Delaware Water Hardness by County
Water in Delaware has a median hardness of 63.9 mg/L as CaCO₃ — "moderately hard" on the USGS scale — based on 38,083 measurements at 1,599 monitoring stations. New Castle County has the state's hardest measured water (99.8 mg/L); Sussex County the softest (41.1). Rank: #32 of the 50 states and D.C.
Delaware water hardness map — what the numbers mean. Data verified 2026-07-22.
Of Delaware's 3 measured counties, 2 moderately hard and 1 soft. The hardest are New Castle County (99.8 mg/L), Kent County (60.8), and Sussex County (41.1). Sussex County is the softest at 41.1 mg/L (soft).
No measured Delaware county reaches the hard threshold, so effectively none of its residents in counties with gate-passing hardness data live in a hard-or-worse county.
Statewide, Delaware's median works out to 3.7 grains per gallon and ranks #32 of the 50 states and D.C. Across 38,083 measurements at 1,599 stations — 35,085 of them since 2000 — the middle 80% of readings fall between 28.7 and 407.1 mg/L. The public record runs from 1931 to 2025.
See where Delaware sits nationally on the national map, or what its number means on the hardness scale.
Download the Delaware water hardness map (PNG) — Water hardness by county in Delaware: 3 counties mapped from USGS/EPA data, statewide median 63.9 mg/L (moderately hard); darker counties have harder water.
By county
Water hardness by county
The table lists all 3 measured Delaware counties, hardest first. Their medians span 99.8 mg/L in New Castle County to 41.1 in Sussex County — a 2.4× range.
| County | Median | Band | Results | Sites | Newest |
|---|---|---|---|---|---|
| New Castle County | 99.8 | Moderately hard | 12,724 | 546 | 2025 |
| Kent County | 60.8 | Moderately hard | 9,981 | 307 | 2025 |
| Sussex County | 41.1 | Soft | 15,378 | 746 | 2025 |
ZIP codes
ZIP codes in Delaware
The ZIP codes below are grouped by the Delaware 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.
New Castle County · 99.8 mg/L · Moderately hard
1970119702†1970319706197071970919710†19711197131971619717197201973019731†19732†1973319734†19735†19736†19801198021980319804198051980619807†198081980919810
Kent County · 60.8 mg/L · Moderately hard
19901†1990219904†19934†1993619938†19943†19946†19952†19953†19954†1995519962†19964†19977†19979†19980†
Sussex County · 41.1 mg/L · Soft
1993019931†19933†19939†19940†19941†19944†1994519947†19950†19951†19956†19958†19960†19963†199661996719968†199701997119973†19975†
† 38 ZIP codes are not served by a mapped community water system — most are private-well areas. See well water hardness.
Source
Groundwater vs surface water
Delaware's surface water measures harder than its groundwater — 64.3 mg/L across 36,663 samples versus 50.4 across 1,419, about 1.3× harder. For well owners that is the better side of a hard bargain: at 50.4 mg/L, Delaware groundwater is still soft, and most of the state's drinking water is pumped from it. Groundwater readings there run 10.7 to 135 mg/L across the middle 80% of samples. The comparison draws on 851 groundwater and 747 surface-water monitoring sites; 5,713 readings above 2,000 mg/L were logged but held out of the medians as outliers.
Groundwater (wells)
50.4 mg/L as CaCO₃ · 1,419 results at 851 stations.
Surface water (rivers, lakes)
64.3 mg/L as CaCO₃ · 36,663 results at 747 stations.
Surface water measures about 1.3× harder than groundwater here.
Chemistry
Water chemistry in Delaware
Beyond the hardness number, Delaware's water carries a fuller chemical signature — here is how its measured parameters compare to national benchmarks.
TDS
313 mg/L · n=2,296
313 mg/L — about 1.4× the national county median of 226, consistent with hard, mineral-rich water.
pH
7.3 pH · n=15,900
pH 7.3 — within the EPA secondary range of 6.5–8.5, on the alkaline side.
Alkalinity
37 mg/L as CaCO3 · n=35,135
37 mg/L — modest buffering capacity.
Calcium : Magnesium
15.7 : 6.1 mg/L ≈ 2.57 : 1 · n=12,646
Calcium is about 61% of Delaware's hardness (15.7 mg/L Ca to 6.1 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 Delaware
Two aquifer systems supply much of Delaware's measured groundwater.
- Piedmont and Blue Ridge crystalline-rock aquifers — median 115 mg/L across 47 samples at 46 sites: moderately hard.
- Northern Atlantic Coastal Plain aquifer system — median 48.1 mg/L across 1,066 samples at 601 sites: soft.
Together these systems account for 1,113 measured samples at 647 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
Delaware's decade median moved from 52 mg/L in the 1940s to 68 in the 2020s. Over the series, the decade median peaked at 68.9 mg/L in the 2010s and bottomed at 37 in the 1970s. Decade medians reflect where and what agencies sampled in each era, not just changes in the water itself.
Scale
Where Delaware sits on the scale
On the USGS scale, Delaware's 63.9 mg/L is moderately hard — 3.7 grains per gallon, between the 60 and 120 mg/L soft and hard cutoffs.
Questions
Frequently asked questions
- Is Delaware's water hard?
- Delaware's median hardness is 63.9 mg/L as CaCO₃, "moderately hard" on the USGS scale and below the national county median of 140 mg/L.
- Which county in Delaware has the hardest water?
- New Castle County, with a median of 99.8 mg/L as CaCO₃ across 12,724 measurements.
- Is well water harder than city water in Delaware?
- Surface water measures harder here — about 1.3× the groundwater — so wells tend to run softer (groundwater 50.4 vs surface water 64.3 mg/L).
- How do I find my exact hardness in Delaware?
- Search your ZIP on the lookup tool, read your utility's Consumer Confidence Report, or test at home with a kit.
- Is Delaware's water harder than the national average?
- No. Delaware's median of 63.9 mg/L is below the national county median of 140 mg/L; it ranks #32 of the 50 states and D.C.
Compare
States with similar hardness
Two states measure closest to Delaware's 63.9 mg/L: Arkansas at 62 mg/L and New Jersey at 59.9 mg/L. Delaware measures harder than 38% 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 — 38,083 Delaware measurements. Data last verified . Methodology.
Cite this page: “Delaware Water Hardness by County.” Hard Water Map. Retrieved 2026-07-22, https://hardwatermap.com/water-hardness/delaware/