thuan province which is characterized by a hyper-arid climate. During the dry season extending from November to April, seawater intrudes through estuaries and threatens groundwater resources. The latter are under increasing pressure due to water extraction for agri- and aquaculture. To evaluate the current state of salinity in the shallow coastal aquifer, 21 electrical resistivity tomography (ERT) measurements were collected along the downstream part of the Luy river based on the previous saltwater intrusion boundary which was estimated from water samples collected from shallow boreholes. The data were inverted to get the resistivity distribution of the subsurface and interpreted in terms of salinity. Comparison with well data shows that resistivity values below 6.5 Ohm.m correspond to the presence of saltwater in the aquifers. On the right bank of the river, a higher elevation dune area contains a freshwater aquifer which limits the intrusion of saltwater. On the left bank dominated by lowland areas, saline water fills almost the entire thickness of the aquifer, except locally for small thin freshwater lenses. At larger distances from the sea, the aquifer displays a complex distribution of fresh and saline lenses. Those variations seem to be correlated with the presence of clay lenses, recharge sources and irrigation practices. ERT data also reveals the depth of the rock basement. The geophysical observations show that the extension of saltwater intrusion is much larger and more complex than expected from existing borehole data and is not limited to interaction with the river.

KEYWORDS: saltwater intrusion, groundwater, electrical resistivity tomography, Luy river.  

*Corresponding Authors. Email: Diep.CongThi@UGent.be; Linh.PhamDieu@Ugent.be; robin.thibaut@ugent.be; marieke.paepen@ugent.be; hohuuhieu@yahoo.com; f.nguyen@uliege.be; thomas.hermans@ugent.be

How to cite: Diep, C.-T.: Imaging the extent of saltwater intrusion in the Luy river coastal aquifer (Binh Thuan) using electrical resistivity tomography (ERT), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4960, https://doi.org/10.5194/egusphere-egu21-4960, 2021.

Discretizing anthropogenic and natural contaminations represents a crucial step in groundwater management and regulation. Natural background levels (NBLs) have a huge impact on groundwater protections and remediation strategies, but it is still an issue on the ground in terms of reliability and accuracy, thus its derivation needs further scientific efforts.

The derivation of local NBLs (LNBLs) is intended to overcome the limitation of considering a groundwater body (GWB) homogeneous, hence accounting hydrogeochemical heterogeneities within the aquifer system.

This work presents a statistical approach assessing LNBLs for sensitive redox species (As, Fe, Mn, NH₄) in 30 GWBs within the Lombardy Region. Under the monitoring network of the Regional Agency for Environmental Protection of Lombardy (ARPA), more than 500 wells were investigated, thus each GWBs were identified within 4 aquifer types: shallow, intermediate, deep Po Plain aquifers and Alpine valley aquifers. The initial dataset underwent preselection and multivariate analyses, appointing at each well a geogenic redox zonation. It leaded to discretize geochemically-homogeneous subgroups and characterize them as function of site-specific natural facies: oxidised (293 wells), reduced (199 wells) and saline (11 wells). Interquartile range criteria, validations’ tests (Mann-Kendall and Shapiro-Wilk), probability density histograms and probability plots inferred temporally and spatially the datasets, one for each target species, discretized for aquifer and natural facies appartenances. This resulted in the identification of the statistical distributions from redox-homogeneous sets of data from which the LNBLs were derived.

Considering the Po Plain aquifer (shallow, intermediate and deep), NBLs derivation for As revealed three subgroups within the oxidised facies, for which the NBLs values are of 2, 3 and 7 μg/L, four subgroups ascribe to the reduced facies with NBLs of 13, 49, 71 and 291 μg/L, and two subgroups for the saline facies with NBLs of 3 and 12 μg/L. According Fe, two are the subgroups within the oxidised facies, with NBLs of 40 and 94 μg/L, four subgroups fall in the reduced facies with NBLs of 653, 1430, 3200 and 6000 μg/L; within the saline facies, two subgroups are identified with NBLs of 1647 and 6000 μg/L. Two subgroups characterize the oxidised facies for NBLs of Mn with values of 8 and 27 μg/L, and NBLs of 34, 216, 485, 912 and 1514 μg/L refer to five subgroups in reduced facies, while within the saline facies fall two subgroups with NBLs of 381 and 921 μg/L. With regards to NH₄, NBLs reach values of 49, 110 and 190 μg/L for the three subgroups within the oxidised facies, whereas values of 834, 2600, 3090, 4480 μg/L are derived for the four subgroups in the reduced facies; the two subgroups ascribed to the saline facies reveal NBLs of 1860 and 6620 μg/L.

