Japan's water facilities are social infrastructure essential to people's lives. At the same time, however, the large investment in resources and energy has an impact on the environment. Moreover, these facilities are currently facing a difficult phase in which large-scale renewal is needed due to their advancing age, having been built largely during past times of high economic growth. In order to continue maintaining water pipelines as lifelines, one important issue to be considered is the transition to a system with as little environmental impact as possible. This paper, focusing on water pipeline systems made up of pipeline facilities and pump facilities, calculates the CO₂ emission amounts, from the standpoint of LCA (Life Cycle Assessment), in each of the processes of construction, operation, maintenance, and renewal (of facilities past their service life). Comparing transport by vehicle and by pipeline, the latter (pipeline) is shown to be superior for water transmission. The effects of population density, differences in pipeline layout, and pipe diameter on CO₂ emissions are further clarified from multiple aspects.