Hot Spring Chemistry and What's in the Water?
Picture this: you're standing at the edge of a steaming pool in the backcountry, mist rising into the crisp mountain air. That water didn't just appear from nowhere—it's been traveling through miles of underground rock, picking up minerals, dissolving elements, and accumulating heat from the Earth's depths. The result? A unique chemical cocktail that gives each hot spring its own character, therapeutic properties, and yes, sometimes its own distinctive smell.
Understanding what's actually in the water can transform your soaking experience from simple relaxation into something more intentional. When you know your lithium from your sulfur, you can seek out springs that align with what your body needs—whether that's relief from sore muscles, calm for a restless mind, or just the silky-soft water that makes you feel like you're bathing in liquid luxury.
This guide will help you decode the science behind the steam, so you can make informed choices about where to soak next.
The Major Minerals
The mineral content of a hot spring—often called its "total dissolved solids" or TDS—is what gives each spring its personality. These minerals come from the rocks and geological formations the water passes through, and their concentrations can vary dramatically from spring to spring. Here's what you'll encounter most often in your hot spring adventures:
Calcium (Ca) — Ever noticed some hot springs leave your skin feeling impossibly soft and silky? You're probably soaking in calcium-rich water. This mineral is commonly found in areas with limestone formations and creates that luxurious "spa feel" many soakers chase after. Calcium deposits often create the stunning travertine terraces you'll see at places like Yellowstone's Mammoth Hot Springs, where mineral-rich water cascades over stepped limestone formations. For your skin, calcium means smoothness—and for your joints, it can mean relief from arthritic discomfort.
Sodium Chloride (NaCl) — This is the salt you've probably tasted if you've ever gotten hot spring water in your mouth (let's be honest, we've all done it). Sodium chloride is the most common mineral found in hot springs worldwide, making up a significant portion of most spring waters. These chloride-rich waters feel gentle on the skin and are often associated with deeper, geothermal sources. The Great Salt Lake region's hot springs and many volcanic-fed pools in places like Colorado and New Mexico frequently feature elevated sodium levels.
Sulfur (H₂S) — Ah, the one everyone recognizes. That unmistakable "rotten egg" smell is actually hydrogen sulfide gas dissolving into the water, and it's one of the most therapeutically valuable minerals you can soak in. Sulfur hot springs have been used for centuries to treat skin conditions, arthritis, and respiratory issues. The smell might take some getting used to, but many devoted soakers insist the benefits are worth it—and your nose adjusts surprisingly quickly once you're submerged. Places like Truth or Consequences in New Mexico and the hot spring creek towns of Colorado's San Luis Valley are famous for their sulfur-rich waters.
Lithium (Li) — Here's where things get interesting. Lithium is relatively rare in hot springs, but when you find it, you're in for a special experience. This lightweight metal has gained attention for its mood-stabilizing and calming properties (it's literally used in medications for bipolar disorder). Natural lithium hot springs are considered by many to promote relaxation, reduce anxiety, and improve sleep quality. The geothermal waters near Boulder, Montana, and some springs in California's Long Valley have notably high lithium content. Finding a lithium spring feels like discovering a secret the Earth has been keeping.
pH and Safety
Beyond which minerals are present, the acidity or alkalinity of a hot spring—measured by its pH level—is crucial for your safety and skin health. The pH scale runs from 0 to 14, with 7 being neutral:
The Safe Soaking Zone: pH 6.5–8.5
Most commercially viable and naturally tolerable hot springs fall within this range. These waters won't irritate your skin or cause discomfort during extended soaks. The majority of developed hot springs you'll visit aim for this sweet spot.
The Danger Zones
Hot springs with pH below 3 are considered highly acidic—think battery acid territory. These acidic waters can cause chemical burns, damage your skin's natural protective barrier, and should be avoided entirely unless you're specifically seeking out one of the rare "acid hot springs" with proper safety protocols in place. Some volcanic areas like the geothermal fields at Lassen Volcanic National Park have pools with pH readings this low.
On the other end, highly alkaline waters (pH above 9) feel slimy or soapy to the touch—but that slippery sensation is actually your skin's oils being chemically stripped away. Extended exposure can cause irritation and discomfort. Many soda springs and carbonated pools fall into this higher pH range.
When visiting developed hot springs, check if water chemistry has been tested and posted. For wild springs, use your senses: if the water stings, smells extremely chemical, or feels wrong on your skin, it's best to move on.
Temperature Rules
Temperature is perhaps the most immediately felt aspect of any hot spring, and it directly affects how long you can safely soak. Here's a practical guide to what those temperature ranges mean for your body:
Below 98°F (37°C) — This is considered cool or tepid, similar to a heated swimming pool. Perfect for longer soaks, especially in summer months or if you're heat-sensitive. These temperatures won't provide significant therapeutic heat benefits but are comfortable for extended relaxation.
98–104°F (37–40°C) — The therapeutic sweet spot. This range mimics your core body temperature closely, allowing you to soak for 20–45 minutes comfortably while still receiving the muscle-relaxing, circulation-boosting benefits of heat therapy. Most developed hot springs aim for this range, and it's where you'll find the majority of commercial soaking pools.
104–112°F (40–44°C) — Hot, but manageable in short bursts. Plan for 10–20 minute soaks maximum at these temperatures, and listen to your body. This range is common in natural springs where the water hasn't been cooled or mixed. You'll feel the heat more intensely, and longer exposure risks overheating, dizziness, or heat exhaustion.
Above 112°F (44°C) — Dangerous territory. At these temperatures, you're at real risk of burns and heat stroke even in just a few minutes of exposure. Some wild hot springs reach these temperatures naturally, and while they might look inviting in photos, the consequences aren't worth the Instagram shot. Always test the water with your hand or foot before submerging—and if it feels anywhere near painful, it is.
Choose Your Soak Intentionally
Now that you understand what you're actually soaking in, you can approach your next hot spring adventure with intention. Want silky skin? Seek out calcium-rich travertine pools. Need deep relaxation? A lithium spring might be calling your name. Dealing with achy joints? Sulfur waters have your name on them.
The beauty of hot spring culture is that there's truly something for everyone—and now you have the vocabulary to find it. Check posted water chemistry when available, test temperatures with your hand before diving in, and remember that the "best" hot spring is the one that matches what your body needs that day.
So go ahead—get curious about the chemistry. Your perfect soak is waiting.
*Have a favorite mineral-rich spring or a chemistry question about your favorite soaking spot? Drop it in the comments below. The Soak the Rockies Team loves hearing about your adventures.