Shallow Alluvial Aquifer Research Articles

Characterising alluvial aquifers in a remote ephemeral catchment (Flinders River, Queensland) using a direct push tracer approach

The availability of reliable water supplies is a key factor limiting development in northern Australia. However, characterising groundwater resources in this remote part of Australia is challenging due to a lack of existing infrastructure and data. Here, direct push technology (DPT) was used to characterise shallow alluvial aquifers at two locations in the semiarid Flinders River catchment. DPT was used to evaluate the saturated thickness of the aquifer and estimate recharge rates by sampling for environmental tracers in groundwater (major ions, 2H, 18O, 3H and 14C). The alluvium at Fifteen Mile Reserve and Glendalough Station consisted of a mixture of permeable coarse sandy and gravely sediments and less permeable clays and silts. The alluvium was relatively thin (i.e. < 20 m) and, at the time of the investigation, was only partially saturated. Tritium (3H) concentrations in groundwater was ∼1 Tritium Unit (TU), corresponding to a mean residence time for groundwater of about 12 years. The lack of an evaporation signal for the 2H and 18O of groundwater suggests rapid localised recharge from overbank flood events as the primary recharge mechanism. Using the chloride mass balance technique (CMB) and lumped parameter models to interpret patterns in 3H in the aquifer, the mean annual recharge rate varied between 21 and 240 mm/yr. Whilst this recharge rate is relatively high for a semiarid climate, the alluvium is thin and heterogeneous hosting numerous alluvial aquifers with varied connectivity and limited storage capacity. Combining DPT and environmental tracers is a cost-effective strategy to characterise shallow groundwater resources in unconsolidated sedimentary aquifers in remote data sparse areas.

Geological and hydrogeochemical controls on radium isotopes in groundwater of the Sinai Peninsula, Egypt

Radium isotopes (226Ra and 228Ra) were analyzed in 18 groundwater samples from the Nubian Sandstone Aquifer System (NSAS) and the shallow alluvial aquifers overlying the basement complex of the Sinai Peninsula, Egypt. Groundwater samples from deep Nubian aquifer wells (total depths 747 to 1250m) have 226Ra and 228Ra activities ranging from 0.168 to 0.802 and 0.056 to 1.032Bq/L, respectively. The shallower Nubian aquifer wells (63 to 366m) have 226Ra and 228Ra activities ranging from 0.033 to 0.191 and 0.029 to 0.312Bq/L, respectively. The basement shallow alluvial aquifers have 226Ra and 228Ra activities ranging from 0.014 to 0.038 and 0.007 to 0.051Bq/L, respectively. Combined Ra activities in most wells were generally in excess of the US Environmental Protection Agency (EPA), the European Union (EU), and the World Health Organization (WHO) maximum contaminant levels (MCL) for drinking water. Radium in groundwater is produced mainly by decay of parent nuclides in the aquifer solids, and observed activities of dissolved Ra isotopes result from a combination of alpha-recoil, adsorption/desorption, co-precipitation/dissolution processes. The observed correlation between Ra activities and salinity indicates that adsorption/desorption processes may be the dominant factor controlling Ra mobility in Sinai groundwater. Radium activities in central and northern Sinai are generally higher than those in southern Sinai, consistent with a gradual increase in salinity and water-rock interaction with increasing groundwater age. Barite is approximately saturated in the groundwater and may limit maximum dissolved Ra concentration. The results of this study indicate that Sinai groundwater should be used with caution, possibly requiring Ra removal from water produced for domestic and agricultural consumption.

Dissolved arsenic in the shallow alluvial aquifers in North Brahmaputra Plain, India: a case study in and around lower Jia Bharali River basin

