Stall Mat Quagmire

We usually try to leave the horses outside for most of the late Spring, Summer and early Fall (I live on the West Coast of BC so it rains pretty much rains non-stop 6 months of the year.) Over the past few days I have been doing a little summer cleaning in preparation for the coming fall/winter drudgery. I pulled up the stall mats, hauled them out, hosed them down and left them to dry in the sun. While the rational (and lazy) side of my brain argued that to clean them at all is an effort in futility, the other side found it enormously satisfying to make something so nasty nice and clean. We last stalled a horse in the stall pictured almost two months ago. You can see in the picture that despite the heat we've had, and despite leaving the stall empty, under the mats were still sticky nasty wet.

Ideally I would like to not stall at all but that isn't my reality... So what do you do to combat the nastiness under the mat?

Ps- I am dying to get my next hoof trimming post written but am having computer issues. It is coming soon! (writing on my iPhone takes for-ev-er!