Instead of having the plastic sheet down into a hole with a stone in the center, it is hung above the water source like a pyramid. The water runs down the plastic toward the outside edge of the plastic.

Use 2 kiddie pools. A 6' diameter one inside of an 8' one. Put the contaminated water in the 6'pool. Hang the plastic sheet over the 6' pool and drape the edge past the 6' pool but inside the 8'pool.

If this is set up at a slight angle, all of the pure water will flow to the low side of the larger pool.