It can be incredibly frustrating to test your water and realize you don't know what causes alkalinity to drop in a pool, especially when you've been keeping up with regular maintenance. You go out there, dip your test strip or fill your titration tube, and suddenly the reading is way below that sweet spot of 80 to 120 parts per million (ppm). It feels like a moving target. Total alkalinity is basically the "bodyguard" for your pH level; it keeps your pH from bouncing around like a pinball. When it drops, your water becomes unstable, your equipment starts to corrode, and your eyes might start stinging the next time you jump in.
So, why does this happen? Is the pool leaking? Is it the weather? Usually, it's a combination of several small things that add up over a week or two. Let's break down the main culprits behind those dipping numbers.
The rain is more acidic than you think
One of the most common reasons your alkalinity takes a dive is simply the weather. We think of rain as "pure" water, but in reality, rainwater is naturally acidic. As rain falls through the atmosphere, it picks up carbon dioxide, which turns into a very weak carbonic acid. In many areas, the pH of rain can sit somewhere between 5.0 and 5.5.
When a heavy storm rolls through and dumps a few inches of water into your pool, that acidic water reacts with the bicarbonates in your pool. To neutralize the incoming acid, the pool "consumes" some of its alkalinity. If you've had a particularly wet week, don't be surprised if your levels are suddenly in the basement. It's basically a chemistry battle happening in your backyard, and the rain is winning.
Using the wrong kind of chlorine
This is the one that catches most pool owners off guard. We all know we need chlorine to keep the water clear, but the type of chlorine you use makes a massive difference. If you use those popular 3-inch tablets (Trichlor), you are essentially adding a slow-release acid puck to your pool.
Trichlor has a very low pH—usually around 2.8. For context, that's almost as acidic as lemon juice. Because Trichlor is so acidic, it constantly eats away at your pool's alkalinity as it dissolves. If you're over-relying on these tabs, especially in a smaller pool, your alkalinity will slowly but surely trend downward. Dichlor granules are also acidic, though not quite as bad as Trichlor. If you notice your alkalinity is always low, you might want to look into switching to liquid chlorine (sodium hypochlorite) or cal-hypo occasionally, as these have a much higher pH and won't tank your alkalinity levels.
The "pH correction" feedback loop
Sometimes, we are the ones responsible for the drop. It's a bit of a catch-22 situation. When your pH gets too high (maybe it's hitting 8.0 or higher), you add muriatic acid or dry acid (sodium bisulfate) to bring it back down. This is standard maintenance, and it works great.
However, muriatic acid doesn't just lower pH; it also lowers total alkalinity. If you find yourself constantly battling high pH—perhaps because you have a new plaster finish or a salt chlorine generator—you're probably adding acid every few days. Each dose of acid nibbles away at your alkalinity. Eventually, you've fixed the pH problem but created an alkalinity problem. It's a delicate balancing act, and it's why we always recommend testing alkalinity before you adjust pH, rather than the other way around.
Heavy use and "bather load"
Believe it or not, the people swimming in the pool are changing the chemistry. When you have a big pool party with ten kids splashing around for five hours, they are bringing a lot of "stuff" into the water. We're talking about sweat, oils, sunscreen, and—let's be honest—a little bit of urine.
All of these organic contaminants are slightly acidic or react with the chlorine to create acidic byproducts. As the chlorine works overtime to oxidize these contaminants, the chemical reactions can cause the alkalinity to dip. If you notice your levels drop significantly after a busy weekend of swimming, it's just the price of having a good time.
Evaporation and the "topping off" cycle
In hotter climates, evaporation is a constant battle. When water evaporates, it leaves behind minerals, but it doesn't leave behind the "buffering" capacity of alkalinity in a way that stays balanced. When you drag the garden hose over to fill the pool back up, you're introducing "fill water."
Depending on where you live, your tap water might have very low alkalinity. If you are constantly replacing evaporated pool water with low-alkalinity tap water, you are essentially diluting your pool's chemistry. Over time, this dilution results in a lower overall reading. It's always a good idea to test your tap water at least once a season so you know exactly what you're adding to the mix every time you top off the pool.
Why does a drop in alkalinity even matter?
You might be thinking, "Okay, so it's a little low, who cares?" Well, your pool cares. Low alkalinity leads to pH instability, often called "pH bounce." Without enough alkalinity to act as a buffer, even a tiny amount of acid or base can send your pH skyrocketing or plummeting.
When alkalinity is low, the water becomes "hungry" and aggressive. It will actually try to pull the minerals it needs from the pool itself. This means it can start etching the grout between your tiles, pitting the plaster on the walls, and corroding metal parts in your pump and heater. Plus, low-alkalinity water is notorious for causing itchy skin and burning eyes. It's just not a comfortable environment for a swim.
How to get your numbers back up
The good news is that fixing low alkalinity is one of the easiest and cheapest things you can do for your pool. You don't need a fancy "Alkalinity Increaser" from the pool store that costs $30 a bucket. You can just go to the grocery store and buy baking soda (sodium bicarbonate). It is the exact same chemical.
Here is a quick rule of thumb: about 1.5 lbs of baking soda will raise the alkalinity of a 10,000-gallon pool by 10 ppm.
- Test your water: Make sure you know your current level.
- Calculate the dose: Figure out how many ppm you need to gain.
- Spread it out: Walk around the perimeter of the pool and pour the baking soda into the water.
- Wait and re-test: Give the pump about 6 to 12 hours to circulate the water before you test again.
Don't try to raise it all at once if you're way off. If you need to go from 40 ppm to 100 ppm, do it in two or three stages over a couple of days.
Keeping things steady
To stop wondering what causes alkalinity to drop in a pool, the best defense is a good offense. Test your water weekly. If you see it trending down toward 80 ppm, add a little bit of baking soda then and there. Don't wait for it to hit 40 ppm.
Also, keep an eye on your chlorine sources. If you use pucks, maybe supplement with a little liquid bleach now and then to take the pressure off your alkalinity. And if a big rainstorm is in the forecast, just know you'll probably be doing some chemical adjustments once the clouds clear.
Pool chemistry isn't about being perfect; it's about staying within the range. Once you understand these five or six main "thieves" that steal your alkalinity, you'll find it much easier to keep your water crystal clear and your equipment in great shape for the long haul. Just keep that bag of baking soda handy, and you'll be fine!