**5. The fate and transport of chemicals in groundwater**

#### **5.1. Volatilization**

Landfills may be grouped according to the type of materials they contain. Municipal landfills accept only non-hazardous materials, but are still likely to contain materials which pose poten‐ tial health risks. Industrial landfills may contain either "hazardous" or "non-hazardous" mate‐ rials. Until recently, little was known about how they were operated or what they contained. Open dumps and abandoned disposal sites generally have no engineering design. Their con‐ nection with the groundwater system and the type of materials present is often unknown. It is often in abandoned disposal sites that large volumes of highly toxic materials are found. The most hazardous solid waste disposal generally results from industrial and manufacturing ac‐ tivities as well as some governmental energy and defense activities. Populations of both devel‐ oped and developing countries, where there is current or historical industrial activity, face potential health risks from solid waste disposal. It is reported that there will be the highest con‐ tent and most types of organic contaminants in groundwater which is near the landfills. If there

In recent years, there has been increasing awareness of the large number of potentially leak‐ ing underground storage gasoline tanks. For much of the twentieth century, underground storage tanks were constructed of unprotected carbon steel. Corrosion causes leaks in such tanks over some period of time, ranging from a few years to tens of years. Although the leakage from individual tanks is often small, it is often enough to contaminate a large vol‐ ume of groundwater. In addition, the large number of buried tanks-several million in the United States-makes them a potentially significant groundwater contamination source. Above ground storage tanks pose less of a threat than underground tanks. Leak detection and maintenance is easier and the connection with the groundwater system is less direct.

However leaks from such tanks may still act as groundwater contamination sources.

Numerous agricultural activities can result in non-point sources of groundwater contamina‐ tion. Fertilizers, pesticides, and herbicides are applied as part of common agricultural prac‐ tice throughout the world. These applications can act as sources of contamination to groundwater supplies serving large populations. Whether or not fertilizers, pesticides, and herbicides become sources of groundwater contamination depends on changing hydrogeo‐ logic conditions, application methods, and biochemical processes in the soil. In developing countries, animal and/or human waste is used for fertilizer. This is an example of the land application of sewage discussed earlier. There are the same concerns with pathogens and ni‐ trates. The manufactured inorganic fertilizers widely used in developed countries, and find‐ ing increasing usage in all countries, also pose the threat of nitrate contamination of groundwater systems. Pesticide and herbicide application provides a source of numerous

Even without the introduction of fertilizers, pesticides, and herbicides, irrigation activities can lead to groundwater contamination. Naturally occurring minerals in the soil can be leached at higher rates leading to hazardous concentration levels in the groundwater. Evap‐

has 1 kilometers distance it still exist in the groundwater [13].

*4.2.3. Petrochemical pollution*

94 Organic Pollutants - Monitoring, Risk and Treatment

*4.2.4. Agricultural activities*

toxic organic chemicals to groundwater supplies.

Volatilization occurs in whether the vadose zone or saturated zone when the dissolved con‐ taminants and non-aqueous phase contaminants exposed to gas. The factors affecting volati‐ lization include solubility of the compound, molecular weight and water-saturated state of the geological media. The evaporation rate must be measured fundamentally in order to de‐ termine pollutions transporting into the atmosphere, changes of the pollution load in the va‐ dose zone and groundwater. The process that the contaminants of deep soil volatilize to the atmosphere can be assumed as one-dimensional diffusion, which can be described with Fick's second law. Volatilization of the water-soluble organic matter, such as benzene dis‐ solved in water is generally described by Henry's Law [15].

