Leslies Pool told us to drain half the pool

[jinxies]

We live in Northern Cal and leave our pool uncovered, with the pool running all winter. We went to just the chlorine tablets in the float for the last 4 months of the winter, due to unusually cold weather this last winter. Took a sample into Leslies to see what needs to be added and they said the water will no longer hold chlorine due to the chemical being out of wack. Here's the summary:

FAC 0ppm
TAC 0.0 difference
CH 500ppm
CYA 100ppm
TA 170ppm
pH 8.0
Acid 3
TDS 2501

Pool is clear, no algae.
In the printout recommendation, it states the CH, CYA and TDS is too high and we should partially drain the pool. I'm concerned with this...ground water is naturally high and it's getting warm out in the day 80-85 and I don't want the plaster to crack.
Do we really have to drain 10,000 gallons?

[TSH Tech]

yes, your Cyanuric Acid is high and your TDS is off the chart.

You mentioned high ground water, that is a big concern, the pool will pop out of the ground like a big concrete boat. You can drain it 1/4 at a time, refill it, test the water, drain it again 1/4th again, refill, test it, until you get your CYA/TDS down.
The problem is, there does not exist any chemical to remove CYA from a pool, so it needs to be rinsed out/diluted with fresh water. Typically, winter rains rinse out pool water, then come spring, CYA readings are zero to very low. Conditioner is added for the summer season, then Autumn sets in, winter rains, the pool water rinses out again and the cycle repeats.
Hope this is helpful

[chem geek]

The following are chemical facts independent of concentration of product or of pool size:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm.

So it doesn't take long for the CYA to build up if you are using Trichlor or Dichlor as your source of chlorine. To prevent this, you need to use a hypochorite source of chlorine such as chlorinating liquid or bleach. Or if you want to use Trichlor and have the CYA go higher, then you need other means of controlling algae such as the use of algaecide or phosphate remover.

If you want to maintain the pool with chlorine alone and no algaecide, then the FC must be at least 7.5% of the CYA level to prevent green and black algae growth. See the Pool School for more info.