Showing papers on "Vadose zone published in 2022"

Integrated variable weight model and improved DRASTIC model for groundwater vulnerability assessment in a shallow porous aquifer

Abstract: Accurate and effective assessment of groundwater vulnerability is very important for ensuring a healthy groundwater ecosystem. We evaluate the vulnerability of shallow porous aquifers by modifying the DRASTIC model and verify the effectiveness of the variable weight model (VWM). Firstly, topography, aquifer media, and the impact of the vadose zone are replaced by land-use type, aquifer thickness, and the hydraulic resistance of the vadose zone. Second, the weighting is optimized using the analytical hierarchy process (AHP) and variable weight theory (VWT). Thirdly, the original and the improved DRASTIC methods were used to evaluate groundwater vulnerability. Finally, three sets of samples hydrochemical parameters (NO3-, Mg2+,COD) were used to verify the improved frameworks using the Receiver Operating Curve (ROC) method. The improved model increases the dispersion degree of groundwater vulnerability. Compared with the original DRASTIC method, the correlation of the VWM is significantly improved in terms of the area under curve (AUC) for NO3- (0.786), COD (0.753), and Mg2+ (0.831). In short, it is necessary to optimize the parameters and weights of the model in order to realize reliable estimations of groundwater vulnerability. In particular, the use of VWM brings the results in line with the actual situation by changing the weights.

Characterization of relevant site-specific PFAS fate and transport processes at multiple AFFF sites

Abstract: The relevance of multiple poly- and perfluoroalkyl substance (PFAS) fate and transport process across multiple sites was established through high-resolution characterization of the spatial distribution of PFAS within aqueous film forming foam (AFFF)-related source areas and the downgradient plumes. The maximum total PFAS concentrations in source area groundwater at the three study sites ranged from 6 to 51 mg/L but consistently decreased by several orders of magnitude with distance from the source area at all sites, indicating that non-destructive attenuation of PFAS occurred along each flow path. The relative distribution of different PFAS classes, including zwitterionic/cationic species, provided site-specific lines of evidence for retardation due to hydrophobic, air-water interfacial, and electrostatic partitioning processes, as well as impacts from biotransformation and matrix diffusion at multiple sites. The only site where one of these processes (air-water interfacial partitioning) was not supported by the data (Site 1) was attributable to disturbance of vadose zone soils as part of historic remedial efforts. In other cases, the magnitude that these processes influenced PFAS transport reflected site-specific conditions. This included apparent salting out of PFAS at Site 2 due to its elevated groundwater salinity, which has implications for plume migration in coastal areas. In addition, PFAS was present in lower-permeability soils at each site, suggesting that longer-term retention of PFAS is occurring in these zones. The finding that multiple processes were active at site-wide scales is consistent with expectations that these are naturally occurring reactions that should be relevant at most AFFF-impacted source zones.

A comparative study of groundwater vulnerability methods in a porous aquifer in Greece

Abstract: Abstract Groundwater vulnerability assessment is critical for the effective groundwater management, particularly in areas with significant anthropogenic activities, such as agriculture. In this study, seven different methods, namely, DRASTIC, Pesticide DRASTIC, SINTACS, Nitrate SINTACS, GOD, AVI, and SI, were implemented using Geographical Information System techniques in Nea Moudania aquifer, Chalkidiki, Greece, to evaluate and identify groundwater vulnerability zones. The study area was classified into five categories: very low, low, moderate, high, and very high vulnerability. The southern and south-western parts of the study area had the highest pollution potential; the corresponding potential is lower in the northern part. Furthermore, single-parameter sensitivity analysis has revealed that the vadose zone and the topography were the most influential parameters of the vulnerability indexes, while the hydraulic conductivity exhibited the lowest effective weight. Finally, nitrate concentrations, taken from 23 observation wells, were employed for the validation of the aforementioned seven methods, using the coefficient of determination ( R 2 ). Results showed that Pesticide DRASTIC and Nitrate SINTACS were the most accurate and efficient methods for the present study area, which is characterized by intense agricultural activities.

