Studies On Water Quality And Ecological Issues In The Lower Mekong River

Water quality deteriorating in the lower Mekong River basin has beenidentified as a priority ecological issue by each of the four member countries.Three particular water quality issues are considered in this paper, i. e. thepotential effects of municipal and industrial wastewater from Phnom Penh; thepotential effects of municipal and industrial wastewater from Vientiane; theinfluence of the degraded water quality in the Mekong Delta.Risk assessment is a general term used to describe an array ofmethodologies and techniques concerned with estimating the likelihood andconsequences of undesired events. Risk is often defined as the product of theprobability or likelihood of a hazard and the consequence if that hazard occurs.There are now a number of initiatives aimed at further developing the ecologicalrisk assessment technique to provide a framework for considering a widernumber of interacting stressors (e. g. nutrients, environmental flows, habitat,sediments, exotic species) within a catchment or river basin context. Thus, theecological risk assessment process is an attractive tool that can assist inassessing the impacts of multiple stressors on complex ecosystems, the situationthat exists within the Lower Mekong River system. A risk assessmentframework has been used to assess these issues. In particular, the risk of adverseeffects on three key values of the Mekong River-ecosystem health(characterized by eutrophication, toxicity due to dissolved oxygen and toxicants,and ecosystem processes), fish migration and human health (drinking, recreation)have been assessed. Unfortunately, the data currently available is not enough to fully assess therisk of water quality issues in the Lower Mekong River. The aim of this paper isto review existing data and information to determine if real or potentialdownstream and ecological impacts exist from effluent discharges in the urbanareas of Phnom Penh and Vientiane, and to assess the effects in the MekongDelta caused by transfers of poor quality water and seawater intrusion;recommend specific studies or actions that should be carried out to make adefinitive assessment. If the present assessment remains inconclusive due to lackof sufficient data or information; advise the Mekong River Commission on stepsthat could be taken to monitor real or potential ecological issues arising fromthis analysis.A summary of the preliminary risk assessment for the three water qualityissues is given in the paper. The present risks are low in all cases where theycould be assessed. While the assessment indicates that the ecological risks dueto water quality are low, this is not the case for local effects. For Phnom Penh inparticular, the preliminary assessment suggests that there are moderate to highrisks of adverse ecological and human health effects in Tonle Sap River and theupper reaches of the Bassac River due to high COD and low DO. The relativelyhigh values of the COD in the two sites close to Phnom Penh reflect organicpollution from Phnom Penh, and the low DO concentrations reflect the input ofstormwater and sewage from Phnom Penh.