**2. Material and methods**

#### **2.1. Experimental site**

The experiment was carried out in the field of lysimeters at the Mashhad research station site, (36°13′ latitude, 59°38′ longitude) in northern east Iran during growing season (2004–2005).

This research was done to investigate the soil capacity to remove impurities when it is irrigated with wastewater and effluent and to study the potential impacts on groundwater quality. For this purpose, the effects of irrigation with four types of water (wastewater, effluent, mixture of freshwater and effluent, and freshwater) on three types of soil (sandy loam, loam, and clay loam) were investigated. A randomized completely blocked design was performed with three replications. The experiment was carried out, using 36 lysimeter (2 × 1.5 m) as experimental units. The number of lysimeters was equal to the number of experimental treatments × replicates (i.e., 4 × 3 × 3 = 36). Barley was planted as a common agricultural crop. A layer of gravel was placed at the bottom of each lysimeter to facilitate drainage. The leachates from lysimeters were collected and sampled at the beginning, middle, and end of the growing season. The samples were analyzed for chemical oxygen demand (COD) [6], phosphate, and nitrate [7]. Physicochemical characteristics of irrigation water, wastewater, and soil used in this study are summarized in **Tables 1** and **2**, respectively.


a Food and Agriculture Organization of the United Nations.

b Iranian Department of Environment.

**1. Introduction**

(P), and potassium (K) content [1].

178 Soil Contamination - Current Consequences and Further Solutions

the form of phosphates (PO4

**2. Material and methods**

**2.1. Experimental site**

algae to grow faster than the ecosystems can handle.

HYDRUS-1D model was based on the experimental results.

Besides wastewater usage and their environmental impact, water shortages are a severe problem in several parts of the world. Many parts of the world are threatened by water scarcity. In the Middle East, the threat of water scarcity is particularly important as it is an arid region with limited fresh water sources. Therefore, seeking for unconventional sources of water is inevitable in this area. The use of treated sewage waterforirrigation ensures the reuse of water resources. Municipal wastewater not only offers an alternative waterirrigation source, but also the opportunity to consider as low price fertilizer because of its high nitrogen (N), phosphorus

Phosphorus is a valuable nutrient contained in wastewater [2]. There is potential for these nutrients present in recycled water to be used as a fertilizer source when the water is recycled as an irrigation source for agriculture [3]. Phosphorus (P) is commonly found in municipal and agricultural waste and wastewater, originating from the digestion of phosphorus-containing food sources. Municipal wastewaters may contain 5–20 mgl−1 of total phosphorus, of which 1– 5 mgl−1 is organic and the rest is inorganic. Phosphorus in natural waters is usually found in

from or retained in the soil or taken up by plants. Too much phosphorus in the water causes

Phosphorus can move into surface water bodies by runoff or erosion and cause water quality problems such as eutrophication. Phosphates are not toxic to people or animals unless they are present in very high levels. The phosphate in wastewater is initially quite soluble and available [4]. Movement of phosphate is slow but may be increased by rainfall or irrigation water flowing through the soil. Due to erosion of soil and when the sediment reaches a body of water it may act as a sink or a source of P in solution. Therefore, to develop effective management practices, there is a need to improve the understanding of P transport in the soil profile through percolation or matrix flow. In the case of blue-green algae, toxic by-products can be produced, which create health issues if a lake or reservoir would be used as a source of drinking water. For this reason, phosphorus removal is an essential role of wastewater treatment plants and testing for phosphorus in the plant effluent is critical. Controlling phosphorus discharged from municipal and industrial wastewater treatment plants is a key factor in preventing eutrophication of surface water bodies. The objectives of this study were, using HYDRUS-1D model [5], as a tool, to develop an understanding of vertical distribution and transport processes PO4 leaching in soil lysimeter condition. Calibration and validation of

The experiment was carried out in the field of lysimeters at the Mashhad research station site, (36°13′ latitude, 59°38′ longitude) in northern east Iran during growing season (2004–2005).

3−). During irrigation with wastewater, phosphorus may be leached

\* The standard is higher than the range of Iranian Department of Environment.

\*\*The standard is higher than the range of FAO.

**Table 1.** Physicochemical characteristics of water and treated wastewater.


**Table 2.** Some chemical properties of soil layers at the experimental field site at initial condition.

#### **2.2. Data collection**

In this model, some physical and soil hydraulic properties, concerning soil moisture retention characteristics, *θ* (*h*), and saturated hydraulic conductivity, *K*sat, were measured in the field. The parameters of van Genuchten's [8] model were evaluated by fitting on *θ* (*h*) data using the curve RETC code. The average values of van Genuchten parameters for lysimeter study at different soil types are given in **Table 3**.


