Vadose Research Articles

Sensitivity of surface water and groundwater contributions to streamflow in a tropical glacierized basin under climate change scenarios

Abstract While mountain water faces threats posed by climate change, particularly in snow-dominated and glacierized systems, the role of groundwater in sustaining streamflow in these systems remains elusive. Changing mountain headwaters, marked by reduced snowpacks, retreating glaciers, shifting precipitation patterns, and rising temperatures, pose a crucial question: what is the resilience of streamflow in these mountains, and what role does groundwater play in this resilience? This is particularly uncertain in tropical high mountains where the seasonality of precipitation and glacier melt govern streamflow generation. A glacio-hydrological model was created using the Cold Regions Hydrological Modelling platform to investigate cryosphere-surface water-groundwater interactions in the Quilcayhuanca Basin, in Peru’s Cordillera Blanca. The model was forced by in-situ meteorological observations and parameterized using numerous data sources and process-based studies in the basin. Model results show that during the dry season, 37% of streamflow is generated from groundwater discharge, increasing to 56% during the lowest flows. Evapotranspiration is the largest mass flux from the basin at the peak of the dry season. Precipitation, temperature, and glacier change scenarios were used to assess the sensitivity of basin hydrology to climate change and glacier retreat. In a warmer, wetter, and nearly deglaciated future, Quilcayhuanca basin streamflow is expected to decrease by 4-19% annually, with a larger volumetric change in overland and vadose zone flow than in groundwater flow. The range in values is more closely linked to uncertainty in precipitation change than temperature change. Despite a strong reduction in snow and ice contribution to streamflow with warming and deglaciation, the concomitant increase in precipitation can limit the changes in streamflow and groundwater flow, showcasing the resilience of the system to shifts in climate and glacier cover.

The impact of dissolved organic nitrogen (DON) retention in the vadose zone on nitrogen leaching losses

Leaching of dissolved organic nitrogen (DON) is a significant pathway for nitrogen (N) loss in agricultural ecosystems. The excessive application of N for enhanceing agricultural productivity often results in the leaching of N into groundwater. Yet not well understood, the extent of retention in the vadose zone has critical implications for risk management and remediation strategies. This study aims to advance simulation techniques for modelling the transport process of reactive DON within a heterogeneous vadose zone. Through a combination of laboratory experiments and numerical simulations, the study firstly examines the extent of DON retention in the vadose zone and quantitatively analyse groundwater contamination risk from this kind of accumulation. Our findings indicate that heavy N fertilizer application and high-intensity rainfall events led to elevated contents of DON in the vadose zone and increased DON leaching fluxes into groundwater. Besides, intensifier rainfall reduced the N storage more quickly in scenarios devoid of DON application with higher mineralization rate, while DON slowly mineralized to other forms, largely accumulated in the top layer and migrated deeper with intensifier rainfall after input of urea. In our scenarios, DON accounted for a substantial portion (33–68%) of the total dissolved nitrogen (TDN) leaching fluxes, with exogenous DON content contributing significantly (25–85%) to the overall DON leaching into the aquifer. These results underscore the need for effective strategies to mitigate groundwater contamination risks associated with agricultural N use.

In situ soil moisture and thermal properties estimated using a dual-probe heat-pulse

In situ monitoring of the temporal and spatial distribution of soil moisture and thermal properties are important for studying the water and energy transport in the vadose zone. The single-probe heat-pulse method based on fiber Bragg grating technology (SPHP-FBG) has become a research focus in field monitoring because of its capability to realize quasi-distributed and real-time monitoring. However, the SPHP-FBG method can only obtain thermal conductivity. This study developed a dual-probe heat-pulse method based on FBG (DPHP-FBG). The DPHP-FBG method can measure thermal conductivity (λ), volumetric heat capacity (Cv), and thermal diffusivity (k). Consequently, volumetric soil water content (θ) can be estimated from its linear relationship with Cv. The accuracy of the DPHP-FBG method in the estimation of Cv, λ, and θ was tested under different heating duration and various soil moisture conditions. In addition, Monte Carlo simulation was performed to investigate the impact of FBG measurement errors on accuracy. Finally, a field test was conducted to verify the effectiveness of the developed DPHP-FBG system. The results show that the DPHP-FBG method allows accurate soil moisture and thermal properties estimation without soil-specific calibration. The mean errors of the Cv and θ decrease with the extended heating duration. When the heating lasts 20 s, the measured Cv and θ have mean errors of 0.02 MJ m−3 K−1 and 0.01 m3/m3, respectively, for various moisture conditions. In the field test, the spatio-temporal distribution of soil moisture and thermal properties can be obtained in real time. Thereby, the proposed DPHP-FBG monitoring system is potential to conduct in situ coupled heat and soil moisture measurements at a large scale.

