Jui-Sheng Chen

Numerical simulation of the evolution of aquifer porosity and species concentrations during reactive transport

While flowing through a porous medium, a reactive fluid dissolves minerals thereby increasing its porosity and ultimately the permeability. The reactive fluid flows preferentially into highly permeable zones, which are therefore dissolved most rapidly, producing a further preferential permeability enhancement. Thus, the reaction front may be unstable. However, other factors, such as diffusion, suppress the instability of a reaction front. This study presents a numerical model to evaluate the interactions between mechanisms that determine the shape of a reactive front. That is, a method is developed to solve a set of nonlinear equations coupled with fluid flow, species transport, and rock-fluid reactions and includes the effects of grain dissolution and the alteration of porosity and permeability due to mineral-fluid reactions. The numerical model enables us to evaluate how a dissolution reaction affects the porosity structure and fluid pressure variation, from which local Darcy flux can then be evaluated. In addition, the model is used to examine how upstream pressure gradient affects the morphological instability of the species concentrations and the aquifer porosity. Simulation results indicate that, although stable for small upstream pressure gradients, the growth of a planar front becomes unstable for large upstream ones. Moreover, the diffusive, advective and resultant species fluxes of both these mechanisms are computed and presented to further elucidate the behavior of the morphological instability for a planar concentration and porosity front that results from the interactions between diffusion and advection.

Analysis of percolation and seepage through paddy bunds

This study investigates percolation and seepage through the bunds of flat and terraced paddies. Field experiments were conducted in Hsin-Pu of Hsin-Chu County, Taiwan, to measure the soil water content of various types of bund. Measurements revealed that the soil was unsaturated along the sloped surface of the terrace. Experimental results also indicated that seepage face flow did not develop even after 2 days of heavy rainfall. A three-dimensional model, FEMWATER, was adopted to simulate percolation and lateral seepage under various bund conditions. In a flat paddy, the rate of percolation of bunds under which a plow sole was located, was 0.40 cm d 21 , close to the average infiltration rate of a flooded paddy. The percolation of the bund without plow sole was 0.85 cm d 21 , or double the average infiltration rate of a flooded paddy. Infiltration in the central area of a terraced paddy is mainly vertically downward, whereas flow near the bund is predominantly lateral. The paddy field near the bund has a high hydraulic gradient. The simulated infiltration flux into the bund (1.47 cm d 21 ) after 85 days of rice cultivation exceeded that into the central area (0.54 cm d 21 ) by a factor of 2.72. The final percolation flux from the bund (1.24 cm d 21 ) also exceeded the final percolation from the plow sole (0.68 cm d 21 ) by a factor of 1.82. The lateral seepage fluxes through the bund, downward and upward along the slope surface, are 2.01 and 22.12 cm d 21 , respectively. However, the lateral seepage flux does not fully saturate the surface of the hillside soil. A simulation clearly shows that the seepage upstream of the paddy field does not move water downstream and is reused as subsurface return flow. Both experimental and simulation results clarify the mechanisms of water movement in the terraced paddy and reveal the existence of an unsaturated seepage face along the sloping surface of the terraced field. q 2003 Elsevier B.V. All rights reserved.

A two-well method to evaluate transverse dispersivity for tracer tests in a radially convergent flow field

This work presents a two-well method to evaluate transverse dispersion coefficients for tracer tests conducted in a radially convergent flow field. A two-dimensional mathematical model is derived and applied to illustrate how transverse dispersion influences tracer transport in a convergent radial tracer test. A curve-fitting procedure is proposed to evaluate simultaneously the longitudinal and transverse dispersivities. A hypothetical experiment is used to demonstrate the application of the method. The proposed method allows one to evaluate the transverse dispersivity from field tracer tests, as long as the observed breakthrough curves at the pumping well and an observing well are known.

Two-dimensional Laplace-transformed power series solution for solute transport in a radially convergent flow field

This paper presents an analytical solution for two-dimensional non-axisymmetric solute transport in a radially convergent flow field. We applied a Laplace-transformed power series (LTPS) technique to solve the two-dimensional advection-dispersion equation in cylindrical coordinates. The solution is compared with a numerical solution to evaluate its robustness and accuracy. The applicable Peclet number range of the developed power series solution is also examined. Results show that the LTPS technique can effectively and accurately handle the two-dimensional radial advection-dispersion equation for a Peclet number up to 60. The two-dimensional power series solution is appropriate for hydrogeologic circumstances where temporally and spatially continuous solutions are demanded.

