Complex sentence: In Windhoek, groundwater was almost the only source of water for until 1958 when a small surface reservoir, the Goreangab Dam, was built downstream from Windhoek and a conventional treatment plant was constructed to treat the surface water from this reservoir.
Split into simpler sentences: In Windhoek, groundwater was almost the only source of water for until 1958 when a small surface reservoir, the Goreangab Dam, was built downstream from Windhoek. A conventional water treatment plant was also constructed to treat the surface water from this reservoir.