Sakata lagoon, a sand-dammed dune lake fed by only spring water and precipitation, is located in Niigata City. The inflow water from the upper lagoon, Kami-sakata, and the outflow water to the general river, were collected biweekly 152 times over a period ranging from February 2011 to January 2017, respectively. The total arsenic (T-As) and other parameters (temperature, pH, ORP, main ionic components including HCO₃⁻, iron, and manganese) were measured. Based on the temporal variations, seasonal comparisons and factor analysis, the following conclusions were obtained: 1) The concentration of T-As increased in the warm season, and sometimes exceeded the environmental standard value of 10 μg L⁻¹. Approximately, 80 % of the T-As is present in the soluble form. 2) The ORP of outflow water decreased during the warm seasons to approximately 86 % of that during the cold seasons. Further, during warm seasons, the concentrations of NO₃⁻ and SO₄²⁻ significantly decreased, and the concentrations of HCO₃⁻ in both the inflow and outflow water remarkably increase during warm season. 3) The increase in the concentration of T-As during the warm season is considered to be caused by the reduced adsorption affinity of As onto the organic matter and/or Fe(III) hydroxide in the lake sediments. Biological activities in the sediments may induce the decomposition of organic matter and a decreased ORP; the increased concentration of HCO₃⁻ and pH occurred mainly in outflow water, which affords the following decreased adsorption affinity of As.