Shallow Aquifer System Research Articles

Active rock glaciers as shallow groundwater reservoirs, Austrian Alps

Rock glaciers are the most prominent landforms of alpine permafrost and comprise complex shallow aquifer systems in (high) alpine catchments. Recession analyses of groundwater discharge of four active rock glaciers that contain permafrost ground ice show that they have a base flow component of the order of a few liters per second, similar to that of a relict rock glacier in which permafrost ground ice is absent. This is related to an unfrozen (fine-grained) base layer with a thickness of about 10 m. Based on a threshold analysis of precipitation events and event water discharge, depressions atop the bedrock or the permafrost table seem to play only a minor role in storing groundwater. This important finding has rarely been documented, but is highly relevant for optimal groundwater resources management in sensitive (high) alpine catchments and ecosystems. All the rock glaciers analyzed here are located in the Austrian Alps and represent the nationwide sites where suitable discharge data are available. The analysis highlights the hydrogeological importance of these discrete permafrost-derived debris accumulations as complex shallow groundwater bodies with important—but limited—storage and buffer capabilities.

Groundwater Throughflow and Seawater Intrusion in High Quality Coastal Aquifers

High quality coastal aquifer systems provide vast quantities of potable groundwater for millions of people worldwide. Managing this setting has economic and environmental consequences. Specific knowledge of the dynamic relationship between fresh terrestrial groundwater discharging to the ocean and seawater intrusion is necessary. We present multi- disciplinary research that assesses the relationships between groundwater throughflow and seawater intrusion. This combines numerical simulation, geophysics, and analysis of more than 30 years of data from a seawater intrusion monitoring site. The monitoring wells are set in a shallow karstic aquifer system located along the southwest coast of Western Australia, where hundreds of gigalitres of fresh groundwater flow into the ocean annually. There is clear evidence for seawater intrusion along this coastal margin. We demonstrate how hydraulic anisotropy will impact on the landward extent of seawater for a given groundwater throughflow. Our examples show how the distance between the ocean and the seawater interface toe can shrink by over 100% after increasing the rotation angle of hydraulic conductivity anisotropy when compared to a homogeneous aquifer. We observe extreme variability in the properties of the shallow aquifer from ground penetrating radar, hand samples, and hydraulic parameters estimated from field measurements. This motived us to complete numerical experiments with sets of spatially correlated random hydraulic conductivity fields, representative of karstic aquifers. The hydraulic conductivity proximal to the zone of submarine groundwater discharge is shown to be significant in determining the overall geometry and landward extent of the seawater interface. Electrical resistivity imaging (ERI) data was acquired and assessed for its ability to recover the seawater interface. Imaging outcomes from field ERI data are compared with simulated ERI outcomes derived from transport modelling with a range of hydraulic conductivity distributions. This process allows for interpretation of the approximate geometry of the seawater interface, however recovery of an accurate resistivity distribution across the wedge and mixing zone remains challenging. We reveal extremes in groundwater velocity, particularly where fresh terrestrial groundwater discharges to the ocean, and across the seawater recirculation cell. An overarching conclusion is that conventional seawater intrusion monitoring wells may not be suitable to constrain numerical simulation of the seawater intrusion. Based on these lessons, we present future options for groundwater monitoring that are specifically designed to quantify the distribution of; (i) high vertical and horizontal pressure gradients, (ii) sharp variations in subsurface flow velocity, (iii) extremes in hydraulic properties, and (iv) rapid changes in groundwater chemistry. These extremes in parameter distribution are common in karstic aquifer systems at the transition from land to ocean. Our research provides new insights into the behaviour of groundwater in dynamic, densely populated, and ecologically sensitive coastal environments found worldwide.

Hydrogeological framework of the volcanic aquifers and groundwater quality in Dangila Town and the surrounding area, Northwest Ethiopia

