Hi Pheonix 16,
Let's see if I can help, firstly the saw dust particals may be a bit out of scale for 1/48. Try plaster of paris. That's what I use for snow. It cures very quickly. To simulate melting snow I mix the plaster with water till it is like thick milk. With a small cup I pour little dribbles here and there and along the side of roads or tarmac. Then with a hand pump I spray water on surfaces that I want snow and sift dry plaster over these areas. I continue adding water and plaster until I'm satisfied with the effect.
Your airfield is situated in mountanous terrain so your base would therefore be mostly sand and rock. For 1/48 use the finest sand you can get (dried mud)and the smallest stones(coarse sand) for your terrain and secure them down with lots of diluted white glue using a pipette.
The sand paper for the tarmac is do-able. Cut it in scale sized squares for concrete slabs. Glue them down properly closely at the joints and make sure there are no dog ears. When dry score the joints with the back of a hobby knife. After painting and dry brushing the surface of the tarmac, add a pin wash of black in between the joints.
Oil spills on the tarmac can be done with the thinned wash and splattered randomly.
Hope this helps.
Cheers,
Richard