This study was carried out to investigate Arsenic (As) contamination in the alluvial aquifers of the lower Jia Bharali catchment and adjoining areas in Sonitpur district of Assam. Samples were collected twice a year (July and February) for three consecutive years from 50 monitoring wells spread into both older and younger alluvium between the Brahmaputra River towards south and Arunachal foothills towards north. The analytical results show that dissolved As content [both As(III) and As(V)] varies from below detection level (BDL) to 7.39 µg/L with a mean value of 1.92 µg/L and standard deviation of 1.37 µg/L during wet season (July). Thus, it remains within the WHO (2004) prescribed limit (10 µg/L) in the study area in the wet season. During the dry season, the range of variation is higher, from BDL to as much as 13.8 µg/L with a mean value of 2.57 µg/Land standard deviation of 2.23 µg/L. About 78% of the wells show a concentration between 1 and 10 µg/L in both the seasons. However, only one of the wells present in foothills of Arunachal Himalaya was found to have As content higher than the WHO limit in the dry season. Along with total As, examination of concentration levels of other key parameters, viz., Fe, Mn, Ca, Na, K, and Mg with pH and SO42− was also carried out. Most of the wells (~92%) showed Fe concentration much higher than the WHO (2004) permissible limit of (0.3 mg/L) particularly during the dry season and it is likely that high Fe concentration was responsible for keeping total As concentration at comparatively low levels. 34% of the samples in the wet seasons and 86% of the samples in the dry seasons have Mn above the permissible limit of 0.1 mg/L.

Re-evaluation of Shallow Floodplain Aquifers Groundwater Potentials and Storage of Sokoto Basin, Northwestern Nigeria

Floodplains of Sokoto Basin with covering about 280,000 hectares of land was assessed for its shallow groundwater potentials. Data on pumping test of tube wells, logs and drilled tube wells were utilized for this study. Storativity was determined from known lithology of tube wells. Results of this study gives the ranges and averages of Hydraulic parameters for the shallow alluvial aquifers of hydraulic conductivity, transmissivity, specific capacity and storativity to be 6.66 - 1316 m/day and 214,044 m/day, 40 – 3950 m2/day and 799.5 m2/day, 12.25 – 5763.6 m3/day and 1,031m3/day and 1×10-5 - 6×10-3 and 5×10-4 respectively, While the Yields range of 0.3- 7 l/s was obtained with 90% of tube wells evaluated having yields of 2.2 l/s and above. Lithologic Aquiferous units vary from medium sands, sands, coarse sands to gravel with thickness range of between 0.3 to 15 m. Computed hydraulic conductivity indicates the aquifer materials to be gravels and alluvial sands of high permeability. Specific capacity values indicated the wells to be of high productivity when compared with standard values, Transmissivity average revealed the area to be of high groundwater transmission potentials. Overall assessment of the yields of tube wells studied indicated that 90% of the wells have yields that are above the minimum required for irrigation; the low average drawdown of 0.41 m recorded is an indication that the aquifers have high efficiency and high performance as regards storage and transmission of water. Recharge of the alluvial aquifers is mainly through surface river flow and precipitation, climate change have recently threaten the recharge potential s owing to the drying up of some of these streams and rivers at the peak of dry season as well reduction in rainfall. Statistical analysis has shown a good linear relationship between Specific capacity with yield, Transmissivity and hydraulic conductivity while Transmissivity is linearly related with yield and Conductivity. Generally the study revealed that the Sokoto basin Fadama land to be of high groundwater potentials, with storage coefficient values of confined and high yielding aquifers with capabilities of sustaining withdrawals for long period without recharge from external sources. Standard procedures are highly recommended to reduce or avoid the case of drilling abortive or low yielding wells.

Characterising Bedrock Aquifer Systems in Korea Using Paired Water-Level Monitoring Data

