**2. Water dynamics**

#### **2.1. Soil water content**

Soil system can be defined as a three‐phase system that can be divided into solid, gas, and liquid phase. The fractions of liquid and gas are located in the voids between soil particles. These voids are defined as pores and their quantification can be defined as ratio of pore volume to the total (bulk) volume of a soil. Soil porosity can be estimated using the following expres‐ sion:

$$P = \frac{V\_p}{V\_s} \tag{1}$$

where *Vp* represents volume of pores [L3 ] and *Vs* volume of the undisturbed soil sample [L3 ].

Soil water content represents the quantity of water contained in soil and is expressed as a ratio, which can range from completely dry to the soil porosity value at the point of saturation. Volumetric soil water content is defined as *θv* [L3 L-3]:

$$
\theta\_v = \frac{V\_w}{V\_s} \tag{2}
$$

where *Vw* is the volume of water in soil pores [L3 ], and *Vs* is the volume of the undisturbed soil sample [L3 ]. Its expression is unitless; however, usually, it is expressed as cm3 /cm-3 (or m3 /m-3) to emphasize its volumetric origin. Soil water content can be also defined as a mass, thus named gravimetric water content *θg* [MM‐1]:

$$
\theta\_s = \frac{m\_w}{m\_s} \tag{3}
$$

where *mw* is the mass of water and *ms* is the mass of soil [M]. It is also unitless but often expressed as gram per gram (or kg per kg), following the same rule as mentioned earlier. Both values can be multiplied by 100 to express it as a percentage.

#### *2.1.1. Soil water content measurements*

**1. Introduction**

134 Groundwater - Contaminant and Resource Management

resources.

resources.

example presented.

**2. Water dynamics**

**2.1. Soil water content**

In the past few decades, more attention is devoted to environmental protection and pollution control in the soil‐plant‐atmosphere continuum. There are growing numbers of potentially harmful substances that are introduced in the environment, especially by agricultural practi‐ ces and industry in arable and urban areas. Such substances can accumulate in the soil (and potentially get into food chain), surface water, or can be transported in a deeper soil zone and eventually reach groundwater. Intensive agriculture uses large amounts of fertilizer (organic and inorganic), plant protection products (pesticides), animal hormones, and include various other substances that may accumulate in environment (e.g. pathogens, bacteria, and trace metals). Although most of the substances applied in agricultural production are useful in the surface layer of the soil, due to their leaching into the deeper soil layers and groundwater and their bioaccumulation potential, they can also cause serious pollution and degrade natural

Water flow and solute (pollutant) transport models can be used as tools for describing and predicting specific processes in variably saturated soil zone or vadose zone. For example, different models can be used for predicting and/or management of irrigation and drainage systems, crop growth, fertilizer application, and pesticide leaching to protect soil and water resources. Models are also equally necessary for the design of waste disposal sites (industrial, municipal) or long‐term management of various harmful substances (e.g. radioactive waste). A large number of models were developed to simulate the numerous simple hydraulic or complex biogeochemical processes and may be used for different purposes. Vadose zone is in focus of many research topics due to its complex nature and also the possibility of elimination and remediation of present/introduced contaminants before leaching to underground water

Transfer of solutes in the soil is closely linked with the flow of water through vadose zone, which largely affects the concentration and biochemical reactions of various substances. Solute transport in soil vadose zone is one of the most demanding problems that occur in numerical modelling. It includes the transport of water and solutes, chemical reactions and microbial transformations. With the development of new numerical models, it is possible to describe more complex processes that are occurring in the soil‐plant‐atmosphere continuum. In this chapter, basic soil physical concepts and numerical modelling procedures (with example) are explained. Trace metal behaviour in the ecosystems, their mobility in soil, and metal chemical forms (species) in the soil solution are shortly discussed with a chemical speciation modelling

