Ah, thanks for the reminder....
For this lake site, it's an interesting story I'll write about for future issues of Pond Boss, might even make a video about this project because there are so many elements to it.
Here's what we learned. As this 56 acre lake bed was excavated to build the dam, the earthmover came across a black dirt vein about 25 feet below natural ground level. He liked how it felt and moved with his dozer, so the team (landowner, contractor, executive assistant & engineer) had the soils tested for plasticity. Ended up being the most compactible soil any of them had ever seen. So, they excavated it and began to use it for the core of the dam for the lake. At the same time, the landowner wanted a smaller pond, which the contractor built. Keep in mind, that on any given day for this project there have been 6 scrapers, 2 bulldozers, an excavator, two water wagons,a sheepsfoot roller and a maintainer going all day, every day...so they can get lots of work done, fast.
When the new, small pond was finished, they lined it with about a foot of this fine "clay" and packed it with the sheepsfoot roller. Turned on the wells and about two weeks later, the pond was full. I put a few fish in it and they were dead the next day. I just thought we had mishandled the fish. As we tested further, and began to discover how low the pH was, we saw it slightly below 3.0. But, the pH wasn't actually accurately measured. The lowest the labs could go was 2.11. We think it was actually lower than that.
Here's what happened. That thick vein of black dirt was highly organic. It was full of sulfur and when contacted with water, the water reacted with the soil. We are pretty sure the Oxygen molecules in the water reacted with iron in this soil and oxidized it under water and the Hydrogen ions reacted with volatile sulfur in the soils to create sulfuric acid. There was no smell to the water at all. But, after the fact, we remembered a couple of contractors who were building a dock speak of their legs "burning and itching" when they were in the water.
We sent some of the soil to several labs for testing and the most confident of the engineers made educated guesses as what to do. They weren't totally sure because they couldn't determine the actual pH.
So, here's what we did.
By the time these tests were being run, we were building another pond and lining it with this "clay". After the clay was added in lifts, we added three tons of aglime per surface acre, tilled it in, compacted it and then added 12 inches of additional "neutral" soils on top of that.
This image shows the stuff being used to line a pond bottom.
This photo shows where the earthmovers stockpiled the black stuff to use later.
Well, back to the first pond. We amended it with aglime to raise the pH. That pond covers about four acres and it took almost 15 tons of lime to raise the pH to 7.0. By that time. the contractors had lined the basin of the 56 acre lake with the black clay, added aglime and then added another 12-18" of "normal" soils on top of that black stuff. We started pumping the water from the smaller pond into the big lake, once we knew the water was safe. It took about three weeks to get all the water out of that pond. The entire periphery and the bottom of that thing was orange, the color of oxidized iron, almost a rusty color. Then, the contractor moved all the existing black clay out of the pond bottom, replaced it with "new" black clay, limed it and then covered it up with nearby soils. The pond is now filling with well water.
The whole process has been expensive, but the landowner saw the necessity.
If it wasn't for losing a few nice fish, we might have figured out there was a problem...way too late. I can't imagine a 56 acre lake, with that black soil coating the bottom of it, filling with water and THEN figuring out there was a problem.