This study focused on characterising aquifer systems based on water-level changes observed systematically at 159 paired groundwater monitoring wells throughout Korea. Using spectral analysis, principal component analysis (PCA), and cross-correlation analysis with linear regression, aquifer conditions were identified from the comparison of water-level changes in shallow alluvial and deep bedrock monitoring wells. The spectral analysis could identify the aquifer conditions (i.e., unconfined, semi-confined and confined) of 58.5% of bedrock wells and 42.8% of alluvial wells: 93 and 68 wells out of 159 wells, respectively. Even among the bedrock wells, 50 wells (53.7%) exhibited characteristics of the unconfined condition, implying significant vulnerability of the aquifer to contaminants from the land surface and shallow depths. It appears to be better approach for deep bedrock aquifers than shallow alluvial aquifers. However, significant portions of the water-level changes remained unclear for categorising aquifer conditions due to disturbances in data continuity. For different aquifer conditions, PCA could show typical pattern and factor scores of principal components. Principal component 1 due to wet-and-dry seasonal changes and water-level response time was dominant covering about 55% of total variances of each aquifer conditions, implying the usefulness of supplementary method of aquifer characterisation. Cross-correlation and time-lag analysis in the water-level responses to precipitations clearly show how the water levels in shallow and deep wells correspond in time scale. No significant differences in time-lags was found between shallow and deep wells. However, clear time-lags were found to be increasing from unconfined to confined conditions: from 1.47 to 2.75 days and from 1.78 to 2.75 days for both shallow alluvial and deep bedrock wells, respectively. In combination of various statistical methods, three types of water-level fluctuation patterns were identified from the water-level pairs: Type I of identical aquifer systems (77.8%), Type II of the different aquifer systems with different recharge flow paths (9.5%), and Type III of unmatched aquifer system pairs and correlations (12.7%). Type I and II could be used as verification of aquifer condition in the paired monitoring system. However, Type III shows the complexity of water-level fluctuation in different aquifer conditions. This study showed that confined or not-confined conditions are not directly related to the depth of wells in the aquifer. Therefore, the utilisation of groundwater as a water-supply source should be carefully designed, tested for its hydrogeologic conditions, and managed to ensure sustainable quantity and quality.

Shallow bedrock limits groundwater seepage-based headwater climate refugia

Groundwater/surface-water exchanges in streams are inexorably linked to adjacent aquifer dynamics. As surface-water temperatures continue to increase with climate warming, refugia created by groundwater connectivity is expected to enable cold water fish species to survive. The shallow alluvial aquifers that source groundwater seepage to headwater streams, however, may also be sensitive to seasonal and long-term air temperature dynamics. Depth to bedrock can directly influence shallow aquifer flow and thermal sensitivity, but is typically ill-defined along the stream corridor in steep mountain catchments. We employ rapid, cost-effective passive seismic measurements to evaluate the variable thickness of the shallow colluvial and alluvial aquifer sediments along a headwater stream supporting cold water-dependent brook trout (Salvelinus fontinalis) in Shenandoah National Park, VA, USA. Using a mean depth to bedrock of 2.6m, numerical models predicted strong sensitivity of shallow aquifer temperature to the downward propagation of surface heat. The annual temperature dynamics (annual signal amplitude attenuation and phase shift) of potential seepage sourced from the shallow modeled aquifer were compared to several years of paired observed stream and air temperature records. Annual stream water temperature patterns were found to lag local air temperature by ∼8–19 d along the stream corridor, indicating that thermal exchange between the stream and shallow groundwater is spatially variable. Locations with greater annual signal phase lag were also associated with locally increased amplitude attenuation, further suggestion of year-round buffering of channel water temperature by groundwater seepage. Numerical models of shallow groundwater temperature that incorporate regional expected climate warming trends indicate that the summer cooling capacity of this groundwater seepage will be reduced over time, and lower-elevation stream sections may no longer serve as larger-scale climate refugia for cold water fish species, even with strong groundwater discharge.

Metatranscriptomic Analysis Reveals Unexpectedly Diverse Microbial Metabolism in a Biogeochemical Hot Spot in an Alluvial Aquifer.

