**2. Flows in unsaturated soils**

The pollution has dramatically impacted the aquatic habitat in many of the world's most important water bodies. The contamination of the subsoil and the aquifers has the peculiarity of being "silent" and not presenting samples that show the same but until its consequences

Groundwater is usually more difficult to contaminate than surface water, but when this happens, reversing the situation is a very complex solution. The subsoil waters have a very slow rate of renovation. It is estimated that while the mean time of water stays in the rivers is of days, in an aquifer it is from years to hundreds of years, which makes it very difficult to

The greatest concern about groundwater contamination has focused on the pollution associated with human activities such as the disposal of waste (liquid and solid waste in landfills of urban waste, waste from the oil industry, waste from the mining industry, radioactive waste, etc.) or not directly related to the emission of waste (agricultural activities, mining, construction and inadequate maintenance of buildings, etc.). On many occasions the situation

Adequate protection of groundwater resources should have as a priority to prevent the entry of harmful elements into groundwater. It is a priority then to carry out geological, hydrologi-

Among the diffuse pollution activities, a very important one is the excessive use of fertilizers and herbicides. This type of pollution can cause situations of concern over time, because it is

Herbicides are substances, usually of organic origin, which are used mainly for the control of weeds in agriculture. To minimize the environmental impact of the same, its application must be made considering the conditions of the plants, the soil, and the environment and the use of procedures for which they were designed and thus obtain an optimal dosage. However rigorous the conditions of use, there is evidence of the presence of traces of herbicides and other pesticides even in nonagricultural areas, in the atmosphere, and in surface

The presence of pesticides, and especially herbicides, in groundwater and aquifers represents a frequent situation, especially in those regions that have had or have a high consumption of pesticides [3]. This would rule out the possibility of using water without a purification treatment, given that concentration levels of pesticides have been detected ranging from only

When agrochemicals exceed the limits for which they were intended, they constitute both a loss to the agrosystem and a source of contamination for adjacent systems. The level of risk of contamination of soil and water results from the combination of the pollutant load and the

In this context, the prediction of the behavior of pesticides released into the environment is necessary to anticipate, and therefore minimize, adverse impacts outside the point of application [5]. This means that we must understand what happens to a pesticide that has been

natural vulnerability of the environment to the said contamination [4].

is aggravated by the late knowledge that the aquifer is deteriorating [1].

cal, and hydrogeological studies and potential sources of contamination.

slow but continuous and in very large areas.

and underground water [2].

traces to high levels of concentration.

are observed in the biotic media.

58 Soil Contamination and Alternatives for Sustainable Development

decontaminate it.

Pesticides, fertilizers, and solutes in general get dissolved or dragged with water from the soil through the pores, so the knowledge and measurement of properties such as hydraulic conductivity and effective porosity within the soil are important. These properties depend on the geometry, interconnection, and distribution of pore size within the soil. The presence of interconnected macropores is directly related to the natural aggregation of the soil and can constitute preferential flow paths within the soil [6], which is considered today the main mechanism for the relatively rapid appearance of contaminants in groundwater.

The term preferential flow refers to the fact that the water that infiltrates does not have enough time to get in balance with the water that remains in the soil matrix moving more slowly. These preferential flows can occur, for example, in structured soils, where macropores (cracks, tunnels caused by macroorganisms such as worms and insects, holes in the roots) dominate the hydrogeology of unsaturated to saturated soil, particularly in fine-textured soils, and operate as routes of high flow conductivity through the densest and least permeable matrix of the soil [7].

These preferential flows can also occur in unstructured sandy soils in the form of unstable wetting fronts, which is caused by heterogeneous profiles such as interfaces in the horizons or by water repellency [8].

A consequence of preferential flows is that a heterogeneous front of solute penetration does not occur in the soil; this contradicts the simple (convection-dispersion) equation, which predicts a homogeneous infiltration front.

#### **2.1. Processes of transporting a solute in the soil**

The transport process of a solute in the soil will present: volatilization (transport of the solute to the atmosphere); runoff to water surfaces; and leaching into groundwater.

These processes are affected by diffusion, convection, and dispersion. In addition, there are other processes that add to the previous ones such as retention (adsorption or sorption) and chemical transformation.

The processes of chemical transformation can be catalyzed by the constituents of the soil or photochemically induced. Most pesticides are transformed mainly by biochemical processes through soil microorganisms with changes in the molecule toward simpler forms that can be of equal, less, or greater toxicity than the original, which determines in what form and for how much the solutes will be present on the ground [9].