#### **5.2. Adsorption**

Adsorption in Soil and sediment makes an important influence on the behavior of organic pollutants. The mobility and biological toxicity reduced as organics are detained in the soil and sediment. Generally, adsorption is affected by sediments and soil properties, such as or‐ ganic percentage, the type and quantity of clay minerals, cation exchange, pH and the physi‐ cal and chemical properties of the contaminants. During the adsorption, the organic contaminants in the water adsorbed on the surface of the soil particles by the simultaneous distribution role of both water and solid, the driving force is mainly based on principle of "like dissolves like" and electrostatic adsorption of the polar group, and the following for‐ mulas is established [15]:

$$\mathbf{C}\_{sa} = \mathbf{K}\_a \mathbf{C}\_{na} \tag{1}$$

 *<sup>T</sup> dc KXC k c*

( )

1 2

The half-life of degradation of residual contamination is determined.

exp exp( ) *t T <sup>C</sup> KX k t*

In many cases, the receptor medium for release of a contaminant will be the unsaturated zone. In contrast to the saturated zone, pores in the unsaturated zone are not completely sa‐ turated with liquid. This fundamentally affects the processes governing flow and chemical transport. A number of processes will affect the contaminant within the unsaturated zone before it enters the saturated groundwater system and potentially is tapped by supply wells. The uncertainties in characterizing releases just described lead to uncertainties in defining the source terms and initial and boundary conditions for modeling unsaturated transport. Analogously, uncertainties in characterizing unsaturated transport processes lead to uncer‐ tainties in defining the source terms and initial and boundary conditions for modeling satu‐

For the most part, computer simulation of contaminant transport has focused on movement in the saturated zone. Assumptions are made regarding the time required for movement through the unsaturated zone. Often some sort of lag between source release and entry of chemicals into the saturated flow system is introduced into source terms. It is important to be aware of the unsaturated processes that are actually occurring, the uncertainty associated

Once a chemical has been released into the ground and has either moved through the unsa‐ turated zone or directly entered the saturated zone, saturated transport processes will deter‐

with these processes, and the role of monitoring in reducing these uncertainties.

bial in the organic matter of soil degradation[106

0

order biodegradation rate constant [g/(d 106

Substituted into with half-life formula,

**5.4. Fate and transport in unsaturated zone [21]**

From the above equation,

rated transport.

**5.5. Saturated transport [21]**

Where, C is the mass fraction of soil organic matter[mg/g]; X is the number of active micro‐

*dt* =- =- (2)

The Investigation and Assessment on Groundwater Organic Pollution

)]; kr is substrate removal constant[d-1].

*<sup>C</sup>* = - =- (3)

2/ *<sup>T</sup> t ln k* <sup>=</sup> (4)

/g]; t is degradation time[d]; K is the one-

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97

The equation (1) is existed when the adsorption systems reach equilibrium. Where, *C*sa is the amount of organic pollutants adsorbed per unit weight of soil particles; *C*wa is concen‐ tration of organic pollutants; *K*a is the total sorption coefficient.

The adsorption of organic contaminants in soil or sediment usually described by *K*a (soil ab‐ sorption coefficient) or *K*oc (organic carbon absorption coefficient). The former refers to the ra‐ tio of the concentration of organic matter in the soil or sediment and its aqueous phase concentration. As well, the latter factor represents the ratio of the concentration of organic mat‐ ter adsorbed by organic carbon in the soil or sediment and its aqueous phase concentration.

#### **5.3. Biochemical processes**

Microorganisms may play an important role in contamination transformations within groundwater and on the soil. They can act as catalysts for many types of reactions. When modeling biochemical reactions in groundwater, additional processes must be considered. These include the changes in the availability of substrate for the microorganisms to utilize and reactions on the particles that the microorganisms are attached to. When microbial reac‐ tions are significant, there is a possibility of clogging of pores due to precipitation reactions or to biomass accumulation [16].

Microorganisms not only influence chemical reactions, but may be contaminants them‐ selves. There is much current uncertainty about the fate and survival time of viruses, bacte‐ ria, and larger enteric organisms in groundwater [17-18]. Distribution of microorganisms will vary greatly with depth. Potential outbreaks of waterborne diseases due to biologic pol‐ lutants are of particular concern where there is land disposal of human waste (often via sep‐ tic tanks) and animal waste. The potential for transmittal of waterborne diseases in groundwater is particularly high in areas of rapid velocities such as karst regions.

