I discussed the things that I drag along with me for fieldwork here. That's my personal stuff - what about equipment for environmental investigations that an office should have?
I don't recommend buying everything instead of renting. Water quality instruments are expensive and finicky and require a level of upkeep that most small environmental firms or offices don't bother with. Submersible pumps are also incredibly finicky and have cords that you either need to engineer or just deal with long lengths of, and although I've jury-rigged them to work well enough, it's easier to just order what you need. Air quality instruments can go either way - they're equally expensive, but more durable and you tend to use them all the time. Examples of what I've used regularly are a photoionization detector (PID) for general VOC detection, or 4-gas (O2, CO, CH4, H2S) for doing confined-space entry or if you have to run heavy machinery inside.
So, equipment that doesn't require a lot of upkeep and is extremely handy for environmental work:
1. Water level meter (fancy option - interface probe) - you end up using this for one thing or another on practically any field job. And as long as you don't wedge the sensor in the well (obviously a problem for rentals as well), tie the cable into knots, or drive into it, it's pretty much indestructible.
2. Peristaltic pump for groundwater sampling - rugged, relatively cheap, hard to screw up. Some jurisdictions allow you to sample anything with it, sometimes you can only use it for non-volatile sampling (metals). Also handy for field filtering samples.
3. Metal detector for finding flush-mount wells under 2 feet of snow or 1 inch of dirt.
4. Rod and transit. Can be really simple (optical surveying) or a laser level, depending on the nature of the work and the accuracy you need. Extremely handy on small sites where you don't have a good base map and you need to figure out the relative elevations of various features and you don't have the need/budget for a professional survey. Also handy for figuring out stream channel contours to determine stream flow. I have many a fond memory of hanging the rod over or cowering under spider-infested bridges and fighting to plant it in a creek without snagging it on a tree branch.
5. Hand auger with several flights for soil sampling.
6. Hammer drill for going through concrete if you're doing a lot of indoor/basement sampling.
7. Miscellaneous hardware: crowbar, pry bar, large bolt cutters, medium and large pipe wrenches, cordless drill with attachments for sockets as well as drill bits, machete for clearing small brush, hand saw that is sharp enough to actually be useful (same thing for the machete), square shovels, spades, pickaxe, long-handled sledgehammer, snow shovels if appropriate.
8. A huge quantity of keyed-alike locks so that every single monitoring well, gate, and chain for large equipment can be opened by anybody from the firm and you don't end up with a fistful of keys for every blessed site.
9. Stuff to move stuff. Wheelbarrow, drum dolly, hand carts. If in a cold climate, plastic sleds for dragging equipment around.
10. Other heavy, durable equipment based on what the office does regularly. Sediment sampling? Various dredges/samplers and a metal boat on a trailer tucked in the back of the parking lot. Surface water stuff? Flowmeters and auto-sampling devices. Air? Weather station and ambient air sample/filter boxes.
... anything else I missed?