（新概念4）第38课：下句中的themselves指的是什么？做什么成分？

（新概念4）第38课：下句中的themselves指的是什么？做什么成分？
Contamination of water supplies is usually due to poor sanitation close to water sources, sewage disposal into the sources themselves, leakage of sewage into distribution systems or contamination with industrial or farm waste.

themselves指的是什么？是sources还是sewage disposal还是其他？做什么成分？相许分析一下这句中的themselves