Biodegradation mainly depends on two factors [19], the intrinsic characteristics of the pollu‐ tants (the structure of organics, physical and chemical properties) and microorganism (the activity of microbial populations), and the environmental factors controlling the reaction rate (temperature, pH, humidity, dissolved oxygen). As the U.S. Environmental Protection Agency researched [20], soil microbial degradation of organic pollutants can be expressed as a one-order response equation:

The Investigation and Assessment on Groundwater Organic Pollution http://dx.doi.org/10.5772/53549 97

$$\frac{dc}{dt} = -\text{KXC} = -k\_T c \tag{2}$$

Where, C is the mass fraction of soil organic matter[mg/g]; X is the number of active micro‐ bial in the organic matter of soil degradation[106 /g]; t is degradation time[d]; K is the oneorder biodegradation rate constant [g/(d 106 )]; kr is substrate removal constant[d-1].

From the above equation,

and sediment. Generally, adsorption is affected by sediments and soil properties, such as or‐ ganic percentage, the type and quantity of clay minerals, cation exchange, pH and the physi‐ cal and chemical properties of the contaminants. During the adsorption, the organic contaminants in the water adsorbed on the surface of the soil particles by the simultaneous distribution role of both water and solid, the driving force is mainly based on principle of "like dissolves like" and electrostatic adsorption of the polar group, and the following for‐

The equation (1) is existed when the adsorption systems reach equilibrium. Where, *C*sa is the amount of organic pollutants adsorbed per unit weight of soil particles; *C*wa is concen‐

The adsorption of organic contaminants in soil or sediment usually described by *K*a (soil ab‐ sorption coefficient) or *K*oc (organic carbon absorption coefficient). The former refers to the ra‐ tio of the concentration of organic matter in the soil or sediment and its aqueous phase concentration. As well, the latter factor represents the ratio of the concentration of organic mat‐ ter adsorbed by organic carbon in the soil or sediment and its aqueous phase concentration.

Microorganisms may play an important role in contamination transformations within groundwater and on the soil. They can act as catalysts for many types of reactions. When modeling biochemical reactions in groundwater, additional processes must be considered. These include the changes in the availability of substrate for the microorganisms to utilize and reactions on the particles that the microorganisms are attached to. When microbial reac‐ tions are significant, there is a possibility of clogging of pores due to precipitation reactions

Microorganisms not only influence chemical reactions, but may be contaminants them‐ selves. There is much current uncertainty about the fate and survival time of viruses, bacte‐ ria, and larger enteric organisms in groundwater [17-18]. Distribution of microorganisms will vary greatly with depth. Potential outbreaks of waterborne diseases due to biologic pol‐ lutants are of particular concern where there is land disposal of human waste (often via sep‐ tic tanks) and animal waste. The potential for transmittal of waterborne diseases in

Biodegradation mainly depends on two factors [19], the intrinsic characteristics of the pollu‐ tants (the structure of organics, physical and chemical properties) and microorganism (the activity of microbial populations), and the environmental factors controlling the reaction rate (temperature, pH, humidity, dissolved oxygen). As the U.S. Environmental Protection Agency researched [20], soil microbial degradation of organic pollutants can be expressed as

groundwater is particularly high in areas of rapid velocities such as karst regions.

tration of organic pollutants; *K*a is the total sorption coefficient.

*C KC sa a wa* = (1)

mulas is established [15]:

96 Organic Pollutants - Monitoring, Risk and Treatment

**5.3. Biochemical processes**

or to biomass accumulation [16].

a one-order response equation:

$$\frac{\mathcal{C}}{\mathcal{C}\_0} = \exp\left(-K\mathcal{X}\_t\right) = \exp(-k\_T t) \tag{3}$$

Substituted into with half-life formula,

$$\begin{aligned} \text{tr}\_{\frac{1}{2}} &= \ln 2 \nmid k\_T \\ \frac{1}{2} \end{aligned} \tag{4}$$

The half-life of degradation of residual contamination is determined.