Volcanic aquifers are sources of groundwater for both urban and rural areas. Their occurrence in geological formations and water quality are the main issues for sustainable utilisation. Dangila town and its surrounding area, northwest Ethiopia, is assessed for hydrogeological framework and groundwater quality based on well and hydrochemical data. The study showed that the area has a multi-aquifer system: an unconfined perched aquifer at shallow depth and; semi-confined and confined aquifers at greater depth in Quaternary basalts.The aquifer has five groundwater facies: Ca-HCO3, Ca-Mg-HCO3, Ca-Na-HCO3, Na- Ca-HCO3 and Na-HCO3. The Ca-HCO3 groundwater types are dominant in the shallow unconfined aquifer system, and the Na- Ca-HCO3 and Na-HCO3 types are dominant in deep aquifer systems. Mixing of waters from shallow and deep aquifer systems is minimal and possible only through interconnected fractures. Groundwater storage and flow is controlled by intensity and interconnection of fractures. Hydraulic conductivity is higher for fracture dominant aquifers than weathered rock aquifers. The groundwater chemistry is mainly controlled by rock weathering and cation exchange. Groundwater quality assessment shows that water from both aquifer sources is potable. Based on sodium adsorption ratio (SAR) and residual sodium carbonate (RSC) values, all groundwater schemes are in the excellent quality class and can be used for irrigation without any problem. However, high Na+ in borehole samples restrict the deep groundwater suitability for irrigation use without adjustment. The combined results of the study are used to construct a hydrogeological conceptual model of volcanic aquifers, which can be used to manage the groundwater resource for sustainable development. This study would be helpful for water resources management in other similar geological settings.

Noble gas as a proxy to understand the evolutionary path of migrated CO2 in a shallow aquifer system

To provide confidence in the safety of a carbon capture and storage (CCS) project, researchers have focused on developing monitoring techniques to trace the unlikely, but potentially possible, migration of CO2 from a deep reservoir. Among the various techniques, noble gas tracing is a beneficial approach, owing to the unique noble gas fingerprints present in injection fluids, the deep reservoir, and the shallow aquifer above the storage area. However, the value of this approach has been limited to demonstrations in a natural analogue CO2-rich reservoir and an artificial injection test site. Therefore, further efforts are required to link those valuable observations to an actual CCS site. In this study, we outline how to use these tracers for actual monitoring work in a shallow aquifer system. First, two artificial injection tests were performed using He, Ar, Kr, and SF6 to understand the behavior of the leaked plume in the shallow aquifer system. In both tests, the noble gas ratio remarkably changed with the solubility-controlled process and the mixing process. To extend and link the valuable findings from the artificial injection tests to an actual CO2 leakage event, we performed a leakage simulation using data from a real CO2 injection site, i.e., the Weyburn–Midale site. This simulation suggested that combinations of 4He with other heavier noble gases can be used to monitor CO2 leakage, as they allow us to separate and explain the major interactions governing the migration of the leaked plume in the shallow aquifer system. Additionally, although the high CO2 density of a dissolved plume is known to add uncertainty in quantitative approaches, the influence of those effects was negligible when compared to the errors arising from the wide variation in the noble gas fingerprints in the leaked CO2. This study, therefore, provides insight into the evolutionary path of the migrated CO2 plume in the shallow aquifer system and to the results can be used to inform the tracing of a leakage source within a shallow aquifer despite various mechanisms complicating the plume distribution.

Evidence of groundwater vulnerability to climate variability and economic growth in coastal Kenya

Climate change and future abstraction regimes will influence the availability of groundwater resources. To alleviate any potential negative effects on aquifer systems and dependent industrial and human uses, it is important to develop long-term water management plans. This study evaluates the effect of climate change and future increased groundwater demand from a coastal aquifer located in Kwale County in southern Kenya. A previously calibrated numerical groundwater flow model has been used as an assessment tool to study how future climate (precipitation and temperature variation) and groundwater abstraction changes will affect the aquifer system. The groundwater flow model was built to simulate the period 2010 to 2017, and eight future model scenarios were developed that cover six hypothetical future years. Future groundwater abstraction has been based on current allocations and future estimates made by Kenya’s Water Resources Authority. Future rainfall scenarios have been constructed based on a long historical data series (from 1959 to 2017) and the Standard Precipitation Index. The main results show that future abstraction increases due to economic growth exerts a minimum impact compared with expected climate variability. Recharge depends on intense rain events with important implications for both dry periods and for an average rainfall year. A succession of extended dry seasons may affect all water users. However, the groundwater level decline in the local shallow aquifer can reach five meters, with important consequences for local community water supplies. The most significant groundwater decline in drought periods is observed in the area surrounding the pumping wellfields in the deep aquifers, where the effects of drought and significant abstraction are multiplied. However, the effect of increased abstraction on the shallow aquifer system is limited. Despite groundwater level decline observed during prolonged dry periods, a dry period followed by a humid period leads to the relatively swift recovery of the groundwater system.

