The correct option of "2 and 3 only" highlights two significant environmental consequences of heavy sand mining in river beds. Firstly, excessive sand extraction can lead to pollution of groundwater, as the disrupted riverbed can allow contaminants to seep into the aquifers, compromising water quality. Secondly, sand mining lowers the water table because it diminishes the natural filtration and retention capacity of the river system, leading to reduced groundwater recharge. However, decreased salinity is not a direct consequence of sand mining; instead, it is influenced by other factors like freshwater inflow and tidal dynamics, making options 2 and 3 the only relevant consequences in this context.