Lower Aquifer Research Articles

Hydrogeochemical Investigation and Estimation of Aquifer Properties at Dhaka University Campus

The University of Dhaka has its own water supply system and entirely relies on groundwater. Ten groundwater samples were collected from 10 production wells of 120-320 m depth across the University campus from upper and lower Dupi Tila aquifers. The pH values revealed that water is mildly acidic to neutral and electrical conductivity (EC) values ranging from 325 to 654 μS/cm, indicating fresh groundwater. The total hardness (TH) of the samples ranges from 76-203 mg/L, suggesting moderately hard to very hard water. Of the ten groundwater samples, nine are of Ca-Mg-HCO3 type, whereas one shows Ca-Na-HCO3 type of groundwater. The saturation indices show that the groundwater samples are near-saturated for calcite, dolomite, gypsum, siderite, and rhodochrosite, indicating the low potential for dissolution-precipitation of these mineral. Notably, the results of WQI indicate that all the wells are safe for drinking. As of July 2023, the groundwater level was 72 m below ground level with a declination rate of 2.8 m/year. The estimated hydraulic conductivity values of 20 to 90 m/day and transmissivity values of 637 to 2740 m²/day suggest better permeability and water transmission potential within the lower aquifer. These results provide essential information for the university authority to implement appropriate strategies for efficient groundwater management. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 12(2), 2023, P 55-67

Groundwater, salt and contaminant transport in a coastal aquifer system with a low‐permeability layer in the intertidal zone

AbstractLow‐permeability layer (LPL), formed by natural deposit or artificial reclamation and commonly found below the intertidal zone of coastal groundwater system, can retard the ingress of seawater and contaminants, and shorten the travel time of the land‐sourced contaminant to the marine environment compared with a homogenous sandy coastal aquifer. However, there is limited understanding on how an intertidal LPL, a condition occurred in a coastal aquifer at Moreton Bay, Australia, influences the groundwater and contaminant transport across the shallow beach aquifer system. We characterized the aquifer hydrological parameters, monitored the in situ groundwater heads, and constructed a 2‐D numerical model to analyses the cross‐shore hydrological processes in this stratified system. The calibrated model suggests that in the lower aquifer, the inland‐source fresh groundwater flowed horizontally towards the sea, upwelled along the freshwater–saltwater interface, and exited the aquifer at the shore below the LPL. Whereas in the upper aquifer, the tidally driven seawater circulation formed a barrier that prevented fresh groundwater from horizontal transport and discharge to the beach above the LPL, thereby directing its leakage to the lower aquifer. A contaminant represented by a conservative tracer was ‘released’ the upper aquifer in the model and results showed that the spreading extent of the contaminant plume, the maximum rate of contaminant discharge to the ocean, and its plume length decreased compared with a simulation case in a homogenous sandy aquifer. Sensitivity analysis was also conducted to investigate the characteristics of the LPL, including its continuity and hydraulic conductivity, which were found to vary along the beach at Moreton Bay. The result shows that with a lower hydraulic conductivity and continuous layer of LPL reduced the groundwater exchange and contaminant transport between upper and lower aquifer. The findings from the combined field and modelling investigations on the impact of an intertidal LPL on coastal aquifer systems highlight its significant implications to alter the groundwater and mass transport across the land–ocean interface.

Influence of Selected Soil Properties on Groundwater Flow around Ariaria Dumpsite, Aba, Southeast, Nigeria

A study on the analysis of soil and groundwater characteristics due to municipal waste is indispensable to the sustenance of the environment and human health. This study is to evaluate the “Influence of selected Soil properties on groundwater flow around Ariaria dumpsite, Aba Southeast, Nigeria using Particle Size Analysis and Vertical Electrical Sounding (VES) configurationˮ. The sieve analysis shows that the soil samples collected consist of 11.95% of Silt, 83.62% of Sand and 4.42% of little Gravel. The mean permeability of the study area ranges from 0.19 (cm/sec) to 0.49 (cm/sec), the mean infiltration rate ranges from 6840mm/hr to 31320mm/hr. The Longitudinal conductance and Transmissivity values range from 0.0060 to 0.0284Ω-1 and 3.8165 to 16.85892/day. The values of longitudinal conductance and transmissivity indicates that the aquifer has poor protection capacity rating and its classification of well is from very low potential to low potential. The soil samples of the study area are well sorted and the movement of leachate from the dumpsite into the subsurface is very fast because the Permeability (K) is good, Coefficient of uniformity (Cu) and Coefficient of curvature classified the soil samples collected from the area as well graded soil. From the geophysical survey (VES) results, the groundwater depths (upper and lower aquifers) within the study area ranges from 30m (98ft) to 60m (197ft) at average depth of 45m(147ft). The curve types are AHA, HKA and AKA. The SWL of the study area ranges from 12m to 19m deep. The recommend drill depth to groundwater of is 50m (164) for domestic and consumption purpose.

