When two oceanic plates collide, the younger of the two plates (because it is less dense)* will ride over the edge of the older plate. *(Oceanic plates grow more dense as they cool or move further away from the Mid-Ocean Ridge). The older, heavier plate bends and plunges steeply through the athenosphere, and descending into the earth, ir forms a trench that can be as much as 70 miles wide, more than a thousands miles long, and several miles deep.
Fertilizers and chemicals are important contaminants of water and enter ground both directly and indirectly. In the case of Karst landscapes, the contaminated water infiltrates into the groundwater through underground streams or percolation.
Oil and gas spills also pose a serious problem for groundwater contamination as they can pollute aquifers near fracking zones and other oil extraction areas.
Animal feces are more of a problem in the modern age due to large-scale commercialized animal farms. Unless the waste from these facilities is controlled, it can runoff into fields and rivers and eventually seep into groundwater reserves.
