Sand like residue on bottom of IG pool, not on walls

[RobQ]

Hi All,

I have an inground pool of 22,000 gallons, Vinyl liner (replaced last year), sand filter and 1.5 HP pump.

Just tested my water wit the Taylor K2006 test kit (thanks to TFP for the info)
PH 7.7
TA 150
CH 200
FC 0.5
CC 0
CYA less than 30

I have a sand like residue on my liner (bottom and sloped surfaces only) , which I can brush off with some severe scrubbing. Easy enough to do in the shallow end, but not in the 9ft deep end
The water is crystal clear and has been for 2 weeks.

Does anyone have any idea what the residue could be, it's brownish and looks like sand.

Any help would be appreciated.

Rob

 

[Texas Splash]

Hello Rob and welcome to TFP! Your situation requires some more intel. Safe to assume this pool was recently opened from winter hibernation. Your FC is low which means algae "might"have tried to settle even though your water may still be cool (unless you use a heater). Organnics could result in a stain. Your CH isn't high, but if the pH got exceptionally high over the winter, that, along with the elevated TA migth have created a bit of scale. It doesn't make sense that it would be sand since you mentioned having to really work at it quite hard. Sand would just brush off. Not sure if you are in the pool yet. If you are able to grab a sample, you could see if it smears at all (I doubt it though) or if it fizzles with some acid poured on it (calcium). But if you can't get in there yet, you might try lowering the pH a bit more to around 7.4 or so and allow the TA to fall more as well. A slightly lower "CSI" (see PoolMath) along with brushing may help. Without a doubt, increase the FC right away. The CYA should be at least a firm 30, maybe even 40 for summer. Always balance the FC to the CYA as noted on the FC/CYA Levels. Hope that helps.

Oh, please update your signature with all of your pool and equipment info.