Diagnosis
Black Marks on Toilets and Sinks
It is not hard water. Charleston’s water is soft. Here is how to tell what it actually is.
It is not hard water. Charleston’s water is soft. Here is how to tell what it actually is.
Quick answer: Black marks on Charleston fixtures are not caused by hard water — calcium scale is white, and Charleston’s water is soft by the utility’s own figure. There are three real causes: airborne black mould, which is the most common and is a cleaning and humidity problem rather than a water problem; manganese, which Charleston Water System’s own monthly data shows exceeding the EPA standard in most summers; and a degrading rubber toilet flapper. You can tell them apart yourself in about a minute. Call (843) 718-8177.
This is one of the most commonly misdiagnosed complaints in the trade, and the usual answer is wrong. Of eighteen national pages we read on black staining, nine blamed hard water or hardness minerals. One large Charleston company publishes an article explaining that local water picks up calcium and magnesium as it moves through soil and rock — which describes groundwater, and Charleston is on surface water.
Hard water cannot be the cause here for a simpler reason: there is barely any hardness in it. Charleston Water System publishes 58.4 ppm, about 3.4 grains, and USGS classifies anything under 60 as soft. And calcium scale is white and chalky, not black.
So what is it? Usually one of three things, and it matters which, because two of them should not cost you a filtration system.
Before anybody sells you anything, spend a minute on this.
| Test | Mould / biofilm | Manganese | Degrading rubber |
|---|---|---|---|
| Texture | Soft, slimy, wipes off | Gritty, needs scrubbing | Soft flakes that smear oily between two fingers |
| Does bleach remove it? | Yes | No | No |
| Does an acid descaler remove it? | No | Yes | No |
| Is it in the tank as well as the bowl? | Usually the bowl only | Yes — it arrives in the water | Flakes in the tank |
| Laundry or dishwasher affected? | No | Yes | No |
| Seasonal? | Worse in humid months | Worse June to September | No |
| Back within days of cleaning? | Yes | Slower | n/a |
The bleach-versus-descaler result is the most reliable single test. ⚠️ You will find a rule online that staining above the waterline is mould and below it is mineral. We can find no authoritative source for that and it contradicts published work on where biofilm actually sits, so we do not repeat it.
Charleston Water System addresses this directly on its own site: “Pink staining and black growth are both are caused by airborne microorganisms … not by anything in the water. Some people observe a black gunk on their faucets and in toilets. This is a type of mold … The best way to combat both is frequent cleaning with a bleach solution. Make sure you have proper ventilation.” (Charleston Water System — common water quality issues).
That is very likely the answer for a black ring that wipes off easily and returns within days of cleaning. Charleston runs about 79% relative humidity in August, and roughly three quarters of all hours in the year sit at or above the 60% that EPA names as the mould threshold. A bathroom here is an extremely hospitable place for it.
It is airborne, not waterborne. No filter will touch it. The answer is cleaning with a bleach solution, running the extractor fan, and getting indoor humidity below about 50 to 60%.
If a company tests your water and then sells you a treatment system for a black ring that bleach removes, they have sold you equipment for a problem it cannot affect. We would rather tell you to buy a dehumidifier.
EPA lists the noticeable effect of manganese as “black to brown color; black staining”. Its secondary standard is 0.05 mg/L (50 µg/L) — a guideline for appearance and taste that EPA does not enforce (EPA secondary drinking water standards).
Charleston Water System’s 2024 annual report prints a manganese annual average of 0.06 mg/L — directly beside EPA’s recommended maximum of 0.05. (CWS 2024 Water Quality Report)
USGS research carried out with Charleston Water System found that Bushy Park Reservoir stratifies thermally at about 20 feet from June to early October, producing an oxygen-free layer below 25 feet and releasing metals and phosphorus from the sediment under those reducing conditions. (USGS Scientific Investigations Report 2018-5010)
Dissolved manganese that gets through treatment is oxidised in the distribution system by chlorine and chlorine dioxide into a black particulate that coats the inside of mains and later sloughs off. CWS says the same thing about its own mains: deposits accumulate inside some water mains and changes in pressure can break them loose.
