Heavy Metals in Soils Following 50 Years of Sewage Sludge Application

*Thandile Mdlambuzi, Pardon Muchaonyerwa and Awonke Mbangi*

## **Abstract**

Heavy metal contamination has increasingly become an environmental problem. While it is found in soils naturally through processes of weathering of parent materials, it is the anthropogenic activities that create the greatest threat. A study was conducted to investigate the vertical distribution of heavy metals in soils after over 50 years of sewage sludge application. Soil samples were collected at 10 cm intervals to a depth of 50 cm from five treated transects and a control. The soils were analyzed for zinc, copper, lead, nickel, cadmium, arsenic and chromium. The concentration of all the metals was higher in the treated soils compared to the control. The results were compared with two parameters: the total maximum thresholds (TMT) and maximum permissible limits (MPL). The TMT is the concentration of the metal beyond which the risk to the environment is unacceptable, while MPL is the concentration beyond which further waste disposal is prohibited. Zinc, chromium, lead and cadmium were above maximum permissible limits, in treated soils. High concentrations of all the metals, including Pb, and organic carbon were measured down to 40–50 cm depth. Only Cd (and Pb only in transect 2) was above the maximum permissible limits beyond the 20–30 cm depth.

**Keywords:** heavy metals, soils, sewage sludge, soil physicochemical properties, contamination

### **1. Introduction**

Large quantities of sewage sludge generated globally present disposal challenges [1, 2]. High energy required for incineration and the scarcity of landfill space have made land application a major disposal option [3]. Land application of sewage sludge could benefit from the contents of organic matter and plant essential nutrients [4]. Sewage sludge from the Vlakplaas Wastewater Treatment Plant, South Africa, was found to contain 20–23% total carbon (C), 1.9–3.1% total nitrogen (N), 40–166 mg available phosphorus (P) kg−1 and 689–3804 mg potassium (K) kg−1 over a 4-year period [5]. Feasibility of using sewage sludge as a nutrient source could be limited by its composition of heavy metals, including cadmium (Cd), lead (Pb), chromium (Cr), mercury (Hg), arsenic (As), nickel (Ni), zinc (Zn) and copper (Cu) [1, 6]. The metals can be sorbed on soil colloids, lost through leaching to ground water or taken up by plants growing on contaminated sites. Soil conditions, sludge metal concentration

and loading rates could determine the accumulation, mobility and fate of these metals, through interaction with soil colloids, pH and P.

A number of laboratory leaching tube and glasshouse studies have been conducted to determine effects of different soil properties, including pH, P and organic matter (OM) content, or other characteristics on mobility of selected heavy metals (HM) in soil [7–11]. Water solubility and phyto-availability of Zn, Cd and Pb were found to be reduced by P additions, with greater effects on Pb [10, 12]. Dissolved OM was found to enhance the mobility of Ni and Cu, whereas Zn mobility was not modified, in soil [13]. Leaching of Cu was found to increase with decline in pH, with the lowest mobility occurring at pH 5–7 [14, 15]. Kumpiene et al., [16] concluded that it was not feasible to make long-term predictions based on short-term standardized laboratory tests.

Long-term field experiments are impractical, and sampling and analysis of soils contaminated decades before, relative to adjacent uncontaminated soil, could be an alternative [16]. The longest studies of this nature were conducted on a site that had received a once-off treatment with sewage sludge 15–20 years before sampling [8, 17] reported no substantial vertical movement of Cd, Cu, Ni and Zn in soil, whereas calculated metal deficits suggested that there could have been leaching losses. Accumulation in the soil could result in high metal concentrations in tissues of volunteer and indigenous vegetables, with serious health risks. Indigenous and volunteer exotic vegetables grow on polluted sites and could be harvested and consumed. Addition of sewage sludge could modify soil pH, available P and OM, and their interactions with HM could determine the fate of the metals under field conditions. Limited studies have been conducted on effects of these soil properties (pH, OM and P) on the mobility of a mixture of HM under field conditions.

Darvill Waste Water Works (DWWW), South Africa, has continuously applied sewage sludge containing different HMs on a dedicated site for over 50 years. This long-term sewage sludge application site provides an opportunity to understand what happens to HMs in the soil under field conditions with time. The continuous application of sewage sludge could have resulted in increases in pH, OM, available P and HM concentrations in the soil. The forms of the HM, which depend on soil pH and their interaction with OM and P, could determine their fate in soil and their accumulation in plants, with risks on human health and plant tissue. It is therefore essential to investigate the effects of the long-term application of sewage sludge on the mobility and concentrations of HM with increasing soil depth. The objective of this study was to determine the effects of 50 years of sewage sludge application on the distribution of heavy metals, in the soil profile and selected physicochemical property composition on different transects of a loam soil.

### **1.1 Study area**

The study was carried out at a dedicated sewage sludge application site at DWWW in Pietermaritzburg (PMB) (29.602500o S to 29.61139o S and 30.433900°E to 30.43861°E), KwaZulu-Natal, South Africa (**Figure 1**). The site has a mean annual rainfall of 680 mm and mean annual temperature of 18.4°C. The soil is formed from Ecca shale, a laminated carbonaceous sedimentary rock formed from the deposition of clastic sediments [18]. Over 250 m3 of thickened sludge (about 3% solids) is produced per day and applied by sprinkler irrigation on 57 ha of land, per day, which translates to an average of about 48,000 kg solid sludge ha−1 year−1.

The area treated with sludge was divided into five transects. A control transect, adjacent and upslope to the study field, was also included (**Figure 1**). Parts of

*Heavy Metals in Soils Following 50 Years of Sewage Sludge Application DOI: http://dx.doi.org/10.5772/intechopen.110009*

### **Figure 1.** *Sampling points at the sewage sludge disposal site.*

transects 1 and 4 were on lower slope positions, and transect 4 received drainage water from transects 3 and 5 and was always wet. Commercial turf grass production is currently practiced on the site, and the harvesting involves removal of the sludge-rich soil attached to the root system.
