Agricultural Meteorology
A. Khedri; A. Saberinasr; N. Kalantari
Abstract
Introduction The comprehension of the hydrogeological conditions of the aquifer and the determination of its hydraulic characteristics, such as hydraulic conductivity, transmissivity coefficient, and specific storage, are crucial for the management and preservation of groundwater resources. Various ...
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Introduction The comprehension of the hydrogeological conditions of the aquifer and the determination of its hydraulic characteristics, such as hydraulic conductivity, transmissivity coefficient, and specific storage, are crucial for the management and preservation of groundwater resources. Various conventional methods, including empirical formulas, laboratory techniques (constant and falling head), tracer tests, field tests (Lugeon, Lefranc, slug, flowmeter, and pumping tests), and groundwater inverse modeling, are employed to establish these characteristics, particularly hydraulic conductivity. Empirical formulas are limited to ideal conditions, and in laboratory methods, the sample must be kept undisturbed. Due to the impracticality of measuring large-scale effective factors, the hydraulic conductivity determined through laboratory methods is also the only representative of the hydraulic conductivity at the sampling point. Tracer studies encounter numerous constraints, such as time, cost, porosity determination, and tracer dispersion in multilayered aquifers. It is also difficult to determine the average hydrodynamic properties of the heterogeneous aquifer based on the data obtained from a specific section of the Lefranc and Slug tests. Consequently, pumping tests are commonly selected for hydraulic parameter estimation. Although costly and time-intensive, these tests provide more precise coefficients. Geophysical methods have been greatly developed during the last two decades and have shown a significant correlation with the hydraulic parameters of the aquifer derived from borehole pumping tests or direct laboratory measurements. This approach minimizes uncertainties in numerical model calibration, improves data coverage, and reduces the time and cost of regional hydrogeological investigations. The conventional approach, known as the electrical resistivity method, is still widely used in global and local research projects for evaluating aquifer hydraulic characteristics (Ige et al., 2018; Arétouyap et al., 2019; Youssef, 2020; Ullah et al., 2020; de Almeida et al., 2021; Lekone et al., 2023). Therefore, this study aims to use the integrated approach of the geophysical method and pumping test as a cost-effective and efficient alternative for estimating the hydraulic parameters of the alluvial aquifer in the northeast of Gachsaran city. Material and Methods The research area is an alluvial aquifer located 5 km to the northeast of Gachsaran, between coordinates 50-52 to 51-09 E longitude and 30-15 to 30-28 N latitude. Using 86 vertical electrical soundings, Archie's equations, and the IPI2win software, the hydraulic characteristics of the aquifer under investigation were estimated. Subsequently, these characteristics were then compared to the coefficients derived from the data of two pumping test wells, which were calculated using the Aquifer test software and obtained via the Cooper-Jacob and Neuman methods. Results and discussion The hydrodynamic coefficients of the aquifer were initially determined using the Cooper-Jacob method in this study. The hydraulic conductivity values for wells one and two are 4.9 m/day and 5.7 m/day, respectively. Correspondingly, the storage coefficient values for wells one and two are 0.015 and 0.021, respectively. Based on the Cooper-Jacob approach, it is deduced that if the storage coefficient values exceed 0.001, the aquifer is classified as unconfined. In this study, the storage coefficient values for both pumping wells suggest that the aquifer is unconfined. Since the vertical flow component and the delayed yield phenomenon should also be taken into account in unconfiend aquifers, the Neuman analytical model has been used in the studied aquifer. The values of specific yield (Sy) for pumping wells one and two, which are related to delayed yield, are 0.05 and 0.04, respectively. These values were calculated by analyzing the first segment of the curve derived from the Neuman logarithmic drawdown-time plot. The storage coefficient values for pumping wells one and two, extracted from the second section of the curve, are 0.015 and 0.021, respectively. Furthermore, the transmissivity value for well number 1 was 323 m2/day, while for well number 2, it was 655.5 m2/day. The vertical electrical sounding (VES) data were subsequently initially analyzed and interpreted using the IPI2win software and the equalization curve method (partial curve matching technique). The coefficients denoted as m and n, indicative of the degree of cementation of the sediments, were determined based on the sedimentary composition prevalent in the area. Archie's equations were employed to calculate the formation factor and porosity parameters. The aquifer exhibits a porosity range of approximately 0.15 in the eastern and southeastern parts (near the outlet of the plain) and around 0.41 in the centeral, northern, and northwestern sections of the area (next to the Asmari Formation). The specific yield (Sy) of the aquifer was calculated using the provided formula: The minimum and maximum specific yield were estimated as 0.006 (in the eastern and southeastern regions) and 0.089 (in the western and northwestern regions of the plain), respectively, with an average value of 0.04. The transmissivity coefficients for the entire aquifer were then calculated based on the fitted relationship between hydraulic conductivity (K) and formation factor (F): The range of transmissivity coefficients varies from a minimum of 63 m2/day (in the western and northwestern sections of the plain) to a maximum of 608.9 m2/day (in the eastern and southeastern areas). The average transmissivity coefficient is calculated as 323.7 m2/day. To ensure the precision of the geoelectric method's coefficients, a comparative analysis was conducted with the hydrodynamic coefficients obtained from the two pumping test wells, as presented in the table below: Well No.K(m/d)T(m2/d)SyPT*VES*PTVESPTVES14.93.63232370.050.0525.75.5655.5632.50.040.03*PT: Pumping Test; VES: Vertical Electrical Sounding Conclusion The evaluation and comparison of the hydrodynamic coefficients derived from the aforementioned methods indicate that the geoelectric method coefficients exhibit acceptable agreement with the pumping test coefficients. In other words, the analysis of the pumping test conducted using the Neuman technique in the unconfined aquifer revealed that well number two displayed a greater transmissivity coefficient, while well number one presented a higher specific yield. These findings are confirmed by the geoelectric approach. Consequently, such hybrid approaches, which include simultaneous analysis of geophysical methods (such as VES) and pumping tests will be a great alternative to multiple costly pumping tests for evaluating the hydrodynamic coefficients of an aquifer. Moreover, employing this hybrid technique enables the generation of dense hydrodynamic coefficients in an aquifer for use as inputs in the groundwater model.