Hydrogeophysical characterization of the Bir Guendouz basin (SW Morocco): Insights into its relations to the Senegal-Mauritanian basin

The Senegal-Mauritanian Basin onshore is the largest among the northwest African Atlantic margin basins, covering approximately 350,000 km2 and extending from Morocco to Guinea Bissau. Although valuable information is provided on the Senegalese part, data on other parts of the basin are scarce. The study aims to visualise the subsurface structure and describe the distribution patterns of Meso-Cenozoic deposits in the Moroccan portion of the Senegalo-Mauritania basin. The dataset integrates information from 750 vertical electrical soundings, hydrogeological drilling data, and wireline logging into a Geographic Information System (GIS) platform and 3D modelling software. A new three-dimensional model has been created to represent the topmost layer of the Palaeozoic substratum. Additionally, thickness maps and geo-electric profiles have been developed for distinct geo-electric intervals, including the lower, middle, upper, and surface layers. The study presents the geometric properties of two aquifers, namely, the ''upper and lower'': the Continental Terminal sandy and sandstones, dating back to the upper Miocene to Pliocene period, and the Paleocene sand to shaly sand. The upper aquifer has a thickness of 20 m in the eastern region, whereas it thickens to nearly 100 m towards the west, approaching the coastline. Similarly, the lower aquifer's thickness ranges from 100 m to 400 m in the same direction. The resistivity values range from 5 to 240 Ω m, providing an average of 120 Ω m. Four discontinuities have been identified in the sections, which may indicate lateral facies modifications or synsedimentary faults, correlating with Atlantic margin structuration and Meso-Cenozoic sea-level variations. The parameters' range for various mappings, models, and quantitative descriptions of reservoir properties. The new structural and hydro-stratigraphic model provides novel insights for researching this critical area.

Characteristics and evolution of karst collapse columns in the Huainan coalfield

Karst collapse columns (KCCs) seriously affect the mining safety of deep coal seams. This study systematically summarizes and analyzes the development of KCCs, and classifies the different development stage to identify their development stages and evolutionary process in the Huainan coalfield. The evolution models for KCC development are given, combining with the exploration strata data from boreholes, the groundwater flow data of regional field, the hydrogeochemical data from the relative aquifers of KCC, and the hydrodynamic parameters. The results show that first types of KCCs are discovered in Liuzhuang and the Pansan mine, which is broken and disorganized, with a high degree of filling and cementation, and with the low permeability and velocity, and lower storage capacity. The KCCs in the Xieqiao and Zhangji mine have various morphologies and size differences. Their internal rocks are broken and semi-cemented, with a coefficient of permeability between 0.2 and 0.5 m/d and a specific discharge between 0.1 and 0.2 L/s•m. The KCCs are located in the cone of depression and are part of a sink area with moderate water-richness, where upper and lower aquifers have close hydraulic connections with mixed water quality. The internal core of the KCCs in the Gubei Mine is close to the center of the cone of depression which is highly broken and disorganized, with a high permeability, and a specific discharge more than 1.5 L/s•m. The intensity of runoff increases as its vertical depth also increases, especially the local areas are a higher hydraulic conductivity. Based on comprehensive hydrogeological characteristics, the KCCs development is divided into three stages: growing stage, declining stage, and dead stage. According to their characteristics of different stages, a series of evolutionary processes are established. Combining the sedimentological, karstological, and hydrogeological theories, some measures have been taken for prevention and control of mine water hazard in the various developmental stages. Hence, this research not only provides a new classified approach for KCC stages, but also an essential reference for a better understanding the mechanism of water inrush of KCCs in Northern China.

