**4. Discussion**

The results of this study indicate that the factory had an impact on the river's physico-chemical parameters. For example, at EDP, pH, Temperature and EC were much higher than at WIP upstream and DSP downstream, with a slight distortion at 1 km from the river bank. Since there was no other point source between WIP and EDP, the effluent discharged from the pulp and paper mill at EDP probably accounted for much of the observed increased levels in the physico-chemical parameters. Higher pH at EDP sites was likely the result of alkaline industrial effluent discharges. In the factory, sodium hydroxide and sodium sulphide (Kraft process) used during the cooking process of wood pulps, [22, 23, 26], might have resulted in an increase in alkaline condition of the water body at the EDP. In addition, after Kraft cooking process at the mill, the wood pulp underwent bleaching through CEHP or CEHH bleaching sequences. It is likely that the strong caustic extraction in the bleaching sequence was not completely neutralized by the final hypochlorite stage, thereby leading to additional alkalinity into the river water.

In addition to pH, high conductivity at EDP was probably the result of the paper mill discharge into the river. Wastewater from pulp and paper mills is known to contain substantial amounts of dissolved ions and electrolytes. The discharge of such electrolytes ultimately increased the electrical conductivity of the liquid effluent and by extension the EC of the receiving river water. For instance, alkaline sulfates dissociate under high temperature to liberate the positively charged alkali metals (X+ , X2+ or X3+) and the sulfate ions (SO4 <sup>2</sup>), which all increase the electrical conductivity of the water body [52]. *Distribution of Potentially Toxic Elements in Water, Sediment and Soils in the Riparian… DOI: http://dx.doi.org/10.5772/intechopen.102440*

The major components of lime mud and recovery boiler ash in the paper mill contain carbonate compounds [53]. Since the mill sometime releases lime mud and recovery ash into its effluent, this practice more likely exacerbated the alkaline conditions of its wastewater and the increase in pH and EC at the EDP when compared with WIP.

The leaching test results (**Table 6**) showed only marginal leachable concentrations of Cd and Cu. This can be explained by the fact the soils around the paper mill are of *Planesol* type, which are of fine texture intensely weathered and negatively charged. *Planesols* tend to attract positively charged ions such as Cd2+, Cu2+, Zn2+, and Pb2+, which will be strongly bound to the soils around the paper mill.

There were significant spatial variations in the potentially toxic elements concentrations in water, sediment and soils near the pulp and paper mill. Reports by Jumbe and Nandini, 2009 [54] had shown that average abundance of Pb in the Earth's crust is 13 ppm, but in natural soils the background level for Pb ranges from 2.6 to 25 ppm. In the same report, Cd in the Earth's crust is 0.16 ppm but in soils, it ranges from 0.1 to 0.5 ppm. The average abundance of Cu in the Earth's crust is 68 ppm while, in soils, it is between 9 and 33 ppm. The average abundance of Zn in Earth's crust is 76 ppm and in soils it ranges between 25 to 68 ppm. Our results show that Pb, Cd, Cu and Zn concentrations in the soils near the paper mill were approximately 5, 2, 0.5 and 2 times higher than the respective natural background levels of these metals in the earth's crust suggesting anthropogenic input of these metals. The high concentrations of Pb and Zn at EDP, suggest possible enrichment from the paper mill effluent as the source of the metals [18, 20, 29, 55–60]. The levels of the PTEs were not consistently high on the (AA) side of the mill, suggesting that the river bank was not an accurate reference point for the factory (AA) and opposite (BB) factory sides. In addition, high PTEs levels away from the river bank on the opposite (BB) side of mill indicate that river water was not the only source of the metals after effluent discharge.

Some of these PTEs may been deposited on soils through air emission from the paper factory, as suggested by Adoli *et al*, 2011 [61] while analyzing moss and topsoil to monitor metal emission from the same mill. Paper mills have been reported as potential hazardous substances emitters of metals. The presence of high Zn concentrations can be related to the use zinc dithionite as a bleaching agent of stone-ground wood pulps produced within the mill. Zn concentrations at EDP was probably influenced by pH, electrical conductivity, and temperature at the same sampling point. The reduced concentration of Cd as we moved from the EDP to DSP is due to absence of Cd compounds during pulp and paper processing. We can postulate that the factory effluents probably neutralized any geochemical Cd and higher Cd levels obtained at the soils at the downstream sites probably came from the recovery boiler system through the mill's gaseous emissions.

Most of the sulfates from industries are often associated with metal complexation causing insoluble inorganic substances [62], which have higher chances of removing Cd from the water. It is possible that the lack of such complexation agents from the paper mill probably resulted in increased Cd at DSP. Because the study area has higher rate of fertilizer use by farmers for sugarcane production, the net impact of fertilizer use on PTEs in the vicinity probably surpassed Cd concentration from the industrial effluents. However, it seems that it was efficiently reducing Cd, and therefore Cd enrichment of water was low. While it has generally been assumed that most of the metals are immobile in soils or sediments [63], there are factors that enhance their mobility resulting in differential metal distribution. These factors include the properties of the metals, soil texture, pH and competing cations in the soil and sediments

solution [52]. Cd retention is reported to be greater in fine-textured soils with high CEC than in coarse-textured soils with lower CEC, while McBride, 1995 [63] noted that potentially toxic elements mobility was most closely associated with metalorganic complexation and soil pH, all these were likely to affect the distribution of Cd and Pb in water, soils and sediments. *Ultisol* soils around the paper mill are generally known to be poor in organic matter and therefore retained little of the Cd which was transported away from the EDP.