After Tropical Storm Debby in 2024, Charleston Water System’s spokesperson told The Daniel Island News that “elevated levels of iron and manganese in the water caused the discoloration. These minerals, stirred up from decaying organic matter in the Bushy Park Reservoir and Edisto River … were released during the storm”.
He added something more useful still: “Our treatment process isn’t designed to effectively eliminate those elements because they’re not usually an issue.” (The Daniel Island News).
CWS’s routine reporting limit for manganese is 0.05 mg/L — exactly equal to the standard. A result printed as “<0.05” cannot distinguish water that is fine from water that is nearly staining. When measured precisely, the average was 9.38 µg/L, and research puts the threshold for deposits forming inside mains at about 10 µg/L.
| Year | Months above the standard | |
|---|---|---|
| 2023 | March 0.06, June 0.10, July 0.07, August 0.05 | |
| 2024 | July 0.065, August 0.133 — the August sheet is footnoted “Tropical Storm Debby” | |
| 2026 | July 0.0535 (data through July) |
Eight of the ten exceedances in CWS’s published monthly data fall in June, July or August. There is a reason for that, and it is specific to where Charleston’s water comes from.
A water softener is the wrong product for this, and the reason is specific. Penn State Extension states that oxidised forms of iron and manganese will foul softener resin, and that it is critical the raw water not meet any oxidising agent before entering the softener. Charleston’s water arrives carrying a chloramine residual, so any manganese in it is already oxidised — the softener will not remove it and the particles will damage the resin. (Penn State Extension — iron and manganese in private water systems)
So the product most likely to be offered for this complaint is the one product that cannot fix it and will be damaged by trying.
The honest product is particulate filtration certified to NSF/ANSI 42, which covers particulate, iron and manganese — either on the main line or at the affected fixture. The whole residential iron and manganese industry is built for wells running 3 to 10 mg/L; Charleston is at 0.05 to 0.13, roughly a hundred times lower, so oxidising media beds are overkill.
And because the utility’s own reporting limit sits exactly at the standard, the sensible first step is a test that can measure below it. That costs about $50 through a state-certified lab.
Chloramine degrades natural rubber faster than chlorine does, and Charleston chloraminates. A toilet tank flapper or a supply hose breaking down sheds soft black flakes that smear oily between your fingers — quite unlike gritty mineral deposit.
If the flakes are in the tank as well as the bowl and they smear rather than scrub, replace the flapper before you consider anything else. Chloramine-resistant flappers exist and are sold as such.
Most often airborne black mould, which bleach removes and which returns within days. Sometimes manganese in the water, which needs a descaler rather than bleach and is worse in summer.
It is not hard water. Charleston Water System publishes 58.4 ppm, about 3.4 grains, which is soft — and calcium scale is white, not black.
No. Hardness minerals leave white, chalky deposits. EPA lists black staining as the characteristic effect of manganese.
Of eighteen national pages we read on black staining, nine blamed hard water. It is the most common wrong answer in this trade.
Yes, and Charleston Water System’s own 2024 annual report prints an annual average of 0.06 mg/L against EPA’s recommended maximum of 0.05.
Its published monthly data shows results above the standard in most summers, peaking at 0.133 mg/L in August 2024 during Tropical Storm Debby. Eight of the ten exceedances fall in June, July or August.
Bushy Park Reservoir stratifies each summer and the water below about 25 feet goes oxygen-free, which releases manganese from the sediment.
That is from USGS research carried out in cooperation with Charleston Water System, and the timing matches the exceedances in the utility’s own monthly data almost exactly.
No — and it is actively the wrong product. Oxidised manganese fouls softener resin.
Charleston’s water arrives carrying a chloramine residual, so any manganese in it is already oxidised. Penn State Extension is explicit that raw water should not meet an oxidising agent before entering a softener.
Bleach removes mould and does nothing to manganese. An acid descaler removes manganese and does nothing to mould.
Also check whether it is in the tank as well as the bowl, whether laundry is affected, and whether it is seasonal. Mould is soft and slimy; manganese is gritty and needs scrubbing.
The mould kind is a nuisance rather than a health hazard, and manganese’s EPA standard is an aesthetic guideline rather than a health limit.
Neither is a reason to panic. Both are a reason to find out which one you actually have before anyone sells you equipment.
Part of Southeastern Renovation's water treatment division. Published prices: Charleston · York County.