#### **5.4. Fate and transport in unsaturated zone [21]**

In many cases, the receptor medium for release of a contaminant will be the unsaturated zone. In contrast to the saturated zone, pores in the unsaturated zone are not completely sa‐ turated with liquid. This fundamentally affects the processes governing flow and chemical transport. A number of processes will affect the contaminant within the unsaturated zone before it enters the saturated groundwater system and potentially is tapped by supply wells. The uncertainties in characterizing releases just described lead to uncertainties in defining the source terms and initial and boundary conditions for modeling unsaturated transport. Analogously, uncertainties in characterizing unsaturated transport processes lead to uncer‐ tainties in defining the source terms and initial and boundary conditions for modeling satu‐ rated transport.

For the most part, computer simulation of contaminant transport has focused on movement in the saturated zone. Assumptions are made regarding the time required for movement through the unsaturated zone. Often some sort of lag between source release and entry of chemicals into the saturated flow system is introduced into source terms. It is important to be aware of the unsaturated processes that are actually occurring, the uncertainty associated with these processes, and the role of monitoring in reducing these uncertainties.

#### **5.5. Saturated transport [21]**

Once a chemical has been released into the ground and has either moved through the unsa‐ turated zone or directly entered the saturated zone, saturated transport processes will deter‐ mine if, how fast, and at what concentration a chemical reaches a supply well. A great deal of research has been carried out on understanding and modeling these processes. There is increasing recognition that chemical transport must be viewed as a stochastic process.

amount of sewage generated, treatment and disposal of the way, main pollutants and their

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99

Agricultural pollution sources investigation mainly include land use history and current sit‐ uation, the varieties, numbers, operations, time of farmland application of chemical fertiliz‐ ers and pesticides, range of sewage irrigation, main pollutants and concentration, the number of sewage irrigation and sewage irrigation amount. The scale of farms and so on.

The surface polluted waters mainly about rivers, lakes, ponds, reservoirs and drains. We survey the distribution of polluted waters, the scales, the utilizations and water quality.

The coastal areas have to survey the situation of seawater invasion and saline water distri‐

The four steps of NAS was proposed by National Academy of Sciences, United States(NAS), was an assessment method on human health risk that led by the accident, air, water, soil and other medium. The method mainly in the following aspects: the hazard identification (quali‐ tative evaluation the degree of hazards of the chemical substances on the human health and ecological); dose-response assessment (quantitative assessment the toxicity of chemical sub‐ stances, established a relationship between the dose of chemical substances and the human health hazard); exposure assessment (quantitative or qualitative estimate or calculate the ex‐ posure, exposure frequency, exposure duration and exposure mode); exposure attribute (us‐ ing the data to estimate the strength of the health hazards in the different conditions or the probability of the certain health effects). This method can qualitative analysis or quantitative analysis of groundwater contamination, or combine them, the results could be quantify and

In 1989, U.S. Environmental Protection Agency (EPA) promulgated the "risk assessment guidance for superfund: Human health evaluation manual", there was a similar assessment method to NAS method [22]. The steps following as data collection, exposure assessment, toxicity assessment, risk characterization. Contrast the two methods, NAS is more common methods, the use range wider, suitable for a variety of health risk assessment; the EPA meth‐

**7. The assessment on groundwater organic pollution**

**7.1. The methods of groundwater organic pollution assessment**

analysis, and provide more detailed information to the decision-makers.

concentration and hazards

*6.2.4. Surface polluted waters*

*7.1.1. The four steps of NAS*

*7.1.2. The four steps of EPA*

bution.

*6.2.3. Agricultural pollution sources*

The same elements of uncertainty are present for saturated transport as for unsaturated transport. The important differences are that in saturated transport, water content equals porosity, hydraulic conductivity is no longer a function of water content or head, gravity rather than suction head is the driving force, and the scale of concern may be much larger
