Groundwater is indispensable water resource in coastal areas of Taiwan and is typically used following simple disinfection. Disinfection by-products (DBP), which are hazardous materials that are biologically toxic, are commonly produced. To elucidate the effect of environmental factors on the formulation of DBPs and arsenic species, and the effect of these factors on the bio-toxicity, data from a one-year monitoring program that was performed in a coastal area of central Taiwan were analyzed using the multivariate statistical method of redundancy analysis (RDA). The-results reveal that the dominant DBP for trihalomethanes (THMs) was CHCl3 and for haloacetic acids (HAAs) was CHClBr2COOH (BDCAA). The formation of these compounds was most affected by the concentrations of humic substances and Br-. As5+ ions are abundant in the area close to the seashore and are the main source of biological toxicity. Cl-, Br- and As5+ concentrations were strongly correlated with biological toxicity as they promoted the formation of DBP. A geographic information system (GIS) and the above results revealed that the area near the seashore is rich in Br- wherever high As5+ concentration and bio-toxicity are detected. (C) 2014 Elsevier B.V. All rights reserved.