Evaluating Freshwater Lens Vulnerability in a Multi-layered, Island Aquifer System in the Tropics

2020 ◽  
Author(s):  
Eddie W. Banks ◽  
Saskia Noorduijn ◽  
Okke Batelaan ◽  
Vincent Post ◽  
Adrian Werner ◽  
...  
Keyword(s):  
Transport Model ◽  
Three Dimensional ◽  
Primary Source ◽  
Variable Density ◽  
Geophysical Data ◽  
Near Surface ◽  
Freshwater Lens ◽  
Calibration Process ◽  
Aquifer System ◽  
Dimensional Variable

<p>Groundwater is the primary source of freshwater supply on remote small islands, where it exists as a freshwater lens. It is extremely vulnerable to over-extraction, pollution and seawater intrusion. Ensuring long-term sustainable management of the groundwater resource is of the utmost importance when there are growing water demands, sea-level rise and/or recharge decline. This study used a three-dimensional, variable-density numerical groundwater flow and solute transport model to investigate vulnerability of a freshwater lens in a multi-layered aquifer system on Milingimbi Island, a small tropical island in northern Australia. The model was used to explore the impacts and possibility of increased groundwater demand on the freshwater lens, its volume, geometry as well as the thickness of the transition zone. The risks of saltwater intrusion, both laterally from the ocean and by localised up-coning from the deeper, more saline aquifers beneath the freshwater lens, were also assessed. Model calibration used observed hydraulic heads and salinity observations from pumping and observation wells. Subsurface bulk conductivity values, which were calculated from inverted airborne electromagnetic (AEM) and near-surface geophysical data, were also used in the calibration process. The results showed that the hydraulic heads and observed salinity achieved the ‘best fit’ in the calibration process, whereas the addition of the geophysical data assisted in constraining the lens geometry in the steady state model and integrated the data poor areas based on traditional hydrogeological datasets. The models’ calibration sensitivity to the range of measured salinities could be enhanced by improving the conversion factor between the AEM-derived conductivity values and the observed salinity data. This would best be accomplished by targeted monitoring wells at discrete depths and locations across the lens and improvements in the sampling/restoration of existing ones. The numerical model provided a framework to evaluate the key underlying hydrogeological processes on the island, as well as an important decision-making tool to ensure a sustainable and reliable water supply for the island community.</p>

scholarly journals Simulations of column-average CO<sub>2</sub> and CH<sub>4</sub> using the NIES TM with a hybrid sigma-isentropic (σ−θ) vertical coordinate

2012 ◽  
Vol 12 (3) ◽  
pp. 8053-8106 ◽  
Author(s):  
D. A. Belikov ◽  
S. Maksyutov ◽  
V. Sherlock ◽  
S. Aoki ◽  
N. M. Deutscher ◽  
...  
Keyword(s):  
Transport Model ◽  
Three Dimensional ◽  
Model Performance ◽  
Vertical Motion ◽  
West Siberia ◽  
Total Carbon ◽  
Tracer Transport ◽  
Chemical Transport Model ◽  
Near Surface ◽  
Vertical Coordinate

Abstract. We have developed an improved version of the National Institute for Environmental Studies (NIES) three-dimensional chemical transport model (TM) designed for accurate tracer transport simulations in the stratosphere, given the use of a hybrid sigma-isentropic (σ−θ) vertical coordinate that employs both terrain following and isentropic parts switched smoothly around the tropopause. The air-ascending rate was derived from the effective heating rate and was used to simulate vertical motion in the isentropic part of the grid (above level 350 K), which was adjusted to fit to the observed age of the air in the stratosphere. Multi-annual simulations were conducted using NIES TM to evaluate vertical profiles and dry-air column-averaged mole fractions of CO2 and CH4. Comparisons with balloon-borne observations over Sanriku (Japan) in 2000–2007 revealed that the tracer transport simulations in the upper troposphere and lower stratosphere are performed with accuracies of ~5% for CH4 and SF6, and ~1% for CO2 compared with the observed volume-mixing ratios. The simulated XCO2 and XCH4 were evaluated against daily ground-based high-resolution Fourier Transform Spectrometer (FTS) observations measured at twelve sites of the Total Carbon Column Observing Network (TCCON) (Bialystok, Bremen, Darwin, Garmisch, Izaña, Lamont, Lauder, Orleans, Park Falls, Sodankylä, Tsukuba, and Wollongong) between January 2009 and January 2011. The comparison shows the model's ability to reproduce the site-dependent seasonal cycles as observed by TCCON, with correlation coefficients typically on the order 0.8–0.9 and 0.4–0.8 for XCO2 and XCH4, respectively, and mean model biases of ±0.2% and ±0.5%, excluding Sodankylä, where strong biases are found. The capturing of tracer total column mole fractions is strongly dependent on the model's ability to reproduce seasonal variations in tracer concentrations in the planetary boundary layer (PBL). We found a marked difference in the model's ability to reproduce near-surface concentrations at sites located some distance from multiple emission sources and where high emissions play a notable role in the tracer's budget. Comparisons with aircraft observations over Surgut (West Siberia), in an area with high emissions of methane from wetlands, show contrasting model performance in the PBL and in the free troposphere. This is another instance where the representation of the PBL is critical in simulating the tracer total columns.

scholarly journals Estimation of effective porosity in large-scale groundwater models by combining particle tracking, auto-calibration and <sup>14</sup>C dating

2018 ◽  
Author(s):  
Rena Meyer ◽  
Peter Engesgaard ◽  
Klaus Hinsby ◽  
Jan A. Piotrowski ◽  
Torben O. Sonnenborg
Keyword(s):  
Particle Tracking ◽  
Large Scale ◽  
Transport Model ◽  
Age Distribution ◽  
Three Dimensional ◽  
Effective Porosity ◽  
Advective Transport ◽  
Transport Modelling ◽  
Aquifer System ◽  
Groundwater Quantity