Data demonstrate how an in depth understanding of aquifers’ redox-zonations turned out to be functional for assessing LNBLs. Regional Legislation (D.G.R. 23novembre2020 n.3903) has been amended on the basis of the outcomes of this work, revealing site redox-specific LNBLs of practical significance.

Funding: this work was granted and carried out in collaboration with Lombardy Region.

How to cite: Rotiroti, M., Caschetto, M., Zanotti, C., Parini, M., Cipriano, G., Bonomi, T., and Fumagalli, L.: Local natural background levels assessment through a groundwater redox zonation, the case of Lombardy Region., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3772, https://doi.org/10.5194/egusphere-egu21-3772, 2021.

In South Transdanubia (Hungary) there are remarkable geothermal and hydrocarbon resources. The area is also characterized by natural thermal water discharge at the boundary of outcropping carbonate hills and adjacent sedimentary basin. These regional discharge areas are favourable sites of hypogenic caves as well. These geofluid systems and groundwater related phenomena are usually investigated separately and their interactions are just neglected. The aim of this study is to give all these groundwater-related resources and phenomena a common framework applying the concept of regional hydraulic continuity, and to complete the basin-scale hydraulic assessment of the area based on preproduction archival measured data. Pressure-elevation profiles, tomographic fluid-potential maps and hydraulic cross-sections were constructed to determine the vertical and horizontal fluid-flow conditions. As a result, two kinds of fluid flow systems could be identified. Within the gravitational flow systems, horizontal flow conditions are dominant and the regional flow direction tends toward the S–SE. In deeper basin regions, an overpressured flow system is prevalent, where fluids are driven laterally from the deeper sub-basins towards their margins. Based on the regional-scale evaluation of fluid flow systems, conclusions could be drawn regarding the geothermal and hydrocarbon potential of the area. Additionally, local-scale phenomena could be explained, and the study emphasizes that knowledge on regional groundwater flow systems is essential to understand local scale groundwater-related phenomena such as recent cave formation in an area. A comparison with the marginal Buda Thermal Karst area allows for generalized conclusions regarding the connections between marginal karst reservoirs and the Pannonian Basin. Furthermore, the results of the study can be directly applied in the exploration and sustainable utilization of groundwater related resources, such as thermal waters and hydrocarbons.

This topic is part of a project that has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 810980. The National Research, Development and Innovation Fund has provided financial support to the research under the grant agreement no. PD 116227.

How to cite: Csondor, K., Csobaji, L., Zentai-Czauner, B., Győri, O., and Erőss, A.: Karst evolution, hydrocarbon and geothermal resources in flow system context (South Hungary), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14896, https://doi.org/10.5194/egusphere-egu21-14896, 2021.

Groundwater originating from great depths provide a valuable geochemical sampling medium for exploring the development of the Earth's crust, geological, and hydrogeological resources. This particularly applies to sites of natural springs, where favorable hydrogeological conditions enabled regional discharge. Despite the numerous occurrences of mineral and thermal waters in Serbia, the current understanding of the regional groundwater flow is associated with many open questions that need to be addressed. From a geological standpoint, Serbia is part of the Alpine-Mediterranean mountain belt. From the middle of the Mesozoic to the present, this area underwent processes of subduction, collision, and extensions with accompanying voluminous magmatism and volcanism. As a result of the mentioned geodynamic events, the Serbian territory was a zone of intensive tectonomagmatic processes which had a significant impact on the formation of the hydrogeological structures for forming groundwater enriched with specific elements and elevated temperatures.

