*3.1.1 Point and non-point sources*

Water pollutants are divided into two categories based on the emission source: point pollutant (point or concentrated pollution) and non-point pollutant source (non-point pollution).

#### *Phosphorus Dynamics in Soil-Water-Sediment Environment DOI: http://dx.doi.org/10.5772/intechopen.113225*

A point source of pollution refers to a specific and identifiable location that releases pollutants into the receiving environment. Examples of such sources include wastewater discharged by industries, power plants, and urban sewage treatment facilities. The Clean Water Act (CWA) in the United States provides a regulatory definition for point sources. The CWA's definition of a point source was updated in 1987 to encompass municipal sewage systems, industrial wastewater, and construction wastewater.

A non-point pollutant source refers to a source of pollution that does not have a specific and identifiable entry point into the receiving environment. Typically, nonpoint sources include water and runoff from agricultural lands, mines, construction sites, roads, and urban areas. Additionally, air pollution settling on water sources also constitutes non-point pollution. Unlike point source pollution, which originates from a single source, non-point pollution is the result of the accumulation of pollution amounts collected from a large basin. Non-point sources of pollution pose the biggest threat to surface and subsurface drinking water sources worldwide. Erosion is also a significant source of non-point pollution due to the large amounts of chemical fertilizers, insecticides, pesticides, and other substances used in agricultural lands, which have an average erosion rate higher than other land uses. Consequently, agricultural lands are one of the primary sources of this type of pollution.

#### *3.1.2 Soil*

Phosphorus is a crucial element for modern agriculture, widely used in chemical fertilizers to enhance crop yields. In recent years, phosphorus consumption has increased due to the depletion of soil phosphorus caused by extensive crop harvesting. The significance of phosphorus in the crop production system is evident from the doubling of its use in fertilizers since 1960, while its annual global production has remained less than 2 million tons over the past decade. As depicted in **Figure 2**, phosphorus primarily leaves the land through runoff in solution and attached to particles, while a small fraction infiltrates the groundwater through preferential flow. The potential for phosphorus to reach groundwater is relatively low compared to nitrogen, owing to its low solubility. However, soil particles have a high capacity to adsorb insoluble forms of phosphorus, and hence, soil functions like a filter. Phosphorus loss from agricultural land is classified into three levels:

**Figure 2.** *Potential routes of phosphorus exit from agricultural lands.*


Animal and chemical fertilizers contain a significant amount of soluble phosphorus, which can enter runoff due to rainfall after application and increase the concentration of this element in the runoff up to 100 times its normal level [25]. The amount of phosphorus that can be stored or adsorbed on the surface of clay depends on the type and quantity of clay, iron oxide, and organic matter. The critical concentration of phosphorus varies in different surface waters, as shown in **Table 1** [26].

Phosphorus is primarily released from agricultural lands in association with sediments. Proper management of plowing can reduce erosion and phosphorus loss. Therefore, in agricultural fields with a steep slope, it is advisable to use protective plows or perform contour cultivation [27]. Studies have demonstrated that total phosphorus losses are higher in fields with Chisel plowing compared to minimum plowing, whereas soluble phosphorus losses are higher in fields with minimum plowing [28]. The retention or release of phosphorus in water depends on the physical, chemical, and biological conditions of the compounds in the water system. The intensity of phosphorus discharge varies from one river to another, and even within different parts of a river, changing with seasonal variations [29]. Phosphorus storage in water is influenced by several factors, such as the biomass of macrophytes, the absorption of this element by aquatic plants, the expansion of the river margin, and nutrient concentration [30].

### *3.1.2.1 Phosphorus leaching*

Some researchers suggest that phosphorus can be absorbed into the subsoil through the vertical movement of water in the soil profile, as the subsoil layers have a high absorption capacity for phosphorus [31]. Thus, the transfer of phosphorus from these soils to underground water and drains is expected to be very low or negligible [24]. However, studies have shown that despite the high absorption capacity of the underlying soils, significant concentrations of phosphorus can still be found in the drains of these fields [11]. These studies indicate the presence of preferential flow,


#### **Table 1.**

*Critical concentration of phosphorus in surface waters.*