\* C: clay loam, L: loam, and S: sandy loam.

**Table 3.** Physical properties and van Genuchten parameters for soil sample with *θ*r, residual water content (cm3 cm−3); *θ*s, saturated water content (cm3 cm−3); *a* (cm−1) and *n*(–), empirical parameters; l(–), pore-connectivity and tortuosity factor and *K*sat, saturated hydraulic conductivity (cm h–1).

#### **2.3. The HYDRUS-1D-flow and transport model**

In this study, HYDRUS-1D software, version 4.14, was used to conduct numerical simulations of one-dimensional water flow and phosphorous transport in vertical profiles of unsaturated soil to simulate the phosphorous transport in the different soil types under municipal wastewater application. The total depth of each soil profile was 200 cm with one soil type in each profile. Raw sewage then passes through the filter mesh, effluents-treated municipal wastewater, obtained daily from the Parkanabad wastewater treatment plants, mixture of 50% effluents and 50% well water, and well water was used as the influent. Irrigation water was applied to the lysimeters at a flow of 0.78–0.21 m3 m−2 day−1 in 2004 and 2005, respectively. Each soil profile was oriented vertically, so that the irrigation water flowed in a vertical direction.


\* Water uptake is assumed to be zero close to saturation (i.e., wetter than some arbitrary "anaerobiosis point" *h*o). Root water uptake is also zero for pressure heads less than the wilting point (*h*3). Water uptake is considered optimal between pressure heads *h*opt and *h*2, whereas for pressure heads between *h*2 and *h*3 (or *h*o and *h*opt), water uptake decreases (or increases) linearly with pressure head.

**Table 4.** Effective root depth, root water uptake parameters, and root distribution\* .

**Soil sample\***

\*

\*

**Parameters Anions solution saturation extract (meql−1)**

**2– HCO3**

C: clay loam, L: loam, and S: sandy loam.

different soil types are given in **Table 3**.

**Clay Silt Sand**

C: clay loam, L: loam, and S: sandy loam.

*θ*s, saturated water content (cm3

**Soil sample\* Particle fraction (%) Texture (–) Bulk**

factor and *K*sat, saturated hydraulic conductivity (cm h–1).

**2.3. The HYDRUS-1D-flow and transport model**

**– SO4**

180 Soil Contamination - Current Consequences and Further Solutions

**CO3**

**2.2. Data collection**

**Total anions** 

**Table 2.** Some chemical properties of soil layers at the experimental field site at initial condition.

**Cations solution saturation extract (meql−1)**

**2– Cl– Ca2+ Mg2+ Na+ K+ C** – 1.8 35 9 45.7 14 20 12 0.1 45.9 4.2 7.4 0.87 **L** – 2.5 4.5 6.3 13.2 4.1 7.2 2.1 – 13.1 1.2 7.7 0.9 **S** – 2.2 5.3 6.1 13.6 4.2 7.3 2.1 – 13.5 1.4 7.8 2.9

In this model, some physical and soil hydraulic properties, concerning soil moisture retention characteristics, *θ* (*h*), and saturated hydraulic conductivity, *K*sat, were measured in the field. The parameters of van Genuchten's [8] model were evaluated by fitting on *θ* (*h*) data using the curve RETC code. The average values of van Genuchten parameters for lysimeter study at

> **density (kgcm−3)**

**S** 22.09 19.19 58.72 Sandy loam 1.51 0.065 0.41 0.075 1.89 0.5 106.1 **L** 20.30 39.68 40.02 Loam 1.43 0.078 0.43 0.036 1.56 0.5 24.96 **C** 48.65 28.75 22.6 Clay loam 1.3 0.095 0.41 0.019 1.31 0.5 6.24

**Table 3.** Physical properties and van Genuchten parameters for soil sample with *θ*r, residual water content (cm3

In this study, HYDRUS-1D software, version 4.14, was used to conduct numerical simulations of one-dimensional water flow and phosphorous transport in vertical profiles of unsaturated soil to simulate the phosphorous transport in the different soil types under municipal wastewater application. The total depth of each soil profile was 200 cm with one soil type in each profile. Raw sewage then passes through the filter mesh, effluents-treated municipal wastewater, obtained daily from the Parkanabad wastewater treatment plants, mixture of 50% effluents and 50% well water, and well water was used as the influent. Irrigation water was applied to the lysimeters at a flow of 0.78–0.21 m3 m−2 day−1 in 2004 and 2005, respectively. Each soil profile was oriented vertically, so that the irrigation water flowed in a vertical direction.

