Could the brackish water in the colonist's wells be killing them? “A lot of the fresh water was dried up by the 17th century.” says document A. So the colonists could have been dying from dehydration because there was no fresh water to drink. The colonists also dug wells but they “were vulnerable to drought and saltwater intrusion.” So that could also have killed a lot of the colonists.
In document B it shows that Jamestown had a lot of droughts. Which probably was the reason for the wells drying up. A lot of people have been dying in Jamestown.