Forgive me, but I saw you were from out of state and entered into the typical "be cautious" frame of mind once you said hike+Kelso.
That's a fair question on arsenic etc. So far as I know, however, there is no real compilation of data for arsenic concentrations in smaller waterways (could be wrong here, but I've never seen one). Myself, I generally am careful around areas that have been mined heavy and prefer sources that are snowmelt based (if available) but this is not necessarily skirting arsenic, admittedly.
As for actually coming in via Grizzly- here's a TR that comes in from there (it's a long read... a shitshow as well) with some pics etc. http://www.14ers.com/php14ers/tripreport.php?trip=10029&parmuser=born215&cpgm=tripmain&ski=Include
I was planning on climbing it this coming week, so if I am able to and am able to via Grizzly, I will. We'll see how the road is, as though Steven's isn't bad, Grizzly might be a bit worse this time of year. I'll get back to you next week, perhaps Sunday about what it's like. Obviously the conditions will not be snow covered when you do it, but at least it's something. I have a feeling you'll have lots of talus/scree though, since our snowpack was light and snowclimbs are in season a good bit early. As I said, coming from Steven's is much easier (you will have less vertical to gain and the trail is easy going up to the split). You have a bit of rock to deal with, but I don't think it'd be as much. You might consider camping at grizzly and starting from Steven's. It's not my personal stomping ground so I can't say for sure, but I'd venture to guess that there is more camping traffic at Steven's and/or early arrivals for climbs because it's the standard/easy route trailhead.