Potential toxic elements in groundwater and their health risk assessment in drinking water of Limpopo National Park, Gaza Province, Southern Mozambique.

Concentrations of trace elements in drinking water affect its safety and acceptability for use. Potentially toxic element (PTE) contaminations are considered extremely hazardous because of toxicity, persistence, and bioaccumulative behaviour. Many areas in the Southern African Development Community are data poor and have poor accessibility. The results of our previous research identified the presence of fossil waters in southern Limpopo National Park. Groundwater and river water are the only sources of drinking water for the villages in the study area. The current study focuses on the understanding of trace element distribution and health perspectives of PTEs (Hg, U, Sr, B, and Mn) in the groundwater and surface water samples (rivers and lakes) collected within the buffer zone of the Limpopo National Park, Southern Mozambique. Two sampling campaigns (October 2016-March 2017) were carried out during the end of the wet season and the end of the dry season to analyse the differences. The results improved our knowledge of the occurrence of trace elements in drinking water in an area where water resources play a fundamental role-because of their scarcity-and where the climate is harsh. ICP-MS results provided information on concentration ranges, highlighting the exceedance of the permissible maximum limit of mercury imposed by the World Health Organization on several groundwater samples. In the buffer zone of Limpopo Park, the highest levels of risk seem to be associated with the presence of Hg and U in drinking water. The use of risk assessment markers such as non-cancer risk value (hazard quotient [HQ]) revealed the exceedance of HQ values for Hg and U. The HQ values are higher in the wet season than the dry season, and most of the exceedance has been found in groundwater. HQ values are higher in exposed children than exposed adults. The water of Lake Massingir seems to be safer than any other source, but people do not currently use it because of the distance between the lake and their villages. Proactive control and research on alternative solutions for the water needs of the population and on creation of water distribution are recommended. In the current study, drinking water was the only route of exposure that was evaluated. Therefore, it would be appropriate to investigate the concentrations of PTEs in crops, livestock, and any other potential pathways.

Assessment of groundwater quality in the buffer zone of Limpopo National Park, Gaza Province, Southern Mozambique.

Many areas in the Southern African Development Community are data-poor and poorly accessible. Water quality assessment in these areas therefore has to rely on the limited available data, coupled with restricted field sampling. This paper documents the first evaluation of the main geochemical processes and impact of anthropogenic and natural sources of contamination on the groundwater quality of the aquifer system used for domestic and agricultural purposes in Limpopo National Park, Gaza Province, Southern Mozambique. Twenty-five groundwater and surface water samples were collected during two field campaigns, one in October 2016 (the end of the arid period) and the other in March 2017 (the end of the wet period). In the field, the researchers analysed chemical-physical parameters such as temperature, pH and EC. In the laboratory, the major ions (boron, Na+, Ca2+, K+, Mg2+) and anions (Cl-, SO42-, NO3-, HCO3-, CO32-) were determined, and almost all groundwater samples fall into the brackish water category. The B/Cl ratio and δ11B concentrations were then measured to better understand the origin of these brackish waters. The direct relationship between boron and chlorine and δ11B concentrations above 40‰ suggests the presence of fossil water in the aquifer. The groundwater in this area was found to be above the limits stated as desirable by the Department of Water and Sanitation (DWS) and the World Health Organisation for domestic and irrigation purposes. The suitability of the groundwater for drinking purposes was also evaluated, with the results indicating that it is not suitable. The values of the sodium absorption ratio, sodium percentage and electrical conductivity show that most of the groundwater samples are also not suitable for agricultural purposes. However, the surface waters have higher quality for both domestic and irrigation purposes. The water of Massingir Lake turns out to be the most suitable resource for a possible rural development plan for the area.

Contaminant back-diffusion from low-permeability layers as affected by groundwater velocity: A laboratory investigation by box model and image analysis.

Low-permeability lenses represent potential sources of long-term release when filled from contaminant solute through direct contact with dissolved plumes. The redistribution of contaminant from low to high permeability aquifer zones (Back-Diffusion) was studied. Redistribution causes a long plume tail, commonly regarded as one of the main obstacles to effective groundwater remediation. Laboratory tests were performed to reproduce the redistribution process and to investigate the effect of pumping water on the remediation time of these contaminated low-permeability lenses. The test section used is representative of clay/silt lenses (k≈1∗10-10m/s/k≈1∗10-7m/s) in a sand aquifer (k≈1∗10-3m/s). Hence, an image analysis procedure was used to estimate the diffusive flux of contaminant released by these low-permeability zones. The proposed technique was validated performing a mass balance of a lens saturated by a known quantity of tracer. For each test, performed using a different groundwater velocity, the diffusive fluxes of contaminant released by lenses were compared and the remediation times of the low-permeability zones calculated. For each lens, the obtained remediation timeframes were used to define an analytical relation vs groundwater velocity and the coefficients of these relations were matched to grain size of the low-permeability lenses. Results show that an increase of the velocity field is not useful to diminish the total depletion times as the process mainly diffusive. This is significant when the remediation approach relies on pumping technology.

Geochemical modeling and multivariate statistical evaluation of trace elements in arsenic contaminated groundwater systems of Viterbo Area, (Central Italy).

Contamination of groundwater by naturally occurring arsenic has recently become a disturbing environmental problem in Viterbo area, Central Italy. Arsenic concentrations in most of the public supply networks exceed the maximum allowable limit of 10 µg/l (WHO) for drinking water. The primary purpose of this paper is to obtain a better understanding of the factors contributing to the high levels of As in water supply networks. This study focuses on (a) the determination of basic hydrochemical characteristics of groundwater, (b) the identification of the major sources and processes controlling the As contamination in public supply networks, (c) to find out possible relationships among the As and other trace elements through principal component analysis (PCA). Groundwater samples from public water supply wells and springs were collected and analysed for physico-chemical parameters and trace elements. Springs and well water samples are predominantly of the Na-HCO3, Na -Ca-HCO3 and Ca-HCO3 types and the highest arsenic concentrations were observed in Na-HCO3 type water. Eh-pH diagrams reveal that H2AsO4 (-) and HAsO4 (2-), As(V) arsenate, are the dominating As species highlighting slightly to moderately oxidizing conditions. Geochemical modeling indicates that arsenic-bearing phases were undersaturated in the groundwater, however most of the samples were saturated with respect to Fe (i.e. magnetite, hematite and goethite) and Al (diaspore and boehmite) oxide and hydroxide minerals. Concentrations of As, Li, B, Co, Sr, Mo, U and Se are highly correlated (r > 0.7) with each other, however in some groundwater samples As show also good correlations (r > 0.5) with Fe and Mn elements reflecting the relationships among the trace elements result from different geochemical processes. Evaluation of the principal component (PCA) analysis and geochemical modeling suggest that the occurrence of As and other trace element concentrations in groundwater are probably derived from (i) weathering and/or dissolution of volcanic source aquifer materials and (ii) adsorption/desorption processes on the Fe and Al oxide and hydroxide minerals.