Assessment of Groundwater Vulnerability to Pollution in the Metro Hilir Watershed Using the SINTACS Method

Abstract Assessment of groundwater vulnerability in the Metro Hilir watershed is necessary because most people use well water to meet basic needs. Identification and spatial analysis are the first steps to determine the potential for groundwater pollution in the area. The SINTACS method was used to assess the intrinsic vulnerability of groundwater using the parameters of groundwater table depth, effective infiltration, material in the unsaturated zone, soil texture type, aquifer media, hydraulic conductivity, and slope. Data processing was carried out by using Geographic Information System (GIS) techniques. Weight scenarios was used based on the hydrological conditions of the study area, namely “Normal Impact” and “Drainage/seepage”. The SINTACS index results was classified into several levels of vulnerability. The high vulnerability class is spread across the east to the south side, while the low vulnerability class is more dominant in the western area. The results of Sensitivity Analysis using the Map Removal technique showed that in both scenarios, soil texture parameters have the highest variation index value of 1.73% in the “Normal Impact” scenario and 2.13% in the “Drainage/Seepage” scenario. Parameters with high variation index values have a sensitive influence on vulnerability class values.

Remedial Conformable Sand-Control Technology Deployed Offshore Colombia

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 23690, “First Horizontal Deployment of Remedial Conformable Sand-Control Technology in Colombia: Qualification and Case Study,” by Suzanne Stewart, SPE, TAQA, and Jose M. Bermudez, SPE, and Camilo Mazo, Hocol, et al. The paper has not been peer reviewed. Copyright 2024 International Petroleum Technology Conference. _ The challenge of remedial sand control lies in the fact that the system must pass through restrictions in the upper completion before ideally conforming to the inner diameter (ID) of the zone requiring sand control. The complete paper describes the qualification of a multilayer, open-cell matrix polymer (OCMP) system for the first horizontal deployment in an offshore gas well. The system is capable of being deployed remedially through tight tubing restrictions while providing conformant sand control in a horizontal wellbore without the need for gravel packing, making the approach particularly valuable for scenarios where surface space is limited. Background During 2006, gas Wells CH-20, CH-19, and CH-18 were originally drilled and completed in the B2 zone in the Chuchupa field offshore Colombia. Wells CH-20, CH-19, and CH-18 experienced water breakthrough from the aquifer in late 2016, 2018, and 2020, respectively. The first workover operation in these wells was performed in 2021. The aim was to isolate the B2 zone with a packer and to perforate the slightly shallower B1 zone to access attic gas. In Well CH-18, an inflatable packer between Zones B2 and B1 was installed successfully, resulting in the ability to produce at full potential, while Wells CH-19 and CH-20 recovered their potential partially but below the calculated target rate. A new operation for the three wells was planned in 2022 to set a mechanical packer to isolate Zone B2 and reperforate Zone B1 to increase flow area, reduce drawdown at the sandface, and diminish the risk of sand production. However, because produced sand had accumulated in front of the perforations in all wells, removing the sand and setting the mechanical packer was impossible. At present, Wells CH-19 and CH-20 are not producing and Well CH-18 is on lower-than-calculated target production. Given the production difficulties, the plan is to isolate Zone B2 successfully in all wells, clean the wells, and install effective remedial sand control with significant surface-space constraints. OCMP: Conformant Remedial Through-Tubing Sand Control The OCMP system evaluated and subsequently selected for this application consists of multiple layers of highly porous (up to 85%) and permeable (40 darcies or greater) OCMP bonded to a specially designed base pipe. The compressibility of the system enables it to be run in hole compressed within a sleeve to allow passage through tight restrictions in the upper completion; then, once in place, the sleeve can be removed, allowing a return to the original shape and conformance to the ID of the sand-producing section. The system can be run on slickline, braided line, coiled tubing, or pipe. If deployed with a hydraulic conduit, it has the capability to perform sand cleanout during deployment and can incorporate an integral hydraulic-set, high-expansion anchor. If run with slickline or braided line, it is necessary to run an anchoring device on a separate trip; then, the first OCMP system can be anchored in place.