Estimation of groundwater recharge in a shallow sandy aquifer using unsaturated zone modeling and water table fluctuation method

Abstract: Quantification of groundwater recharge is one of the most important issues in hydrogeology, especially in view of the ongoing changes in climate and land use. In this study, we use numerical models of 1D vertical flow in the vadose zone and the water table fluctuation (WTF) analysis to investigate local-scale recharge of a shallow sandy aquifer in the Brda outwash plain in northern Poland. We show that these two methods can be jointly used to improve confidence in recharge estimation. A set of preliminary numerical simulations based on soil water content measurements from 4 grassland and pine forest profiles provided a wide range of recharge estimates (263 mm to 839 mm for a 3-year period). Additional simulations were performed with the lower boundary condition specified as a functional relationship between the groundwater table elevation and the rate of groundwater outflow from the vertical profile (horizontal drains boundary condition). In this way, we could reproduce the water table fluctuations resulting from recharge and lateral discharge to nearby lakes. The agreement between simulated and observed groundwater levels differed depending on the specific set of parameters characterizing vadose zone flow, which allowed us to find the most representative parameter sets and refine the range of plausible recharge estimates (501 mm to 573 mm per 3 years). The recharge rates from WTF (410 mm to 606 mm per 3 years) were in good agreement with numerical simulations, providing that the effect of the natural recession of groundwater table due to lateral outflow was considered (master recession curve method). Our results show that: (i) the proposed approach combining 1D vadose zone modeling and WTF improves recharge estimation, (ii) multiple types of observations, including groundwater table positions, are needed to calibrate and validate vadose zone flow models, and (iii) extended periods of observations and simulations are necessary to capture year-to-year variability in the recharge rates.

Estimation of Transport Parameters of Perfluoroalkyl Acids (PFAAs) in Unsaturated Porous Media: Critical Experimental and Modeling Improvements.

Abstract: Predicting the transport of perfluoroalkyl acids (PFAAs) in the vadose zone is critically important for PFAA site cleanup and risk mitigation. PFAAs exhibit several unusual and poorly understood transport behaviors, including partitioning to the air-water interface, which is currently the subject of debate. This study develops a novel use of quasi-saturated (residual air saturation) column experiments to estimate chemical partitioning parameters of both linear and branched perfluorooctane sulfonate (PFOS) in unsaturated soils. The ratio of linear-to-branched air-water interfacial partitioning constants for all six experiments was 1.62 ± 0.24, indicating significantly greater partitioning of linear PFOS isomers at the air-water interface. Standard breakthrough curve analysis and numerical inversion of HYDRUS models support the application of a Freundlich isotherm for PFOS air-water interfacial partitioning below a critical reference concentration (CRC). Data from this study and previously reported unsaturated column data on perfluorooctanoate (PFOA) were reevaluated to examine unsaturated systems for transport nonidealities. This reanalysis suggests both transport nonidealities and Freundlich isotherm behavior for PFOA below the CRC using drainage-based column methods, contrary to the assertions of the original authors. Finally, a combined Freundlich-Langmuir isotherm was proposed to describe PFAA air-water interfacial partitioning across the full range of relevant PFAA concentrations.

Tracing the Sources and Fate of NO3- in the Vadose Zone-Groundwater System of a Thousand-Year-Cultivated Region.

Abstract: Excess nitrate (NO3-) loading in terrestrial and aquatic ecosystems can result in critical environmental and health issues. NO3--rich groundwater has been recorded in the Guanzhong Plain in the Yellow River Basin of China for over 1000 years. To assess the sources and fate of NO3- in the vadose zone and groundwater, numerous samples were collected via borehole drilling and field surveys, followed by analysis and stable NO3- isotope quantification. The results demonstrated that the NO3- concentration in 38% of the groundwater samples exceeded the limit set by the World Health Organization. The total NO3- stock in the 0-10 m soil profile of the orchards was 3.7 times higher than that of the croplands, suggesting that the cropland-to-orchard transition aggravated NO3- accumulation in the deep vadose zone. Based on a Bayesian mixing model applied to stable NO3- isotopes (δ15N and δ18O), NO3- accumulation in the vadose zone was predominantly from manure and sewage N (MN, 27-54%), soil N (SN, 0-64%), and chemical N fertilizer (FN, 4-46%). MN was, by far, the greatest contributor to groundwater NO3- (58-82%). The results also indicated that groundwater NO3- was mainly associated with the soil and hydrogeochemical characteristics, whereas no relationship with modern agricultural activities was observed, likely due to the time delay in the thick vadose zone. The estimated residence time of NO3- in the vadose zone varied from decades to centuries; however, NO3- might reach the aquifer in the near future in areas with recent FN loading, especially those under cropland-to-orchard transition or where the vadose zone is relatively thin. This study suggests that future agricultural land-use transitions from croplands to orchards should be promoted with caution in areas with shallow vadose zones and coarse soil texture.

