The first step to completing A Toxic Trail in Arc Raiders is to head back to the drain where you previously found the barrel for the Source of the Contamination quest. This barrel can be found outside the Water Treatment Center on the Dam Battlegrounds map by approaching from the north via the swamp. Check the map below for the exact spot.
Doherty's story began when he noticed a rise in bizarre medical conditions among the county's 45,000 residents, linked to toxins in the local water. Working with the county health office, the rancher-turned-official began a survey of 70 wells throughout his jurisdiction - 68 of which, his testing found, violated the federal limit for nitrates in drinking water. Of the first 30 homes he visited, Doherty told RS that 25 residents had recently had miscarriages, while six had lost a kidney.