544 A-Grade Islands Surrounded by Failing Neighbors

Data source: ZipCheckup analysis of EPA SDWIS, safety score percentiles, neighbor comparisons

Barksdale Air Force Base in Bossier City, Louisiana (ZIP 71110) scores a 87 — an A grade. All five of its neighboring ZIP codes have D or F grades.

How does one community achieve excellent water safety while every community around it struggles? That question led us to analyze 544 "island" ZIP codes — outliers that defy their geographic context, for better or worse.

What the Data Shows

Our anomaly engine compares every ZIP code's safety score against its geographic neighbors. When a ZIP code's grade differs by 2+ letter grades from the majority of surrounding ZIPs, it gets flagged as an outlier.

Of the 544 island anomalies:

• 19 are A-grade islands — scoring 85+ while surrounded by D/F neighbors
• The remaining are D/F outliers — scoring poorly while surrounded by A/B neighbors
• Grade gaps range from 2 to 4 letter grades compared to the nearest neighbors

The A-grade islands are the more instructive group. They prove that excellent water quality is achievable even in regions where most communities struggle.

The 19 A-Grade Islands

These ZIP codes stand as proof points — communities that figured out what their neighbors haven't:

Columbia, TN (38402) and Memphis, TN (37544) both score 99 out of 100 — nearly perfect — while surrounded by D and F grade neighbors. Whatever these communities are doing with their water systems, it's working.

Why Islands Exist: The Water System Boundary Effect

The most common explanation for islands of safety is deceptively simple: different ZIP codes are served by different water systems.

American drinking water is managed by approximately 148,000 public water systems, each with its own:

• Source water (groundwater well vs. surface water reservoir)
• Treatment plant (age, technology, capacity)
• Distribution infrastructure (pipe material, age, maintenance history)
• Regulatory compliance record

Source water itself differs dramatically between neighbors: across the 5,572 public utilities in the CCR Rich Dataset, the disclosed source-water-type splits between groundwater, surface water, blended, and purchased systems — each with distinct contamination profiles and treatment requirements.

ZIP code boundaries don't align with water system service areas. Two homes on the same street can receive water from different utilities, treated at different plants, delivered through different pipes. One might score an A. The other, an F.

What Makes Islands Succeed

Analyzing the A-grade islands reveals several patterns:

1. Independent Water Sources

Some islands succeed because they tap a different aquifer or reservoir than their neighbors. Geological formations create natural water quality boundaries — a shallow aquifer contaminated with agricultural runoff may sit next to a deep aquifer producing pristine water.

Nezperce, ID (83543) scores 99 in a region of D/F neighbors. The community likely benefits from Idaho's deep basalt aquifers, which are naturally filtered and protected from surface contamination.

2. Military and Federal Infrastructure

Barksdale AFB (71110) illustrates another pattern: federal installations often operate their own water systems with dedicated funding, regular upgrades, and strict compliance requirements. The military maintains its water infrastructure to standards that surrounding municipal systems can't afford.

3. Recent Infrastructure Investment

Some islands became islands through investment. A community that replaced its lead service lines, upgraded its treatment plant, or switched water sources can jump from a D to an A while neighbors who deferred maintenance stay behind.

4. Smaller System Advantages

Counter-intuitively, some small water systems outperform large ones. A system serving 2,000 people from a single well can maintain tighter quality control than a system serving 200,000 through miles of aging distribution pipes.

The Dark Side: D/F Outliers

The 525 non-A-grade islands tell the opposite story — communities that fail while their neighbors succeed. These D/F outliers amid A/B neighbors include:

• ZIP codes with a single contaminated well that drags down an otherwise clean region
• Communities that deferred infrastructure investment while neighbors upgraded
• Areas with legacy contamination (industrial sites, landfills) that doesn't affect neighboring ZIP codes

These outliers are arguably more actionable — the surrounding success proves that good water quality is achievable in the area. The outlier community just needs to do what its neighbors already did.

What This Means for Homebuyers

The island effect has a practical implication for anyone buying or renting: don't assume your ZIP code's water quality matches the area's reputation.

1. Always check the specific ZIP. A city's overall reputation tells you nothing about your particular ZIP code. Use ZipCheckup to check the exact address you're considering.

2. Compare with neighbors. Use our compare tool to see how your target ZIP stacks up against surrounding ZIP codes. If it's an outlier in either direction, investigate why.

3. Ask about the water system. Find out which utility serves the property and check its compliance record. Two homes on the same block may be on different systems.

4. Islands can change. A D-grade outlier today could become a B-grade ZIP after infrastructure investment. Check the score trends for trajectory — is the score improving or declining?

The Lesson for Communities

The existence of A-grade islands surrounded by failure proves a fundamental point: water quality is a choice, not a destiny. The geology, climate, and economic conditions of a region create a baseline — but individual community decisions about infrastructure investment, source water selection, and proactive maintenance determine whether that community thrives or struggles.

Every D/F outlier surrounded by successful neighbors is a community that could be doing better. The blueprint is literally next door.

Methodology: Island anomalies are identified by comparing each ZIP code's safety grade against the grades of its 3-5 nearest geographic neighbors. A ZIP qualifies as an island when its grade differs by 2+ letter grades from the majority (>50%) of neighbors. The 544 islands include both positive outliers (high grades amid low) and negative outliers (low grades amid high). Data from EPA SDWIS, ECHO, Census ACS. Current as of March 2026.