What could cause powder residues to be washed away after a shot?

The presence of excessive blood at a crime scene can indeed affect powder residues that may be left after a gunshot. Blood can absorb or wash away these residues, leading to a decrease in the amount of gunshot residue that can be collected and analyzed later. When gunfire occurs, the discharge creates microscopic particles that settle in the vicinity, including on surfaces and clothing. If there is significant blood present, it could physically interact with these residues, either through displacement or dilution, ultimately diminishing their detectability.

In contrast, rainfall can immediately wash away residues from various surfaces, while immediate law enforcement interaction might disrupt evidence but does not directly result in the washing away of powders. Non-porous materials would tend to preserve residues rather than wash them away, making the connection to blood an essential factor in understanding how various elements of a crime scene can impact the forensic evidence.