The first consistent inventory of rock glaciers and their hydrological catchments of the Austrian Alps

Abstract A first consistent and homogenized polygon-based inventory of rock glaciers of the Austrian Alps is presented. Compiling previous inventories and updating them by using digital elevation models (1 m grid resolution) derived from airborne laser scanning yield a dataset of 5769 rock glaciers in a ca. 48400 km2 large area. A consistent methodological approach for assigning attributes, stored in a detailed attribute table, was developed and applied here to improve comparability and reproducibility. The majority (60 %) of the studied landforms is considered to be relict (no permafrost); the remaining 40 % may still contain permafrost ice and are thus classified as intact. Rock glaciers range in elevation from 476 to 3312 m a.s.l. and cover a total area of 303 km2. The distribution of rock glaciers is mainly related to the topography of the Austrian Alps and related effects such as past glaciation history. In addition, a comprehensive analysis of the hydrological catchment areas of all individual rock glaciers was carried out. A hydrological catchment analysis in rock glacier areas is of great interest for sustainable water management issues in alpine catchments as these landforms represent shallow aquifer systems with a relatively high storage and thus buffer capability, especially in crystalline bedrock areas. A total area of almost 1280 km2 is drained through rock glaciers. The presented rock glacier and rock glacier catchment inventories provide an important basis for further research, particularly for a better understanding of the hydrogeology and geomorphology of alpine catchments and their potential alteration in the light of climate change, but also in terms of paleoglaciation and deglaciation in the Alpine Lateglacial to Holocene period. As such, the inventories are seen as an important base to stimulate further research.

Seasonal Groundwater Quality Status and Nitrogen Contamination in the Shallow Aquifer System of the Kathmandu Valley, Nepal

: The increasing concentration of nitrogen compounds in the groundwater is of a growing concern in terms of human health and groundwater quality. Although an excess of nitrogen compounds in the groundwater of the Kathmandu Valley has been reported, the seasonal variations of the fate of the nitrogen compounds and their relationships to the subsurface sediments are unknown. In this study, spatially distributed shallow dug well samples were collected during both the dry and wet seasons of 2016, and the nitrogen compound, chloride (Cl−), and iron (Fe2+) concentrations were analyzed. Two shallow dug wells and one deep tube well were monitored monthly for 2 years. Although NH4-N concentrations were similar in the clay-dominated areas during both seasons (1 and 0.9 mg-N/L), they were lower in the gravel-dominated areas during wet season (1.8 > 0.6 mg-N/L). The NO3-N concentration differed depending upon the soil type which increased during the wet season (clay 4.9 < 13.6 mg-N/L and gravel 2.5 < 6.8 mg-N/L). The Fe2+ concentration, however, was low during the wet season (clay 2.7 > 0.4 mg/L and gravel 2.8 > 0.3 mg/L). Long-term analysis showed higher fluctuation of nitrogen compounds in the gravel-bearing areas than in the clay-bearing areas.

Numerical Simulation of the Influence of Geological CO2 Storage on the Hydrodynamic Field of a Reservoir

CO2 geological storage in deep saline aquifers is an effective way to reduce CO2 emissions. The injection of CO2 inevitably causes a significant pressure increase in reservoirs. When there exist faults which cut through a deep reservoir and shallow aquifer system, there is a risk of the shallow aquifer being impacted by the changes in reservoir hydrodynamic fields. In this paper, a radial model and a 3D model are established by TOUGH2-ECO2N for the reservoir system in the CO2 geological storage demonstration site in the Junggar Basin to analyze the impact of the CO2 injection on the deep reservoir pressure field and the possible influence on the surrounding shallow groundwater sources. According to the results, the influence of CO2 injection on the reservoir pressure field in different periods and different numbers of well is analyzed. The result shows that the number of injection wells has a significant impact on the reservoir pressure field changes. The greater the number of injection wells is, the greater the pressure field changes. However, after the cessation of CO2 injection, the number of injection wells has little impact on the reservoir pressure recovery time. Under the geological conditions of the site and the constant injection pressure, although the CO2 injection has a significant influence on the pressure field in the deep reservoir, the impact on the shallow groundwater source area is minimal and can be neglected and the existing shallow groundwater sources are safe in the given project scenarios.