Understanding groundwater origin and characterization of a deep circulation is a big challenge since the groundwater pathways and aqueous chemistry are significantly influenced by various factors. To contribute to the characterization of the hydrogeological systems in which the mineral and thermal waters of Serbia are formed, a general hydrochemical study was conducted. During this research 190 of the most significant sources of mineral and thermal waters were sampled, belonging to different geological (geotectonic) units all over Serbia. The applied hydrochemical approach of recognition of deep circulation patterns is based on an analysis of rare earth elements (REE) and natural radioactivity. REE and long-lived radionuclides ⁴⁰K, ²³⁸U, ²³²Th, ^226,228Ra, gross alpha, and beta radioactivity, have proven to be significant fingerprints of water-rock interaction as well as groundwater flow tracers.

The integrated approach of the hydrogeochemical analysis and multivariate statistical method, including spatial mapping of obtained results, was an important process for meaningful interpretation of the data set. The applied approach summarized the complex hydrochemical properties on a general level defining specific hydrochemical fingerprints of hydrogeological systems with distinct geochemical characteristics and flow patterns. Geochemical behavior of natural tracers (REE) and radioactivity contributed to further characterization of deep hydrogeological systems in basins structures, hard rocks (igneous and metamorphic rocks), as well as carbonate environments.

Rare-earth element data (including abundances and fractionation patterns along with anomalies of Ce and Eu and interelement ratios), relationships of U and Th as elements with different geochemical behavior, and the content of Ra in groundwaters have been singled out as important indicators of deep hydrogeological systems. The results showed that the isolated regional hydrogeological systems are in the function of significant tectonic structures/dislocations, but also hydrogeological characteristics and circulation conditions. Further use of the proposed methodology will provide important data from the assessment of the origin of hydro-geofluids in Serbia and contribute to the wider picture in the understanding of the hydrogeological evolution of regional groundwater flow.

Keywords: natural radioactivity, rare earth elements, hydrogeochemical fingerprints, regional groundwater flow

How to cite: Ćuk Đurović, M., Todorović, M., Jemcov, I., and Papić, P.: Long-lived Radioactive Elements and REE as Fingerprints of Deep Groundwater Flow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7079, https://doi.org/10.5194/egusphere-egu21-7079, 2021.

A 3-D Model of Surface Water and Groundwater Interaction in the Central Passaic River

Basin, New Jersey.

Duke U. Ophori

Department of Earth and Environmental Studies

Montclair State University

Upper Montclair

New Jersey 0743

Ph: (973) 464 6238, Email: Ophorid@montclair.edu

Abstract

Hydrogeologists have believed, over the years, that the development of a groundwater basin results in increase in the basin’s groundwater recharge and decrease in discharge. This basin’s response to development is intricately tied with the natural recharge-discharge behavior of regional groundwater flow systems. A 3-D regional groundwater flow model is developed for the Central Passaic River Basin, New Jersey to evaluate its surface water-groundwater interactions. The models show that groundwater pumping increases recharge and decreases discharge in the basin. Groundwater-fed wetlands are reduced in size by groundwater pumping. These findings will enhance groundwater management in the basin.

How to cite: Ophori, D.: A 3-D Model of Surface Water and Groundwater Interaction in the Central Passaic River Basin, New Jersey., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13782, https://doi.org/10.5194/egusphere-egu21-13782, 2021.

Groundwater plays an active role in certain geologic processes that has been recognized in numerous subdisciplines for a long time. According to Toth (1963, 2009), gravity-driven regional groundwater flow is induced by elevation differences in the water table and its pattern is self-organized into hierarchical sets of local, intermediate and regional flow systems.  Convergence of two flow systems results in a stagnant zone called hydraulic trap which is under the discharge area, and diverge of two flow systems results in a stagnant zone called quasi-stagnant zone which is under the water divide. These stagnant zones have been found to be critical to accumulation of transported mineral matter. Based on analytical and numerical solutions, some researchers reported that the local stagnant point or zone that are located under the local counter directional flow system. There is a question that whether hydraulic trap and quasi-stagnant zone is separate or integrate, and whether they are located under the discharge area or water divide or counter directional flow systems.

