Wilmington water runs 94 to 228 mg/L hard. Lower panes take winter road salt and street spray, and mature northern canopy drips honeydew and tannin.
Wilmington treats its own Brandywine water, averaging 136 mg/L from the Brandywine plant and 154 from Porter, hard either way, so a plain tap rinse will mark glass that dries in the sun. Mature trees in the northern neighbourhoods drip honeydew and tannin, and lower panes on busy streets take winter salt spray. April and August to October are the best months; pane count and access both work against the crew in the older city, and a single-story exterior runs $250 to $425.
The city's 2025 report gives hardness for each filter plant separately: the Brandywine plant averages 136 mg/L across a 94 to 190 range, and Porter averages 154 across 100 to 228. Both sit in the hard band, and the top of Porter's range is close to very hard, so a rinse left to dry on dark glass will spot. The private utility's published numbers describe groundwater from a different system and would have been the wrong water to plan from. Even so, mineral is the secondary problem here; access and pane count are the first.
Figures by plant, as reported.
| Hardness | 94–228 mg/L as CaCO₃ 136 on average at the Brandywine plant and 154 at Porter, hard on the USGS scale either way, as published by City of Wilmington Department of Public Works. |
|---|---|
| pH | 7.6 (range 6.7–8.7) |
| Alkalinity | 67–73 mg/L as CaCO₃ |
| Iron | 0.021 mg/L (range 0.02–0.04) |
| Manganese | 0.011–0.013 mg/L |
| Sodium | 22–23 mg/L |
| Chloride | 75–96 mg/L |
| Disinfectant | free chlorine |
| Source and treatment | Surface water on the city's own system, per the EPA SDWIS federal record · Coagulation, sedimentation and rapid sand filtration, pre and post chlorination, fluoridation, with an orthophosphate inhibitor and pre pH adjustment · No softening step. |
Not in the report: corrosion index and total dissolved solids.
SOURCE · City of Wilmington 2025 Water Quality Report, Conventional Physical and Chemical Parameters table for the Brandywine and Porter filter plants · How we source these numbers
Winter sets the heaviest work of the year. Road salt on narrow streets with constant traffic throws spray onto everything below the second storey from December to March, and that accumulated film is what the first spring visit is really removing. Through summer the canopy neighbourhoods run on a shorter cycle than the centre because of honeydew, while the riverfront and business core stay on contract intervals driven by tenancy rather than weather. Autumn leaf fall is the one event that fills glazing channels and repays clearing before it sits.
DELAWARE SEASON BY SEASON →Wilmington treats its own surface water from the Brandywine rather than buying from the private groundwater company that serves much of the surrounding county — which is why figures published for that company describe a different supply entirely. The city's 2025 report prints hardness plant by plant: 136 mg/L on average from the Brandywine plant and 154 from Porter, both in the hard band. What actually governs the work is the rowhouse fabric: no side access, very high pane counts per frontage, and four distinct building eras inside a short walk.
Wilmington runs its own water system rather than buying from the regional private utility that serves much of the surrounding county, and it draws surface water from the Brandywine rather than from wells.
That is an important distinction in this state, because the private company serving northern New Castle County is a groundwater operation and publishes figures that have nothing to do with what a Wilmington address receives.
The city's own report settles the hardness question plant by plant: 136 mg/L on average from Brandywine, 154 from Porter, so which plant feeds a street moves the figure by more than a tenth.
Figures quoted elsewhere put Wilmington at roughly 120 parts per million, a band softer than either plant actually delivers — a small miss, but the kind that decides whether a pure-water final rinse is worth carrying.
Rowhouse density is the physical fact that shapes the work. Whole blocks share party walls and have no side access at all, so every pane has to be reached from the street or from inside, and ladder placement on a narrow pavement is a real constraint.
Window counts per building are correspondingly high. A modest three-storey rowhouse front can carry more individual panes than a suburban house four times its floor area, which is why pricing on square footage fails badly in this city.
The building stock spans an unusual range for a small city — early stone along the Brandywine, Victorian and Edwardian brick through the residential quarters, post-1990 curtain wall on the riverfront — and the right method changes across a few blocks.
