Diesel Migration and Distribution in Capillary Fringe Using Different Spill Volumes via Image Analysis

Laboratory-scale column experimSents were conducted to assess the impact of different LNAPL volumes on LANPL migration behavior in capillary zone in porous media. Three different volumes of diesel (50 mL, 100 mL, and 150 mL) were released in different experiments using a 1D rectangular column filled with natural sand. The water table was set at 29 cm from the bottom of the column. The image analysis results provided quantitative time-dependent data on the LNAPL distribution through the duration for the experiments. Results demonstrated that the higher diesel volume (150 mL) exhibited the faster LNAPL migration through all experiments. This observation was due to the high volume of diesel as compared to other cases which provides high pressure to migrate deeper in a short time. In all experiments, the diesel migration was fast during the first few minutes of observation and then, the velocity was decreased gradually. This is due to pressure exerted by diesel in order to allow the diesel to percolate through the sand voids. Overall, this study proved that the image analysis can be a good and reliable tool to monitor the LNAPL migration in porous media.

Numerical and experimental study of the vertical distribution of velocity and hydraulic gradient in the capillary fringe

<p>Capillary fringe plays an important role in the fate and transport of infiltrated solutes from agricultural lands. In this study, flow patterns and the vertical distribution of the velocity and hydraulic gradient inside the capillary fringe were investigated using FEFLOW calibrated by experimental data. An experimental box along with a real sample of capillary fringe from the study area (Sand and clay pit Brüggen, Germany) was used for the experiments. The dimension of the filled part of the box was 0.75 m long, 0.55 m high, and 0.150 m wide. To maintain a constant hydraulic gradient throughout the experiments the upstream and downstream groundwater levels were fixed to 7 cm and 3 cm, respectively. The horizontal velocity at different points inside the capillary fringe and the vadose zone was measured by injecting the fluorescent dye tracer (Uranin). At the end of the experiments, the soil samples are collected from different parts of the box for water content measurement. The results indicate that FEFLOW successfully estimates water content, overall flow pattern, and more importantly horizontal movement inside the capillary fringe. The streamlines are parallel to the groundwater table in the middle part.  Based on both experimental and numerical results, while there is a downward movement near the outflow, an upward movement was seen near the inflow. In previous studies, the velocity profile inside the capillary fringe was estimated using Darcy’s law, unsaturated hydraulic conductivity, and constant hydraulic gradient. The detailed comparison of measured water content and velocity with numerical modeling results showed that the constant hydraulic gradient assumption above the water table in previous studies is not valid. The vertical hydraulic gradient profile calculated by FEFLOW showed that the hydraulic gradient at the middle part of the box changes from 0.042 to 0.03. Moreover, the shape of the vertical hydraulic gradient profile is a function of the location in the box and soil type.</p><p><strong>Keywords: </strong>Solute transport, Unsaturated zone, Streamline, Pore velocity, Hydraulic conductivity, FEFLOW</p>

Inverted channels, polygonal fractures and layered mounds from the Makgadikgadi Pan of Botswana: possible analogues for Martian morphologies

<p>Inverted channels with polygonal fractures and layered mounds from the Ntwetwe pan in the Makgadikgadi Basin (central Botswana) have been herein investigated. These morphologies are from an evaporitic basin (the Makgadikgadi Basin) that is the remnant of an ancient Plio-Pleistocene lake and is currently part of the world’s largest evaporitic system.</p> <p>The mounds in the Ntwetwe pan are characterized by a layered structure and low relief (max. 5 m above the pan floor) and can be in excess of 2 km wide. The mounds consist mainly of loose (non-lithified) sand and silt with high moisture contents, even during the dry season. Geophysical investigations have shown that groundwater processes, particularly those related to the capillary fringe that rises and conveys moisture through the mounds, are factors that make mound sediments resistant to wind erosion.</p> <p>The inverted channels, identified in the southern part of the Ntwetwe pan, are characterized by gentle reliefs and depressions, which depend upon the distribution of calcretes and indurated sediments. Large scale (up to 100 m wide) polygonal fractures localized at the front of the fan, disappear at the transition with the present-day pan floor.</p> <p>We consider that these particularmounds, withinthe Ntwetwe pan, are remnants of the strandline of the paleo-Makgadikgadi Lake, and that the inverted channels representdistributary channels of a relict fan delta,formed by an ephemeral river, most likelythe paleo-Boteti River, during a Lake Paleo-Makgadikgadihighstand stage. We consider that largescale (up to 100 m wide) polygonal fractures, located on thechannel-mouth lobes, representlarge-scale desiccation cracks formed byrapid water evaporation from deltadeposits.</p> <p>The results of this investigation highlight the importance of the paleo-drainage system and itsinteractions with thewater table and wind-deflation as main geomorphological factors within salt pan environments. The mounds in the Makgadikgadi pans also show strong geomorphic similarities to spring mounds on the surface of Mars, localized in equatorial layered deposits (ELDs). These ELDs mounds are considered to result from cyclical groundwater upwelling, evaporation and wind deflation. The geological processes that resulted in the formation of mounds within the Makgadikgadi may, therefore, help to explain how similar layered deposits formed on Mars and confirm existing theories.</p>

Distinguishing Capillary Fringe Reflection in a GPR Profile for Precise Water Table Depth Estimation in a Boreal Podzolic Soil Field

Relative permittivity and soil moisture are highly correlated; therefore, the top boundary of saturated soil gives strong reflections in ground-penetrating radar (GPR) profiles. Conventionally in shallow groundwater systems, the first dominant reflection comes from the capillary fringe, followed by the actual water table. The objective of this study was to calibrate and validate a site-specific relationship between GPR-estimated depth to the capillary fringe (DCF) and measured water table depth (WTDm). Common midpoint (CMP) GPR surveys were carried out in order to estimate the average radar velocity, and common offset (CO) surveys were carried out to map the water table variability in the 2017 and 2018 growing seasons. Also, GPR sampling volume geometry with radar velocities in different soil layers was considered to support the CMP estimations. The regression model (R2 = 0.9778) between DCF and WTDm, developed for the site in 2017, was validated using data from 2018. A regression analysis between DCF and WTDm for the two growing seasons suggested an average capillary height of 0.741 m (R2 = 0.911, n = 16), which is compatible with the existing literature under similar soil conditions. The described method should be further developed over several growing seasons to encompass wider water table variability.