What are the main contaminants to worry about, and can I test for them myself?  

The most reliable data we have comes from research carried out on similar major urban-wildfire incidents, like the Camp Fire in California. In previous studies of wildfires, the harmful substances detected in water systems typically belong to two groups: Volatile Organic Compounds (VOCs) and Semi-Volatile Organic Compounds (SVOCs). See this link for an in-depth list. Water quality laboratories using EPA approved analysis methods are the most trustworthy and best way to test for these contaminants. Standard home test kits do not work to detect these contaminants. Commercial labs on the mainland offer VOC and SVOC analytical services for $200-$750 per sample.