Organic matter deposits in alluvial aquifers have been shown to result in the formation of naturally reduced zones (NRZs), which can modulate aquifer redox status and influence the speciation and mobility of metals, affecting groundwater geochemistry. In this study, we sought to better understand how natural organic matter fuels microbial communities within anoxic biogeochemical hot spots (NRZs) in a shallow alluvial aquifer at the Rifle (CO) site. We conducted a 20-day microcosm experiment in which NRZ sediments, which were enriched in buried woody plant material, served as the sole source of electron donors and microorganisms. The microcosms were constructed and incubated under anaerobic conditions in serum bottles with an initial N2 headspace and were sampled every 5 days for metagenome and metatranscriptome profiles in combination with biogeochemical measurements. Biogeochemical data indicated that the decomposition of native organic matter occurred in different phases, beginning with mineralization of dissolved organic matter (DOM) to CO2 during the first week of incubation, followed by a pulse of acetogenesis that dominated carbon flux after 2 weeks. A pulse of methanogenesis co-occurred with acetogenesis, but only accounted for a small fraction of carbon flux. The depletion of DOM over time was strongly correlated with increases in expression of many genes associated with heterotrophy (e.g., amino acid, fatty acid, and carbohydrate metabolism) belonging to a Hydrogenophaga strain that accounted for a relatively large percentage (~8%) of the metatranscriptome. This Hydrogenophaga strain also expressed genes indicative of chemolithoautotrophy, including CO2 fixation, H2 oxidation, S-compound oxidation, and denitrification. The pulse of acetogenesis appears to have been collectively catalyzed by a number of different organisms and metabolisms, most prominently pyruvate:ferredoxin oxidoreductase. Unexpected genes were identified among the most highly expressed (>98th percentile) transcripts, including acetone carboxylase and cell-wall-associated hydrolases with unknown substrates (numerous lesser expressed cell-wall-associated hydrolases targeted peptidoglycan). Many of the most highly expressed hydrolases belonged to a Ca. Bathyarchaeota strain and may have been associated with recycling of bacterial biomass. Overall, these results highlight the complex nature of organic matter transformation in NRZs and the microbial metabolic pathways that interact to mediate redox status and elemental cycling.

Impact of Macropores and Gravel Outcrops on Phosphorus Leaching at the Plot Scale in Silt Loam Soils

Abstract. In response to increased nutrient loads in surface waters, scientists and engineers need to identify critical nutrient source areas and transport mechanisms within a catchment to protect beneficial uses of aquatic systems in a cost-effective manner. It was hypothesized that hydrologic heterogeneities (e.g., macropores and gravel outcrops) in the vadose zone play an integral role in affecting flow and solute transport between the soil surface and shallow alluvial aquifers. The objective of this research was to characterize phosphorus (P) leaching through silt loam soils to alluvial gravel aquifers in the floodplains of the Ozark ecoregion at the plot scale. Solute injection experiments used plots (1 m × 1 m, 3 m × 3 m, and 10 m × 10 m) that maintained a constant head for up to 52 h. Solutes in the injection water included P (highly sorptive), Rhodamine WT (slightly sorptive), and chloride (conservative). Electrical resistivity imaging identified zones of preferential flow. Fluid samples from observation wells indicated nonuniform subsurface flow and transport. The surface soil type, ranging from silt loam to clean gravel outcrops, had a significant impact on P leaching capacity, with gravel outcrops resulting in high infiltration rates and rapid solute detection in wells (e.g., 4 min). Even in silt loam soils without gravel outcrops, macropore flow resulted in rapid transport of P. Maximum transport velocity for soluble reactive P in one silt loam plot was 810 cm h-1, compared with a mean pore water velocity in the range of 25 to 130 cm h-1. Soluble reactive P concentrations in observation wells reached up to 0.54 mg L-1 in silt loam plots and 1.3 mg L-1 in gravel outcrop plots, demonstrating that a highly sorbing solute can be mobile. Keywords: Gravel outcrop, Leaching, Macropores, Ozark ecoregion, Phosphorus.

Groundwater quantitative status assessment for River Basin Management Plan 2015-2021 in Slovenia

The improved methodological approach of the groundwater quantitative status assessment in Slovenia and the results of the assessment period 2010-2013, taking into account the new reference thirty-year period 1981- 2010, are presented. Within the assessment period quantitative status in all shallow alluvial aquifers of 21 groundwater bodies in Slovenia is assessed as good, with a medium to high level of confidence. Groundwater quantitative status assessment methodology considers the processes of the whole hydrological cycle and the results of groundwater recharge modelling. The methodology incorporates the concept of sustainable groundwater use to preserve the quantities not causing environmental and other harm (unacceptable environmental and other consequences). Legislative baseline for assessing the impacts of groundwater abstraction on renewable and available quantities of groundwater introduces new methodology by abandoning obsolete mining concept of "calculation of groundwater reserves".