#### **2.2. Transportation of contaminants**

The one-dimensional solute transport equation in an unsaturated medium, which is similar to that of saturated medium, is written as shown in Eq. (1):

$$\frac{\partial \mathbf{C}}{\partial t} = -\boldsymbol{\sigma} \frac{\partial \mathbf{C}}{\partial z} + \boldsymbol{D}\_{\mu} \frac{\partial^2 \mathbf{C}}{\partial z^2} - \frac{\rho}{\hbar} \frac{\partial \mathbf{S}\_i}{\partial t} \tag{1}$$

**2.3. Measurement of solute concentration**

maximum possibility of interaction [11].

**3. Measurement of moisture in the soil**

processes in the entire soil-plant system [13].

vary more with the depth that with the area.

will depend on the range of interest required for the data.

function and the nonlinearity of the moisture/suction function [12].

adjusting the model as close to reality is what takes most of time and money.

The concentration of fertilizers and pesticides in different environmental matrices is estimated based on sampling based on statistical methods. In general, due to the great spatial variability of the concentrations of the different pesticides in the environment, the values obtained in a sample are only an approximation to reality. Therefore, the greater or lesser degree of certainty in obtaining the collected data and their interpretation depends to a large extent on an adequate sampling, on the collection of the sample and on the preservation of these [10]. In the study of the adsorption of a pesticide in a soil, two laboratory techniques are used: batch experiences and experiences in columns. Batch experiences are designed to study the equilibrium of adsorption in a continuously stirred soil suspension. This situation is presented as a physical model of particles of completely dispersed soil, where the entire surface of the particles is exposed and available for interaction with contaminants. Experiences in batches are not representative of the natural conditions that represent the conditions of a closed system and offer adsorption to the greatest possible surface area and, therefore, the

Flow in Unsaturated Soils and Transport of Herbicides in Agricultural Areas

http://dx.doi.org/10.5772/intechopen.82329

61

The infiltration process in the soil can be modeled by the Richards equation whose solution implies knowing the hydraulic functions of the soil. These functions depend on some parameters they need for their calibration of the hydraulic properties, determined by means of measurements. The measurements of the hydraulic properties of the soil present numerous complications due to two important factors: the nonlinearity of the conductivity/ suction

Given the problem of the heterogeneity of the porous medium, the modeling of the watersolute-soil-plant system is more complicated. Therefore, obtaining the parameters that allow

The adequate measurement of the water content in soil plays a critical role for the estimation of water and energy balances, as well as for understanding the biological and chemical

The evaluation of soil moisture content at different suction conditions in the field requires considerable time and effort, as well as equipment. The effort, time, and equipment needed

The surface and depth to study must be carefully defined. The surface to study will depend on the variability existing in the place. In certain occasions the characteristics of the soil-water

The moisture in the soil depends mainly on the texture or the particle size distribution. On the other hand, the content of organic matter and the composition of the solution phase can play a determining role in soil moisture function or retention function. Organic matter has a direct

where *v* is the linear velocity, *DH* is the dispersion coefficient, *C* is the concentration of the pollutant, *z* is the distance along the direction of the flow, *Si* is the concentration of the pollutant adsorbed, *ρ* is the volumetric density, *n* is the porosity, where the first, the second, and the third terms of the right hand side refer to advection, dispersion, and adsorption, respectively.

To model the concentration of a pollutant substance subjected to adsorption processes, the advective-diffusive transport equation is used in one dimension expressed by Eq. (2):

$$\frac{\partial \mathbf{C}}{\partial t} = \frac{1}{R} \frac{\partial \mathbf{C}}{\partial z} \Big( -\nu + D\_H \frac{\partial \mathbf{C}}{\partial z} \Big) \tag{2}$$

*DH*: dispersion coefficient

*C*: concentration of the pollutant

*R:* delay factor

*v*: linear velocity

The variation of the solute is difficult to know, so in order to solve the equation, different isotherms can be considered. For the isotherms of variation, linear, second order, or exponential approximations can be considered.

If we assume that the concentration of the solute in the solid and liquid phases is related by a linear adsorption isotherm, the delay factor *R* takes the following expression Eq. (3):

$$R = \left(1 + \frac{\rho \, K\_{\text{D}}}{\theta}\right) \tag{3}$$

*ρ*: volumetric density

*θ*: volumetric moisture content

*KD:* distribution or adsorption coefficient that characterizes the linear isotherm.

