Occurrence and ecological risk assessment of heavy metals in agricultural soils of Lake Chilwa catchment in Malawi, Southern Africa

Abstract

Understanding the levels and associated ecological risk caused by heavy metals is important for the sustainable management and utilization of Lake Chilwa catchment, an important ecosystem in Malawi providing fertile lands for agriculture and a designated wetland ratified by the Ramsar convention in 1997. Concentrations of chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), zinc (Zn), arsenic (As), mercury (Hg) and cadmium (Cd) were analyzed from 22 soil sampling locations. Extraction and quantification were achieved by microwave digestion and Inductively Coupled Plasma Optical Emission Spectrometer, respectively. Mean values were detected in the following order; Zn > Cr > Ni > Cu > Pb > As. Strong correlations were observed between As and Pb (r = 0.85), Cr and Ni (r = 0.82), Cu and Ni (r = 0.81), Cr and Cu (r = 0.8), and Pb and Zn (r = 0.73) suggesting similar sources of input. Principal component analysis revealed that Cu, Pb, Zn and As originate from anthropogenic activities, while Cr and Ni were geogenic. The ecological risk caused by these metals, calculated by the Ecological Risk Index (RI) method, showed a low to moderate ecological risk. The wetland areas had higher overall concentrations and RI values compared to the rest of the catchment. It is therefore important to enforce measures to manage and control these levels to avoid their damaging effects.

Groundwater Quality Assessment in Urban Areas of Malawi: A Case of Area 25 in Lilongwe

“…Te pH values were well within the permissible limits set by WHO and MBS from all sampling locations. Te variations are usually associated with the natural process and anthropogenic activities [14]. Dissolution of organic matter, carbonates, and hydroxides in bedrock and soils and other pollutants can create an imbalance in the water's natural pH of 7.…”

A synthesis of mercury research in the Southern Hemisphere, part 1: Natural processes

“…In Africa, a few studies on Hg in background soils in Tanzanian oxisols suggest that ion exchange may be involved in Hg translocation and retention (Semu et al 1986(Semu et al , 1987(Semu et al , 1989; however, these studies are limited to the analysis of the theoretical soil adsorption capacity under laboratory conditions and neither consider the in situ accumulation of Hg nor compare Hg concentrations in the different horizons. Background Hg concentrations measured in situ in Africa ranged between 13 and 48 ng g -1 in soils around northern Lake Victoria (East Africa) (Campbell et al 2003); between 15 and 50 ng g -1 in sediments of the Berg River (South Africa) (Kading et al 2009); and below 20 ng g -1 in soils collected around Lake Chilwa (Malawi) (Mussa et al 2020). Mercury concentrations in more remote soils in Dar es Salaam (Tanzania) were also low (below 15 ng g -1 ) (Nipen et al 2022).…”

Ecological Risk Assessment for Heavy Metals Occurrence in Rice Cultivation Areas in An Giang Province, Vietnam

“…Some heavy metals in the soil are sources of micronutrients within appropriate concentrations. However, other heavy metals such as Pb, Cd, Hg and As have no beneficial effects on plants and animals and are considered toxic [3], [4]. In addition, many previous studies have demonstrated that agricultural cultivation is considered a significant source of additional heavy metal content in soil [5]- [8].…”

“…In addition, many previous studies have demonstrated that agricultural cultivation is considered a significant source of additional heavy metal content in soil [5]- [8]. The presence of heavy metals in agricultural soil was mainly attributed to mining activities, waste disposal, application of organic manure, inorganic fertilizers, herbicides and pesticides [4], [9]. This suggests that heavy metal accumulation in plants may increase by plant root respiration in soils containing harmful heavy metals [3], [10].…”