Why Your Pool’s Salinity Level Keeps Dropping
It can be a puzzling and costly problem for a saltwater pool owner: you feel like you're constantly buying and adding bags of salt just to keep your chlorinator happy. You know salt doesn't evaporate, so where is it all going? Understanding the mechanics of salt loss is key to diagnosing the issue and saving money on unnecessary additions.
This guide will walk you through the primary reasons why your pool's salinity level might be dropping, from the obvious to the hidden. The fundamental principle to remember is simple: **salt only leaves the pool when water leaves the pool.**
The Core Concept: Salt Doesn't Evaporate
This is the most important scientific principle to understand. When water evaporates from your pool's surface on a hot day, only pure H₂O molecules turn into vapor and exit, leaving the heavier salt and mineral molecules behind. In fact, during long periods of hot, dry weather with no new water added, your salt concentration (PPM) will actually increase slightly because the same amount of salt is now dissolved in a smaller volume of water. We explain this in more detail in our guide about how environmental factors affect salt levels.
Therefore, if your salt level is dropping, it's a certainty that salty water is being physically removed from your pool and replaced with fresh, unsalted water.
Cause #1: Backwashing and Filter Maintenance
This is one of the most common and significant causes of salt loss for pools with sand or Diatomaceous Earth (D.E.) filters.
The process of backwashing involves reversing the flow of water through the filter to flush out trapped dirt and debris. This process sends **hundreds of gallons** of chemically-treated, salty water out to waste. When you refill the pool with fresh water from your hose, you are directly diluting the remaining salt, causing a significant drop in the overall PPM. If you find yourself backwashing frequently (e.g., weekly), this will be a primary reason for your salt loss.
Cause #2: Splash-Out and Drag-Out
Never underestimate how much water can be lost during an active swim season. This happens in two main ways:
- Splash-Out: Every cannonball, jump, and splash sends salty water out of the pool and onto the deck. A pool that's used heavily by active kids and adults will lose far more water this way than a quiet relaxation pool.
- Drag-Out: This is the water that leaves the pool on swimmers' bodies, hair, and swimsuits. It might seem like a small amount per person, but with multiple swimmers over many days, it adds up to a surprising volume.
When you top up the pool to compensate for this lost water, you're adding fresh water, which again dilutes the salt concentration.
Cause #3: Heavy Rain and Overflow
Rainwater is fresh water with zero salt content. A steady or heavy rainstorm adds a large volume of this fresh water to your pool, directly lowering your salt PPM. If the storm is severe enough to cause the pool to overflow, it's a double whammy: salty water is physically forced out of the pool while being replaced by fresh rainwater, causing an even more dramatic drop in salinity.
Cause #4: A Leak (The Hidden Culprit)
If you've ruled out the causes above and your salt level continues to drop faster than expected, it's highly probable that your pool has a leak. A leak is a constant, silent drain of salty water.
How to Tell if You Have a Leak:
- You're adding fresh water frequently: Are you having to top up your pool more than once a week to keep the water level at the middle of the skimmer? (Excluding periods of very heavy use/splash-out).
- There are wet or soggy spots in your yard: Check around the pool and the equipment pad for unusually green or marshy areas.
- You lose more water than normal evaporation: A pool typically loses about 1/4 inch (0.6 cm) of water per day to evaporation. If you're losing more, it's time to investigate.
You can confirm a leak using the **bucket test**. Our guide on finding pool leaks provides step-by-step instructions for this simple but effective diagnostic test. Finding and fixing a leak is essential to stop the constant loss of water, salt, and other expensive chemicals.
What Should I Do?
- Test Accurately: Get a reliable salt reading using a digital tester or quality test kit. Don't trust your system's display.
- Identify the Cause: Go through the list above. Have you backwashed recently? Had a lot of rain or pool parties? If none of these apply, perform a bucket test to check for leaks.
- Calculate and Add Salt: Once you've identified and, if necessary, fixed the cause (like a leak), use our Pool Salt Calculator to determine the precise amount of salt needed to return to your target level.
By understanding why your salinity is dropping, you can move from constantly treating the symptom (adding more salt) to solving the root cause, saving you money and frustration.
