District of Columbia Water Hardness by County
Water in District of Columbia has a median hardness of 113 mg/L as CaCO₃ — "moderately hard" on the USGS scale — based on 119 measurements at 61 monitoring stations. Its single measured area, District of Columbia County, medians 113 mg/L. That is 0.81× the national county median of 140. Rank: #25 of the 50 states and D.C.
District of Columbia water hardness map — what the numbers mean. Data verified 2026-07-22.
District of Columbia is a single measured jurisdiction: its water medians 113 mg/L as CaCO₃, moderately hard. Because it is moderately hard — below the 120 mg/L hard threshold — none of its measured residents live in a hard-or-worse county.
See where District of Columbia sits nationally on the national map, or what its number means on the hardness scale.
Download the District of Columbia water hardness map (PNG) — Water hardness by county in District of Columbia: 1 counties mapped from USGS/EPA data, statewide median 113 mg/L (moderately hard); darker counties have harder water.
By county
Water hardness by county
The table below shows District of Columbia's single measured area.
| County | Median | Band | Results | Sites | Newest |
|---|---|---|---|---|---|
| District of Columbia County | 113 | Moderately hard | 119 | 61 | 2022 |
ZIP codes
ZIP codes in District of Columbia
The ZIP codes below are grouped by the District of Columbia county they fall in. Each ZIP takes its county's measured median — shown in the group header — so this is a ZIP-to-county guide, not a separate reading for every ZIP. To check the water system serving a specific address, use the lookup.
District of Columbia County · 113 mg/L · Moderately hard
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Source
Groundwater vs surface water
District of Columbia's surface water measures harder than its groundwater — 120 mg/L across 41 samples versus 108.5 across 78, about 1.1× harder. For well owners that is the better side of a hard bargain: at 108.5 mg/L, District of Columbia groundwater is still moderately hard, and most of the state's drinking water is pumped from it. Groundwater readings there run 25.4 to 311.2 mg/L across the middle 80% of samples. The comparison draws on 38 groundwater and 23 surface-water monitoring sites.
Groundwater (wells)
108.5 mg/L as CaCO₃ · 78 results at 38 stations.
Surface water (rivers, lakes)
120 mg/L as CaCO₃ · 41 results at 23 stations.
Surface water measures about 1.1× harder than groundwater here.
Chemistry
Water chemistry in District of Columbia
Beyond the hardness number, District of Columbia's water carries a fuller chemical signature — here is how its measured parameters compare to national benchmarks.
TDS
209.5 mg/L · n=80
209.5 mg/L — below the national county median of 226.
pH
7.4 pH · n=510
pH 7.4 — within the EPA secondary range of 6.5–8.5, on the alkaline side.
Alkalinity
56 mg/L as CaCO3 · n=132
56 mg/L — modest buffering capacity.
Calcium : Magnesium
27.6 : 9.2 mg/L ≈ 3 : 1 · n=104
Calcium is about 65% of District of Columbia's hardness (27.6 mg/L Ca to 9.2 mg/L Mg), a little more magnesium-rich than the national ~68% calcium share.
Parameter medians may include a small share of quality-control samples; the hardness lane is fully QC-filtered.
Aquifers
Major aquifers under District of Columbia
Two aquifer systems supply much of District of Columbia's measured groundwater.
- Piedmont and Blue Ridge crystalline-rock aquifers — median 157.5 mg/L across 16 samples at 10 sites: hard.
- Northern Atlantic Coastal Plain aquifer system — median 89.2 mg/L across 56 samples at 25 sites: moderately hard.
Together these systems account for 72 measured samples at 35 sites. Aquifer medians pool samples across a whole system and can mask local variation — see the national aquifer map for how these compare across states.
Trend
Hardness over time
District of Columbia's decade median moved from 152.5 mg/L in the 2000s to 91.3 in the 2010s. Decade medians reflect where and what agencies sampled in each era, not just changes in the water itself.
Scale
Where District of Columbia sits on the scale
On the USGS scale, District of Columbia's 113 mg/L is moderately hard — 6.6 grains per gallon, between the 60 and 120 mg/L soft and hard cutoffs.
Questions
Frequently asked questions
- Is District of Columbia's water hard?
- District of Columbia's median hardness is 113 mg/L as CaCO₃, "moderately hard" on the USGS scale and below the national county median of 140 mg/L.
- Which county in District of Columbia has the hardest water?
- District of Columbia County, with a median of 113 mg/L as CaCO₃ across 119 measurements.
- Is well water harder than city water in District of Columbia?
- Surface water measures harder here — about 1.1× the groundwater — so wells tend to run softer (groundwater 108.5 vs surface water 120 mg/L).
- How do I find my exact hardness in District of Columbia?
- Search your ZIP on the lookup tool, read your utility's Consumer Confidence Report, or test at home with a kit.
- Is District of Columbia's water harder than the national average?
- No. District of Columbia's median of 113 mg/L is below the national county median of 140 mg/L; it ranks #25 of the 50 states and D.C.
Compare
States with similar hardness
Two states measure closest to District of Columbia's 113 mg/L: Wisconsin at 110 mg/L and Kentucky at 120 mg/L. District of Columbia measures harder than 52% of states. Browse the full ranking on the statistics page.
More: hardness scale · check your address · national statistics · the national map.
By Marko Visic, MPharm, BSc Physics. Source: USGS/EPA Water Quality Portal — 119 District of Columbia measurements. Data last verified . Methodology.
Cite this page: “District of Columbia Water Hardness by County.” Hard Water Map. Retrieved 2026-07-22, https://hardwatermap.com/water-hardness/district-of-columbia/