Abstract. Effective porosity plays an important role in contaminant management. However, the effective porosity is often assumed constant in space and hence either neglected or simplified in transport model calibration. Based on a calibrated highly parametrized flow model, a three-dimensional advective transport model (MODPATH) of a 1300 km2-large coastal area of southern Denmark and northern Germany is presented. A detailed voxel model represents the highly heterogeneous geological composition of the area. Inverse modelling of advective transport is used to estimate seven, spatially distributed, effective porosity units based on apparent groundwater ages inferred from 11 14C measurements in Pleistocene and Miocene aquifers, corrected for the effects of diffusion and geochemical reactions. The match between the observed and simulated ages is improved significantly by the calibration of seven porosity units by a reduction of ME of 99 % and RMS of 82 % compared to a uniform porosity approach. Groundwater ages range from a few hundred years in the Pleistocene to several thousand years in Miocene aquifers. The advective age distributions derived from particle tracking at each sampling well show unimodal (for younger ages) to multimodal (for older ages) shapes and thus reflect the heterogeneity that particles encounter along their travel path. The estimated effective porosity field, with values ranging between 4.3 % in clay and 45 % in sand formations, is used in a direct simulation of distributed mean groundwater ages. Although the absolute ages are affected by various uncertainties, a unique insight into the complex three-dimensional age distribution pattern and potential advance of young contaminated groundwater in the investigated regional aquifer system is provided, highlighting the importance of estimating effective porosity in groundwater transport modelling and the implications for groundwater quantity and quality assessment and management.

scholarly journals Estimation of effective porosity in large-scale groundwater models by combining particle tracking, auto-calibration and <sup>14</sup>C dating

2018 ◽  
Vol 22 (9) ◽  
pp. 4843-4865 ◽  
Author(s):  
Rena Meyer ◽  
Peter Engesgaard ◽  
Klaus Hinsby ◽  
Jan A. Piotrowski ◽  
Torben O. Sonnenborg
Keyword(s):  
Particle Tracking ◽  
Large Scale ◽  
Transport Model ◽  
Age Distribution ◽  
Three Dimensional ◽  
Effective Porosity ◽  
Advective Transport ◽  
Transport Modelling ◽  
Aquifer System ◽  
Groundwater Quantity

Abstract. Effective porosity plays an important role in contaminant management. However, the effective porosity is often assumed to be constant in space and hence heterogeneity is either neglected or simplified in transport model calibration. Based on a calibrated highly parametrized flow model, a three-dimensional advective transport model (MODPATH) of a 1300 km2 coastal area of southern Denmark and northern Germany is presented. A detailed voxel model represents the highly heterogeneous geological composition of the area. Inverse modelling of advective transport is used to estimate the effective porosity of 7 spatially distributed units based on apparent groundwater ages inferred from 11 14C measurements in Pleistocene and Miocene aquifers, corrected for the effects of diffusion and geochemical reactions. By calibration of the seven effective porosity units, the match between the observed and simulated ages is improved significantly, resulting in a reduction of ME of 99 % and RMS of 82 % compared to a uniform porosity approach. Groundwater ages range from a few hundred years in the Pleistocene to several thousand years in Miocene aquifers. The advective age distributions derived from particle tracking at each sampling well show unimodal (for younger ages) to multimodal (for older ages) shapes and thus reflect the heterogeneity that particles encounter along their travel path. The estimated effective porosity field, with values ranging between 4.3 % in clay and 45 % in sand formations, is used in a direct simulation of distributed mean groundwater ages. Although the absolute ages are affected by various uncertainties, a unique insight into the complex three-dimensional age distribution pattern and potential advance of young contaminated groundwater in the investigated regional aquifer system is provided, highlighting the importance of estimating effective porosity in groundwater transport modelling and the implications for groundwater quantity and quality assessment and management.

Wake behind a three-dimensional dry transom stern. Part 2. Analysis and modelling of incompressible highly variable density turbulence

2019 ◽  
Vol 875 ◽  
pp. 884-913 ◽  
Author(s):  
Kelli Hendrickson ◽  
Dick K.-P. Yue
Keyword(s):  
Mass Flux ◽  
Algebraic Closure ◽  
A Priori ◽  
Three Dimensional ◽  
Variable Density ◽  
Phase Region ◽  
Reynolds Stresses ◽  
Near Surface ◽  
Variable Density Turbulence ◽  
Turbulent Mass Flux

We analyse the turbulence characteristics and consider the closure modelling of the air entraining flow in the wake of three-dimensional, rectangular dry transom sterns obtained using high-resolution implicit large eddy simulations (iLES) (Hendrickson et al., J. Fluid Mech., vol. 875, 2019, pp. 854–883). Our focus is the incompressible highly variable density turbulence (IHVDT) in the near surface mixed-phase region ${\mathcal{R}}$ behind the stern. We characterize the turbulence statistics in ${\mathcal{R}}$ and determine it to be highly anisotropic due to quasi-steady wave breaking. Using unconditioned Reynolds decomposition for our analysis, we show that the turbulent mass flux (TMF) is important in IHVDT for the production of turbulent kinetic energy and is as relevant to the mean momentum equations as the Reynolds stresses. We develop a simple, regional explicit algebraic closure model for the TMF based on a functional relationship between the fluxes and tensor flow quantities. A priori tests of the model show mean density gradients and buoyancy effects are the main driving parameters for predicting the turbulent mass flux and the model is capable of capturing the highly localized nature of the TMF in ${\mathcal{R}}$.

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