FATE OF NITRATE AND PHOSPHATE THROUGH AGRICULTURAL SUB-SOIL ENVIRONMENT

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FATE OF NITRATE AND PHOSPHATE THROUGH AGRICULTURAL

SUB-SOIL ENVIRONMENT

Sadashiva Murthy.B.M

, Ramesh. H.S.

& Mahadevaswamy. M.

1,*) Assistant Professor, Department of Environmental Engineering,

2) Professor & Head, Department of Environmental Engineering

3) Professor, Department of Environmental Engineering

Sri Jayachamarajendra College of Engineering, MYSORE – 570 006, KARNATAKA, INDIA

FAX: +091 821-2548290 EMAIL: bmsada@yahoo.com

Abstract:

The connection of the groundwater pathway in the hydrologic cycle to the land surface

provides the opportunity for anthropogenic impacts on groundwater and devalues the

resource. Several sources of groundwater pollution have been identified, including leachate

from sanitary landfills, industrial waste seepage from storage basins, industrial waste

introduced through groundwater recharge, domestic waste from septic tanks, agro-

chemicals (fertilizer and pesticides) irrigation salts leached. So, with concern increasing

over nitrates and phosphate contamination of ground water, today research is oriented

towards various processes affecting the transport of chemicals in soils, which is ultimately

determined by the relative rates of percolation, leaching, sorption and degradation within

soil profile and is evaluated by different methods. As accurate quantification of solute

under field conditions is inherently difficult, leaching characteristics of pollutants (nitrates

and phosphates) have been analyzed by laboratory soil column and channel studies. The

analysis of the breakthrough curves showed that, the dispersion co-efficient for sandy soil

was 0.202 m

/d for column and 0.276 m

/d for channel studies. The study of leaching

characteristics of nitrates and phosphates conducted upto 40 days through agricultural soil

column revealed that the concentration of nitrates and phosphates increased steadily in the

first few days (around 5 days) and then it increased rapidly after 8 to 10 days. However, it

attained equilibrium after 20 days. Similarly, the channel studies revealed steady increase

in concentration of nitrates and phosphates in the first few days (1 to 2 days) and rapid

increase upto 15 days and thereafter it attained equilibrium. In the contemporary study,

one-dimensional analytical model has been developed which include the advection,

dispersion, and a characteristic term called “total elimination rate”. The comparative study

of soil column and channel studies, and model output (in which the term total elimination

rate “K” is considered as zero) exhibited a variation of 30 to 50 % in leaching

characteristics of pollutants (nitrates and phosphates).

Keywords: Vadoze Zone, Contaminant, Transport, Plume Model, Total Elimination

Rate.

INTRODUCTION

In the last few decades groundwater contamination has increased dramatically. Among the various

land uses, agriculture is reported to be the main source of groundwater contamination. Therefore, it is vital to

understand the various complex factors such as dispersion, advection, sorption, degradation and quantity of

seepage water, which control the movement of contaminants in the porous medium (soil). The most evident

broadly based scientific advances groundwater hydrology includes the transport of fluid, energy, mass in porous

media, ground-water microbiology, theoretical and practical knowledge related to its contamination, etc.

Groundwater is one of the earth’s widely distributed and most important resources. The existing demand for

groundwater as a source of conventional water supply will continue to grow (Metry, 1976). The merits of

groundwater over surface water includes, little or no need of treatment to ensure potable quality as natural

groundwater has a minimal suspended solids, small concentration of bacteria and viruses, and often meager

concentrations of dissolved mineral salts, easy availability and natural annual replenishment by rainfall (Agrawal

et al., 1999).To understand the mitigation and control of various pollutant sources, development and application

of a wide range of mathematical modeling techniques are essential. Modeling techniques are thus used for

assessing the probable impact on groundwater quality. It is also used to assess remedial measures, which is cost

effective and sufficient to prevent serious degradation of groundwater quality (Rudraiah and Vinay, 2005). Thus,

mathematical models are important tool, which plays a vital role in understanding the mechanisms of

groundwater pollution problems.

OBJECTIVE OF THE STUDY

In this study a one dimensional analytical model has been developed for multi component transport

(nitrates and phosphates) through unsaturated zone and to check the validity of the laboratory experimental

leachate data for nitrates and phosphates with the analytical model equations. For this, physico-chemical

characteristics of agricultural soil are determined and also one-dimensional leaching characteristics of pollutants,

(nitrates and phosphates) through column and channel studies were assessed.

MATHEMATICAL FORMULATIONS OF THE MODEL

The advective-dispersive equation provides a comprehensive framework for quantitatively describe

mass transport with and without accompanying chemical reactions (Schwartz and Zhang, 2004). The general

one-dimensional equation, a non-reactive compound, in a steady-state condition, homogeneous media, with

constant dispersion coefficient is described by Schwartz and Zhang, (2003); Pachepsky, (2000) and Metry

(1976) as:

(C / t) = D

(

C /z

) – V (C / z) (1)

Where, C is the solute concentration (ML

-3

), D

is the solute dispersion coefficient (L

-1

), V is the average pore

water velocity (LT

-1

), z is the distance (L) and t is the time (T). The first term on the right hand side, describes

mass transport by dispersion, and the second term describes mass transport by advection.

The governing one-dimensional transport equation for the transient transport of contaminants with

advection, adsorption, and dispersion under a steady state condition is given by Domenico and Schwartz, (1998).

(C / t) = D [(

C /z

)/ R

] –[V (C / z)/ R

] – K C (2)

Where, D = Hydraulic Dispersion Coefficient (m

/d)

V = Average Linear Velocity, (m/d), K = Total Elimination Rate, (d

-1

) R

= Retardation Factor

The Retardation factor is estimated as follows,

= [1+ (

/n) K

] (3)

Where, 

= Bulk mass density of the porous medium (g/ cm

= the distribution Coefficient (cm

/g). n = Effective porosity.

The total elimination rate (K) in the governing equation includes the rate constants of the four

transformation reactions namely, degradation, volatilization, leaching and plant uptake. Mathematically these

processes are incorporated as shown,

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FATE OF NITRATE AND PHOSPHATE THROUGH AGRICULTURAL SUB-SOIL ENVIRONMENT Sadashiva Murthy.B.M1, Ramesh. H.S.2 & Mahadevaswamy. M.3 1,*) Assistant Professor, Department of Environmental Engineering, 2) Professor & Head, Department of Environmental Engineering 3) Professor, Department of Environmental Engineering Sri Jayachamarajendra College of Engineering, Mysore – 570 006, Karnataka, India Fax: +091 821-2548290 Email: bmsada@yahoo.com Abstract: The connection of the groundwater pathway in the hydrologic cycle to the land surface provides the opportunity for anthropogenic impacts on groundwater and devalues the resource. Several sources of groundwater pollution have been identified, including leachate from sanitary landfills, industrial waste seepage from storage basins, industrial waste introduced through groundwater recharge, domestic waste from septic tanks, agro-chemicals (fertilizer and pesticides) irrigation salts leached. So, with concern increasing over nitrates and phosphate contamination of ground water, today research is oriented towards various processes affecting the transport of chemicals in soils, which is ultimately determined by the relative rates of percolation, leaching, sorption and degradation within soil profile and is evaluated by different methods. As accurate quantification of solute under field conditions is inherently difficult, leaching characteristics of pollutants (nitrates and phosphates) have been analyzed by laboratory soil column ...
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