I love my pool and I take great pride in maintaining it but recently I’ve come across a contaminant that flummoxed me for days until I finally figured it out and when my wife heard the solution to this mystery, she vowed to not get in the pool for the rest of the season.

It started with this black sludge looking substance that settled at the bottom of the pool.

I have a robot that cleans the pool every day and does a fantastic job but the sludge it picked up was not like anything I had seen before (dirt, leaves, bugs, etc.).  Emptying the bag filter of the robot was disgusting and every day there’d be a fresh deposit of sludge at the bottom despite not having any storm/rain event (which normally deposits dirt in the pool due to runoff).  Where is this shit coming from?!?!

Then I noticed that the large oak tree by the pool that overhangs the pool had a large amount of some brown substance that it was dropping on the concrete. 

After further inspection these items were actually small pellet shaped objects.  Some were smashed and turned into powder but here is a pic of the pellet and my pinky finger is included in the pic for scale.

And after further observation of these pellets, these pellets turned into this sludge after they hit the water and settled to the bottom of the pool.  But what were these pellets?  Oak trees drop ‘aborted’ acorns in the summer but tiny acorns are very clearly not pellet shaped and you can see one of these aborted acorns beside my hand on the left of the image above.   

These pellets appear not to originate from the tree…..

Then I looked up in the tree and saw many of the leaves appeared to be eaten and I also remember that many black and yellow caterpillars have been also falling on my pool deck and pool around this same time. 

Well….after further research I discovered that the mystery pellets are really caterpillar turds!  These caterpillars had infested the tree, had a great meal on the leaves and then dropped their shit below!  Pic below of one of the critters toward the end of his life with a caterpillar turd circled in red. 

That’s right, I’ve been swimming in caterpillar poop for a couple weeks and I didn’t even know it.  

No worries…That’s what chlorine is for.