Numerical tank for suspended sediment transport under strong nonlinear waves in shallow water zones

Numerical tank for suspended sediment transport under strong nonlinear waves in shallow water zones

Assessment of the impact of greenhouse rainwater harvesting managed aquifer recharge on the groundwater system in the southern Jeju Island, South Korea: Implication from a numerical modeling approach

Managed aquifer recharge (MAR) is increasingly being adopted worldwide to mitigate groundwater depletion and ensure the sustainability of water resources. Rainwater harvesting (RWH)-MAR can augment aquifer storage and reduce flood damage in rural areas with dense greenhouse facilities. This study has assessed the feasibility of greenhouse RWH-MAR in Namwon agricultural areas in the southern part of Jeju Island, South Korea, by considering the injection rate and location of MAR using a numerical model. The model results showed that groundwater level increases were directly related to the infiltration rate, although spatial differences in head rise were observed owing to the spatial variability of hydraulic conductivity. In addition, installing the RWH-MAR in highland areas (>100 masl) enhanced the water level rise when compared to the expected values, indicating that a higher hydraulic gradient and thick unsaturated zone facilitated more effective MAR outcomes than in lowland areas. To optimize the contribution of source water to the agricultural water demand, placing the RWH-MAR near the pumping well improved the availability of injected rainwater to agricultural wells. This study highlights the importance of designing RWH-MAR schemes considering MAR objectives and the topographic and hydrogeological characteristics of the site.

Impact of Magnetic Fields on the Efficiency of Dense Non-Aqueous Phase Liquid Removal from Unsaturated Zones Using Steam Injection

Unsaturated zone is of great importance in providing water and nutrients that are vital to the biosphere and the main factor controlling water movement from the land surface to the aquifer. Contamination of unsaturated zone by Dense Non-Aqueous Phase Liquids (NAPLs) has becoming major threat to human environment as a result of increasing concern with industrialization. The use of steam injection for remediation of porous media which are contaminated by DNAPLs has not given the desire recovery efficiency, hence the need for improvement in recovery efficiency has been a subject of continuous study. This study investigated the effect of magnetic field on the removal of DNAPL from unsaturated zone using steam injection. An unsaturated zone of a sand box of interior dimensions 110 x 74 x 8.5 cm was polluted at different periods with 200 ml of Carbon tetrachloride. Steam injection experiment with flow rate of 0.01 m3/s was performed to determine the recovery efficiencies of Carbon tetrachloride in an unsaturated zone containing sand of porosity 0.42 and permeability of 0.001163779 cm/s. Magnetic field in step of 1 Tesla (T) was introduced from 1-3 Tesla (T) into experiment. The effects of magnetic field on removal of DNAPL from unsaturated zone using steam injection only and steam injection with magnetic field were compared using descriptive statistic. The recovery efficiency of Carbon tetrachloride using steam injection only was 82.05 %, while that with varying magnetic field at 1T, 2T and 3T were 89.45%, 94.95% and 95.35% respectively. The recovery efficiency of steam injection with varying magnetic field at 1T, 2T and 3T were 9.02, 15.72, 16.21% higher than the result of steam injection only for Carbon tetrachloride. The result demonstrated the ability of steam injection to recover contaminants from the subsurface. The application of magnetic field as an aid to facilitate system activities has been signifi-cantly effective due to its ability to overcome the constraints of conventional treatment processes. A combined application of steam injection with magnetic field appreciably enhances the removal of Non Aqueous phase liquids from Unsaturated Zone.