A Multiscale Assessment of Shallow Groundwater Salinization in Michigan

Managing nonpoint-source (NPS) pollution of groundwater systems is a significant challenge because of the heterogeneous nature of the subsurface, high costs of data collection, and the multitude of scales involved. In this study, we assessed a particularly complex NPS groundwater pollution problem in Michigan, namely, the salinization of shallow aquifer systems due to natural upwelling of deep brines. We applied a system-based approach to characterize, across multiple scales, the integrated groundwater quantity-quality dynamics associated with the brine upwelling process, assimilating a variety of modeling tools and data-including statewide water well datasets scarcely used for larger scientific analysis. Specifically, we combined (1) data-driven modeling of massive amounts of groundwater/geologic information across multiple spatial scales with (2) detailed analysis of groundwater salinity dynamics and process-based flow modeling at local scales. Statewide "hotspots" were delineated and county-level severity rankings were developed based on dissolved chloride (Cl- ) concentration percentiles. Within local hotspots, the relative impact of upwelling was determined to be controlled by: (1) streams-which act as "natural pumps" that bring deeper (more mineralized) groundwater to the surface; (2) the occurrence of nearly impervious geologic material at the surface-which restricts fresh water dilution of deeper, saline groundwater; and (3) the space-time evolution of water well withdrawals-which induces slow migration of saline groundwater from its natural course. This multiscale, data-intensive approach significantly improved our understanding of the brine upwelling processes in Michigan, and has applicability elsewhere given the growing availability of statewide water well databases.

A simulation model for estimating root zone saturation indices of agricultural crops in a shallow aquifer and canal system

Shallow aquifers significantly impact crop growth as saturated soil conditions may occur. Canals are widely constructed in such areas to mitigate groundwater saturation or flooding. We applied a simulation model to estimate the occurrence of root zone saturation [root zone saturation index (RZSI)] for agricultural crops and to identify factors that influence root zone saturation in a shallow coastal aquifer and canal system. Results indicated that groundwater modeling combined with multiple linear regression can relate the influencing factors and root zone saturation durations in low lying farmland adjacent to canal systems. In our study, most areas had a low RZSI, but areas towards the northwest and southeast where the land surface elevation is generally low were predicted to have a greater RZSI. In general, positive correlations were found between the root zone saturation durations and rainfall amount, antecedent groundwater table elevation and average canal stages in areas where the higher RZSIs were predicted. Rainfall amount played a more important role than antecedent groundwater table elevation and canal stage in determining the root zone saturation during the wet season, while antecedent groundwater table elevation and canal stage played a more important role than rainfall amount during dry season. Correlations between the predicted root zone saturation duration and land surface elevation were negative and stronger during the wet season than the dry season, and the correlations were stronger in the deep (0–61 cm) root zone than with the shallow (0–18 cm) root zone. In area where the land surface elevation is relatively high, the root zone saturation duration was not influenced by rainfall amount, antecedent groundwater table elevation or canal stage, at least under the current management practices and climate conditions.

Application of Inert Gas Tracers to Identify the Physical Processes Governing the Mass Balance Problem of Leaking CO2 in Shallow Groundwater System

The CO2, leaked from a deep storage reservoir, can arrive at the shallow groundwater system. In shallow aquifer, the leaked CO2 forms a multi-phase plume and its mass balance will be controlled by various physical, chemical and biological processes. Inert gases such as sulfur hexafluoride (SF6) and noble gases are biochemically stable in groundwater system which can separate and explain the physical process governing the mass balance of the multi-phase plume. In this study, two pilot tests were conducted using inter gas tracers to evaluate the influence of physical processes on multi-phase plume migration. First injection test was made in Wonju, Korea. Three different tracers including chlorine, helium and argon tracer were jointly injected in 7.5 – 10.5 m below water table and recollected at same point after 1 day drift time in groundwater system. The mass recovery of the Single-Well Tracer Test (SWTT) was 49.3% for SF6, 58.1% for helium, 78.2% for argon, and 73.1% for chlorine. The recovery of inert gas tracers was relative to their solubility. Second injection test was made in Eumseong, Korea. Four tracers such as chlorine, helium, argon, and krypton were released along an induced pressure gradient where 0.0135% of helium, 0.0137% of argon, 0.0602% of krypton, and 75.0% of chlorine were retrieved. In this Inter-Well Tracer Test (IWTT), the recovery of inert gas tracer was also relative to tracer’s solubility. This study suggested the applicability of inert gas tracers to evaluate the mass balance of leaking CO2 plume in shallow aquifer system.

