Last weekend I bought several red mulch bags after a massive rain storm. I thought it would be okay but was surprised with all the water that came out of the bags before I reached my house. The issue was that the water found its way to every corner inside my car. It worried me to find water even in the 12V battery compartment. It took me a long time to dry the inside. I got most of it with towels but at least was glad to find a drain plug under my spare tire. This was the area that accumulated the most.
A cargo area tray might have deterred some of that. The dead giveaway on mulch is the bag weight. Soggy, they will weigh as much as three times what a dry bag should weigh.