Mapping the intrinsic potential of water infiltration in urban subsurface: feedback from France

In a context of increasing urbanization, with strong soil sealing, and with an increase in extreme weather events, the management of rainwater in urban areas is becoming a major issue. In order to improve water resource management and to prevent urban floods, more and more cities are considering or already implementing water infiltration systems. Infiltration of water is not possible anywhere due to natural and/or anthropic reasons. In this frame, the intrinsic infiltration capacity of the subsurface is one key natural parameter. Global maps are needed to build territorial strategies. In France, a lot of studies are available but a national methodological framework for mapping this index does not exist. In this paper, we analyse various studies carried out in France for such mapping and compare the methods with examples in other countries. Most of the French studies combine a Multi-Criteria Analysis (MCA) and a Geographic Information System (GIS) spatial analysis. The criteria include geological, hydrogeological and geomorphological parameters. The PHOEBUS method developed on Rennes Metropolis seems the most relevant and replicable one to provide a common framework at French scale. It takes into account 7 criteria, including rock/soil permeability, topographic slope and thickness of the unsaturated zone. The obtained maps may be used as such, modified by integrating sealed surfaces or crossed with other criteria linked to the urban environment (e.g. sealed surfaces, soil pollution, heat islands). They provide a decision support tool for urban planning. In particular, they are useful to improve rainwater management, but also in the elaboration of desealing or renaturation strategies. It seems essential to act in concertation with local stakeholders to identify their needs and the specificities of the territory.

Numerical evaluation of vertical and horizontal distances for nitrogen attenuation from Onsite Sewage treatment and disposal systems (OSTDS)

Onsite Sewage Treatment and Disposal Systems (OSTDS) used for domestic wastewater treatment are a source of nitrogen (ammonium and nitrate) contamination to groundwater and surface water. One way to manage the contamination is to establish setback distances for nitrogen attenuation through nitrification and denitrification reactions within the setback distances. Determining the setback distances requires investigating nitrogen attenuation as a function of distance from OSTDS. While field investigations have been conducted, there have been few modeling investigations. Using an ArcGIS-based Nitrogen Load Estimation Toolkit, this study simulates nitrogen attenuation for over 20,000 OSTDS at five study areas across Florida, where a third of households are served by OSTDS. Different from conventional studies that only consider the horizontal distances (between an OSTDS and a surface water body that receives OSTDS effluents), this study also considers the vertical distances (between an OSTDS drainfield and water table beneath the drainfield) to account for the nitrogen reactive transport processes in the vadose zones and groundwater aquifers, making the assessment of nitrogen attenuation more comprehensive. Modeling results yield the following three empirical but quantitative relations: (1) vertical attenuation rate as a function of vertical distance and saturated hydraulic conductivity (Ks) available in the Soil Survey Geographic Database, (2) horizontal attenuation rate as a function of horizontal distance and Ks, (3) total attenuation rate as a multiplication of the vertical and horizontal attenuation rates. These relations can be used to determine nitrogen attenuation within vertical and horizontal distances to OSTDS for practical management of OSTDS nitrogen contamination.

Infiltration of secondary treated wastewater into an oxic aquifer: Hydrochemical insights from a large-scale sand tank experiment

To mitigate groundwater level decline, managed aquifer recharge (MAR) with secondary treated wastewater (STWW) is increasingly considered and implemented. However, the effectiveness and potential risks of such systems need evaluation prior to implementation. In this study, we present a large-scale sand tank experiment to analyse processes related to the infiltration of real STWW through the vadose zone and subsequent mixing with oxic native groundwater. The varying composition of STWW from 15 infiltration cycles over six months of operation and the retention times were the main drivers of the observed processes, which were characterized by a wide range of analytical techniques such as in situ high-resolution oxidation–reduction potential (ORP) measurements, closed mass balances of solutes, characterization of dissolved organic carbon (DOC), stable nitrate isotopes analysis, as well as numerical flow and transport modelling. Depending on the composition and infiltration rates of the STWW, both nitrification and denitrification could be observed, even simultaneously at different locations in the tank. Furthermore, due to the variability of the real STWW we observed enhanced arsenic mobilisation during times of elevated phosphate concentrations of the infiltrating STWW. Additionally, uranium was mobilised in our experimental system via carbonate mineral dissolution caused by the infiltrating STWW which was undersaturated of calcite for all infiltration cycles. Overall, our results showed the importance of conducting studies with waters of complex matrix, such as real STWW, and considering mixing with groundwater to assess the full range of possible processes encountered at MAR field sites.