A Rapid Method for Analyzing Well Log Records in Preparation for Water Resource Mapping.

The needs of subregional and local entities for water resource management are immediate, and required information is not forthcoming in a timely and relatively inexpensive manner. The following method allows one to use all available well record data to produce baseline hydrogeologic unit maps using geographic information system querying and mapping techniques. Subjective borehole and geophysical strata descriptions are converted to objective equivalent numerical values by establishing the average hydraulic conductivity of the combined soils making up the stratum of interest. These hydraulic conductivity values are then used to assign each stratum to a hydrogeologic unit based on typical ranges of hydraulic conductivity for aquifers, aquitards and aquicludes. The resulting data are queried and mapped to provide a preliminary description of a shallow aquifer system composed of unconsolidated materials. This method can be used to provide information to make initial estimates of the configuration of the shallow aquifer system and to plan for groundwater sustainability. Two examples of maps for an area in northeastern Illinois that were prepared using the resulting database are presented.

Origin and Dynamics of Saltwater Intrusion in a Regional Aquifer: Combining 3‐D Saltwater Modeling With Geophysical and Geochemical Data

AbstractWorldwide, aquifers in low‐lying coastal areas are threatened by saltwater occurrence, as a result of small head gradients, high groundwater abstraction rates, and drain management of the landscape, which is likely to intensify with climate change. Numerical models can serve as tools to identify the sources of the salt and thus to increase understanding of the driving mechanisms and important parameters controlling the extent of saltwater intrusions. This way, areas vulnerable to sea level rise can be identified and managed. Challenges include unknown initial salt concentrations, heterogeneous geology, and anthropogenic alterations. In this study, hydrogeological, geophysical, and geochemical data are used to develop a numerical density‐dependent groundwater flow and transport model with the objective to understand the history of a saltwater‐affected groundwater system and its likely response to historic and future changes. The extent of the simulated saltwater intrusion compares well with Airborne Electromagnetic data that show salt water up to 20 km inland. The results reveal that the salt water originates from a combination of laterally intruding seawater and vertically infiltrating transgression water. Main features controlling the progression of the modern seawater into the coastal aquifers are high permeable, deep Miocene sand aquifers, buried valleys that provide preferential flow paths in combination with extensive Miocene clay layers that delay saltwater intrusion. Anthropogenic activity enhances the saltwater inflow from the ocean and induces transient conditions. Future scenarios show that saltwater progression due to nonstationarity leads to enhanced contamination of the deeper aquifers. Climate change affects primarily the shallow aquifer systems.

Seasonal microbial variation accounts for arsenic dynamics in shallow alluvial aquifer systems.

Determining the temporal variation of microbial communities in groundwater systems is essential to improve our understanding of hydrochemical dynamics in aquifers, particularly as it relates to the fate of redox-sensitive contaminants like arsenic (As). Therefore, a high-resolution hydrobiogeochemical investigation was conducted in the As-affected alluvial aquifer systems of the Jianghan Plain. In two 25 m-deep monitoring wells, the seasonal variation in the composition of groundwater microbial communities was positively correlated with the change in groundwater level (R = 0.47 and 0.39 in NH03B and NH05B, respectively, P < 0.01), implying that the latter could be a primary driver of the seasonal microbial dynamics. In response to the fluctuating groundwater level, iron (Fe) reducers within the Desulfuromonadales were dominant (9.9 ± 4.7% among different sampling sites) in groundwater microbial communities during the monsoon season and associated with high concentrations of Fe(II) and As, while the predominance (16.7 ± 15.2% among different sampling sites) of iron-oxidizers the Gallionellaceae was accompanied by low Fe(II) and As in the non-monsoon season. These results suggest that microbially-mediated iron reduction/oxidation may have governed the seasonal mobilization/scavenging of As in groundwater. Our results provide new insights into mechanisms responsible for seasonal variations in groundwater As concentrations in similar aquifer systems.

