Charleston's water is soft at 54 to 56 mg/L, far below the regional assumption, so the enemy on the peninsula is airborne sea salt that re-wets itself.
Charleston's water is soft, far softer than people in the region expect, so mineral deposits are not a problem; airborne sea salt that keeps re-wetting itself on the peninsula is. Salt means waterfront homes need frequent visits, and February, April, May and October to December give the best weather. Period glazing in the historic district carries a real skill premium, and a single-story exterior runs $300 to $550.
At 54 to 56 mg/L the supply is soft, and the practical reading is that mineral deposit is simply not a Charleston failure mode. If a pane here dries badly, the cause came out of the air rather than out of the tap, and no change of rinse water will fix it. The one genuine consequence of soft water is that detergent goes a long way, so over-soaping is an easy mistake and a far more common source of film on this glass than anything the utility delivers.
| Hardness | 54–56 mg/L as CaCO₃ 3.15 to 3.27 grains per gallon, printed by the utility alongside the mg/L figures, as published by Charleston Water System. |
|---|---|
| pH | 8.7 (range 8.6–8.8) |
| Alkalinity | 30 mg/L as CaCO₃ (range 26–35) |
| Iron | 0.11 mg/L (range 0.1–0.15) |
| Manganese | 0.06 mg/L (range 0.05–0.13) |
| Sodium | 8 mg/L (range 5–12) |
| Chloride | 14 mg/L (range 11–17) |
| Total dissolved solids | 101 mg/L (range 76–140) |
| Disinfectant | chloramine |
| Source and treatment | Bushy Park Reservoir as the primary source and the Edisto River as the secondary, treated at the Hanahan Water Treatment Plant and distributed through 1,912 miles of water mains fed by 33 miles of raw water tunnels · Raw water pumps, rapid mix with alum, powdered activated carbon, lime and polymer, then coagulation and flocculation, sedimentation in plate settlers, filtration, and clearwell storage where chlorine dioxide, chloramine, fluoride, orthophosphate and lime are dosed before the finished-water pumps · No softening step. |
Not in the report: corrosion index.
SOURCE · Charleston Water System, Water Quality Results (Monthly Averages) 2024, YTD average/min/max (charlestonwater.com), read with the treatment-process diagram in the operator's 2026 Water Quality Report (charlestonwater.com) · How we source these numbers
Salt is continuous rather than seasonal and its intensity is set by distance from open water, heaviest on the peninsula tip and the island developments and easing inland. Humidity keeps it chemically active all year instead of letting it dry to a liftable powder. Pollen adds a heavy spring layer under the live oaks in the western suburbs. The shortest-lived wash of the year is one taken immediately before a coastal blow; the most durable follows a settled spell of offshore wind.
SOUTH CAROLINA SEASON BY SEASON →Charleston is a city where the water turns out not to be the problem, and the published numbers are unambiguous about it: Charleston Water System reports 56 mg/L, printed as 3.27 grains per gallon, in one year and 54 mg/L the next, with alkalinity under 30 and chloride around 15. That is soft water, and tap is a perfectly adequate final rinse anywhere on the system. The assumption to discard is the regional one — a coastal Southern city is not automatically hard, and here the operator's own document says the opposite. What does govern the work is salt. The peninsula sits in tidal water on three sides and airborne sea salt is hygroscopic, so it stays damp on the glass, keeps the surface tacky enough to collect everything else, and holds moisture against metal fittings where it accelerates corrosion. Flood first, then wash. On top of that sits some of the oldest and most fragile glazing in the country.
Charleston Water System publishes hardness plainly and the number is far lower than the Lowcountry's reputation suggests: 56 mg/L, which the utility prints as 3.27 grains per gallon, in its 2021 report, and 54 mg/L or 3.15 grains the following year. That is soft water by any standard classification.
The supporting figures agree. Alkalinity runs at 29 to 30 mg/L, chloride at 15 to 16, sodium at 9 to 10 and conductivity at 179 to 184 microsiemens. Nothing in that profile describes a mineral-loaded coastal supply.
For anyone cleaning glass here the consequence is direct and worth stating flatly: tap water in Charleston is clean enough to be the last thing that touches a pane. Deionised rinse equipment solves a problem this city does not have, and a quote built around it is selling capacity rather than results.
That matters because the regional assumption runs the other way. A crew arriving from almost anywhere else in the South would reasonably expect hard coastal water, and nothing about a humid tidal city discourages the guess. The operator's own measurements contradict it.