In this study, two-dimensional numerical cross-sectional model is used to investigate the effect of climate change on local stagnant zones and whether the hydraulic trap and quasi-stagnant zone is separate or integrate. Considering the climate change of basin and the change of rainfall infiltration intensity, a flux upper boundary is used to simulate the rainfall recharge. Then a synthetic homogeneous sandbox with three potential sinks is used to validate the evolution of the hierarchical nested groundwater flow systems considering different rainfall infiltration intensity. Salt tracer test is used to investigate the effect of stagnant zones on solute transport.

According to numerical results, we concluded that the hydraulic traps and quasi-stagnant are possible to be separate only for simple local systems and the two local stagnant zones are located on two sides of the counter directional flow system. When nested flow systems occur, such as local-intermediate, local-intermediate-local, local-regional, the local hydraulic traps and quasi-stagnant zones are always integrated under the local counter directional flow systems. Laboratory results show that when the rainfall infiltration intensity reduce, the groundwater flow pattern will change and the penetration depth and scope of counter directional local flow system will decrease. The corresponding local stagnant zone will slowly be closing to the discharge area of that counter directional local flow system. Salt tracer tests show that there are obvious non-fickian phenomenon in the local stagnant zones in hierarchically nested flow systems even in the homogeneous aquifer.

How to cite: Sun, R., Jiang, L., Liang, X., and Jin, M.: The evolution and non-fickian flow of local stagnant zones in hierarchically nested groundwater flow system under different rainfall infiltration intensity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3929, https://doi.org/10.5194/egusphere-egu21-3929, 2021.

The distribution of groundwater ages under transient conditions are investigated by a numerical model coupled groundwater flow and age, and the nested pattern of groundwater flow are determined by the probability density function of residence time. The variation of local groundwater flow system to the fluctuation of upper boundary head evolves rapidly. During the process from the initial steady to the unsteady state, the groundwater age field evolves with simulation time and gradually reaches a new dynamic equilibrium after about 50 years. The age abrupt interface between the local and intermediate flow systems gradually shifts upward, and the scale of the local flow system gradually decreases. The groundwater ages of the regional and intermediate flow systems are mainly controlled by the long-term dynamic component of the upper boundary head, while the local flow systems are mainly influenced by the transient periodic fluctuation. The location of the stagnation points are mainly controlled by the upper boundary head. The larger head difference between recharge and discharge area is, the greater penetrated depth of the stagnation point is. The location of the stagnation point indicates the penetrated depth of the local flow system. The larger head fluctuates, the deeper stagnation point is, leading to a greater penetration depth of the local flow system. Molecular dispersion causes the scatters of residence time probability density function to aggregate near the inflection point, and the aggregation area mainly locates at the junction of basin-scale flow systems. The transition of groundwater flow field will intensify the mixing of old and new water, leading to the blurring or even disappearance of the residence time abrupt interface. The dispersion of groundwater mixing is poor in steady state, and the convective-dispersive effect gradually increases with time in unsteady state. Traditional hydraulics methods based on flow nets and stagnation points can effectively identify the groundwater flow system, but the differences in groundwater chemical characteristics and ages at long-term scales cannot be clearly described by these methods, as well as the evolution of groundwater flow system at long time scale. The groundwater residence time distribution expressed by the probability density function, which comprehensively involves the spatial and temporal information of groundwater interaction, can help accurately distinguish different groundwater flow systems at long time scales. The methods proposed in this study will act as a meaningful guidance for the delineation of groundwater flow system in the real world.

How to cite: Jin, M., Li, Y., Wang, J., and Liang, X.: Groundwater flow pattern and age distribution under transient conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7083, https://doi.org/10.5194/egusphere-egu21-7083, 2021.