If it makes you feel better, the original version of Environmental (for Binding, B40) read "from -20% to -40%". It's really a scale, not discrete categories with hard boundaries. As with all modifiers, the game you have in mind affects the probability of being in particular environments ("underground" isn't unlikely in a dungeon crawl game; "vacuum" isn't all that rare in an SF game).
I'd be happy with -30% for this one.
You probably want to come to an agreement between GM and player before the game starts on what amount generally counts as "a sufficient quantity" of sand. Frex, things you might find in a city:
- Child's sandbox / landscaping
- Stream/river bed
- (Rail)road bed (sand-clay, ballast, often underneath or mixed with other stuff)
- Artificial turf football field (often has several inches of sand underneath)
- Home improvement store
- Construction sites
- Pond / lake bed
- Concrete plant
- Glass factory
- Beach
RAW examples for comparison:
-5%:
Air
In a gravity field
On a planet
-10%:
Contact with dust
In the presence of microbes
Light
Pebbles or equivalent (TK Bullet)
Inhabited areas
Mechnically coupled to the ground
-20%:
Must be touching ground
Touching the ground (and not flying or standing on an artificial surface)
Requires vegetation or soft ground nearby
Trees or tall buildings (for Brachiator)
Attack shapes existing earth
Strong, direct light
Turns the ground to goo (Binding)
In a city
In the wilderness
Outdoors
-40%:
Swamp
In the desert
Furniture, tree limbs, etc
Dense vegetation
Light sources or electricity energetic enough to do damage
Violent storm in area
Underground
-80%:
In quicksand
In lava
In vaccuum