What is Rapid City water hardness in grains per gallon?

Rapid City's water hardness is 12.9 grains per gallon (GPG), equivalent to 221 parts per million (PPM). 1 GPG = 17.1 PPM.

Water Quality in Rapid City, SD

Rapid City's water has very hard water (221 PPM) and PFAS above EPA limits (5 ppt). A water softener is recommended to protect plumbing and appliances. An activated carbon filter can reduce PFAS levels. Read below for a full breakdown.

Rapid City Water · Source: Pactola Reservoir, groundwater · Serves 77,503 residents

221

Hardness (PPM)

314

TDS (PPM)

0.9

Chlorine (mg/L)

PFAS (ppt)

Lead (ppb)

Hardness Scale: Where Rapid City Falls

Soft 060120180250400+

Very hard water

Rapid City has very hard water at 221 PPM from the Great Plains limestone and glacial geology. Water softeners are considered standard household equipment in this region — most residents view them as a necessity, not a luxury.

What Rapid City's Water Means for Your Home

Rapid City residents deal with some of the hardest municipal water in the country. At 221 PPM (12.9 grains per gallon), the water contains high concentrations of dissolved calcium and magnesium picked up from underground limestone and mineral formations. This is not a safety issue. The minerals are safe to consume. But they create real, measurable costs for homeowners. Hard water at 221 PPM costs the average household $800 to $1,200 per year in hidden expenses, including higher energy bills from scale-insulated water heaters, increased soap and detergent usage, and premature appliance failure. Many Rapid City homeowners discover this only after a water heater replacement or plumber visit.

PFAS contamination is a significant concern in Rapid City. At 5 parts per trillion, levels exceed the EPA's 2024 maximum contaminant level of 4 ppt. PFAS are synthetic chemicals that do not break down in the environment or the human body, which is why they are called "forever chemicals." Long-term exposure has been associated with increased cancer risk, thyroid disruption, and immune system effects. A whole-house activated carbon filter or point-of-use reverse osmosis system can reduce PFAS to safe levels. Look for NSF P473 certification when comparing products.

Contaminant	Detected	Health Guideline	Legal Limit	Status
Hardness (as CaCO₃)	221 PPM	< 60 PPM	No federal limit	⚠ Very Hard
Total Dissolved Solids	314 PPM	< 300 PPM	500 PPM	⚠ Elevated
PFAS (total)	5 ppt	4 ppt (EPA)	4 ppt (2024)	⚠ Above limit
Lead	4 ppb	0 ppb (no safe level)	15 ppb (action level)	✓ Low
Chlorine / Chloramine	0.9 mg/L	Taste threshold ~1.0	4.0 mg/L	✓ Normal
Nitrate	2 mg/L	5 mg/L	10 mg/L	✓ OK

Recommendations for Rapid City Homes

🧂 Water Softener

At 221 PPM (12.9 GPG), Rapid City's water is very hard. Scale will accumulate in pipes, water heaters, and on fixtures. A salt-based ion exchange softener removes the calcium and magnesium causing the problem. For a typical Rapid City household of 2 to 4 people, a 48,000-grain softener is the right starting size.

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🔬 Activated Carbon Filter for PFAS

PFAS detected at 5 ppt, above the EPA maximum contaminant level of 4 ppt. PFAS are linked to cancer, thyroid disease, and immune system effects with long-term exposure. A whole-house granular activated carbon (GAC) filter or a point-of-use reverse osmosis system can reduce PFAS. Look for NSF P473 certification, which specifically tests for PFAS removal.

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About Rapid City's Water Supply

Water Utility: Rapid City Water

Water Source: Pactola Reservoir, groundwater

Population Served: 77,503

Hardness: 221 PPM (12.9 grains per gallon)

Water quality can vary by neighborhood and season. The values above represent typical averages reported by Rapid City Water. To get exact numbers for your address, request a copy of the most recent Consumer Confidence Report (CCR) from your utility, or have your water independently tested.

Compare Rapid City to Other South Dakota Cities

Sioux Falls

250 PPM · Very Hard

Aberdeen

204 PPM · Very Hard

Frequently Asked Questions About Rapid City Water

Is Rapid City tap water safe to drink?

Rapid City's water meets EPA legal standards for drinking water safety. However, legal limits and health-optimal levels are different things. PFAS levels exceed the 2024 EPA health standard, which is a concern for long-term exposure. Review the contaminant table above for specific numbers and how they compare to both legal limits and health guidelines.

Do I need a water softener in Rapid City?

Yes. At 221 PPM (12.9 GPG), Rapid City's water is very hard. Without a softener, expect visible scale buildup on faucets and shower heads, reduced water heater efficiency (15 to 25 percent higher energy costs), spotted dishes and glassware, and soap that does not lather well. A salt-based water softener is the standard solution. For a typical Rapid City home with 2 to 4 residents, a 48,000-grain system is the most common starting size.

What water filter is best for Rapid City?

The best filter depends on which contaminants you want to address. For PFAS, look for a filter with NSF P473 certification. Granular activated carbon (GAC) and reverse osmosis systems are both effective. Standard carbon pitcher filters may not remove PFAS adequately. See our softener vs. filter guide to understand the difference between these categories.

Where does Rapid City's water come from?

Rapid City's drinking water is sourced from Pactola Reservoir, groundwater and treated by Rapid City Water. Rapid City has very hard water at 221 PPM from the Great Plains limestone and glacial geology. Water softeners are considered standard household equipment in this region — most residents view them as a necessity, not a luxury.

What is the hardness of Rapid City water in grains per gallon?

Rapid City's water hardness is 12.9 grains per gallon (GPG), which equals 221 parts per million (PPM). This is classified as very hard. Most water softener manufacturers recommend treatment above 7 GPG. To convert: 1 GPG = 17.1 PPM.

Does Rapid City water damage tankless water heaters?

Yes. Most tankless water heater manufacturers recommend water hardness below 7 to 11 GPG (120 to 188 PPM). At 12.9 GPG, Rapid City's water exceeds this threshold. Scale buildup inside the heat exchanger reduces efficiency and can void the manufacturer's warranty. A water softener installed upstream of the tankless heater is the standard solution.

Is Rapid City water safe for babies and infants?

Rapid City's water meets all EPA safety standards and is generally considered safe for preparing infant formula. The hardness minerals (calcium and magnesium) are not harmful. If your home has older plumbing, running cold water for 30 seconds before use is a good precaution to flush any lead from pipes.

How much does hard water cost a Rapid City household per year?

At 221 PPM, hard water costs the average Rapid City household an estimated $800 to $1,200 per year in hidden costs. This includes 15 to 25 percent higher water heating bills from scale insulation, 50 to 75 percent more soap and detergent usage, shorter appliance lifespans (water heaters, dishwashers, washing machines), and increased plumbing maintenance. A water softener typically costs $600 to $1,500 installed and $50 to $100 per year in salt, which means most Rapid City households see a return on investment within 1 to 2 years.

Data sources. Water quality data compiled from EPA SDWIS, EWG Tap Water Database, USGS water hardness studies, and the Rapid City Water Consumer Confidence Report. Hardness and contaminant levels represent typical average values and may vary by neighborhood and season. Last updated: 2026-02-12.