Question

When aquifer sediment contains dead organic matter, which of the following can be a major groundwater pollutant?
(Points : 3)

DDT


mercury


toxic lead


natural arsenic

Answer 1

My first guess was going to be mercury, but then that sounded wrong. This is what I found on wikipedia without a citation, so take it with a grain of salt.

Another example of widespread groundwater pollution is in the Ganges Plain of northern India and Bangladesh where severe contamination of groundwater by naturally occurring arsenic affects 25% of water wells in the shallower of two regional aquifers. The pollution occurs because aquifer sediments contain organic matter (dead plant material) that generates anaerobic (an environment without oxygen) conditions in the aquifer. These conditions result in the microbial dissolution of iron oxides in the sediment and thus the release of the arsenic, normally strongly bound to iron oxides, into the water. As a consequence, arsenic-rich groundwater is often iron-rich, although secondary processes often obscure the association of dissolved arsenic and dissolved iron.[citation needed]

From this, the answer would be arsenic. I know that the source of this knowledge is questionable, especially without a source, but maybe it'll help stir some memories.