Assessment of the water and mass balance in the landfill body by modelling using the example of the Dubna city landfill

To predict the migration of pollutants in groundwater from the landfill site, it is necessary to know the infiltration rate and the concentration of pollutants in the moisture entering the groundwater level below the landfill. It is shown that the HELP program for calculating moisture transport in the unsaturated zone can be used to estimate the infiltration rate by the example of the municipal solid waste in Dubna, To estimate chloride ion concentration in the leachate entering the groundwater level, the VS2DT program for calculating the transport of pollutants with moisture in the unsaturated zone can be used. It is shown how uncertainty and insufficiency of information about the landfill site affects the estimation of these parameters, in what cases it is possible to use information about a similar dump, how test calculations help to verify hypotheses of formation of groundwater pollution under the landfill. The ranges of infiltration rate and chloride ion concentrations in moisture entering the groundwater level under the landfill body obtained as a result of calculations of the water and mass balance of the landfill in Dubna were further used to calculate the chloride ion transport in groundwater.

Effects of Xerophytic Vegetation-Salix on Soil Water Redistribution in Semiarid Region

Xerophytic vegetation re-regulates and allocates water resources through canopy interception, root water uptake and transpiration, and changes the water budget among precipitation, runoff, interception and infiltration, thus having a significant impact on the processes of the hydrological cycle. In this study, we investigated the effect of xerophytic shrub-Salix on soil water redistribution and water budget through an in situ monitoring experiment combined with two-dimensional vegetation water consumption modeling. The results showed that, due to the interception effect of root water uptake, it was difficult for precipitation infiltration to recharge deep soil water and groundwater. The measured data of soil moisture content, hydraulic head and precipitation were used to verify and calibrate the performance of the soil water flow model in the vadose zone by HYDRUS-2D. The effect of roots system on soil water was simulated, and the appropriate spacing of Salix replanting was estimated. Combined with the relationship between the transverse roots system and the crown width obtained by the investigation, it was determined that the spacing between the Salix should be greater than five times the crown width, so that the balance between the water consumption of Salix and the water supply of deep soil by precipitation could be considered. The results of this study are important for estimating groundwater recharge in arid areas and provide practical vegetation replanting options for similar regions.

Soil Water Flow Affected by Vegetation Root Water Uptake in the Semiarid Region of Mu Us Sandy Land, China

ABSTRACTVegetation plays an important role in the management of water resources and of the terrestrial vegetation degradation in semiarid regions of China. Detailed descriptions of the actual hydrological process of vegetation root water uptake (RWU) are lacking, and the dynamics of interfaces involved in the process of RWU have not received enough attention. A field in situ experiment with bare land and vegetated land was implemented to investigate soil water flow in vadose zones affected by vegetation RWU. The results revealed that, with the influence of RWU, the soil moisture content was redistributed and impacted evaporation and infiltration; therefore, even the roots were driven to utilize groundwater under intense potential evapotranspiration and water stress. Notably, the thicker vadose zones were beneficial for preserving water resources for bare land, but Salix reduced soil water storage by 1191.13 L compared with bare land, which was adverse for water resource development in semiarid regions. The results provide the basis for the protection of the vegetation environment and water resource management in arid regions.

A comparison of sea-level rise and storm-surge overwash effects on groundwater salinity of a barrier island

