Neelarun Mukherjee

Neelarun Mukherjee

Ph.D. Candidate @UT Austin

Talks and presentations

Characteristics of the Coastal Groundwater of Celestún, Mexico on the West Coast of the Yucatan Peninsula for Extreme Conditions During the Dry Season

December 12, 2024

Conference proceedings talk, American Geophysical Union Fall Meeting 2024, Washington DC

Abstract:

Yucatan, Mexico relies entirely on groundwater due to the absence of suitable surface water. Overconsumption of groundwater puts the region at risk of seawater contamination, jeopardizing the drinking water source. This study investigates groundwater-seawater interactions at a beach near Celestún, Mexico. Its goal is to contribute local knowledge regarding Celestún’s water resources which would broadly advance understanding of the coastal hydrogeology of the Yucatan Peninsula. Knowledge of the groundwater flow patterns, water table dynamics, and saltwater intrusion near the coast will provide information useful for water resource management. Water sampling and water table mapping were conducted using 15 piezometers along a 300 ft-long beach-perpendicular transect. The water table was measured using water level meters and pressure transducers, and the transect was surveyed with a total station. Chemical measurements were performed in the field and the lab to determine pH, conductivity, oxidation-reduction potential, sulfide, and major ion concentrations. Saturated hydraulic conductivity of sediment samples as well as electrical resistivity imaging (ERI) was measured along the transect.The hydrological, geophysical, and geochemical observations revealed that the beach transect is saltwater-dominated with a relatively thick vadose zone around 4.5 m. The site has an upper unconfined aquifer composed of sand, a caprock confining layer, and a lower confined aquifer. There was pronounced variability in the permeability of the surface sands ranging from 0.026 m/day to 50.3 m/day, as well as in topography and vegetation, which caused differences in infiltration rates. The tidally influenced unconfined coastal aquifer showed that groundwater flow is primarily landwards, with seawater mostly infiltrating, likely due to the dry season. Geochemical data from the wells are comparable to that of seawater and provide evidence of intrusion. The ERI data indicate that the caprock plays a role in protecting the deeper aquifer from local seawater intrusion. Thus, the shallow, unconfined aquifer at the beaches of Celestún may not be optimal for domestic use, but the deeper aquifer may host substantial fresh groundwater and should be further explored. However, no samples were collected in the confined aquifer.

Investigation of groundwater flow and reactive transport in supra-permafrost aquifers

December 12, 2024

Conference proceedings talk, American Geophysical Union Fall Meeting 2024, Washington DC

Abstract:

Groundwater flow and storage in aquifers on top of permafrost are very sensitive to climate conditions. These supra-permafrost aquifers (SPAs) exhibit high concentrations of dissolved organic carbon (DOC) due to rapid leaching from peat, the main material comprising SPAs. Thus, SPAs have an important role in carbon cycling. The biogeochemical transformation of the DOC into carbon dioxide and methane and DOC transport into aquatic water bodies depend on hydrologic conditions. How climate variability, such as variability in extreme weather events and freeze-thaw cycles, affects the coupled hydro-bio-geochemical processes within SPAs and the connectivity of SPAs with surface water bodies and the atmosphere are still unknown. This study addresses this gap through combining observations with modeling. First, we developed an ensemble of steady-state groundwater flow models which combined information on active layer hydro-stratigraphy, high resolution topography, and DOC concentrations from a first-order stream watershed, the Imnavait Creek in Alaska. The model ensemble showed that the range of groundwater discharge going into Imnavait Creek is the same as the range of summer streamflow. However, this approach does not explicitly represent SPA hydro-bio-geochemical dynamics. To address this, this study will be applying a multiphysics model, the Advanced Terrestrial Simulator (ATS), which integrates coupled surface and subsurface permafrost hydrology, energy transport, and biogeochemical reactions, to investigate the impact of different climate forcing. We will develop a 2D ATS model to first assess the impact of climate variability. The 2D domain is a 100 m-long and 40 m-thick transect beginning from the foot of a hillslope, through the riparian zone, and ending at Imnavait. Transect-specific observations determine the model geometry, flow and transport parameters, and boundary conditions. High-resolution hydro-stratigraphy and temperature observations (water table and ice table elevations) will be used for model assessment. The modeling will ultimately address how future climate, variable freeze-thaw cycles, and extreme weather affect lateral groundwater flow and overall biogeochemistry along the terrestrial-aquatic continuum.