Characterization of groundwater aquifers using hydrogeophysical and hydrogeochemical methods in the eastern Nile River area, Khartoum State, Sudan

The primary goals of this research are to detect the spatial variation of the hydrogeological characteristics and evaluate the groundwater quality in the eastern Nile River, Khartoum state, Sudan, using geophysical and hydrochemical methods. Thirteen Vertical electrical soundings (VES), using Schlumberger configuration, were measured along three profiles to characterize the groundwater aquifer. VES findings denoted that the study area comprises two hydraulically connected aquifers. The upper aquifer of sand has an average thickness of 50 m, and the lower aquifer is composed of sandstone of a thickness of up to 300 m. The results of VES inversion were further used to measure aquifer characteristics, including transverse resistance, longitudinal conductance, hydraulic conductivity, and transmissivity. The detected average values of these parameters were 6690 Ωm2, 1.4 Ω−1, 264 m2/d and 4 m/day, respectively. In addition, regression analysis was performed to suggest local relationships for estimating aquifer characteristics within the study area. On the other hand, total longitudinal conductance was used to predict the protective strength of the hydrogeological columns, ranging from 1.7 to 5.8 Ω−1; as a result, the protective capacity of the aquifer ranged from good to very good, suggesting potable water quality. This result was subsequently confirmed by the groundwater quality index (GWQI) model. Eleven physiochemical parameters analyzed for nine boreholes were used in GWQI estimation to assess groundwater quality in the study area. The primary analysis of the hydrochemical parameters indicated that almost all parameters are below permissible limits prescribed by the World Health Organization (WHO). The computed GWQI varies between 34.8 and 148, and the majority of groundwater samples, precisely 55.5%, are good water types, while 22.2% of the samples are in an excellent quality state. This research concluded that the groundwater aquifer in the study area is ideal for groundwater exploitation. However, applying a detailed geophysical and hydrochemical survey is recommended to reduce the uncertainty of the resulting models.

Application of the RAPS Method for Determining the Dependence of Nitrate Concentration in Groundwater on the Amount of Precipitation

Protecting groundwater from contamination is today’s most current environmental protection topic. What can man do in his environment to reduce the harmful impact of contamination on the environment, and thus the immediate effect on groundwater? Agricultural production is an ongoing source of groundwater contamination due to the increasingly frequent use of nitrates in fertilizers, which are washed out from the soil into groundwater due to precipitation. This paper investigates three wellfields in the north of the Republic of Croatia near the town of Varaždin. With the application of the RAPS method, the dependence of nitrate concentration in groundwater on the amount of precipitation was established. The analysis results show the connection of the observed parameters, especially in the upper aquifer layer. In this layer, the coefficients of correlation are greater than 0.80 at all locations, which shows a strong positive connection between the parameters. In the lower aquifer, the values of the coefficients of correlation are lower, and the results mostly indicate a weak correlation. The obtained results will serve as a starting point for future studies, which will aim to precisely determine the factors that influence groundwater quality in the observed area.

Modelling Interactions between Three Aquifer Thermal Energy Storage (ATES) Systems in Brussels (Belgium)

Shallow open-loop geothermal systems function by creating heat and cold reserves in an aquifer, via doublets of pumping and reinjection wells. Three adjacent buildings in the center of Brussels have adopted this type of aquifer thermal energy storage (ATES) system. Two of them exploit the same aquifer consisting of Cenozoic sands, and started operation in 2014 and 2017, respectively. A previous hydrogeological model developed by Bulté et al. (2021) has shown how the thermal imbalance of one of the systems jeopardizes the thermal state of this upper aquifer. Here, the interactions with a more recent third ATES system located in the deep aquifer of the Palaeozoic bedrock are studied and modelled. After being calibrated on groundwater flow conditions in both aquifers, a 3D hydrogeological model was used to simulate the cumulative effect of the three geothermal installations in the two exploited aquifers. The results of the simulations showed that although the hydraulic interactions between the two aquifers are very weak (as shown by the different observed potentiometric heads), heat exchanges occur between the two aquifers through the aquitard. Fortunately, these heat exchanges are not sufficient to have a significant impact on the efficiency of the individual geothermal systems. Additionally, this study shows clearly that adding a third system in the lower aquifer with a mean power of 286 kW for heating between October and March and an equivalent mean cooling power between April and September is efficient.