Coastal fresh groundwater is threatened by salinization due to both sea-level rise and storm-surge overwash. Most studies of subsurface saltwater intrusion focus on sea-level rise, but storms that are becoming more frequent and intense with climate change could have more imminent and permanent effects on aquifer salinity. Few studies directly compare saltwater intrusion due to sea-level rise with saltwater intrusion due to storm-surge overwash, and no studies address this issue on barrier islands. In this study, a hydrological model simulating coupled, variable-density, surface and subsurface flow and salt transport was developed and calibrated to water level and specific conductance data at Assateague Island, MD. The calibrated model was used to calculate the mass of salt and the total volume of aquifer salinized in 2080 due to both sea-level rise and more frequent storm surges. The results suggest that the total mass of salt that intrudes into the aquifer due to changes in the 2-year storm surge height is approximately equal to that of sea-level rise (∼300 kg), and the surges salinize nearly the entire aquifer, exceeding the volume salinized by sea-level rise (∼80 %). The influence of aquifer properties and unsaturated zone thickness was investigated by reducing aquifer hydraulic conductivity, resulting in greater salinization from storm-surge overwash (100 % of aquifer volume) than from sea-level rise (∼20 %). Higher storm surges are expected to overtop the dunes on Assateague by 2080 causing a 75 % decline in areas above the 2-year storm event while sea-level rise will only inundate ∼ 40 % of the land area on Assateague. This analysis suggests that groundwater is likely more vulnerable to storm-surge overwash than to sea-level rise induced salinization, even in systems where sea-level rise is expected to be most severe.

Improving the water table fluctuation method to estimate groundwater recharge below thick vadose zones

ABSTRACT The water table fluctuation (WTF) method is popular for groundwater recharge (GR) estimation, but its accuracy is challenged when applied in areas with thick vadose zones because of the signal lag and attenuation with depth and uncertainties from barometric pressure effect and lateral flow. Improvement of the WTF method used the linear regression method and Darcy’s law, and has been assessed to give satisfactory results. In particular, the improved method presented lower GR (20–34%) relative to the conventional method. GR decreased from the centre to the edge of the tableland. The regional average GR was 63–81 mm year−1, equivalent to 11–14% of annual average rainfall. Lag times between recharge and rainfall ranged from 1 to 9 months. Rainfall and vegetation dominated the spatiotemporal variability of GR. Our study provides reference and technical support for GR estimation with the WTF method in regions with a thick vadose zone.

Model simplification to simulate groundwater recharge from a perched gravel-bed river

Gravel-bed rivers are an important source of groundwater recharge in some regions of the world. Their interactions with groundwater are complex and highly variable in space and time, with considerable water storage in the riverbed sediments. In losing river sections, where most of the groundwater recharge occurs, the river can be separated from the regional groundwater system by an unsaturated zone (i.e., perched). The complexity of groundwater–surface water interactions in these environments calls for the use of 3D fully integrated hydrological models to represent them, but their computational intensity limits their practicality for parameter inference, uncertainty quantification and regional scale problems. On the other hand, the simple groundwater–surface water exchange functions currently implemented in regional scale groundwater models are not suited to represent complex gravel-bed river systems such as braided rivers. There is therefore a need for developing groundwater–surface water exchange functions tailored to gravel-bed rivers that can be used in regional scale models.To address this issue, we developed a model simplification framework that combines a 3D integrated surface and subsurface hydrological model, a 2D cross-sectional river-aquifer model and a 1D conductance-based analytical model. We aim at broadly simplifying the 3D model while ensuring the appropriate simulation of groundwater recharge. We demonstrate our modelling approach on the Selwyn River (New Zealand) using piezometric data and groundwater recharge estimates derived from field observations and satellite imagery.Our results indicates that groundwater recharge from this river can be simulated using a simple 1D analytical model, which can easily be implemented in regional groundwater models (e.g., MODFLOW models). However, to represent properly the time variability of groundwater recharge, it is essential to use the groundwater level in the shallow aquifer associated with the river as input to the regional groundwater model. Our approach is generally transferable to other gravel-bed rivers but requires some observations of river losses for proper calibration.

