Arlington gets federally treated Potomac water from the Washington Aqueduct at a 2025 average of 142 mg/L, and busy storefronts reload with every crowd.
Arlington's water is hard, Potomac River water treated by the federal Washington Aqueduct, so rinse water left to dry on glass will spot. Busy storefronts reload with every crowd, and April and August to October are the best months to wash. The densest part of the county is also the part no ladder reaches, so height and access set the price; a single-story exterior runs $350 to $600.
Arlington County's 2025 average is 142 mg/L, or 8 grains per gallon, which is hard on the standard scale, so rinse water left to dry on glass will spot. The supply is treated by a federal engineering organisation, the Washington Aqueduct, rather than by a municipal utility, and the county reports the averages for the water it receives. Two things for whoever closes it: one document covers several jurisdictions on the same treated water, and the source is a large river whose chemistry moves with flow and season, so a point value is a snapshot rather than a constant.
Figures by zone, as reported.
| Hardness | 142 mg/L as CaCO₃ 2025 county average, printed as 8 grains/gal or 142 mg/L; hard on the USGS ladder (2024 average 129), as published by Washington Aqueduct (Arlington DES). |
|---|---|
| pH | 7.5 |
| Sodium | 34 mg/L |
| Chloride | 38 mg/L |
| Disinfectant | chloramine |
| Source and treatment | Potomac River water treated by the Washington Aqueduct (U.S. Army Corps of Engineers) at its Dalecarlia plant and bought by the county · Permanganate and powdered activated carbon, coagulation, rapid mix, flocculation and sedimentation, rapid sand filtration, pre and post chlorination with chloramines as the residual, fluoridation, and orthophosphate with lime for corrosion control, per the Aqueduct's SDWIS record · No softening step. |
Not in the report: alkalinity, corrosion index, iron, manganese and total dissolved solids.
SOURCE · Arlington County DES Water Quality Reports page, 2025 average compound levels, with the county's 2025 Water Quality Overview and the EPA SDWIS federal treatment records for Arlington County (VA6013010) and the Washington Aqueduct (DC0000001) · How we source these numbers
Corridor retail runs on footfall and reloads within a fortnight regardless of month. Residential streets follow canopy and season — pollen through spring, sap and leaf cast through the growing season, salt on lower storeys through a moderate winter. High-rise work is weather-gated by wind rather than by temperature, which is a different calendar from everything below it.
VIRGINIA SEASON BY SEASON →Arlington drinks from the only federally owned and operated public water supply in the country. The Washington Aqueduct, run by the Army Corps of Engineers, draws from the Potomac and wholesales treated water to the county alongside the District and a neighbouring city — so treatment questions go to a federal engineering body and distribution questions go to the county. The county's 2025 average hardness is 142 mg/L (129 the year before), and because a river supply moves with the seasons, that average describes a moving quantity. The working reality is two businesses in one county: high-rise residential along the corridor where most of the population lives above ladder height, and inter-war brick neighbourhoods behind it that are ordinary two-storey work. Federal and defence-adjacent contracts dominate the commercial side and are stable enough to plan years around.
Arlington's drinking water comes from the only federally owned and operated public water supply in the United States. The Washington Aqueduct is run by the US Army Corps of Engineers, draws from the Potomac and treats the water before wholesaling it to the county's distribution system alongside the District and a neighbouring city.
That structure is worth knowing because it changes where the answers live. The body that treats the water is a federal engineering organisation rather than a municipal utility, and its reporting is organised accordingly — so a question about treatment goes somewhere quite different from a question about the pipe in the street, which the county handles.
Arlington County publishes a 2025 average hardness of 142 mg/L, which it gives as 8 grains per gallon, along with sodium at 34 ppm and chloride at 38. Those are county averages for water from the Aqueduct's Dalecarlia plant, and the 2024 average was 129 mg/L, so the river-fed figure moves from year to year.
A large river supply is seasonally variable in a way a lake or an aquifer is not. Potomac chemistry moves with rainfall, runoff and flow, so even a correctly sourced point value from this system describes a moving quantity rather than a fixed property.
Arlington is unusually dense for a place with no incorporated city, and the housing stock divides sharply. High-rise residential along the corridor means most of the population lives above ladder height, while the inter-war brick neighbourhoods behind it are ordinary two-storey work — two entirely different businesses inside a few blocks.
The corridor's mid-rise retail frontage reloads on footfall rather than weather, which makes it genuine interval work. Selling a seasonal clean on that frontage is selling something that will look tired within a fortnight, and the client will notice.