Stochastic simulation of soil particle‐size curves in heterogeneous aquifer systems through a Bayes space approach

AbstractWe address the problem of stochastic simulation of soil particle‐size curves (PSCs) in heterogeneous aquifer systems. Unlike traditional approaches that focus solely on a few selected features of PSCs (e.g., selected quantiles), our approach considers the entire particle‐size curves and can optionally include conditioning on available data. We rely on our prior work to model PSCs as cumulative distribution functions and interpret their density functions as functional compositions. We thus approximate the latter through an expansion over an appropriate basis of functions. This enables us to (a) effectively deal with the data dimensionality and constraints and (b) to develop a simulation method for PSCs based upon a suitable and well defined projection procedure. The new theoretical framework allows representing and reproducing the complete information content embedded in PSC data. As a first field application, we demonstrate the quality of unconditional and conditional simulations obtained with our methodology by considering a set of particle‐size curves collected within a shallow alluvial aquifer in the Neckar river valley, Germany.

Isotopic and geochemical studies of groundwater from the Ramganga basin and the middle Ganga Plains: implication for pollution and metal contamination

Hydrochemical and isotopic studies in groundwater were carried out along the entire Ramganga Basin and parts of middle Ganga Plain to examine the quality of shallow groundwater accessed by hand pumps. The aim of the study was to determine the spatial distribution of arsenic (As) in groundwater of the Middle Ganga (Haridwar–Banaras–Saidpur region) and of the Ramganga (Ramnagar–Kannauj) basins. Groundwater samples were collected from a shallow depth because most of the population in the study area use locally drilled hand pumps for obtaining drinking water. A large area encompassing the Rampur, Bareilly, Bilsi, Faridpur, Badaun, Farrukhabad and Varanasi districts were found to have levels of arsenic above the drinking water criterion of 10 ppb. Sampling has indicated that arsenic (As) and nitrate (NO3) present in groundwater in elevated levels are the chemical constituents of most health concern in the shallow alluvial aquifers of Ramnagar, Bareilly, Badaun and Jaunpur districts of Uttar Pradesh in the middle Ganga Plain. These chemical constituents in groundwater are likely to have been derived from both geogenic and anthropogenic sources. The geochemical characteristics of groundwater may be influenced by factors like changes in climatic conditions, mineral weathering (silicate, carbonate), mixing of polluted surface water at shallow depths, ion exchange and anthropogenic activities. Stable isotopes of oxygen (δ18O), hydrogen (δD) and carbon (δ13CDIC) suggest that the process of carbonate weathering during the post-monsoon season has increased the input of chemical constituents into shallow groundwater. A shift in δ18O suggests soil–water interaction at shallow depths, which is reflected in elevated metal concentrations. The results indicate that arsenic concentrations in groundwater increase in the dry season and are decreased by recharge during the monsoon season.

Spatial distribution of triazine residues in a shallow alluvial aquifer linked to groundwater residence time

At present, some triazine herbicides occurrence in European groundwater, 13years after their use ban in the European Union, remains of great concern and raises the question of their persistence in groundwater systems due to several factors such as storage and remobilization from soil and unsaturated zone, limited or absence of degradation, sorption in saturated zones, or to continuing illegal applications. In order to address this problem and to determine triazine distribution in the saturated zone, their occurrence is investigated in the light of the aquifer hydrodynamic on the basis of a geochemical approach using groundwater dating tracers (3H/3He). In this study, atrazine, simazine, terbuthylazine, deethylatrazine, deisopropylatrazine, and deethylterbuthylazine are measured in 66 samples collected between 2011 and 2013 from 21 sampling points, on the Vistrenque shallow alluvial aquifer (southern France), covered by a major agricultural land use. The frequencies of quantification range from 100 to 56% for simazine and atrazine, respectively (LQ=1ngL-1). Total triazine concentrations vary between 15 and 350ngL-1 and show three different patterns with depth below the water table: (1) low concentrations independent of depth but related to water origin, (2) an increase in concentrations with depth in the aquifer related to groundwater residence time and triazine use prior to their ban, and (3) relatively high concentrations at low depths in the saturated zone more likely related to a slow desorption of these compounds from the soil and unsaturated zone. The triazine attenuation rate varies between 0.3 for waters influenced by surface water infiltration and 4.8 for water showing longer residence times in the aquifer, suggesting an increase in these rates with water residence time in the saturated zone. Increasing triazine concentrations with depth is consistent with a significant decrease in the use of these pesticides for the last 10years on this area and highlights the efficiency of their ban.