Study on the Law of Hydraulic Fracture Propagation in Low Permeability Thin Interbed Reservoir

The third member of Weixinnanliu in the west of the South China Sea develops thin interbeds, and the vertical extension of fracturing fractures is excessive. Once the fractures extend vertically to the upper and lower aquifers, it is easy to cause water flooding of oil wells, and the effect after fracturing is not obvious. The present work aims to explore the longitudinal extension law of fractures in Low Permeability Thin Interbed Reservoir based on the finite element calculation platform. A three-dimensional expansion model of hydraulic fractures in the target reservoir was established, and the displacement, fracturing fluid viscosity, minimum horizontal principal stress difference, vertical stress, interlayer thickness, perforation point separation were studied. The interlayer distance and other factors affect the crack propagation law. The research results show that the thin interbed fractures have three forms: T-shaped fractures, through-layer fractures, and I-shaped fractures; for the target layer, the overlying stress is relatively large, and the minimum principal stress is along the horizontal direction. Vertical cracks; the farther the perforation point is or the greater the stress difference, the smaller the thickness of the interlayer required to control the fracture height; the stress difference is 3 MPa, and the distance between the perforation points exceeds 10 m, the thickness of the interlayer is required to be ≥4 m; In order to ensure that the width of the fracture in the middle spacer does not affect the placement of the proppant, it is recommended that the displacement be controlled within 3 m3/min and the viscosity of the fracturing fluid is 150 mPa·s; in addition, the thickness of the spacer required to control the fracture height is different due to different geological parameters. Different, different wells need targeted analysis.

The Mio-pliocene aquifer system of the South Auresian Trough, Low Septentrional Sahara, Algeria: aquifer geometry and groundwater characterization

In the South auresian trough, Low Septentrional Sahara, Algeria, the sands aquifer is contained in the Mio-pliocene deposits in which several sedimentary sets have identified and contain two independent aquifer levels. Their relationship is determined by data correlation of several cross-sections surveyed on outcrops and boreholes that permit us a 2D construction of the multi-layer aquifer formations and to specify its geometry. The latter shows an asymmetrical synclinal structure with greater depth to the south and an unconformity of the upper unit (upper aquifer) on the base unit (lower aquifer) with a reduction in thickness from south to north. The upper aquifer, commonly designated “undifferentiated Mio-pliocene aquifer” is the most exploited. Its water is moderate to strongly charged and presents two families at evolutional hydrochemical facies. Their spatial distribution is strongly influenced by the natural context and the aquifer geometry. The hydrochemical analysis reveals that the dissolution of carbonate (CaCO3 or CaMgCO3), sulfate (CaSO4) and salt (NaCl) were the major sources of ions in the studied groundwaters. In this region deprived of treatment means and softening of water, the exploitation of the study results can help the decision makers, preferentially, to choose the captive fields principally destined for potable water supply in sectors containing the water less charged.

Exploring hydrogeological parameters by integration of geophysical and hydrogeological methods in northern Khartoum state, Sudan

The recent study aims to detect the spatial distribution of the hydraulic parameters and evaluate the protective capacity of the groundwater aquifers in northern Khartoum state using geophysical and hydrogeological methods. The study area is located in the transition zone between the sedimentary basin and the area dominated by crystalline basement rocks. Vertical electrical sounding (VES) measurements, using Schlumberger configuration, were taken along three profiles to delineate the thicknesses of the sediments and groundwater aquifers. The VES measurements were combined with the lithology logs to reduce the uncertainty of the resulting models. VES findings denoted that the study area comprises two hydraulically connected aquifers. The upper aquifer is composed of fine sandstone with an average thickness of 50 m, and the lower aquifer has a thickness of up to 150 m. These two aquifers are confined to semi-confined, separated by relatively thick aquitards. To evaluate the aquifer quantitively, pumping test analysis using Cooper-Jacob method was applied to estimate the aquifer's hydraulic parameters, including transmissivity and hydraulic conductivity. The pumping test approach is costly and time-consuming, and extrapolation of the results is restricted to homogeneous boundary conditions. Since groundwater aquifers are mostly heterogeneous, a geophysical-based method called Dar Zarrouk parameters was applied to give a continuous estimate for the hydraulic parameters. Dar Zarrouk parameters include longitudinal conductance and transverse resistance, combining the true resistivity and thickness obtained from the inverted VES data to determine the hydraulic parameters. The calculated hydraulic parameters from the pumping test and Dar Zarrouk parameters showed high compatibility. Furthermore, longitudinal conductance was used to predict the protective capacity of the aquifers. Generally, it ranged from good to very good protective capacity suggesting potable water quality. The findings of this research indicated that the groundwater aquifer in the study area is ideal for groundwater exploitation. However, this study recommends applying a detailed geophysical survey to reduce the uncertainty of the resulting geological and hydrogeological models.