Hydro-stratigraphy of the active layer in riparian valley bottoms of an arctic watershed

December 10, 2024

Conference proceedings talk, American Geophysical Union Fall Meeting 2024, Washington DC

Abstract:

The active layer describes the ground above permafrost that seasonally thaws, allowing for groundwater flow in an otherwise frozen subsurface. The transport of carbon through groundwater flow in the active layer influences its decomposition to greenhouse gases. Thus, the groundwater flow and storage regime of soil carbon has consequences on the global climate because of the large amount of carbon stored in arctic permafrost soils. Understanding the factors that affect groundwater within the active layer is therefore important, but observations regarding the hydrology of the active layer are lacking. Previous studies have identified three main strata within the active layer, acrotelm, catotelm, and mineral soil, which have different hydrologic properties. This research characterizes active layer soils of an arctic watershed through the measurement of saturated hydraulic conductivity, porosity, and bulk density, all of which affect the flow and storage of groundwater. Soil samples were collected throughout a portion of the Imnavait Creek Watershed on the North Slope of Alaska and classified by their stratum type. The thickness of each stratum was observed directly in the field and samples were collected from each stratum. Falling-head tests were done to estimate saturated hydraulic conductivity of the samples. Porosity and bulk density were measured gravimetrically through sequentially saturating and oven-drying the samples. Statistical analysis will be employed to compare how the measured values vary between soil types and across the spatial extent of the study site. The results of this study will help bound soil parameters needed for groundwater and Earth system models to predict the fate of carbon in permafrost.

Hydrologic, Geophysical, and Geochemical Characterization of an Aquifer along the Beach of a Barrier Island

December 15, 2022

Conference proceedings poster presentation, American Geophysical Union Fall Meeting, 2022, Chicago, California

Abstract:

Coastal aquifers are of global importance. They nurture marine ecosystems and support billions of people living near the coast. Coastal groundwater resources are particularly important for small island communities like Mustang Island, Texas, where rising sea levels, violent storm surges, and urbanization seriously threaten the island’s aquifer. Mustang Island is a barrier island formed by sediment deposition during the last ice age. The permeable island foundation supports a small freshwater aquifer that perches atop saltwater, i.e., a freshwater lens. Freshwater lenses rely on rainfall for recharge and are susceptible to changes in sea level, including from storm surges. Consequently, freshwater lens aquifer systems frequently experience significant fluctuations in shape and extent. Here, as part of a field methods class at the University of Texas at Austin, we report the results of geophysical (electrical resistivity [ER]), geochemical, and hydraulic observations along a beach-perpendicular study transect at Port Aransas beach on Mustang Island. We mapped a water table inclined towards shore, and that changed with the tide. Our observations suggest the water table and unconfined aquifer were responding to a storm surge which occurred immediately prior to our field study. Groundwater salinity (and water electrical conductivity) increased toward the shoreline. ER imaging showed distinct zonation within the water table, measuring groundwater resistivity ranging from 2.0 - 3.6 Ohm-m between 1.5 to 3 m below the surface and groundwater resistivity of 1.1 - 1.7 Ohm-m within 1.5 m of the surface and below 3 m. Measurements of aquifer hydraulic conductivity (K) displayed distinct spatial heterogeneity, with the highest K-values measured near the shore, dunes, and 15 cm beneath the surface of the beach. The analyses of ER, geochemical, and K-value data were used to generate geochemical and geophysical models of the groundwater to better understand the evolution of the freshwater lens in the presence of a dynamic, saline tide.

Supra-permafrost groundwater’s contribution to stream flow and organic matter chemistry in the Arctic: estimation using combined mechanistic and statistical approaches

December 12, 2022

Conference proceedings talk, American Geophysical Union Fall Meeting, 2022, Chicago, California

Abstract:

“Seasonally warm summers in the Arctic produce supra-permafrost aquifers within the active layer. However, the magnitude of groundwater flow, the amount of dissolved carbon and nutrients, and the solute flow paths are largely unknown, but critical to quantifying downgradient contributions to surface waters (lakes and rivers). To develop approachable methods to quantify groundwater inputs in continuous permafrost watersheds, we selected Imnavait Creek watershed on the North Slope of Alaska as a representative headwater drainage. We conducted 1000 groundwater flow simulations based on topography of the watershed and varying aquifer hydraulic conductivity and saturated thickness values. We fitted a lognormal distribution to the resulting 1000 model outputs, and we derived n=1e6 possible discharge values based on Monte Carlo random sampling on the model outputs. The groundwater discharge values integrated across the watershed generally agree with observed streamflow in Imnavait Creek over 2 months. When groundwater discharge estimates were combined with in-situ measurements of groundwater dissolved organic carbon and nitrogen concentrations, we found that Imnavait Creek’s organic matter load is also dominantly sourced from groundwater. Thus, riverine, and lacustrine ecological and biogeochemical processes relate strongly to groundwater phenomena in these continuous permafrost settings. As the Arctic warms and the active layer deepens, it will become more important to understand and predict supra-permafrost aquifer dynamics.”

Characterizing Rayleigh Taylor Instability and Convection in a Porous Medium with Geoelectric Monitoring

December 21, 2021

Conference proceedings poster presentation, American Geophysical Union Fall Meeting 2021, New Orleans, California

Abstract:

The use of geophysical tools for subsurface characterization is a common practice in environmental studies and georesources engineering. The electrical conductivity of the subsurface is strongly influenced by the different properties of the subsurface such as pore fluid chemistry, and consequently, by subsurface processes that affect the spatial distribution of that chemistry, such as the mixing dynamics of pore fluids. In the context of freshwater-saline water interaction in coastal areas, changes in solute spatial distribution are coupled to density-driven flow, which can thus be monitored via geoelectrical measurements. Here, we study the Rayleigh Taylor instability and subsequent convection occurring due to the density difference between two miscible liquids when the lighter one is positioned on top of the denser one, a configuration that is relevant for saltwater-freshwater interactions in coastal aquifers. We simulate the convective process and monitor it numerically by computing the transverse apparent conductivity of the medium in time, as the convection develops. We then look for correlations between the geoelectrical signal and a global scalar measure of the convective process’ advancement, namely the variance of the solute concentration field.

Gravitational Instability and Convection in a Granular Porous Medium: Pore Scale Experimental Study and Implications for Solubility Trapping Of CO2

December 15, 2019

Conference proceedings talk, American Geophysical Union Fall Meeting 2019, San Francisco, California

Abstract:

Geological sequestration of CO2 is considered as one of the few efficient ways to mitigate the increased global warming scenario. Solubility trapping stands as one of the key trapping mechanisms in CO2 sequestration. It dictates the typical timescale for safely storing CO2 in deep aquifers. A less dense layer of supercritical CO2 collects over a denser brine solution on CO2 injection under the cap rock. As CO2 mixes with brine, it incidentally forms a mixture heavier than pure brine, thus triggering a gravitational instability. Solubility trapping efficiency can be estimated by how fast this heavy layer is removed by the convective instability, thereby fuelling subsequent dissolution of CO2 into brine. Solubility trapping results in irreversible storage of CO2 at the bottom of the aquifer where dissolved CO2 is eventually converted into carbonate minerals through chemical trapping. Although most experimental studies of this phenomenon have been carried out in Hele-Shaw setups, as analogues to two dimensional (2D) porous media, few existing 3D numerical simulations indicate that its convection structure may be different from its 2D counterpart. Further, the effect of pore scale detail on the instability and subsequent convection is still unknown. To unravel these aspects, we have developed a laboratory scale experiment based on refractive index matching (RIM) of the liquid to the granular solid matrix. The setup does not currently involve continuous dissolution of CO2 at the top boundary but uses an analogue solute instead. It thus aims to unravel the miscible Rayleigh-Taylor instability dynamics in presence of a granular medium, and how it is differs from the dynamics predicted by Darcy-scale models. We focus on the onset time, non-linear time, mixing time, plume amplitude and plume speeds for the instability. The granular medium, with grains of diameter 3 mm, has dimensions 45x45x1 cm3. The 2D visualization in based on a solute dye that differentiates between the heavier (top) and lighter (bottom) fluid. The 3D visualization relies on a horizontal laser sheet that scans across the setup and triggers fluorescence in the lighter fluid whereas the heavier fluid is RIM to the solid grains. Upscaling the results from these experimental measurements provides insights into the trapping efficiency in geological porous media.