Federal and defence-adjacent institutional work dominates the commercial book and it operates on its own terms — security clearance for access, detailed specification, slow procurement, and contract stability that no private commercial client offers. It is the reason a crew here can plan years rather than seasons.
Height and access dominate, because the densest part of the county is also the part no ladder reaches. Frequency drives the retail frontage. Contract specification drives the institutional work, where the documentation and clearance requirements are a real cost that has to be in the number rather than absorbed.
The inter-war brick neighbourhoods are reasonable homeowner work at two storeys on level ground. Everything on the corridor is not, for the simple reason that most of it starts above the height a ladder should ever reach. Federal-adjacent frontage requires cleared access and is not available to casual operators at all.
OPEN COST ESTIMATOR →2025 annual mean at the Mcmillan NCore-Pams monitor, 2500 1ST Street, N.W. Washington DC, 9 km north-east of the city (EPA site 11-001-0043).
2025 annual mean at the Aurora Hills Visitors Center monitor, S 18TH and Hayes ST, 4.3 km south-east of the city (EPA site 51-013-0020).
Largest air release: Allied Aviation Fueling Co Inc, Ronald Reagan National Airport (petroleum bulk terminals), inside the city limits, 1,266 lb in 2024.
Arlington County's utilities chapter makes it unlawful for anyone the County Manager has not authorized to operate, connect to or otherwise interfere with a hydrant, valve or main of the public water system, and when a main break, pump failure or other shortage hits, the County Manager can impose immediate emergency limits on water use announced by radio, television or door to door.
Arlington County Code §26-4: public water system (Code Updated July 2021) · checked September 2026
In Arlington it is unlawful to discharge, directly or indirectly, into the storm sewer system or state waters any substance the County Manager judges likely to harm them, and depositing dirt, leaves or other material into a gutter or drainage structure is banned too; a discharge posing an immediate threat to the environment or public health counts as serious.
Arlington County Code §26-5: storm sewer system (Code Updated July 2021) · checked September 2026
In Arlington's local historic districts a Certificate of Appropriateness is required for every exterior alteration, new construction and demolition. Routine maintenance, repairs and replacements in the same materials, interior work and paint colours are exempt; everything else goes through the Design Review Committee, then the Historical Affairs and Landmark Review Board.
Arlington County Historic Preservation: making changes to a historic building (certificate of appropriateness) · checked September 2026
Late-summer low-flow stretches concentrate dissolved solids in the Potomac supply. Citric prerinse with extended dwell (3-4 minutes) handles the calcium-carbonate-plus-organic composite.
Pine, oak, and grass pollen wave through the metro and the Piedmont. Standard wet-only handling.
Arlington County's 2025 average is 142 mg/L, or 8 grains per gallon, which is hard on the standard scale, so rinse water left to dry on glass will spot. The supply is treated by a federal engineering organisation, the Washington Aqueduct, rather than by a municipal utility, and the county reports the averages for the water it receives.
Residential window cleaning in Arlington typically runs $12–18 per pane or $350–600 for a standard single-story exterior. Height and access dominate, because the densest part of the county is also the part no ladder reaches.
Arlington's drier, frost-free months are April and August to October, by NOAA's 1991–2020 normals for the Washington Reagan Airport station, where May brings the most rain. The calendar on this page marks August (after the April 12 bloom), September (after smoke season) and October (before frost). Frost nights usually run from November 18 to March 24.
Not as the final rinse: Washington Aqueduct (Arlington DES) publishes hardness of 142 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 Potomac River water treated by the Washington Aqueduct (U.S. Army Corps of Engineers) at its Dalecarlia plant and bought by the county.
In Arlington the dominant residue patterns include federally treated potomac river supply: 2025 average hardness 142 mg/l (hard), varying year to year and footfall-driven reload on corridor retail frontage, indifferent to season. Corridor retail runs on footfall and reloads within a fortnight regardless of month.
The inter-war brick neighbourhoods are reasonable homeowner work at two storeys on level ground. Everything on the corridor is not, for the simple reason that most of it starts above the height a ladder should ever reach. Federal-adjacent frontage requires cleared access and is not available to casual operators at all.
High-rise office and residential glazing at the river bluff facing the capital. Curtain walling on contracted access-equipment schedules, with wind at height a genuine constraint on working days. Clarendon and Ballston and Lyon Village and Lyon Park have their own housing stock and glazing as well.
Just over the Virginia line