The dorsum of the propodeum is really hard to see in most wasps with their wings at rest, the only way to get a shot of it is when the wasp has its wings spread (or catching, chilling, and manually doing it). I can see a bit of it here, but not the top of it.

I found a good dorsal shot of a Chalybion propodeum here. I think the only way to get a sure ID from just the wasp is if you get a shot of it with open wings (which is difficult with these) or you could catch a specimen and chill it in the fridge and get a macro shot of it.

There are some other ways to get an ID though. If you see it carrying a host, Chalybion and Chlorion use different hosts. Chalybion uses spiders and Chlorion uses field crickets (Gryllus sp.).

Chalybion also provisions its hosts in mud nests, which are usually old Sceliphron nests. Chalybion take water from the ponds and not mud. They only rework the existing mud nest with water and give the old Sceliphron nest a rough texture. The frequent trips to the pond are making me think Chalybion, but it's not enough for an ID.

Chlorion are ground nesters and burrow rather than utilize a mud nest.

Hopefully these alternate identification methods help some! If you catch the wasp in any of these acts, it would give a definite genus ID (this does look like the same species of wasp in all of the photographs).