Spring Well Cover

I have a gravity fed well coming into my house from a small pool of spring water (approximately 6 feet by 4 feet) coming off of a stream on the mountain behind my house. Until recently, the pool of water was covered by some old timber and pieces of corrugated metal roof. Some time over the last year and a half the structure collapsed, causing my water to smell like sulfur. Any suggestions on what I can use / build to cover the pool of water. We have a lot of snow in the winter, so it needs to withhold some weight. I am thinking of use polycarbonate roof panels, held up by pressure treated 2 x 4s.
Any help would be much appreciated.

Pressure treated boards contain poisons that I wouldn't want in my water supply. Perhaps redwood would hold up to the environment?

I would use an opaque covering so that algae doesn't get the light necessary to grow in the water.

Perhaps use this as an opportunity to build a spring-box, and cover it with a slab of stone...

I agree with Joseph. To me, pressure treated timber is nothing but toxic gick and there are better/safer methods of protecting your spring point than that, specifically the method that Joseph offered up, but others as well. I would definitely want to keep the light off of my spring point as well, not just algal growth but all kinds of microbial critters love light and a pool of still water.

Thanks for the input! Didn't really think about the chemical treatment of the wood...I will definitely not be using that. I was planning on going for solid color sheet so that bacteria won't grow

Would a spring house be useful to you? Perhaps this could be an opportunity to build a concrete block foundation for one. Put on your cover.
As funds or time allows, build a timber or block structure high enough to enter. Think, just enough room for some shelves and standing room.

https://en.wikipedia.org/wiki/Spring_house