Boom Clay pore-water geochemistry at the Mol site: Chemical equilibrium constraints on the concentrations of major elements

It is important to understand the mechanisms controlling the chemistry of the pore water in the Boom Clay, which has been studied as a reference host rock for the geological disposal of radioactive waste. The chemistry of the pore water and the related properties of the Boom Clay were investigated experimentally. This paper presents the results of equilibrium calculations and discusses the probable chemical constraints that control the concentrations of major elements in the pore water. The present-day Boom Clay pore water is a dilute NaHCO3 solution of 10–20 mM, and the concentration increases with the depth of the Boom Clay over a thickness of 65 m around the underground laboratory at approximately 200 m below sea level. The present-day NaHCO3 water is likely a result of freshwater infiltration, and the concentration profile observed must have been affected by waters from upper and lower aquifers. Among the minerals found in the Boom Clay, equilibrium calculations indicate that the pore water is in equilibrium with calcite, dolomite, siderite, pyrite, and quartz. Aluminium silicate minerals, kaolinite and chlorite, are also possibly in equilibrium, but their saturation states cannot be fully demonstrated mainly because of uncertainties associated with the pore-water concentration of aluminium. Cation exchange, an important property of clays, seems to be controlling only the pore-water concentration of potassium. Concentrations of other cation exchangeable elements, such as calcium and magnesium, are more likely regulated by the solubility of carbonate minerals. On clay surfaces, however, cation-exchange reactions are evidenced and control occupancies of all exchangeable cations. The cation-exchange pool on clay responds to the increase in NaHCO3 concentration with depth, and the surface occupancies vary according to the selectivity of exchangeable cations. Equilibrium calculations allow us to propose a Boom Clay reference model that is able to explain the observed pore-water chemistry. This model is compared to other models in the literature, such as those for the Opalinus Clay in Switzerland and Callovo-Oxfordian Clay in France, which have also been studied for the purpose of radioactive waste disposal. Future studies to improve our understanding of the Boom Clay pore-water geochemistry include i) accurate determination of the pore-water concentration of aluminium, which is crucial for studying the mineral saturation state of aluminium silicates; ii) improving understanding of the mineralogical composition of dolomite in the Boom Clay and its dissolution mechanisms; iii) reassessment of magnesium cation-exchange occupancy on clay surfaces; and iv) improving pore-water sampling and analysis techniques with respect to pH and pCO2 measurements to avoid perturbations to water samples so that the in situ conditions can be preserved to a maximum extent.

Hydrogeological models for the conditions causing the occurrence of manganese in wells for groundwater extraction from the terrace of the Vacha River, Southern Bulgaria

The presented study examines the problem caused by the groundwater pollution in the alluvial aquifer complex formed in the terrace of the Vacha River. Two working hypotheses are considered. In the first one is accepted that the aquitard is regionally well sustained and limits the penetration of Mn-contaminated waters into the lower aquifer. In the second one is assumed that there are hydrogeological windows in the aquitard, through which active mass exchange between the two aquifers can take place and manganese can occur in the pumping wells after their exploitation is started.

Tidal fluctuations relieve coastal seawater intrusion caused by groundwater pumping.

