Chemical data obtained in 1983/84 and 1994 from groundwater monitor wells at Wairakei are reviewed and interpreted in conjunction with physical changes in the deeper system. The composition of the waters is controlled by the proportion of deep chloride, shallow meteoric and steam-heated components. The deep component dominates the central production area, a bicarbonate meteoric water is predominant in the peripheral areas, and steam-heated water is more evident in between, particularly near surface steam discharge and hot ground. The major change observed is a decrease of the deep component. Some wells show effects of evaporation.