The Chianan coastal plain in southwestern Taiwan has long been well known for the prevalence of black-foot disease during the period 1961 to 1985. This disease resulted from drinking groundwater rich in arsenic and dissolved humic substances. However, the occurrence of arsenic and its controlling factors have not yet been studied in detail. Water samples from a total of 86 newly established monitoring wells and sediment samples from 28 fully cored boreholes were analyzed in this study to investigate major chemical [such as SO₄^2-, Cl^-, redox potential (Eh), pH, and ¹⁴C_DIC ages] and geological (such as depositional environment, grain size and arsenic content of sediments) factors which may control the distribution of arsenic in groundwater of the upper ~300-m thick strata. This study shows that arsenic concentrations of almost all groundwater samples exceed the 0.01 mg L^-1 limit of the WHO guidelines, with the highest values up to ~1.2 mg L^-1. Only the deep aquifers (> 50 m) deposited before Holocene transgression contain groundwaters with relatively high arsenic contents (> 0.1 mg L^-1). All the arsenic-rich groundwaters are under strongly reducing conditions with low Eh (< 110 mV) and low SO₄^2- (nearly all < 2 mg L^-1). The different combination of As, Fe, and SO₄^2- concentrations of groundwater reflect various redox potentials. The reductive dissolution of As-rich Fe-(hydr)oxides is believed to be the major source of As in the groundwater, but the contents of SO₄^2-, humic substances and residence time of water are also responsible for the variation of dissolved As in the Chianan coastal plain.