Soil amendments and water management to improve attenuation and recovery of wastewater originated nutrients through a vegetation filter

Vegetation filters (VFs) are on-site wastewater (WW) treatments that can be considered as a nature-based solution (NbS). They are green infrastructures that provide several environmental benefits such as non-potable water reuse, contamination reduction, biomass production, landscaping improvements and CO2 fixation, among others. However, nutrient leaching, especially nitrate, partially exists. To overcome this limitation, operational parameters related to the irrigation water management and soil amendments were tested in a real system receiving WW from an office building operated along 4 years.The attenuation of N is improved (up to 83%) in the vadose zone by boosting biodegradation. Lower hydraulic loads and more frequent irrigation events using drippers and the incorporation of woodchips as a layer above the topsoil promote denitrification processes. Changes in organic carbon characteristics also confirm that biodegradation is enhanced.P attenuation is a result of abiotic processes, mainly driven by chemical equilibriums between the liquid and the sorbed and/or precipitated phase and, when uncontrolled changes in the WW quality occurs, removal efficiency is negatively affected. However, only 10% of the samples collected at 45 cm depth present concentrations above 2 mg/L. The woodchips application does not seem to ameliorate P removal regardless of the application method.The implemented measures allow higher soil water content, infiltration and groundwater recharge and prevents aquifer contamination.

Application of the DRASTIC-LU/LC method combined with machine learning models to assess and predict the vulnerability of the Rmel aquifer (Northwest, Morocco)

The Rmel aquifer, located in the Tangier-Tetouan-Al Hoceima region of northwest Morocco, covers approximately 240 km2 and faces increasing pollution threats due to population growth and economic development. This study assesses aquifer vulnerability to pollution, and compares the performance of various machine learning models integrated with the DRASTIC-LU/LC method. The research used a dataset of 52 water samples analyzed for nitrate concentrations, considering eight factors influencing vulnerability: aquifer depth, net recharge, aquifer lithology, soil texture, topography, vadose zone impact, hydraulic conductivity, and land use. An information gain test was applied to evaluate the importance of these factors. Four machine learning algorithms were used with the DRASTIC-LU/LC method: multilayer perceptron (MLP), the bagging algorithm (BA), K-nearest neighbors (KNN), and extremely randomized trees (ERT). Model performance was assessed via the area under the ROC curve (ROC-AUC) to measure accuracy. The ERT model combined with DRASTIC-LU/LC achieved the highest accuracy (AUC = 0.929), followed by BA (AUC = 0.925), MLP (AUC = 0.852), and KNN (AUC = 0.787). In comparison, the original DRASTIC-LU/LC model had an AUC of 0.530. The results highlight significant vulnerability variation across the Rmel aquifer, with high to very high levels in the southern and northwestern regions, and moderate to low levels in the northeast and central areas. Vulnerability maps were validated by comparing the observed nitrate concentrations in the water samples, confirming model accuracy. Groundwater depth, net recharge, and hydraulic conductivity were identified as the most significant factors influencing vulnerability. This study demonstrates the effectiveness of integrating machine learning models with the DRASTIC-LU/LC method for accurate aquifer vulnerability assessment, offering valuable tools for public policy and groundwater management.

Tracking abiotic transformation of 1,1,1-trichloroethane to 1,1-dichloroethylene in contaminated groundwater on a national scale

1,1,1-trichloroethane (TCA) is a chlorinated aliphatic compound that was increasingly applied as a replacement for trichloroethylene. TCA can simultaneously follow two abiotic degradation processes: hydrolysis and dehydrochlorination, where the latter lead to the formation of a relatively recalcitrant product 1,1-dichloroethylene (DCE). The abiotic processes are relatively rapid and can be assessed based on the ratio between TCA and DCE. Using national data collected in Israel since the year 2000, this work aimed to examine the abiotic degradation extent of TCA nationally and whether it is affected by the aquifer type and unsaturated zone thickness. We have also examined temporal shifts in TCA and DCE concentrations and tested whether they follow expected trends. Our results show that the abiotic degradation of TCA is significant on a national scale, with higher DCE over TCA concentrations in ≈ 89 % of the wells. Furthermore, in ≈ 85 % of the data points, TCA over DCE concentrations indicate that TCA underwent three or more half-lives. Comparing the different lithologies of contaminated groundwater, higher concentrations are observable in karst relative to unconsolidated aquifers. Nevertheless, the ratio between the two and correspondingly the degradation rates are not affected by lithology. Finally, temporal shifts in TCA and DCE concentrations, as well as the ratio between the two, are different than expected in an idealized closed system. Complexities such as lithological heterogeneity, multiple sources, transport parameters, and more must be considered in interpreting these trends when quantification of the degradation rate is attempted.