Road salt through the winter and the spray it generates on narrow streets put a persistent film on lower elevations from December through March, which makes the first spring service the heaviest of the year.
Mature canopy in the northern neighbourhoods brings honeydew and leaf tannin into frame channels, and on divided-light sash that means working the glazing bars rather than just the glass.
Pane count and access dominate, and both run against the operator in this city. A rowhouse frontage carries far more windows than its footprint suggests and can only be reached from the street, so quoting on area loses money reliably. Heritage handling along the Brandywine and in the Highlands adds time. The riverfront is the counterweight — modern, accessible and efficient — and is where a route recovers the margin the old city costs it.
Interior work and ground-floor exteriors are a reasonable homeowner job. Upper storeys on a rowhouse genuinely are not, and the reason is structural rather than a matter of nerve — there is no side access, the pavement is too narrow for safe ladder angles, and the alternative is working from inside a sash that may be painted shut. Original divided-light glazing in the older quarters adds a second reason to leave it alone: the putty and the glass are both older than they look.
OPEN COST ESTIMATOR →2025 annual mean at the MLK Corner of MLK Blvd and Justison ST monitor, MLK Blvd and Justison ST., 2.2 km west of the city (EPA site 10-003-2004).
Largest air release: Edge Moor/HAY Road Energy Centers, 200 HAY RD (electric utilities), 0.3 km outside the city limits, 18,213 lb in 2024.
Letting water run unnecessarily from a private pipe or fixture in Wilmington, inside or outside a building or in a street or alley, is a misdemeanor carrying a $100 fine.
Wilmington City Code §45-96: wasting water; leaks · checked September 2026
Wilmington prohibits starting or keeping up any discharge to its storm drain system other than storm water, whether the pollutants are thrown, drained or let in by someone under your control.
Wilmington City Code §11-307.1: prohibition of illegal discharges · checked September 2026
Anyone planning work that affects the exterior of a property in one of Wilmington's historic areas or districts, alterations, additions and removal of character-defining features included, must first apply for a certificate of appropriateness.
Wilmington Zoning Code §48-424: application process, certificate of appropriateness · checked September 2026
Wet-only handling. Peak mid-April. Pollen lifts cleanly with water plus light alkaline soap; do not scrape.
Piedmont iron-clay fraction in well water. Same citric-protocol approach Tony Petruzzi documents for western Loudoun County, Virginia. Extended dwell required.
Traffic-film composite typical of I-95 corridor commercial stock. Standard alkaline-soap dwell handles it with no special protocol needed.
The city's 2025 report gives hardness for each filter plant separately: the Brandywine plant averages 136 mg/L across a 94 to 190 range, and Porter averages 154 across 100 to 228. Both sit in the hard band, and the top of Porter's range is close to very hard, so a rinse left to dry on dark glass will spot.
Residential window cleaning in Wilmington typically runs $8–14 per pane or $250–425 for a standard single-story exterior. Pane count and access dominate, and both run against the operator in this city.
Wilmington's drier, frost-free months are April and August to October, by NOAA's 1991–2020 normals for the Wilmington New Castle County Airport station. The calendar on this page marks August (after the April 20 bloom) and October (after smoke season and before frost). Frost nights usually run from November 1 to April 7.
Not as the final rinse: City of Wilmington Department of Public Works publishes hardness of 94–228 mg/L, enough to spot as it dries, so finish with pure water. Water left to dry leaves mineral scale; fresh spots lift with a mild acid. It draws on surface water on the city's own system.
In Wilmington the dominant residue patterns include winter road salt and street spray on lower elevations and honeydew and tannin under mature northern canopy. Winter sets the heaviest work of the year.
Interior work and ground-floor exteriors are a reasonable homeowner job. Upper storeys on a rowhouse genuinely are not, and the reason is structural rather than a matter of nerve — there is no side access, the pavement is too narrow for safe ladder angles, and the alternative is working from inside a sash that may be painted shut.
The banking and legal centre, mid-rise office around a civic square. Curtain wall above, heavy pedestrian scrutiny at street level, and building-management scheduling throughout. Trolley Square and Brandywine Village have their own housing stock and glazing as well.
Just over the Delaware line