Groundwater pollution: a) comes only from non-point pollution sources, like fertilizer runoff from farmland. b) is readily detectable because the presence of pollutants is obvious from the water's taste or smell. c) is very difficult to clean up once it has occurred. d) all of the choices are correct.

It is difficult to clean up once it occurred.

Explanation:

Groundwater is a underground water reservoir. The water enters under the surface of earth crust through rain, melting snow, river, ocean, agricultural irrigation or any other water source.

The groundwater can be contaminated by both non-point source pollutants and point source pollutants.

The groundwater can be a large volume source of freshwater. If it get contaminated it cannot be clean up as it will contain many toxic chemicals which come from different sources. Making the water as wastewater, which cannot be reclamed and use further.