Question: How can I identify where plastic manufacturing or processing industries and companies are located in my (1) region, (2) state, and/or (3) at county levels?

It looks like EPA's Enviromapper tool doesn't allow you to specify for plastics, but it does list around 400 manufacturing sites in the Tacoma area, including a few dozen in the Port of Tacoma. That might be a good place to start!

I'm learning a little about the EPA's National Emissions Inventory dashboard, which can show emissions point sources by industry. The interactive report from 2014 is here: https://gispub.epa.gov/neireport/2014/. And the dashboard for the report is here: https://edap.epa.gov/public/extensions/nei_report_2014/dashboard.html#point-db.

This screenshot from the dashboard is for several pollutants reported for plastic, resin, synthetic fiber, or rubber products plants.

The map doesn't show much for WA, but maybe you can cross-reference with @kgradow1's suggestion to start with EnviroMapper?

LDEQ doesn't know the answer to this question on the Lower Mississippi River. here are the ways we are searching for the answer, in Louisiana, in order to help them.

Keep in mind, there are many transportation-related plastic sources, but until plastic is considered pollution per se by the United States, there's inconsistent requirements to report plastic spills.

1) Texas' methods? Texas keeps track of this, see the new plastics rulemaking. Maybe they use the HW-1 NAICS codes? But LDEQ doesn't do this, the HW-1 for TCI Plastics doesn't include Plastic!

2) EIA data for "Ethylene Crackers" 5 facilities

3) we could make a public records request to LDEQ for any air permits along the MS River which mention "pellet" or some other code for "pellet handlers", which would be a pain unless we are sure about the code. --returned 7 plastic facilities that handle pellets, and make tons of PM 2.5 while handling them. https://www.google.com/maps/d/u/1/edit?hl=en&mid=1b7P418KaK7xY-DJutbiJTow-xXWzGI71&ll=30.264748001139765%2C-90.84402800000001&z=10

4) we could make a PRR LDEQ for water permits on the lower river that mention "pellet" LDEQ doesn't list pellets in water permits By the way, EPA has a GIS layer for NPDES_MAJOR which includes many facilities https://erma.noaa.gov/gulfofmexico/#/layers=3+408&x=-90.89512&y=30.05808&z=10.5&panel=legend This is the list LDEQ gives you when you ask them this question. inane.

5) We could search for PVC, Ethylene, Polystyrene, and polypropylene in the Chem Plants .pdfs => 40 plants https://www.google.com/maps/d/u/1/edit?mid=1PS_gydKyioQOMsJ-59XKyOtzfT6wWFZ2&usp=sharing

Here's the link to NAICS codes for Plastic. I would make a State Records Request to your state EPA for all facilities coded 3261. Unless your state is extremely dumb, like mine, the HW-1 (Hazardous Waste form #1) NAICS codes are among the first things that the facility sends the agency before getting a permit.

Using @eustatic's tip on the NAICS code for plastic, this map and facilities list comes up for Pierce County on the EPA ICIS-AIR search (screenshot highlighting one example of a facility currently in operation).