Robbo- you obviously have some experience here. Prepared an SWPPP or two?
While I generally agree with your premise I believe some things jump out immediatly.
The number of rain events should have little effect on total TSSL accumulation but rather higher ppm counts in spot samples during dryer weather and vise-versa.

-I should have added the "on average" portion of the study, my bad. Your absolutely right, though, in that there will be higher/lower counts per storm event. But, in the end, you have X lbs of product.

Rain in and of itself should not create TSSL. If the rain did have an effect it would indicate that the rain is carrying airborne solids into the equation that may or may not be attributable to vehicles. I suppose it could be suspended solids from tailpipe emissions but could just as easily be dirt stirred up from the travel or winds.

-These particles accumulate on the roadway and are subsequently washed into the storm system during a rain event. This study was done on the Westside where dust storms seldom occur and airborned particulates would have little effect on it. I don't believe I ever mentioned that the fact that rain in and of itself creates TSSL, it merely acts as a medium for it.

Much of the TSSL count could also be attributed to ambient or atmospheric generation could it not. (much like dust accumulates in our homes)
While copper does have health risks I am not aware of any associated with magnesium and Zinc is harmless unless you get hit with a large chunk of it.

-The MOTCA standard for Zinc is 5 ppm, and yes, it is toxic. Most of it comes from the oxidation process that occurs from UV rays hitting the tires of our vehicles. Seems inocuous, but it isn't.

As I am sure you know, homeowners with miracle grow are a much larger contributor of copper to the environment than all industry combined. No felony charges for feeding your petunias. I am not aware of the use of toluene, xylene or benzene in the auto industry outside of non-street legal racing gasoline and some fuel additives (aftermarket) I have not however looked at gas formulations lately.

-Benzene, Toluene, Ethylbenzene, and Xylene (BTEX) are still widely used in the petroleum industry, especially in gasoline. BTEX sampling is one of the main tracking methods for meeting MOTCA cleanup standards in petroleum product spills.

There are LOTS of filtering systems that will remove contaminants from stormwater to much better than 99.99 percent purity and RO or nanofiltration can also remove the disolved metals well beyond motca levels.

-Show me a STORMWATER treatment system that can effectively do this at a reasonable cost, and I will show you a costomer willing to buy it, maybe more.

Ro and nano are slow now but the technology is moving quickly. I am currently involved with an oil absorbent firm that is far and away the best I've ever seen. Oil on water is absolutely no trouble. It floats without aid and makes a pretty good hog fuel. Currently testing on some military bases and certain private industries. The only sorbent that will actually meet and exceed ALL astm requirements. The best part, 100% post consumer recycled content. We get paid to haul it away.

-What is the name of your firm?