In water supply facilities such as distribution reservoirs, the inner surface of reinforced concrete (RC) tanks is often coated with resin paint or the concrete cover depth is increased to protect the concrete from surface deterioration. However, there is little published research on carbonation – one of the main causes of concrete deterioration – in relation to RC tanks, leaving room for improvement with regard to methods for preventing surface deterioration. This study discusses the impact of surface coating and environmental conditions on the carbonation rate of concrete based on a survey of the present state of RC tanks of the Sapporo Waterworks Bureau. Coating was found to have an inhibitory effect on carbonation. Because the carbonation rate of concrete used for the inside and outside of RC tanks was slow, the sufficient concrete cover depth to protect rebar from carbonation while in service was found to be 75 mm for RC tanks without coating.