Heat tracer test in an alluvial aquifer: Field experiment and inverse modelling

Using heat as an active tracer for aquifer characterization is a topic of increasing interest. In this study, we investigate the potential of using heat tracer tests for characterization of a shallow alluvial aquifer. A thermal tracer test was conducted in the alluvial aquifer of the Meuse River, Belgium. The tracing experiment consisted in simultaneously injecting heated water and a dye tracer in an injection well and monitoring the evolution of groundwater temperature and tracer concentration in the pumping well and in measurement intervals. To get insights in the 3D characteristics of the heat transport mechanisms, temperature data from a large number of observation wells closely spaced along three transects were used.Temperature breakthrough curves in observation wells are contrasted with what would be expected in an ideal layered aquifer. They reveal strongly unequal lateral and vertical components of the transport mechanisms. The observed complex behavior of the heat plume is explained by the groundwater flow gradient on the site and heterogeneities in the hydraulic conductivity field. Moreover, due to high injection temperatures during the field experiment a temperature-induced fluid density effect on heat transport occurred. By using a flow and heat transport numerical model with variable density coupled with a pilot point approach for inversion of the hydraulic conductivity field, the main preferential flow paths were delineated. The successful application of a field heat tracer test at this site suggests that heat tracer tests is a promising approach to image hydraulic conductivity field. This methodology could be applied in aquifer thermal energy storage (ATES) projects for assessing future efficiency that is strongly linked to the hydraulic conductivity variability in the considered aquifer.

Smart pilot points using reversible‐jump Markov‐chain Monte Carlo

AbstractPilot points are typical means for calibration of highly parameterized numerical models. We propose a novel procedure based on estimating not only the pilot point values, but also their number and suitable locations. This is accomplished by a trans‐dimensional Bayesian inversion procedure that also allows for uncertainty quantification. The utilized algorithm, reversible‐jump Markov‐Chain Monte Carlo (RJ‐MCMC), is computationally demanding and this challenges the application for model calibration. We present a solution for fast, approximate simulation through the application of a Bayesian inversion. A fast pathfinding algorithm is used to estimate tracer travel times instead of doing a full transport simulation. This approach extracts the information from measured breakthrough curves, which is crucial for the reconstruction of aquifer heterogeneity. As a result, the “smart pilot points” can be tuned during thousands of rapid model evaluations. This is demonstrated for both a synthetic and a field application. For the selected synthetic layered aquifer, two different hydrofacies are reconstructed. For the field investigation, multiple fluorescent tracers were injected in different well screens in a shallow alluvial aquifer and monitored in a tomographic source‐receiver configuration. With the new inversion procedure, a sand layer was identified and reconstructed with a high spatial resolution in 3‐D. The sand layer was successfully validated through additional slug tests at the site. The promising results encourage further applications in hydrogeological model calibration, especially for cases with simulation of transport.

Metatranscriptomic evidence of pervasive and diverse chemolithoautotrophy relevant to C, S, N and Fe cycling in a shallow alluvial aquifer.

Groundwater ecosystems are conventionally thought to be fueled by surface-derived allochthonous organic matter and dominated by heterotrophic microbes living under often-oligotrophic conditions. However, in a 2-month study of nitrate amendment to a perennially suboxic aquifer in Rifle (CO), strain-resolved metatranscriptomic analysis revealed pervasive and diverse chemolithoautotrophic bacterial activity relevant to C, S, N and Fe cycling. Before nitrate injection, anaerobic ammonia-oxidizing (anammox) bacteria accounted for 16% of overall microbial community gene expression, whereas during the nitrate injection, two other groups of chemolithoautotrophic bacteria collectively accounted for 80% of the metatranscriptome: (1) members of the Fe(II)-oxidizing Gallionellaceae family and (2) strains of the S-oxidizing species, Sulfurimonas denitrificans. Notably, the proportion of the metatranscriptome accounted for by these three groups was considerably greater than the proportion of the metagenome coverage that they represented. Transcriptional analysis revealed some unexpected metabolic couplings, in particular, putative nitrate-dependent Fe(II) and S oxidation among nominally microaerophilic Gallionellaceae strains, including expression of periplasmic (NapAB) and membrane-bound (NarGHI) nitrate reductases. The three most active groups of chemolithoautotrophic bacteria in this study had overlapping metabolisms that allowed them to occupy different yet related metabolic niches throughout the study. Overall, these results highlight the important role that chemolithoautotrophy can have in aquifer biogeochemical cycling, a finding that has broad implications for understanding terrestrial carbon cycling and is supported by recent studies of geochemically diverse aquifers.

