Consider the following statements regarding fluoride in groundwater:

1. Fluoride which is naturally occurring in groundwater has a national drinking water limit of 1.50 milligrams/litre.

2. Excessive fluoride can cause skeletal fluorosis, accelerate dental decay, and pose health risks, particularly for children.

3. Fluoride contamination is highest during the dry, summer pre-monsoon months.

4. The highest fluorine contamination in groundwater is found in Bihar.

How many of the above statements are correct?