A novel analytical power series solution for solute transport in a radially convergent flow field

The concentration breakthrough curves at a pumping well for solute transport in a radially convergent flow field are governed by an advective ‐ dispersive second order partial differential equation with a radial distance-dependent velocity and dispersion coefficient. The Laplace transform is generally first employed to eliminate the temporal derivative to solve the partial differential equation. The Laplace transformed equations are then converted to the standard form of the special Airy function through successive applications of variable change. This study presents the solution of the Laplace-transformed equation without using the special Airy function. A direct power series method and a power series method with variable changes to eliminate the advection term that usually results in numerical errors for large Peclet numbers are applied to obtain an analytical solution in the Laplace domain. The obtained solutions are compared to other Airy function-formed solutions to examine the method’s robustness and accuracy. Analytical results indicate that the Laplace transform power series method with variable change can effectively and accurately handle the radial advection ‐ dispersion equation of high Peclet numbers, whereas the direct power series method can only evaluate the solution for medium Peclet numbers. The novel power series technique with variable change is valuable for future quantitative hydrogeological issues with variable dependent differential equation and can be extended to higher dimensional problems. q 2002 Elsevier Science B.V. All rights reserved.

Probabilistic health risk assessment for ingestion of seafood farmed in arsenic contaminated groundwater in Taiwan

Seafood farmed in arsenic (As)-contaminated areas is a major exposure pathway for the ingestion of inorganic As by individuals in the southwestern part of Taiwan. This study presents a probabilistic risk assessment using limited data for inorganic As intake through the consumption of the seafood by local residents in these areas. The As content and the consumption rate are both treated as probability distributions, taking into account the variability of the amount in the seafood and individual consumption habits. The Monte Carlo simulation technique is utilized to conduct an assessment of exposure due to the daily intake of inorganic As from As-contaminated seafood. Exposure is evaluated according to the provisional tolerable weekly intake (PTWI) established by the FAO/WHO and the target risk based on the US Environmental Protection Agency guidelines. The assessment results show that inorganic As intake from five types of fish (excluding mullet) and shellfish fall below the PTWI threshold values for the 95th percentiles, but exceed the target cancer risk of 10−6. The predicted 95th percentile for inorganic As intake and lifetime cancer risks obtained in the study are both markedly higher than those obtained in previous studies in which the consumption rate of seafood considered is a deterministic value. This study demonstrates the importance of the individual variability of seafood consumption when evaluating a high exposure sub-group of the population who eat higher amounts of fish and shellfish than the average Taiwanese.

Characterizing hydrochemical properties of springs in Taiwan based on their geological origins

This study was performed to characterize hydrochemical properties of springs based on their geological origins in Taiwan. Stepwise discriminant analysis (DA) was used to establish a linear classification model of springs using hydrochemical parameters. Two hydrochemical datasets—ion concentrations and relative proportions of equivalents per liter of major ions—were included to perform prediction of the geological origins of springs. Analyzed results reveal that DA using relative proportions of equivalents per liter of major ions yields a 95.6% right assignation, which is superior to DA using ion concentrations. This result indicates that relative proportions of equivalents of major hydrochemical parameters in spring water are more highly associated with the geological origins than ion concentrations do. Low percentages of Na +  equivalents are common properties of springs emerging from acid-sulfate and neutral-sulfate igneous rock. Springs emerging from metamorphic rock show low percentages of Cl −  equivalents and high percentages of HCO

Effect of well bore mixing volume on non-axisymmetrical transport in a convergent radial tracer test

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Spatial Analysis of Human Health Risk Due to Arsenic Exposure through Drinking Groundwater in Taiwan’s Pingtung Plain

equivalents, and springs emerging from sedimentary rock exhibit high Cl − /SO

Zonal management of multi-purposes groundwater utilization based on water quality and impact on the aquifer.

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