I have assessed and remediated many ready mix plants. Ready-mix plants often use petroleum-based form-release compounds (they make blocks out of unused concrete). They also have boilers with fuel USTs. Many older plants have or had gas/diesel pumps. There are often repair shops with hoists, service pits, and waste oil ASTS. Generally, mix plants aren't too contaminated, but you never know

If its an old plant, look at old air photos. Check for waste ponds and large whitish areas. They often washed out trucks and dumped the slurry onto the ground. This can be a high-PH material that does not harden. I had a plant site that had a whole gully filled with slurry back in the 60s and 70s. Groundwater migrating off-site had high pH. -Rob Roberts

Do they service their trucks and equipment on site?  Fuels? 

If they have on-site rock crushing, their crushing equipment is no doubt serviced/lubricated where it stands, and staining or other signs of contamination are likely to be obscured by dust.  Same is probably also true of whatever conveyor system they use to move cement and aggregate into the batch plant for mixing.  There's likely a fair quantity of grease and oil stored somewhere on site, and I'd just about bet that their housekeeping isn't great.

Some of the concrete admixtures may be regulated.  They may have large containers of them piped into the batch plant from somewhere, most likely pretty nearby.

Somewhere they will have a washdown area where the returning concrete trucks rinse and discharge whatever residue is left in the drum.  Could be wastewater discharge issues there, possibly issues with soil alkalinity, if that's regulated where you are. 

Acid washing has the issues others mentioned above.  I'd suspect it's muriatic (hydrochloric) acid, and could create soil acidity or wastewater discharge issues, depending on how they are using it, and also the obvious storage/safety concerns.

As someone noted above, the surface impoundments have all the usual concerns.

But one big one often overlooked....check the clinker dust that usually covers everything... Hex Chrome is a common contaminant.

And don't forget to look off site while you're at it.  The stuff tends to blow all over the neighbor's houses, yards, pets and kids.

If you note the latter, look for lawyers lurking in the alleys.

Something that I haven't seen mentioned here that I encountered at a concrete plant in Houston was fly ash.  Fly ash is the ash produced during combustion of coal. It was used as a replacement of Portland cement and as a mineral filler at the plant I was inspecting.  The theory was that it added to the chemical resistance and durability of the concrete.  It is also thought to reduce the greenhouse gas "foot-print" of concrete as the production of one ton of Portland cement produces approximately one ton of CO2 as compared to zero CO2 being produced using existing fly ash.  Huge amounts were stored in big piles on the soil at the plant I inspected.  Since coal contains trace levels of arsenic, barium, beryllium, boron, cadmium, chromium, thallium, selenium, molybdenum and mercury, its ash will continue to contain these traces and therefore cannot be dumped or stored where rainwater can leach the metals and move them to the aquifer.  A Phase II on this Project showed that not only were there high levels of contamination in the groundwater at the Project, but also the adjacent bayou.  It was a serious problem.