In order to find the numerical solution of the transport equation, it is necessary to know the form of the concentration of the solute in the solid phase (porous matrix) and liquid phase (water) and how is the variation of the solute and the physical properties of the soils and their moisture content.

#### **2.3. Measurement of solute concentration**

of equal, less, or greater toxicity than the original, which determines in what form and for

The one-dimensional solute transport equation in an unsaturated medium, which is similar to

<sup>∂</sup>*<sup>z</sup>* <sup>+</sup> *DH*

where *v* is the linear velocity, *DH* is the dispersion coefficient, *C* is the concentration of the pol-

adsorbed, *ρ* is the volumetric density, *n* is the porosity, where the first, the second, and the third terms of the right hand side refer to advection, dispersion, and adsorption, respectively. To model the concentration of a pollutant substance subjected to adsorption processes, the

The variation of the solute is difficult to know, so in order to solve the equation, different isotherms can be considered. For the isotherms of variation, linear, second order, or exponential

If we assume that the concentration of the solute in the solid and liquid phases is related by a

In order to find the numerical solution of the transport equation, it is necessary to know the form of the concentration of the solute in the solid phase (porous matrix) and liquid phase (water) and how is the variation of the solute and the physical properties of the soils and their

linear adsorption isotherm, the delay factor *R* takes the following expression Eq. (3):

*KD:* distribution or adsorption coefficient that characterizes the linear isotherm.

advective-diffusive transport equation is used in one dimension expressed by Eq. (2):

<sup>∂</sup>*<sup>t</sup>* <sup>=</sup> \_\_1 *R* \_\_\_ ∂*C* <sup>∂</sup>*<sup>z</sup>* (−*<sup>ν</sup>* <sup>+</sup> *DH*

∂<sup>2</sup> \_\_\_*C* <sup>∂</sup> *<sup>z</sup>* <sup>2</sup> <sup>−</sup> *<sup>ρ</sup>*\_\_ *n* ∂ *S* \_\_\_\_\_\_\_\_*<sup>i</sup>*

\_\_\_ ∂*C*

<sup>∂</sup> *<sup>t</sup>* (1)

is the concentration of the pollutant

<sup>∂</sup>*z*) (2)

*<sup>θ</sup>* ) (3)

<sup>∂</sup>*<sup>t</sup>* <sup>=</sup> <sup>−</sup>*<sup>v</sup>* \_\_\_ <sup>∂</sup>*<sup>C</sup>*

how much the solutes will be present on the ground [9].

60 Soil Contamination and Alternatives for Sustainable Development

that of saturated medium, is written as shown in Eq. (1):

lutant, *z* is the distance along the direction of the flow, *Si*

**2.2. Transportation of contaminants**

\_\_\_ <sup>∂</sup>*<sup>C</sup>*

\_\_\_ <sup>∂</sup>*<sup>C</sup>*

*DH*: dispersion coefficient

*R:* delay factor *v*: linear velocity

*ρ*: volumetric density

moisture content.

*θ*: volumetric moisture content

*C*: concentration of the pollutant

approximations can be considered.

*<sup>R</sup>* <sup>=</sup> (<sup>1</sup> <sup>+</sup> *<sup>ρ</sup> <sup>K</sup>*\_\_\_\_*<sup>D</sup>*

The concentration of fertilizers and pesticides in different environmental matrices is estimated based on sampling based on statistical methods. In general, due to the great spatial variability of the concentrations of the different pesticides in the environment, the values obtained in a sample are only an approximation to reality. Therefore, the greater or lesser degree of certainty in obtaining the collected data and their interpretation depends to a large extent on an adequate sampling, on the collection of the sample and on the preservation of these [10].

In the study of the adsorption of a pesticide in a soil, two laboratory techniques are used: batch experiences and experiences in columns. Batch experiences are designed to study the equilibrium of adsorption in a continuously stirred soil suspension. This situation is presented as a physical model of particles of completely dispersed soil, where the entire surface of the particles is exposed and available for interaction with contaminants. Experiences in batches are not representative of the natural conditions that represent the conditions of a closed system and offer adsorption to the greatest possible surface area and, therefore, the maximum possibility of interaction [11].
