The correct answer is Option 2: 2 and 3 only.

Heavy sand mining in riverbeds can have several negative consequences on the environment and local communities. The process of sand mining involves extracting sand and gravel from the riverbed, which can result in significant alterations to the river system and surrounding environment. The following are the possible consequences of heavy sand mining in riverbeds:

1. Decreased salinity in the river: Sand mining can cause a decrease in the salinity of the river. Salinity is the measure of the salt concentration in water, and it is an essential parameter for maintaining the ecological balance of the river. The removal of sand from the riverbed can alter the flow of water and cause a change in the salinity levels of the river.

However, this consequence is not a possible outcome of heavy sand mining in riverbeds alone. Salinity levels can also be affected by factors such as rainfall, drought, and agricultural practices. Hence, Option 1 is incorrect.

2. Pollution of groundwater: Heavy sand mining can lead to the pollution of groundwater. The removal of sand and gravel from the riverbed can cause the water table to drop, leading to the drying up of wells and other water sources. In addition, sand mining can also cause sedimentation, which can lead to the contamination of groundwater.

The pollution of groundwater can have significant impacts on human health and the environment. It can cause water-borne diseases, affect soil quality, and lead to the destruction of aquatic habitats. Hence, Option 2 is correct.

3. Lowering of the water-table: Heavy sand mining can cause a lowering of the water table. The water table is the level below the ground where the soil and rocks are saturated with water. The removal of sand from the riverbed can cause the water table to drop, leading to the drying up of wells and other water sources.

The lowering of the water table can have significant impacts on agriculture, as it can lead to a decrease in crop yields and even crop failure. It can also affect the availability of drinking water, leading to water scarcity. Hence, Option 2 is correct.

In conclusion, the possible consequences of heavy sand mining in riverbeds are the pollution of groundwater and the lowering of the water table. Therefore, the correct answer is Option 2: 2 and 3 only.