Plan on no. On most public land where you would boondock, dumping a grey tank on the ground is prohibited, and everywhere else it is a bad idea that gets camping areas closed. Empty the tank at a dump station.
Here is what the actual rules say, verified July 17, 2026.
National Forest land: haul it out
Forest Service sanitation regulations at 36 CFR 261.11 prohibit “failing to dispose of all garbage, including any paper, can, bottle, sewage, waste water or material, or rubbish either by removal from the site or area, or by depositing it into receptacles or at places provided for such purposes.” Waste water is named explicitly. Your two legal options on National Forest land are to carry it out or put it in a facility provided for it, and a patch of dirt behind your rig is neither. The same section separately prohibits placing anything in or near a stream or lake that could pollute it.
BLM land: read the local rule, and the sign
BLM is messier. The bureau’s baseline camping regulations, as summarized on BLM Colorado’s camping and occupancy fact sheet, prohibit “draining sewage or petroleum products or dumping refuse or waste other than wash water from a trailer or other vehicle.” Yes, that wording carves out wash water. Do not build your plan on it. Individual BLM sites close that gap with their own posted rules: the supplementary rules for Hot Well Dunes in Arizona, for example, state flatly that “dumping sewage or gray water is not allowed,” and BLM’s Long-Term Visitor Areas require rigs to be fully self-contained. State water laws apply on top of all of it, and they vary. We are not going to give you a state-by-state table of where the carve-out might survive, because the answer changes site by site and the posted rule at the entrance beats anything on this website.
Why the rule exists
Grey water is not just soapy water. It carries food particles, grease, detergent, and whatever was on your dishes, and it does two reliable things on the ground: attracts animals to campsites and fouls the places people camp most. Heavily used dispersed areas get closed over exactly this kind of accumulated damage. The tank-to-dump-station norm is what keeps free camping open.
The part that makes this easy
Dump stations are cheap and everywhere. Developed campgrounds, truck stops, many fuel stations, and some municipal facilities have them, and the cost ranges from free to a few dollars, less than you will spend on lunch that day. Apps and the campground listings you already use flag them. A 30-gallon grey tank lasts most careful couples several days of boondocking; time your dump for the drive between camps and it costs you ten minutes.
If your rig has a sink that drains straight onto the ground instead of into a tank, fix that before boondocking on public land: a portable grey water tote is the standard workaround.
New to camping on public land? Start with what boondocking actually is, then find where it is legal near you with our state-by-state free camping pages. And whatever this page says, the land manager’s current rule and the posted sign win.