Groundwater characterization from an ecological and human perspective: an interdisciplinary approach in the Functional Urban Area of Parma, Italy

In the Parma Functional Urban Area, hydro-geo-ecology was investigated through an interdisciplinary approach, with emphasis on the shallow aquifer system. The study pointed out that domestic wells and fontanili are both fed by shallow groundwater affected by PCE and nitrate contamination, upgradient of the rural area located north of Parma City. Moreover, Folsomia candida tests suggested the possibility that other types of contaminants (not analysed in this study) can affect the shallow groundwater. Nowadays, PCE concentrations in the city centre are slightly higher than the limit set by law. Moreover, PCE aerobic biodegradation can be due to the local microbial community and then an effective natural attenuation can be expected along the groundwater flow pathway. These results suggest a very low risk for human health, linked to the groundwater consumption in the rural area north of Parma City. Conversely, no forecasts can be made at present about the possible impact of low PCE concentrations on the aquatic ecosystem observed at the fontanili. Concerning nitrate contamination, the higher concentrations detected in some wells and fontanili suggest a high risk for both human health and aquatic ecosystems. In a wider context, thanks to the interdisciplinary approach that combines successfully well-established investigation methods, the present study allows a better knowledge of the hydro-geo-ecological behaviour of groundwater-dependent ecosystems. At the same time, through purpose-designed experimental investigations and simulation models, this approach could be used as a sort of guideline useful in studying such complex environmental systems.

Assessments of seasonal groundwater recharge and discharge using environmental stable isotopes at Lower Muda River Basin, Malaysia

An accurate estimation of groundwater recharge is required to properly manage aquifers, especially for riverbank filtration method (RBF) purposes. The isotopes correlations and differences in different water bodies were studied to assess the sources of groundwater recharge and preliminary tools in understanding of the surface water and groundwater interactions in the Lower Muda River Basin. The environmental isotope and hydrochemical sampling results had emphasised that the area near Lower Muda River Basin had a connection with the river and was actively recharging the near-river shallow alluvial aquifer, via RBF method. Furthermore, the shallow groundwater that was close to Muda River from groundwater signatures had indicated the recharge of the shallow aquifer system by Muda River based on the plots along LMWL on a δ2H versus δ18O. The comparisons between like δ2H and δ18O isotopes in the rainwater revealed the variations in the rainfall amount and the 18O-depleted water of those isotopes for wet season precipitation as compared to dry seasons. Furthermore, the groundwater δ2H and δ18O isotopes exhibited a slight deviation from the δ2H and δ18O isotopic meteoric water line in Lower Muda River. Therefore, in this basin, the groundwater could be a combination of river water and precipitation, which had led to the recharge of river water being more than the recharge of rainfall infiltration.

Fractionation processes of δ13C in a soil-shallow groundwater system of a potential CO2 sequestration site: Wandoan, QLD, Australia

Near-surface leakage detection is often a crucial part of verifying the success of CO2 sequestration projects, and it cannot be achieved without a detailed description of the natural state of an operational site prior to injection. Without baseline studies, it is difficult to define CO2 anomalies outside natural variation in the subsurface that may be related to leakage. CO2 concentrations and δ13C-CO2 values (of DIC, dissolved gas, and soil gas) were investigated to establish a baseline modelling of natural chemical processes leading to CO2 occurrence and δ13C-CO2 fractionation in the soil-groundwater interface on an example at the Glenhaven site in Queensland, Australia. This was analysed within the context of the hydrogeology and hydrogeochemistry of a shallow aquifer system (<20 m depth). The Glenhaven site and its five monitoring station are located in an area which is a candidate for a future small scale carbon storage pilot project. We combine major ion, stable isotope data of groundwater and soil gas as well as physical hydrogeology data with extensive isotope models to explain the pathways and origins of groundwater and CO2. The results indicate shallow aquitard claystone units at the site restrict vertical groundwater leakage and inter-aquifer mixing. Groundwater chemistry is controlled by transpiration of plants, cation exchange, weathering and meteoric recharge to produce Na-Cl and Na-HCO3-Cl type waters. δ18O and δ2H values suggest a shallow meteoric recharge source largely unaffected by evaporation. CO2 concentrations in soil and dissolved in groundwater range between 631 and 47,516 ppm and δ13C values average −17 and −21‰. The mixing of three potential CO2 sources is discussed: 1) the aerobic decay of C3 plant matter, 2) the aerobic decay of C4 plant matter and, 3) shallow coal related CO2. Despite this, the variability of CO2 concentrations and equivocal sources is not likely to hinder leakage detection in the future of site monitoring.