*θ***r (cm3 cm−3)** 

*θ***s (cm3**

cm−3); *a* (cm−1) and *n*(–), empirical parameters; l(–), pore-connectivity and tortuosity

 **cm−3)** 

*a* **(cm−1)** 

**Total cations**  **EC (dsm−1)** 

**pH (–)**

*n* **(–)** *l* **(–)** *K***sat (cm day–1)**

cm−3);

**SAR (meql–1) 1/2**

> The initial condition for volumetric soil water content was between 0.1 and 0.2 for different soil types in all simulations. In case of water flow, the upper water flow boundary condition was atmospheric boundary condition with surface layer, given by the following equation:

$$-K\left(\frac{\partial h}{\partial \mathbf{x}} + \cos(\alpha)\right) = q\_o(t) - \frac{dh}{dt} \qquad \text{at } \mathbf{x} = L\left(Soil \text{ surface}\right) \tag{1}$$

where *q*0 is the net infiltration rate (precipitation minus evaporation).


**Table 5.** The amount of nitrogen and phosphate in different irrigation water (mg l-1).

In this study, the lower water flow boundary condition was free drainage. The minimum allowed pressure head at soil surface is the wilting value and was set at the value of 100,000 cm provided by HYDRUS-1D. The root water uptake by plants is described by the macroscopic approach of Feddes et al.'s [9] model. Information on root water uptake with compensation is available in Ref. [5]. The coefficients of Feddes et al.'s [9] model are presented in **Table 4** [5]. The maximum root depth, seeding depths, and the root growth ratio of barley were 100, 5, and 5 cm, respectively.

To investigate the concentration of nitrogen and phosphate in wastewater, effluent, and well water, at any time of sampling from the Parkanabad wastewater treatment plants, quality of the water/wastewater in terms of total nitrogen, ammonia, nitrate, total phosphate, and chemical oxygen demand (COD) were tested based on standard methods [6]. Mean concentration of nitrogen and phosphate in different irrigation water are presented in **Table 5**.


1\*Input *COD* in terms of milligrams per liter, the pollution load of wastewater, and water used in irrigation.

2\*Drainage *COD* in terms of milligrams per liter, contamination of water is drained from the lysimeters.

\*\*W1: freshwater, W2: mixture of and effluent, W3: effluent, W4: wastewater.

\*\*\*S: sandy loam, L: loam, C: clay loam.

**Table 6.** The amount of chemical oxygen demand (COD) in different irrigation water (mgl−1).

As shown in **Table 5**, about 42% of phosphate in raw wastewater is removed during the treatment process. According to Mojid et al. [10], the maximum permissible level of phosphate in wastewater for irrigation should not be more than 4.1 mg l−1. In our study, the amount of phosphate in raw wastewater was more than FAO's standard. About effluent, however, the average of phosphate was less than 4.1 mg l−1 [11], but in some samples, its concentration was higher than the standard amount. Results of the analysis of chemical oxygen demand (COD) and irrigation water are presented in **Table 6**. This table includes the average results from three similar lysimeters in each irrigation (irrigation water and the type of soil) and through this we can observe the relative change transfer of contamination by COD index into the deep soil during the irrigation season.

In this study, the lower water flow boundary condition was free drainage. The minimum allowed pressure head at soil surface is the wilting value and was set at the value of 100,000 cm provided by HYDRUS-1D. The root water uptake by plants is described by the macroscopic approach of Feddes et al.'s [9] model. Information on root water uptake with compensation is available in Ref. [5]. The coefficients of Feddes et al.'s [9] model are presented in **Table 4** [5]. The maximum root depth, seeding depths, and the root growth ratio of barley were 100, 5, and

To investigate the concentration of nitrogen and phosphate in wastewater, effluent, and well water, at any time of sampling from the Parkanabad wastewater treatment plants, quality of the water/wastewater in terms of total nitrogen, ammonia, nitrate, total phosphate, and chemical oxygen demand (COD) were tested based on standard methods [6]. Mean concentration of nitrogen and phosphate in different irrigation water are presented in **Table 5**.

