Pool backwash kills grass--why, what to do?

[jcrva]

I've had my vinyl in-ground pool for many years. Several years ago I shifted from using trichlor tabs to bleach/liquid chlorine.

Starting last summer, the backwash from the pool is killing the grass. Has anyone else had this problem? What could be the cause? I'm wondering if it's salt from the liquid chlorine. Should I test the water for salt, and if so, how would I do that?

I haven't drained the pool in four years. For about three years, I've primarily used Pool Essentials 10% Chlorinating Liquid. I use a Taylor test kit, borate test strips, and TFP Pool Math calculator. My typical levels are:
2.5 to 5 chlorine
30 CYA
70 TA
7.4-7.5 ph
up to 50 ppm borates (35 at present)
CH is very low--I rarely test it

 

[mknauss]

Welcome to the forum!
How often do you backwash? It is only necessary when your filter pressure rises by 25% over clean pressure.
I would think with your rainfall and winter closure water replacement that salt levels should not be excessive. But, you can take a sample to a pool store for a salt test. Just ignore all the other stuff.
I suggest you read through Pool Care Basics - Trouble Free Pool and even look at a few of our videos TFP-TV - Trouble Free Pool

 

[jcrva]

Thanks mknauss. About backwashing, I only backwash when the pressure rises substantially, and typically this is just in the late spring and early summer when there's heavy pollen. So I don't have to backwash this time of year, but I want to figure out why the pool water is killing the grass so that I can re-plant the grass this fall with confidence that it too won't be killed next time I have to backwash or drain the pool to replace the liner.

It does seem like the salt level shouldn't be excessive given our abundant rainfall (almost 4'/year) here in the Piedmont area of Virginia. I will get the salt level tested, or buy a test. Any suggestions for a good, cheap kit/tool for testing salt?

Has anyone else experienced grass die-off from pool water? I've had my pool for 25 years and never before has the pool water killed any grass.

 

[mknauss]

Any suggestions for a good, cheap kit/tool for testing salt?

We use the K1766 for salinity testing.
But for you, I would take a sample to a pool store.

 

[arff]

Should not be the salt unless you are backwashing a lot and you have a lot of salt in your water.
- test the water for salt
- test to make sure it is the back wash (put some pool water in another small corner of the grass with the same frequency you backwash)
- plant salt resistant grass if that is the problem?

 

[Shirker]

What is your grass species?

 

[jcrva]

Thanks to all for your help. Here's a little more info in case anyone has any other thoughts or suggestions.

I had the water salinity tested as suggested by knauss, and it was 2200 ppm. That's lower than salt water pools, but still high. I believe that the salinity has built up over four years of using bleach as a sanitizer.

My grass is Kentucky 31 plus lots of weeds such as violets, crabgrass, dandelions, and spurge. I only backwash once in a while, such as when pollen is very heavy in the spring, but occasionally I need to drain water from the pool due to heavy rain here in Virginia. My concern is not just for my own turf when I occasionally backwash or lowerr the water level a little, but also concern that drainage from the pool might kill the grass in my neighbor's yard when I have to empty the entire pool (22,500 gallons) in a few years to replace the liner.

The turf die-off started In the summer of 2021 when I emptied some water from the pool due to lots of rain. Everything green died in the location (about 2' x 25') where the water drains out onto the turf and downhill in my yard. In fall 2021, nothing would grow in that area. During this spring 2022, only crabgrass has sprouted, no turf grass.

My working theory is that the salinity of the pool water, while not extremely high, is too high for Kentucky 31, which is not salt tolerant. I'm tentatively planning to construct a french drain where the water drained from the pool would go so that water pumped out of the pool dissipates underground below the roots of the turf. And as belt and suspenders, I would plant grass that's salt tolerant (certain types of tall fescue) in the area where water will be draining.

I'm surprised that increasing salinity from long use of bleach as sanitizer doesn't seem to be a problem for others using that approach.

 

[Bperry]

I have a salt pool and salt is around 3500ppm. I’ve backwashed several times over a spot planted with “southern gold” blend which I think is mostly fescue? Haven’t noticed any sign of damage on the grass where it drains over.

 

[jcrva]

Thank you! Good to know Southrn Gold blend doesn't seem too sensitive to salt.