Fate and transport modelling framework for assessing risks to soil and groundwater from chemicals accidentally released during surface operations: An Australian example application from shale gas developments

Abstract: Shale and tight gas developments in the Beetaloo (28,000 km2) and Cooper (139,000 km2) basins of Australia are subject to stringent State and Federal Government controls and assessments. Several scientific investigations are ongoing to improve the scientific basis of the risks from unconventional gas developments to water and the environment. In this study a framework was developed to derive estimates of chemical dilution associated with leakage to groundwater from accidental release of chemicals used for shale and tight gas extraction in Australia. The quantitative assessment accounted for key landscape parameters that determine natural attenuation: soil type, depth to groundwater and groundwater velocity. Both basins were discretised into 1000 × 1000 m2 grids for which the unsaturated zone and groundwater dilution factors were derived. Migration of chemicals through deep unsaturated zones was calculated with the HYDRUS-1D simulator, taking account of best-available hydraulic properties from a digital soil database. A three-dimensional analytical solution of the advection–dispersion equation provided estimates of dilution in groundwater after solutes travelled 500 m from the centre (source location) to the edge of every grid cell. The combined vadose zone-groundwater dilution factors were used to determine under which conditions concentrations of hydraulic fracturing chemicals or flowback water accidentally released into the environment would decrease to levels that are no longer considered harmful to the environment. When the method was applied to 39 hydraulic fracturing chemicals scheduled for stimulation of a shale gas well, ecotoxicological risk quotients (RQ) were calculated to indicate which chemicals were of no environmental concern. This work contributes to increasing the efficiency of quantitative impact assessments and provides a framework to develop dedicated monitoring and management practices to support regulation and management of the gas industry in Australia.

Utilizing stable water isotopes (δ2h and δ18o) to study soil-water origin in sloped vineyard: first results

Abstract: The diversity of processes taking place in hillslope agro-ecosystems makes the estimation of vadose zone dynamics rather challenging. This paper presents the first insight into the research of volumetric water content, granulometric composition, meteorological data, precipitation and soil-water isotopic composition conducted within the SUPREHILL project at its vadose zone observatory. The main goals of this research are related to the evaluation of soil-water origin at the hillslope vineyard, but also to the estimation of depths until which precipitation infiltrates and where the occurrence of preferential flow is possible. For that purpose, hydrometeorological data, granulometric composition and stable isotopesof hydrogen (δ2H) and oxygen (δ18O) from precipitation and sampled soil water have been used. The results indicate the existence of a different isotopic signature in soil water, which suggests different infiltration patterns in the investigated area. Also, the results point out that surface runoff, subsurface runoff, and most of the passive wick lysimeters respond to precipitation, while the response of suction probes located at deeper depth is not that evident. This corresponds to the results related to the variation of water content at different depths. All the results indicate the possible existence of a low permeable layer at an approximate depth of 60 cm. Furthermore, preferential flow, if it exists, can be expected from the shallowest depths of the vineyard to a maximum depth of 80 cm. It is expected that an established long-term monitoring network at the SUPREHILL Observatory will give a more precise definition of soil-water behaviour and the existence of preferential flow.

Desorption Isotherms for Poly- and Perfluoroalkyl Substances in Soil Collected from an Aqueous Film-Forming Foam Source Area

Abstract: Bench-scale experiments were performed to evaluate poly- and perfluoroalkyl substance (PFAS) desorption from vadose zone soils collected at a site historically impacted with aqueous film-fo...

Natural Source Zone Depletion (NSZD): From process understanding to effective implementation at LNAPL-impacted sites