Coastal aquifers are vulnerable to seawater intrusion and salinization due to groundwater pumping. Most research on salinization vulnerability and pumping rate optimization considers constant seaside boundary. However, tidal fluctuations may propagate through aquifers and affect pumping-induced seawater intrusion. We numerically tested the combined effects of tidal fluctuations and groundwater pumping on the variable-density groundwater flow and salt transport in aquifers with pumping wells close to the coast. The simulations showed that tidal fluctuations relieved seawater intrusion due to groundwater extraction. Stronger tidal fluctuation created larger upper saline plume which inhibited the seawater intrusion in the lower aquifer. Compared with those without tides, the tidal fluctuations with a tidal amplitude of 1m could increase maximum pumping rate by 35.2% and 28.7% in flux- and head-controlled systems, respectively. These results suggest that combined impacts of tides and pumping should be considered in assessing seawater intrusion and designing more sophisticated pumping optimization.

Arnold Verruijt (1940-2022).

Arnold Verruijt (1940-2022).

Insight into Groundwater Resources along the Coast of Benin (West Africa) through Geochemistry and Isotope Hydrology; Recommendations for Improved Management

Along the West-African coast, groundwater is under several threats coming from both human activities and climate change. However, hydrogeological studies have so far been conducted in a piecemeal way, city by city. In this paper, a regional study was conducted along the Beninese coast, combining hydrogeochemistry and water stable isotopes. Monthly rainfall samples were analyzed in terms of chemistry and isotopes as well as groundwater from Holocene (upper aquifer) and Mio-Plio-Pleistocene (lower aquifer). This allowed to determine the recharge timing of aquifers (April to October, excluding August). Rainwater then infiltrates the soil with a slight evaporation. The upper aquifer, more heterogeneous, is displaying many different water types while the lower aquifer shows mainly a Na-Cl water type. While the upper aquifer shows many signs of contamination from human activities and saltwater intrusion from lakes and lagoons, the deeper aquifer is more influenced by a geogenic signature. These results are then interpreted regarding the demographic trends and climate change scenario. In the long-term, the groundwater level of the lower aquifer is expected to decrease as the rate of abstraction increases and recharge rate decreases. It is therefore recommended to develop adapted and urgent protection measures of the water resource to ensure sustainable and healthy groundwater exploitation.

Tracing sources and fate of nitrate in multilayered karstic hydrogeological catchments using natural stable isotopic composition (δ15N-NO3− and δ18O-NO3−). Application to the Toulon karst system (Dordogne, France)

• Nitrate contents and isotopes are measured in a large multilayered karst aquifer. • Primary and secondary sources of nitrate are determined, at the local scale, using nitrate isotopes. • Origins of nitrate, over a karstic hydrogeological catchment, are assessed by nitrate isotopes. • The fate of nitrate in a karst aquifer is specified/ elucidated/ quantified/ combining isotopes and hydrogeological approaches. In groundwater management, the determination of the origins of nitrate (NO 3 – ) endmember sources is the first step in improving the management over a hydrogeological catchment. The generally multiple source contamination by nitrate over the karst systems makes it difficult to identify the exact nitrate origins in surface water and groundwater. This work aims to assess nitrate vulnerability of a karst system fed by several karst aquifers. The Toulon karst system offers the opportunity to study the relationships between two multilayered karst aquifers. It shows an increase in mean NO 3 – concentrations over decades. However, the origins of the NO 3 – remain unclear. Natural stable isotopic composition of NO 3 – (δ 15 N -NO3 – and δ 18 O -NO3 – ) measurements combined with concentrations were used to identify the origins of nitrogen causing this increase. This information was also integrated into the local hydrogeological situation to improve knowledge of the contamination of both confined and unconfined karst aquifers. δ 15 N -NO3 − and δ 18 O -NO3 − in potential nitrate sources over the hydrogeological catchment were analysed in order to identify the range of values that can be measured in both rivers and groundwater. Over the Toulon Springs catchment, rivers are contaminated with nitrate coming from N synthetic fertilizer mineralized in agriculturalsoils. Most of the springs are also contaminated by this nitrate source, but some record an isotope composition of nitrate coming from sewage effluents. Toulon Springs are only contaminated with nitrate from N synthetic fertilizer mineralized in agricultural soils. Finally, from a hydrogeological point of view, the analysis of δ 15 N -NO3 − and δ 18 O -NO3 − values improves knowledge on the functioning of the study area and shows that the upper Jurassic karst confined aquifer has nitrate of the same origins (sources) as the upper Cretaceous unconfined aquifer. The results here highlight that the lower aquifer can be fed by the Upper Cretaceous unconfined aquifer, contrary to previous belief that the lower aquifer was confined. From a more fundamental standpoint, this work provides a set of parameters adapted for karstic hydrogeological catchment. It lays the foundation for the use of isotope biogeochemistry as a powerful tool for understanding the recharge of drinking water catchments and beyond the efficient management of water quality at the scale of karstic hydrogeological catchment.