Redox conditions and groundwater quality issues of selected alluvial aquifers in Serbia

Redox conditions are an important regulator of aqueous biogeochemistry and knowledge about them is crucial for an understanding of the fate and occurrence of nitrates, emerging substances, and organic compounds in shallow groundwater. The aim of this paper is to determine redox categories, along with prevailing redox processes, and the differences in groundwater quality between shallow alluvial aquifers of Serbia's major rivers. The research was conducted on public water-supply wells in the alluvial plain of the Velika Morava River, the Danube River, the Tisa River and the alluvial aquifer of the Sava River. The study period is 2010–2014 and the number of collected and analysed samples 243. The redox categories and processes are characterised by concentrations of aqueous redox species (dissolved oxygen, nitrate, dissolved iron, manganese, and sulphate and sulphide concentrations). Parameters such as Eh, total organic carbon, HCO3−, pH, total iron concentrations, temperature and specific conductivity were also analysed. Different geological and hydrogeological settings, groundwater abstraction, distances between the wells and the river, well depths, and anthropogenic impacts result in diverse redox conditions. Groundwater in selected alluvial aquifers is mostly defined as anoxic, with one predominant process, or mixed anoxic, with various redox processes occurring simultaneously.

Studies on the Seasonal Variation and Behaviour of Hydraulic Head along River Benue Floodplain, Yola, North Eastern Nigeria

The aim of this research is to determine availability of water for irrigational practice during dry season farming along River Benue floodplain. Water table heights were estimated by automatic piezometer instrument and manual measurements. The results of the water level measurements help to quantify the variable response of hydraulic head and also help to indicate the direction of water movement. The results from the two monitored piezometers show a switch in the hydraulic gradient. In the dry season, the water level in the river is higher than the surrounding area and therefore, the river loses water to the surrounding area. Conversely, in the rainy season the water level in the surrounding area rises and therefore loses water to the river. The relationship between the water levels in the two piezometers suggests a linear relationship when P1>174.25 m, which shows time of flow to river. It was also observed that, in the northeast, southeast and southwest the direction of groundwater flows are from the river to the floodplain, whilst in the northwest the direction of groundwater flow is from the floodplain to the river. Hydrological data demonstrate that River Benue is the main source for recharge of the shallow alluvial aquifers of the floodplain during the dry season period.

Nitrate contamination sources in aquifers underlying cultivated fields in an arid region – The Arava Valley, Israel

Nitrate contamination of shallow aquifers poses a major challenge in the development of sustainable agriculture, particularly in (semi-)arid regions, where groundwater is also used as potable water. In this study we estimate the relative contribution of the various nitrate sources contaminating the shallow aquifers underlying agricultural fields in the extremely arid region of the Central Arava Valley (CAV), southern Israel. The estimates are based on a chemical and isotopic mixing-reaction model describing the evolution of the δ18ONO3 and δ15NNO3 isotopic compositions from the sources to the aquifer. The model indicates that all fertilizers used in the agricultural fields: synthetic NO3−, synthetic NH4+ and manure, contaminate the CAV groundwater with nitrate. In most of CAV groundwaters, the contribution of each fertilizer to the groundwater contamination is relative to its proportion in the fertilization scheme of the cultivated fields. Two exceptions of this general observation were found: 1. One field showed a direct nitrate contamination from a “leaking” sewage reservoir, as indicated by its exceptionally high δ15NNO3 value that reached 9.2‰; and 2. A very shallow alluvial aquifer, recharged by winter floods and not by the irrigation water, showed no nitrate isotopic “fingerprint” of manure, which is applied to the cultivated fields only during summer. Groundwater nitrate contamination is actuated by infiltration of irrigation water, which in addition underwent salinization as a result of evapo-transpiration in the cultivated fields. Based on the relatively uniform and depleted groundwater δ18OH2O compositions and the depleted δ18ONO3 values, within the possible range of fertilizers-derived nitrate as deduced by the model, the water-loss in the cultivated fields is transpiration-controlled.The isotope mixing-reaction model used here to identify the major nitrate contamination sources maybe used to help design a sustainable cultivation management strategy in similar arid regions. It can also be used to assess the fertilization efficiency and the relative impact of each fertilizer type. Such models may be particularly useful in evaluating highly heterogeneous and complicated aquifer systems, for which the formulation of absolute water and nitrate mass balances are impossible.

