Geochemical distribution of heavy metals and implications for human health in floodplain area along Mekong River, Lao PDR

Abstract

Arsenic and heavy metal accumulation in the environment have significant impacts on human health. With economic development and population growth, there is an increasing discharge of pollutants into the environment, particularly in floodplain areas, which makes them more vulnerable and dangerous. Despite these risks, there is a lack of research on the geochemical characteristics and the distribution of heavy metals that pose a threat to the health of Lao citizens. Therefore, this study aims to investigate the physicochemical characteristics and distribution of heavy metal contamination associated with the geographical differences in the floodplain areas along the Mekong River, as well as to identify the hydrogeochemical facies and mechanisms controlling the distribution of dissolved ions in groundwater. In addition, the potential human health risks associated with exposure to the heavy metals contaminated in food chains through various pathways (i.e., oral ingestion and dermal contact) of residents living in the study area of Champasak (CH) and Attapeu (AT) provinces were investigated. In order to investigate the physicochemical characteristics of groundwater, along with the distribution of heavy metals in groundwater, surface water, and soil in floodplain areas along the Mekong River and its tributaries in the Lao People's Democratic Republic (Lao PDR), a total of 152 groundwater samples were obtained from Champasak (n = 60) and Attapeu (n = 92) provinces, along with 18 surface waters and 40 topsoil samples. The results showed that groundwater is under oxidizing conditions, with pH ranging from 4.03 to 8.00, while surface water varied from 6.22 to 8.08. Calcium (Ca2+) and bicarbonate (HCO3 -) were the major cations and anions, respectively. The nitrate (NO3 -) concentration was significantly high in AT groundwater. Significantly poor water quality was observed, with high electrical conductivity (EC), arsenic (As), iron (Fe), and manganese (Mn) concentrations exceeding national (Lao) and international (WHO) permissible limits, especially in the AT study area. In contrast, soil in the CH area was found to be more contaminated with As, Cr, and Cu, with hotspot areas observed in soil formed from granite bedrock, indicating geographical differences resulting in distinctions between the quality of the two regions according to pollution index assessment. This preliminary work can be applied as important information for policy and regional environmental management, especially in floodplain areas in the Lao PDR. The mechanisms controlling the enrichment and mobility of major ions, arsenic (As), and barium (Ba) in groundwater in the floodplain regions along the Mekong River in Lao PDR have not yet been fully explained. Therefore, the investigation of the occurrence and spatial distribution of these dissolved ions in the two floodplain areas is needed. Graphical techniques (i.e., Piper plot, Gibbs diagram, and ionic ratio), statistical analysis (i.e., correlation analysis and principal component analysis (PCA)), and geochemical modeling (i.e., saturation index) were employed. The results suggested that Ca-HCO3 is the dominant water type in the study areas, with water- rock interaction being the main mechanism controlling the availability of major cations and anions in the groundwater, particularly the dissolution of carbonate minerals and cation exchange. Human activities, such as the application of chemical fertilizers, domestic sewage, and manure, influence nitrate (NO3 ⁻) concentrations. In the CH aquifer, the oxidation of sulfide minerals and the dissolution of evaporite minerals contribute to the mobility of arsenic (As), while the dissolution and precipitation processes of barium-bearing minerals (i.e., barite (BaSO4) and witherite (BaCO3)) control the presence of barium (Ba) in the region. In the AT aquifer, adsorption and desorption onto the solid phase of carbonate minerals, and the dissolution of witherite, primarily control the presence of As and Ba in the groundwater, respectively. To assess the demographical characteristics and the bioaccumulation of heavy metals in food chains leading to the human health risk in the floodplain areas, field questionnaires and environmental sample collection, along with the calculation of human health risk, were conducted. Among the 155 participants, approximately 23.9% had an elementary level of education, with farming being the main occupation and earning 20,000 kip per day on average. According to the groundwater usage behavior, most of the residents used their own private tube wells for various purposes, including drinking, bathing, washing, cooking, and irrigating. About 12.9% of the residents utilized groundwater as their main drinking water source, while 77.4% consumed bottled water. The areas that did not treat the groundwater before consumption experienced higher rates of waterborne diseases. The elevated levels of heavy metal contamination in groundwater, staple foods (rice grain), and vegetables (leafy and herb) posed potential non-carcinogenic and carcinogenic risks to the residents, especially through the ingestion pathway in the descending order of vegetables > rice grains > groundwater > soil.