Evaluation of water quality using hydrochemistry, stable isotopes, and water quality indices in the Lower Volta River Basin of Ghana

The Lower Volta River Basin of Ghana is an important international and local tourism destination. Communities in the basin depend on shallow aquifer systems, the Volta River, and nearby streams as their sources of water for domestic purposes, cattle watering, and irrigation. Due to increasing human activities and limited information on water quality in the area, a holistic assessment of the quality of the water resources in the basin was conducted using hydrochemistry, δ2H and δ18O, principal components analysis, and selected indices. EC ranged from 141 to 19,370 μS/cm and 60.9–986 μS/cm for shallow groundwater and surface water respectively. NO3− levels above 45–1049.9 mg/l were observed in 45% of analyzed wells. Na–Cl- and HCO3–Ca-dominant facies are identified. Silicate weathering, evaporite dissolution, cation exchange, and salinity are the hydrochemical processes influencing the shallow groundwater in the area. δ2H–δ18O‰ plot showed meteoric shallow groundwater recharge. Ion ratios, EC, and Cl− versus δ18O‰ relationships suggest dissolution and seawater intrusion as causes of groundwater salinity. Hydrochemistry, principal components analyses, and NO3− versus δ18O showed evidence of anthropogenic pollution on water in the basin. All analyzed surface water and 79% of well samples were within suitable range for irrigation. Based on the CCMEWQI model criteria, 85% of the surface water were in good-to-excellent categories while majority of the shallow groundwater were in fair, marginal, and poor categories. The combined approach provided vital information on water quality in the basin. Strategic control measures were proposed.

Hydrogeochemical characterization of a shallow alluvial aquifer: 1 baseline for groundwater quality assessment and resource management

Considering the progressive depletion of groundwater quantity and the alteration of superficial and groundwater quality in the Madrid region, a highly populated area with extended urbanizations in the north and agricultural activities in the south, we conducted a monitoring study over a period of 2.5 years (2015–2017). The study was conducted in the Jarama-Tajuña shallow alluvial aquifer system located southeast of Madrid, where the exploitation of the aquifer and the Jarama river have increased exponentially in the last decade being affected by both urban and agricultural activities. Our aims were to provide: i) a geochemical characterization of the surface and groundwater properties; ii) identify the process responsible for the geochemical evolution and mineralization of the waters; iii) assess the water quality (i.e. nitrates, ammonia, sodium and chloride as potential contaminants) and the water suitability for irrigation (SAR, Wilcox, KI and MH indexes); and iv) identify the main sources of contamination in the area.A set of plots, ion ratios, correlation coefficients, multivariate statistical analyses and indexes were performed. Results indicated that rock weathering largely controls the hydrogeochemistry of the system and that wastewater treatment plant discharges and agricultural practices significantly affect the composition of the water, causing an important decline of both surface and groundwater chemical quality. Nonetheless, water suitability for irrigation is admissible. Thus, taking additional measures to increase its quality are not necessary.With this study we aimed to establish a base line to evaluate future changes in the groundwater properties from the Madrid region enabling the planners and policy makers to develop a strategy to mitigate the impact of the exponential increase of urban and agriculture activities on groundwater resources.

Electrical Resistivity Imaging and the Saline Water Interface in High-Quality Coastal Aquifers

Population growth and changing climate continue to impact on the availability of natural resources. Urbanization of vulnerable coastal margins can place serious demands on shallow groundwater. Here, groundwater management requires definition of coastal hydrogeology, particularly the seawater interface. Electrical resistivity imaging (ERI) appears to be ideally suited for this purpose. We investigate challenges and drivers for successful electrical resistivity imaging with field and synthetic experiments. Two decades of seawater intrusion monitoring provide a basis for creating a geo-electrical model suitable for demonstrating the significance of acquisition and inversion parameters on resistivity imaging outcomes. A key observation is that resistivity imaging with combinations of electrode arrays that include dipole–dipole quadrupoles can be configured to illuminate consequential elements of coastal hydrogeology. We extend our analysis of ERI to include a diverse set of hydrogeological settings along more than 100 km of the coastal margin passing the city of Perth, Western Australia. Of particular importance are settings with: (1) a classic seawater wedge in an unconfined aquifer, (2) a shallow unconfined aquifer over an impermeable substrate, and (3) a shallow multi-tiered aquifer system over a conductive impermeable substrate. We also demonstrate a systematic increase in the landward extent of the seawater wedge at sites located progressively closer to the highly urbanized center of Perth. Based on field and synthetic ERI experiments from a broad range of hydrogeological settings, we tabulate current challenges and future directions for this technology. Our research contributes to resolving the globally significant challenge of managing seawater intrusion at vulnerable coastal margins.