Abstract: This paper describes the proceedings of a Special Session on Natural Source Zone Depletion (NSZD) at the AquaConSoil conference, held virtually in June 2021. It spans research across Europe, the United States of America and Australia.NSZD has been described as the “combination of processes that reduce the mass of [light non-aqueous phase liquid] LNAPL in the subsurface”. LNAPL NSZD research and investigations have been focused on a range of hydrocarbon products, such as gasoline, diesel, jet fuel, as well as crude oil. Key NSZD processes include aerobic biodegradation, fermentation and methanogenesis of LNAPL constituents, dissolution of LNAPL constituents into groundwater and volatilization of LNAPL constituents into the unsaturated zone. In turn the generated methane, carbon dioxide and dissolved / volatilized constituents can be cycled and biodegraded in the unsaturated and saturated zones. Importantly these physical, chemical, and biological processes can act without human intervention to reduce the NAPL mass and toxicity. Over time NSZD can both reduce LNAPL mass, and change its chemical composition resulting in risk reduction, and ultimately source depletion.Methanogenesis of organic materials has long been recognised in municipal landfills and natural anoxic environments, such as peat and wetlands. Recognition of similar processes in LNAPL source zones in the past decade along with high rates of aerobic biodegradation observed in unsaturated zones above LNAPL-impacted areas, has significantly revised the conceptual model of LNAPL source zone behaviour and persistence. Several NSZD monitoring approaches have been developed and are being applied in field studies. While the quantitative NSZD rates derived can vary between techniques, they all demonstrate that NSZD LNAPL removal can exceed that delivered by engineered LNAPL recovery techniques, particularly for mature LNAPL bodies.

The Use of the DRASTIC-LU/LC Model for Assessing Groundwater Vulnerability to Nitrate Contamination in Morogoro Municipality, Tanzania

Abstract: Groundwater is a useful source of water for various uses in different places. The major challenge in the use of this resource is how to manage and protect it from contamination. The current study was conducted in Morogoro Municipality to identify vulnerable groundwater areas by using DRASTIC-LU/LC model. The study applied eight input parameters, i.e., depth to water table, net recharge, aquifer media, soil media, topography, impact of vadose zone, hydraulic conductivity and land use/land cover patterns, which were overlaid in GIS to generate groundwater vulnerable map. The model used rating (R = 1–10) and weighting (W = 1–5) techniques to assess the effect of each parameter on groundwater contamination. The DRASTIC-LU/LC Vulnerability Index map was classified into low- (area = 29.2 km2), moderate- (area = 120.4 km2) and high-vulnerability zones (area = 124.4 km2). Nitrate analysis was conducted using the cadmium reduction method (DR 890) to assess the validity of the model and it was observed that 55%, 15% and 50% of the samples with unacceptable (>50 mg/L), high (29–50 mg/L) and moderate (14–28 mg/L) nitrate concentrations, respectively, fall into the high-vulnerability zone. Furthermore, 45%, 70% and 50% of the samples with unacceptable, high and moderate nitrate concentrations, respectively, fall into the moderate-vulnerability zone. In the low-vulnerability zone, only 15% of samples were found with a high nitrate concentration.

Column-Test Data Analyses and Geochemical Modeling to Determine Uranium Reactive Transport Parameters at a Former Uranium Mill Site (Grand Junction, Colorado)

Abstract: The long-term release of uranium from residual sources at former uranium mill sites was often not considered in prior conceptual and numerical models, as contaminant removal focused on meeting radiological standards. To determine the reactive transport parameters, column tests were completed with various influent waters (deionized water, site groundwater, and local river water) on sediment from identified areas with elevated uranium on the solid phase in (1) vadose-zone (VZ) sediments, (2) saturated-zone sediments with higher organic carbon content, and (3) both vadose- and saturated-zone sediments with additional gypsum content. The gypsum was precipitated when low-pH, high-sulfate, tailings fluids or acidic waste disposal water were buffered by natural aquifer calcite dissolution. In general, the resulting uranium release was higher in the sediments with greater uranium concentrations. However, the addition of deionized water (DI) to the VZ sediments delayed the uranium release until higher-alkalinity groundwater was added. Higher-alkalinity river water continued to remove uranium from the VZ sediments for an extended number of pore volumes, with the uranium being above typical standards. Thus, river flooding is more efficient at removing uranium from VZ sediments than precipitation events (DI water in column tests). Organic carbon provides a stronger uranium sorption surface, which can be explained with geochemical modeling or a larger constant sorption coefficient (Kd). Without organic carbon, the typical sorption in sands and gravels is easily measurable, but sorption is stronger at lower, water-phase uranium concentrations. This effect can be simulated with geochemical modeling, but not with a constant Kd. Areas with gypsum create situations in which geochemical sorption is more difficult to simulate, which is likely due to the presence of uranium within mineral coatings. All the above mechanisms for uranium release must be considered when evaluating remedial strategies. Column testing provides initial input parameters that can be used in future reactive transport modeling to evaluate long-term uranium release rates and concentrations.