W1 S 20 13 20 30 20 30 20 20 20 27 20 24 W1 L 20 12 20 20 20 28 20 18 20 21 20 19 W1 C 20 10 20 28 20 30 20 20 20 28 20 21 W2 S 26 18 27 24 29 33 25 29 27 37 24 34 W2 L 26 17 27 26 29 35 25 28 27 31 24 39 W2 C 26 13 27 27 29 37 25 27 27 38 24 31 W3 S 35 17 45 35 25 42 30 28 29 35 27 40 W3 L 35 20 45 38 25 35 30 25 29 28 27 33 W3 C 35 35 45 37 25 40 30 25 29 27 27 34 W4 S 400 25 430 47 380 53 385 38 392 51 381 45 W4 L 400 27 430 48 380 57 385 40 392 50 381 41 W4 C 400 25 430 50 380 52 385 37 392 48 381 42

1\*Input *COD* in terms of milligrams per liter, the pollution load of wastewater, and water used in irrigation. 2\*Drainage *COD* in terms of milligrams per liter, contamination of water is drained from the lysimeters.

As shown in **Table 5**, about 42% of phosphate in raw wastewater is removed during the treatment process. According to Mojid et al. [10], the maximum permissible level of phosphate in wastewater for irrigation should not be more than 4.1 mg l−1. In our study, the amount of phosphate in raw wastewater was more than FAO's standard. About effluent, however, the average of phosphate was less than 4.1 mg l−1 [11], but in some samples, its concentration was higher than the standard amount. Results of the analysis of chemical oxygen demand (COD)

**Table 6.** The amount of chemical oxygen demand (COD) in different irrigation water (mgl−1).

\*\*W1: freshwater, W2: mixture of and effluent, W3: effluent, W4: wastewater.

\*\*\*S: sandy loam, L: loam, C: clay loam.

**22.6.2004 29.6.2004 8.7.2004 18.7.2004 29.7.2004 6.8.2004 Chemical oxygen demand (***COD***) (mgl−1) 1**\* **2**\* **1 2 1 2 1 2 1 2 1 2**

**Irrigation water\*\* Soil sample\*\*\* Data of sampling**

5 cm, respectively.

182 Soil Contamination - Current Consequences and Further Solutions

The HYDRUS-1D model was also used to simulate PO4 transport under different irrigation treatments and soil types in one-dimensional vertical lysimetrs. The HYDRUS-1D was run for the main processes of water flow and general solute transport. No hysteresis was considered in the simulations. A total of three simulations (one for each soil types) were performed. Each simulation modeled one-dimensional unsaturated water flow, root water uptake, and phosphate transport. In each simulation, the precipitation and irrigation water were applied to the soil surface of lysimeter. The soil surface in each simulation was covered with barley crop. The initial values for the longitudinal dispersivity (*λ*) were derived from HYDRUS-1D dataset and from a study done by [12, 13]. HYDRUS-1D model was then calibrated manually by using these initial values for the *λ* parameter. The *λ* parameter was calibrated against the concentration of PO4-P in drainage water from lysimeters throughout the experiment. The final value of *λ* was determined by using several iterations when the mass balance errors were minimized to <1%. We assumed the molecular diffusion coefficient in free water (DW) was set to zero, therefore the transport of solute through diffusion was considered negligible. The initial water conditions were specified in terms of water content between 0.1 and 0.2 for different soil types in all simulations. The upper water flow boundary condition at the surface (*x* = *L*) was specified as the atmospheric boundary condition with a surface layer. This boundary condition imposed time-dependent conditions to specify the atmospheric conditions at the top of the lysimeter. Initial concentration of PO4 on the top node of the lysimeter was specified equivalent to the amount of PO4 wastewater added on top of the lysimeter before running the experiment. The lower water flow boundary conditions were prescribed using gravitational free draining. As for solute (PO4) transport, concentration flux boundary conditions were implemented at the upper boundary, and a zero gradient boundary condition was set at the lower solute boundary condition. The reaction parameters required by the HYDRUS-1D model were derived from the adsorption experiment reported by Abou Nohra et al. [14]. The reaction parameters (*kd* and *β*) required by the HYDRUS-1D model were derived based on Eq. (2):

$$\mathbf{s} = \mathbf{K}\_d \log \mathbf{c}^\vartheta \tag{2}$$

where *s* is the concentration of PO4 adsorbed to the soil (M M−1), *c* is the concentration of PO4 in solution (M L−3), *kd* is the equilibrium constant (L3 M−1), and *β* is a shape-fitting parameter [15]. The solute transport and reaction parameters considered in the simulations for different soil samples are listed in **Table 7**. The HYDRUS-1D models were run for phosphorous transfer into two stages: calibration and validation. Results obtained from 2004 were used to calibrate the parameters to improve the fit between the simulated and measured data. Similarly, the results obtained from 2005 were used to validate the output from the model.


\* S: sandy loam, L: loam, C: clay loam.

**Table 7.** Transport and reaction parameters for different soil samples.