A. Saberinasr; M. Nakhaei; M. Rezaei; Seyed Mousa Hosseini
Abstract
Introduction: nZVI particles are strong reducing agents, capable of degradation and detoxification of a wide range of organic and inorganic pollutants in contaminated aquifers. Understanding the transport and retention of these nanoparticles in subsurface environments is required for treatment systems ...
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Introduction: nZVI particles are strong reducing agents, capable of degradation and detoxification of a wide range of organic and inorganic pollutants in contaminated aquifers. Understanding the transport and retention of these nanoparticles in subsurface environments is required for treatment systems and in situ groundwater remediation. During the last decade, several studies have been conducted to investigate the effect of different physicochemical conditions on the transport and retention of nZVI in saturated porous media. This study aimed to evaluate the effect of sand grain size and nanoparticle concentration on fundamental processes governing CMC-nZVI nanoparticle transport and retention in saturated porous media.
Materials and Methods: nZVI (NANOFER STAR, NANOIRON, s.r.o. Czech Republic was employed in this study. To prepare CMC-nZVI, nanoparticle, suspension and polymer solution was added by the relative dose of CMC to nZVI mass 1:2, in a 250-ml flask reactor. pH was fixed at 9.5 by NaOH and the solution was mixed for 144 h under ambient temperature condition at the absence of oxygen. Quartz sands with ∼ 99.38% SiO2 and 0.27 Fe2O3 based on XRF analysis, was used as the porous medium. The experiments were conducted using a cylindrical Plexiglas column 30 cm in length and 2.5 cm in inner diameter. In order to capture the effect of particle concentration and grain size, 12 tests were conducted with four different concentrations (C = 10, 200, 3000, 10000 mg/l) and three sizes of grain (dc = 0.297–0.5 mm, 0.5–1 mm, 1–2 mm). In each test, ∼4 PVs of nZVI suspension were introduced into the columns and to complete the test, ∼6 PVs of deionized water were flushed. The column effluent was collected every 2 min and analyzed for total Fe using UV-Vis. The normalized effluent iron concentration (C/C0) for each transport test was plotted as a function of pore volumes. The spatial distributions of retained CMC-nZVI in the sand columns were determined to right after the breakthrough experiment. The quartz sand in each column was carefully excavated in ~3 cm increments, transferred into 50 mL vials and analyzed for total Fe. The concentration of retained CMC-nZVI in all the sand columns was also plotted as a function of travel distance.
Results and Discussion: The breakthrough curves indicate that both grain size and nanoparticle concentration had a relevant impact on CMC-nZVI mobility, even if the influence of nanoparticle concentration was more evident. In all experimental conditions, the BTCs were not symmetrical, which indicates that attachment and detachment phenomena occurred in different modes. The breakthrough curves can be interpreted in two steps: injection and flushing times. The maximum relative concentration (C/C0) decreased, during injection time, for three different grain sizes while influent concentration increased from 10 to 10,000 mg/L, which can be attributed to the increase in particle-particle interaction (aggregation) and particle-sand interaction (attachment). The breakthrough curves, after the initial increase, showed a strong decline, which is a clear indication of the ripening phenomenon. This phenomenon affected the porous medium properties such as porosity and hydraulic conductivity. Moreover, At higher influent CMC-nZVI concentration, Na+ ion and subsequently ionic strength increases because of higher doses of Na-CMC. As a result, aggregation and deposition will occur under a shallower secondary energy minimum well, that they are reversible. At the same nZVI concentrations, the breakthrough curve decreased by a decrease in grain size. Decreases in grain size can lead to an increase in surface area, decrease in pore throat size; and consequently, retention of nanoparticles by straining phenomena. However, another behavior was governed during the flushing time. During the flushing time, a narrow sharp increase in C/C0 was observed called flushing peak. In this study, CMC-nZVI aggregates deposited onto surfaces of sands due to secondary energy minimum were eventually released during the flushing period of the column with DI water. The results suggest that the grain size and particle concentration can have a positive effect on this peak. The results of retention profiles demonstrate that the CMC-nZVI retention in low concentration (10 mg/L) is consistent with filtration theory; whiles the highly concentrated polymer-modified nZVI dispersions (especially 3000 and 10000 mg/l) contradicts filtration theory. Based on filtration theory (Elimelech et al. 1995; Tufenkji et al. 2004), if all factors affecting the transport of colloids are kept constant, grain size increase can lead to a decrease in surface area and attachment efficiency (α). The contradiction at high concentration can be explained by considering the effect of hydrodynamic forces (especially fluid shear) on agglomeration and disagglomeration and deposition and detachment. The size of stable aggregates formed in the pores of finer sands is smaller than when they are formed in the pores of larger sands because the magnitude of local shear is higher for narrower pores. This led to decreased retention in finer sand.
Conclusion: The results of this research show that during the injection time, ripening and straining phenomena are key retention mechanisms of nanoparticles by decreasing the sand size and increasing particle concentration. While during the flushing time, secondary energy minima and hydrodynamic forces play critical roles in the deposition and transport mechanisms of CMC-nZVI. At high particle concentration (3000 and 10000 mg/l), these factors can lead to an increase in nanoparticle mobility by decreasing sand size. However, the results of retention profiles were consistent with colloid filtration theory at low particle concentration (10 and 200 mg/l).
