How to translate "leach" (as in "toxic chemicals leach into ground water")?
If you look at what Chinese website say about fracking, you can see for example:

PetroChina: 　　


  报告认为，水力压裂可能出现的危险在于在压裂过程中可能出现液体溢出，压裂直接渗透到地下水资源中以及对废水的不当处理和排放等方面。


So CNPC uses 渗透 to say that the chemicals get into the ground water.

As for your other example regarding PVC:


  　　生物污染主要是由于管材氧渗透和透光使管内的水介质滋生细菌；化学性污染主要来自于管道接触材料的渗透。


Again they use 渗透 to indicate how the chemicals/bacterial get into the water.

Note: As the comments note, some other ways you  might say this could include 渗入 and 进入.