Complex sentence: Rio's government has been promising a cleanup for years, but nothing has been done thus far, so the poorest inhabitants of the region are still exposed to unacceptable water, in spite of complaining about the situation.
Split into simpler sentences: Rio's government has been promising a cleanup for years, but very little has been done thus far. So, the region's inhabitants, particularly the poorest, are still exposed to unacceptable water, in spite of complaining about the situation.