Complex sentence: Coupled with the abundant local rainfall, these pits tend to fill very fast and overflow into the nearest streams, which easily get contaminated with faecal matter, as they are often quite small and there is not much dilution with clean river water.
Split into simpler sentences: Coupled with the abundant local rainfall, these pits tend to fill very fast and overflow into the nearest streams. The streams get easily contaminated with fecal matter, as they are often quite small and there is not much dilution with clean river water.