The utility is a regional supplier at real scale — around 1,800 miles of water main and 9,800 hydrants serving roughly 450,000 people — and it sells wholesale to a long list of neighbouring systems, which is why one document settles the figure for more than one city on this site.
What actually sits on Charleston glass is salt, not scale. The peninsula is surrounded by tidal water on three sides and the airborne sea salt that reaches it is hygroscopic: it holds moisture against the pane rather than drying to a removable dust, and it re-wets itself on every humid night.
The method follows from that and it is the reverse of hard-water practice. Flood the glass with plain water first to dissolve and carry the salt away, and only then wash. Going straight to a detergent pass drags salt crystals across the surface under a cloth, which scratches and redistributes rather than removes.
Proximity to open water sets frequency, and frequency sets the annual bill far more than pane count does. Historic-district work carries a real skill premium because period glazing cannot be worked at production speed and the downside of haste is permanent. Narrow peninsula streets restrict access equipment in ways that add setup time. Nothing in an honest Charleston quote should be paying for treated rinse water.
Suburban stock across the Ashley is straightforward homeowner work, and the flood-first rule on salted glass is the single thing most people get wrong. The historic peninsula is a different matter and the reason is the glass itself: original crown and cylinder panes are irreplaceable, thin and uneven, and they do not tolerate a scraper or an aggressive alkaline cleaner anywhere near the putty. That is conservation work and should be bought as such.
OPEN COST ESTIMATOR →2025 annual mean at the North Charleston Fire Station monitor, 2800 Carner Avenue (NCFS), 2.3 km north of the city (EPA site 45-019-0020).
Largest air release: Ineos US Chemicals Co - Cooper River Plant, 1306 Amoco DR (chemicals), 0.3 km outside the city limits, 103,438 lb in 2024.
Wasting the city's water supply, which the Commissioners of Public Works control, is unlawful in Charleston.
Charleston Code §30-195: wasting the city water supply · checked September 2026
Charleston makes it unlawful to hook up any pipe, open channel or conveyance that carries anything other than stormwater or approved discharges into a city stormwater system or receiving waters.
Charleston Code §27-40: illicit connections, illicit discharges, and improper disposal · checked September 2026
In Charleston's Old and Historic District and Old City District, building permits for construction or demolition wait on the Board of Architectural Review's approval, which takes the form of a Certificate of Appropriateness.
Charleston Zoning Ordinance §54-232: construction or demolition of structures in districts · Charleston Zoning Ordinance §54-907: certificate of appropriateness · checked September 2026
Three to four week yellow film deposition on horizontal and east-facing vertical glass. Wet-only handling. Do not scrape. Heaviest pollen-deposition state in the country alongside Georgia and Alabama.
At 54 to 56 mg/L the supply is soft, and the practical reading is that mineral deposit is simply not a Charleston failure mode. If a pane here dries badly, the cause came out of the air rather than out of the tap, and no change of rinse water will fix it.
Residential window cleaning in Charleston typically runs $10–16 per pane or $300–550 for a standard single-story exterior. Proximity to open water sets frequency, and frequency sets the annual bill far more than pane count does.
Charleston's drier, frost-free months are February, April and May and October to December, by NOAA's 1991–2020 normals for the Charleston City station, where August brings the most rain. The calendar on this page marks April (after the February 28 bloom), October (after smoke season) and December (before frost). Frost nights usually run from December 30 to February 4.
Yes, if the rinse is squeegeed off before it dries: Charleston Water System publishes dissolved solids of 101 mg/L, between this site's two dissolved-solids flags. Its manganese, up to 0.06 mg/L, is at the staining level, and acid cleaners will not lift that film. It draws on Bushy Park Reservoir as the primary source and the Edisto River as the secondary.
In Charleston the dominant residue patterns include soft published supply at 54–56 mg/l (3.15–3.27 grains), far below the regional assumption and airborne sea salt across the peninsula, hygroscopic and self-re-wetting rather than loose.
Suburban stock across the Ashley is straightforward homeowner work, and the flood-first rule on salted glass is the single thing most people get wrong.
The oldest residential fabric in the city, much of it eighteenth and early nineteenth century, with original crown and cylinder glass surviving in places and piazzas shading the long south elevations. Conservation work in every sense. The French Quarter and Rainbow Row and Cannonborough-Elliotborough have their own housing stock and glazing as well.
Just over the South Carolina line