Water treatment
Free Water Isn't Free: Our Underground Lake Is Hard Water
People hear that there’s an underground lake on site and assume the cooling towers drink for free. It’s a fair guess. The water is right there, and a pump is cheaper than a water bill.
I run an underground freezer, a two-stage ammonia plant feeding a calcium chloride brine loop, with rooms held anywhere from about -15 to 8 °F. The heat that plant pulls out of the rooms has to go somewhere, and for us that’s evaporative cooling towers. Towers lose water to evaporation all day, so they need makeup water all day. This is why the lake isn’t the easy answer it looks like.
What the lab said
The lake water has been tested more than once. A 2020 analysis came back like this:
| Test | Result |
|---|---|
| Total hardness | 288 ppm |
| Calcium | 214 ppm |
| M-alkalinity | 251 ppm |
| pH | 8.36 |
| Conductivity | 499 µS/cm |
| Silica | 15.4 ppm |
| Sulfate | 36 ppm |
A later test, in December 2025, put calcium at about 235 mg/L and suspended solids at about 10 mg/L. So across both tests, calcium sits somewhere in the low-to-mid 200s.
In plain words: it’s hard, it’s alkaline, and it carries some grit. That’s what you’d expect from water sitting in limestone. It’s great for a lake. It’s not great for a heat exchanger.
Why hardness matters: cycles of concentration
This is the part our water treater explained better than any textbook I’ve read.
A cooling tower rejects heat by evaporating water. The water leaves, but the minerals stay behind. So the tower water gets more concentrated than the makeup water over time. How many times more concentrated is called the “cycles.”
Using our own lake numbers: with about 235 mg/L of calcium coming in, 2 cycles means about 470 mg/L in the tower. 3 cycles means about 705 mg/L. Every cycle you add, the scale risk climbs.
Because our lake water starts out hard, the treater limits the towers to about 2.5 cycles on it. City water is softer, so it can be cycled higher. Higher cycles means less blowdown (less water dumped to keep minerals down) and less treatment chemistry for the same job.
That was the treater’s bottom line: even if you filter out the solids, city water is better from a chemical treatment standpoint, because you can run more cycles and use less chemistry.
The other problem: what’s floating in it
Hardness is the dissolved part. The suspended solids are the gritty part.
About 10 mg/L of suspended solids in the lake water doesn’t sound like much, and the treater’s cooling engineer said 10 to 20 isn’t unusual in a tower. But cycle the tower up to about 2.5 and you’d likely have 20 to 25 mg/L in the system, plus whatever dust the towers pull out of the air.
What those solids are made of matters. If it’s iron, that’s a real problem. If it’s mostly silt, dirt and sand, which is the treater’s best guess for our lake, it’s less of a chemistry problem and more of a wear problem. Grit gets into gasket areas and tight spots and can erode piping and gaskets over time. That’s why good filtration is the price of admission for using lake water at all.
The question we actually put to the treater was whether we could run on lake water with the filter out of service. The answer was that it could be OK if you’re willing to watch the tower components for erosion, but the preference was still city water.
The trade-off, honestly
This is the part nobody puts in a brochure. “Free” water from the ground still costs you something:
- More chemistry, because hard water needs more help to stay off the tubes.
- Fewer cycles, which means more blowdown water going down the drain.
- Filtration, and the upkeep that comes with it.
- Wear on the tower, if the grit gets past the filters.
City water costs money too. The treater said it plainly: depending on what you pay for city water, it might be worth spending a little more on chemistry to avoid the water bill. There isn’t one right answer. There’s your water bill, your chemistry bill, and how closely you’re willing to watch the tower.
Why I watch tower water closely now
Hard water leaves you less room for mistakes. In August 2026 a plugged strainer on the supply pump for our tower’s water-treatment controller killed pH and blowdown control. Conductivity spiked past 7,000 and pH went over 9, and scale built up on the tower in about four days. That wasn’t a lake-water problem by itself, but it shows how fast scale wins once the chemistry stops being controlled.
If you have a “free” water source
- Get a full lab analysis first. Hardness, calcium, alkalinity, pH, conductivity, silica and suspended solids at a minimum. Test it more than once; ours moved between tests.
- Ask your treater what cycles they’d run on it. That one number drives your blowdown, your chemistry and your scale risk.
- Find out what the suspended solids are. Iron and silt are very different problems.
- Count filtration as part of the source. If the filter is down, you’re running a different water than the one you planned for.
- Compare the whole cost. City water bill on one side, chemistry, blowdown, filter upkeep and tower wear on the other.
- Check the treatment controller every day. Our daily, weekly and monthly checklist has the water-treatment check on the daily list for a reason.
The cave gives you a lot for free: stable rock, steady temperatures, protection from the weather. Clean makeup water isn’t on that list.
Related: Myth: Caves Are Naturally Freezing, on what the cave does and doesn’t give you, and Retro-Commissioning an Ammonia Plant, which covers condenser staging and head pressure on the same towers.
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