Soil system can be defined as a three‐phase system that can be divided into solid, gas, and liquid phase. The fractions of liquid and gas are located in the voids between soil particles. Measurement of the soil water content can be direct or indirect depending on the used method. Direct measurements include the estimation of water quantity by removal from soil through evaporation, leaching or by chemical procedure. They include destructive soil sampling, and hence, additional soil samples need to be taken in order to achieve more reliable results. Therefore, small undisturbed cores are usually taken to determine water content and bulk density. Indirect methods rely on monitoring soil properties that are directly affected by soil water content (e.g. electrical conductivity). These methods require instrumentation placed in the soil or sensors placed over the soil. For this approach, the calibration is needed in addition to precise installation. The advantage of the *in situ* water content measurements are repeated measuring at the same location during given time period (e.g. years) without disruption of the soil system. There are various methods used for soil water content determination; here, the most common methods are shortly explained. More in depth explanation of a given methods could be found in Refs. [1, 2].

#### *2.1.1.1. Direct measurement*

The gravimetric method for the water content measurement represents simplest way to gather accurate soil water content. A soil sample of a known mass is placed in the container, dried in an oven, removed from the container, and allowed to cool in desiccator, then reweighted. The drying procedure is done by placing the sample in convection oven at 105°C for 24 h. From the measurement, gravimetric water content can be calculated and also converted into volumetric water content using soil bulk density.

#### *2.1.1.2. Indirect measurement*

**Neutron probe** uses radioactive material for measuring soil water content. A neutron meter is placed at the soil surface above the access tube in which probe is lowered into the soil to the desired measurement depth. The probe contains an americium 241/beryllium pellet that emits fast high‐energy neutrons. These high‐speed neutrons pass through the accesses tube and collide with hydrogen atoms in the surrounding soil and water. When neutrons hit H nuclei, they slow down and some are reflected back to the source tube and counted by the neutron detector. Because soil water is the primary source of hydrogen atoms, the count is directly related to the soil water content. This method can produce reliable results and can be used for measurements at multiple depths in few minutes. Its application might be questionable for shallow measurements (>15 cm), since the neutrons might escape from the soil instead of being detected. However, the main disadvantage is the use of radiative source that can be a potential health hazard for the device operator.

Time domain reflectometry (TDR) is a technique that involves measuring the travel time of an electromagnetic wave along a wave guide. The bulk soil properties affecting electromagnetic wave are electrical conductivity and dielectric permittivity. Electrical conductivity is a measure of the free electrons flow when exposed to an electric field. The dielectric permittivity is a measure of the displacement of constrained charges when exposed to an electric field. The speed of the electromagnetic wave in soil is dependent on the dielectric permittivity of soil matrix. The fact that water (80) has much larger dielectric constant than air (1), soil (3–7), or its organic components (2–5) is used to determine the volumetric water content of the soil. The TDR instrument consists of 2–3 parallel rods that are inserted into the soil and act as a waveguides. Electronics in the TDR instrument generate and sense the return of high‐energy signal that travels through the soil along the waveguide, that is, stainless steel rods. The high‐ frequency signal is then converted to the volumetric water content. Readings can be affected by high clay content, high organic matter, or high soil salinity. The rods need to be inserted fully into the soil and have good contact with surrounding soil particles. The TDR probes are widely used due to its simplicity, accurate readings, and minimal soil disturbance.

**Capacitance devices** are used to determine the resonance frequency of a given soil. The capacitor is connected to an oscillator to form an electrical circuit; changes in soil moisture can be detected by changes in the circuit operating frequency. Probes usually consist of two or more electrodes (e.g. plates, rods, or metal rings) that are inserted into the soil. With the ring configuration, the probe is introduced into an access tube installed in the field. When parallel rod configuration is used, the probes are buried at the required depth into the soil and the soil represents a medium between capacitor electrodes. Compared to TDR, frequency domain sensors are relatively inexpensive and have a faster response time. However, because of the complex electrical field around the probe, the sensor needs to be calibrated specifically for different soil types.