Dynamic Monitoring of Mining Destruction on Coal Seam Floor With Constrained Time-Lapse Resistivity Imaging Inversion

With the increase in coal mining depth, water hazards in deep, strong karst aquifers have seriously threatened the safety of coal mine production. Dynamic monitoring of floor damage during coal seam mining under complex conditions is of great significance for the prediction of water inrush at the working face and for improving coal mining methods. Based on parallel electrical monitoring technology, a constrained time-lapse resistivity inversion method was adopted to process the electrical data of the working face floor. The background resistivity inversion results were used as an a priori model to act on the inversion of subsequent monitoring resistivity data, and the subsequent inversion results of resistivity were fed back to the prior model, which greatly eliminated inversion artifacts. Finally, the dynamic change process of the resistivity profile before and after the failure of the bottom floor of the mining face was determined, and the maximum depth of the failure zone of the floor is 25m, which is located above the lower aquifer. The field monitoring data show that compared with the method of directly comparing the differences between consecutive monitoring images, the inversion profiles obtained by the constrained time-lapse resistivity inversion method have clearer differences in the spatial and temporal dimensions. The depth of the water-conducting damage zone of the floor was clarified, and the basic development characteristic of the floor damage zone of the working face was obtained, which has practical application value for ensuring the safe mining of the working face.

Comparative Water Qualities and Blending in the Ogallala and Dockum Aquifers in Texas

Within the US Southern High Plains, it is known that the Ogallala Aquifer (OA) has been over pumped since large-scale agriculture began making use of the water in the 1950s. One option to address the decline is to find new water sources. The last 10–15 years have seen an increase in drilling large capacity, deeper wells in the co-located Dockum Aquifer in the Texas Panhandle. This lower aquifer is separated from the OA by low hydraulic conductivity sediment and is thus generally considered independent from the OA. We examined the suitability of the Dockum to supplement OA water by comparing recent water chemistries where the aquifers coexist. We also examined historical information on well yield, well development, and water quality. We found that water quality is equivalent to the Ogallala in some places but in others it is saltier, softer, and more sodic. Use of PCA and hydrochemical facies revealed that even in this small area Dockum water quality is highly variable. We used USGS-PHREEQC to model water blending at ratios of 0–>100% Ogallala. We show that there is irrigation water quality risk no matter the blend, that risks differ according to location, and that the most frequent risks are salinity, sodicity, and nitrate. We conclude that growers can manage these risks if they use blending to choose the risks they feel most apt to mitigate.

On the Propagation of Reaction Fronts in a Sandy Aquifer Over 20+ Years: Lessons From a Test Site in Northwestern Germany

AbstractDespite reduction measures, nitrate and aluminum concentrations remain high in aquifers in northwestern Europe. To evaluate the effectiveness of groundwater protection policies, the long‐term fate of these contaminants in groundwater needs to be understood. The groundwater catchment of the Haren water works, NW Germany, was characterized hydrogeochemically in the late 1990s, which provides an opportunity to study the solute fronts over a two‐decade period and conduct a post‐audit of the predicted front movement. Results indicate that, despite a significant reduction of the atmospheric acid loads, the acidification of soil and groundwater at the forest site persists. Removal of sorbed aluminum is required to induce a noticeable improvement, which will take at least several decades. The unexpected appearance of nitrate at the site, caused by a land use change in 1998, highlights the need for long‐term monitoring. Core data at the agricultural site show that the denitrification front has moved very little between 1998 and 2017, in accordance with previous forecasts. Denitrification by‐products, mainly sulfate and nitrogen, have migrated from the upper into the lower aquifer. A reactive transport model demonstrated how the link between the regional groundwater flow, pyrite oxidation, and the temporal variability of the nitrate concentration in recharge water, as reconstructed from age tracers, result in the observed vertical distribution of sulfate and nitrogen. This study demonstrates how long‐term monitoring, aided by model‐based data interpretation, can be used to successfully study and predict the fate of contaminants in groundwater.