Groundwater Characteristics and Pollution Assessment Using Integrated Hydrochemical Investigations GIS and Multivariate Geostatistical Techniques in Arid Areas

In arid areas, groundwater quality is an important concern for human life and natural ecosystems. Rapid economic development impacts greatly on limited groundwater resources. This study examines the impacts of natural and anthropogenic contaminations on groundwater quality by integrating hydrochemical investigations, GIS and multivariate geostatistical techniques. Major and trace elements in groundwater were analyzed to evaluate the shallow alluvial aquifer connected to the fractured basement aquifer in the Jazan area of Saudi Arabia (KSA). Results show that the groundwater of the aquifer inland is generally under free conditions and is declining with time, while in the coastal areas, there is a disturbance of the groundwater balance resulting in upward leakage of deep saline water and seawater intrusion. Groundwater types are varied from calcium bicarbonate to sodium chloride, reflecting meteoric water recharge to seawater influence. The pollution risk is high, since the water table is shallow and the aquifers are highly permeable. Urbanization has put stress on groundwater quality due to wastewater contamination from pit latrines and leakage from the unrehabilitated sewage system. Chemical equilibrium and saturation indices were calculated, showing that most of the water samples were under-saturated with respect to anhydrate, aragonite, calcite, dolomite, gypsum and halite mineral phases. This affects carbonate and evaporite minerals in the aquifer, causing ground subsidence and sinkholes in the coastal area. Factor and cluster analyses were used to classify and examine the processes affecting groundwater quality. The main factors impacting on the groundwater hydrochemistry were identified as: dissolution of several minerals, evaporation, human impact, mixing between geothermal and other waters as well as rainfall recharge.

Hydrogeophysical Characterization of Shallow Unconsolidated Alluvial Aquifer in Yenagoa and Environs, Southern Nigeria

Groundwater of the Quaternary shallow alluvial aquifer is the only safe and reliable water resource in Yenagoa and environs, Southern Nigeria. The determination of the aquifer hydrogeophysical parameters is thus critical and constitutes the basic foundation for optimizing the short- and long-term groundwater utilization strategies. In this study, 29 Schlumberger vertical electrical soundings, borehole data, pump test, hydrochemical analysis, and measured and/or calculated hydrogeophysical parameters were integrated to reconnaissance the local aquifer conditions. Aquifer qualitative and quantitative properties in terms of electric and hydraulic conditions were also investigated. The freshwater aquifer was delineated with true resistivity and thickness ranges of 134–664 Ωm and 6.7–41.4 m respectively. Transmissivity and hydraulic conductivity values vary between 189–2609 m2/day and 28–118 m/day, respectively. Porosity ranges between 0.40 and 0.45 and tortuosity ranges between 1.28 and 1.31. Correlation of the results of the geochemical analysis of the groundwater samples with the layer resistivities shows that in the locations where the aquifer is overlain by clay layer (ρ ∼ 10 Ωm) have higher concentrations of dissolved iron (Fe > 0.1 mg/L), whereas the iron concentrations are remarkably low (Fe < 0.1 mg/l) in areas the aquifer is underneath relatively thin silty sand layer (ρ ∼ 349 Ωm). The near-surface lithology acts as a controlling factor for spatial distribution of groundwater iron within the very shallow depths (<20 m). The geochemistry of the groundwater was observed to be controlled mainly by weathering of silicate group of minerals.