As for introducing contaminates,there's a definate possiblilty that it could occur.Personally,I collect NSW from the Atlantic when i can.I dont filter it,let it settle,etc..just straight from the ocean to my tank within 2 hrs.I've had nothing but positive experiences with it.However,your talking about collecting sand,beach sand
(at least here)is not a pure aragonite sand and is full of silicates.The only place i would consider collecting sand from in the wild is near a reef.