Passive Sampling Approach to Identify Contaminants in a Tropical Freshwater River System

Abstract

This study aimed to understand labile metal distribution and water quality associated with agro-industry and farming activities along the Namphong River, a sub catchment of the Mekong River located in NE Thailand. An integrated sampling program was designed to identify the range of potential contaminants in the Namphong River by incorporating active or grab sampling along with passive sampling using the diffuse gradients in thin films technique (DGTs) for the bio available heavy metal forms, field measurement of pH, electrical conductivity, temperature and dissolved oxygen and laboratory-based measurement of total solids, total alkalinity, hardness, nutrients and dissolved organic carbon concentration together with heavy metals (total and filtered (<0.45 µm) fractions). The DGTs were deployed at 10 different sites along approximately 50 km of the Namphong River for 4 days to enable sufficient integrative sampling of heavy metals. One liter of water samples were collected from the sites before and after deployment of the DGTs. Although total and filtered (<0.45 µm) concentrations of cadmium, chromium, copper, lead and zinc in the Namphong River from active sampling significantly exceeded the trigger values of water quality guidelines for protection of freshwater aquatic species, the concentration of metals estimated from DGT data indicated more accurately that the bio available metal levels were below the trigger values and that there were no predicted effects on aquatic species. The agro-industrial area (pulp and sugar mills) was a major potential source of contamination of the Namphong River associated with increased levels of heavy metals (aluminium, chromium, cobalt, iron and manganese) but diluted to insignificant levels in the Namphong River. The release of copper, nickel and lead to the downstream Namphong River occurred in the area of rice and mixed vegetable cultivation and was most likely related to extensive pesticide use. The outcome of this study will help to develop more focused monitoring of specific toxic heavy metals at particular locations on the Namphong River.