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Determination of Solute-transport Parameters in Three Different Textured Soils by Using Time-domain Reflectometry

A.S.M.D. Hossain
Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh-2202 Bangladesh

M.A. Mojid
Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh-2202 Bangladesh

M.J. Faruque
Department of Irrigation and Water Management, Bangladesh Agricultural University, Mymensingh-2202 Bangladesh

Abstract/ Executive Summary:

Breakthrough curves (BTC) of sodium chloride (NaCI) were measured in three different textured soils by time-domain reflectometry (TOR) under saturated condition in laboratory. The BTCs were analyzed by an analytical solution of the classical convection-dispersion equation (CDE) to determine NaCI-transport parameters. The measured BTCs were fitted to the estimated BTCs by non-linear optimization, which minimized the sum of the squared deviations between the two BTCs and resulted in the optimum values of the solute transport parameters. The fitting between the measured and estimated BTCs was found the best in fine textured soils and better in coarse textured soils. The salt solution moves slowly in fine textured soil and hence the breakthrough period is longer than in coarse textured soils. The flow of sail solution in coarse sand under saturated condition is unstable and causes a very sharp and quick breakthrough of salt solution. A linear relationship between the velocity of pore-water and dispersion coefficient indicates increased solute dispersion at low water contents as compared to high water contents at the same water flux. This is because, the velocity of pore water and dispersion coefficient are greater in coarse textured soils than in fine textured soils. So, a solute requires much less time to travel in coarse textured soils and more time in fine textured soils. Thus, coarse textured soils contaminate groundwater very quickly, but removal of contaminants (salt, toxic materials etc.) from these soils is easier than from fine textured soils.

Keywords: Solute-transport parameters, Soil, TOR

Location: BAU Farm and Mymensingh

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Soil fertility

Objectives:

(i) To measure breakthrough curves of NaCI (a non-reactive solute) in three different textured soils.

(ii) To determine solute-transport parameters from the measured BTCs.

(iii) To compare the solute-transport parameters for the three soils.

Methodology:

Solute-transport experiments were done with three different textured soils collected from the Bangladesh Agricultural University (BAU) farm, Mymensingh, and nearby area. The soils were dried in air, sieved with a 2 mm square mesh sieve before determining their textural classes.The BTCs were analyzed by an analytical solution of the classical convection-dispersion equation (CDE) to determine NaCI-transport parameters. The measured BTCs were fitted to the estimated BTCs by non-linear optimization, which minimized the sum of the squared deviations between the two BTCs and resulted in the optimum values of the solute transport parameters. During computation, the values of the solute-transport parameters are optimized to obtain the best fit between the measured and fitted BTCs. A total of seven parameters, which govern the transport of solute through soils, were determined. These are: velocity of pore water, V, mean travel time, 7: mass-dispersion number, Nct. dispersion coefficient, D, retardation factor, Ro Peclet number, P and dispersivity of solute, A . All these parameters cannot be readily determined directly. The primary parameters (V, D and R1) were determined by analyzing the BTCs and the other parameters (secondary parameters) were calculated from them. Since NaCI is a non-reactive chemical, R1 was fixed at unity and the values of V and D were optimized.

Source of Information: Bangladesh J. Agril. Sci. 33(1): 51-60, January, 2006

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The fitting between the measured and estimated BTCs was found the best in fine textured soils and better in coarse textured soils. The salt solution moves slowly in fine textured soil and hence the breakthrough period is longer than in coarse textured soils. The flow of sail solution in coarse sand under saturated condition is unstable and causes a very sharp and quick breakthrough of salt solution. A linear relationship between the velocity of pore-water and dispersion coefficient indicates increased solute dispersion at low water contents as compared to high water contents at the same water flux. This is because, the velocity of pore water and dispersion coefficient are greater in coarse textured soils than in fine textured soils. So, a solute requires much less time to travel in coarse textured soils and more time in fine textured soils. Thus, coarse textured soils contaminate groundwater very quickly, but removal of contaminants (salt, toxic materials etc.) from these soils is easier than from fine textured soils.

Knowledge Management: Journal