EPA system GU0000006 · Guam

Gwa Northern Water System: PFOS at or above EPA's 4.0 ppt limit

System ID GU0000006; community water system; Guam; source: groundwater; serves 139,207 people via 39,541 connections; status: active

In focus139,207people served, EPA registry

Public records. Clearer context.

Gwa Northern Water System is a community water system in Guam serving about 139,207 people, matched to 4 ZIP codes.

PFAS treatment declared to EPA: Granular activated carbon (2 of 102 sampling events), another treatment (2 of 102 sampling events) · PFAS found before UCMR 5: Yes, at 102 of 124 sampling events

PFAS and lithium: what EPA's national survey found

EPA tested this water for 30 substances and found 11. PFOA averaged 13.4 ppt where EPA's limit is 4.0 ppt, and PFOS averaged 67.5 ppt where EPA's limit is 4.0 ppt. EPA's limit applies to a running yearly average and is enforced from 2029. That PFOS result is higher than 99.9% of the 10,307 water systems EPA tested for it.

Between September 2023 and December 2025, EPA's UCMR 5 program tested 3,630 samples from 61 sampling locations for 30 contaminants (29 PFAS plus lithium). 11 were detected above the reporting level in at least one sample. Averaged over a complete monitoring set, the highest sampling location for PFOA was 13.4 ppt, at or above the MCL of 4.0 ppt that applies from 2029 (highest single sample 18.3 ppt). Averaged over a complete monitoring set, the highest sampling location for PFOS was 67.5 ppt, at or above the MCL of 4.0 ppt that applies from 2029 (highest single sample 135 ppt).

Sampling-point average: mean of a location's two (ground water) or four (surface water) UCMR 5 results with non-detects counted as zero, EPA's method for comparing UCMR 5 data to the PFAS MCLs. EPA rounds to the reference value's significant digits before deciding whether an average is "greater than" an MCL (PFOA/PFOS averages count as above 4.0 ng/L only from 4.05 ng/L; lithium results count as above the 10 ppb reference only from 15 ppb). This site does not apply that rounding: "at or above" here means the measured value is numerically at or above the reference.

Utility-reported context (UCMR 5 questionnaire, aggregated across sampling events): PFAS previously found at this system: yes at 102 of 124 sampling events, no at 22 of 124 sampling events; potential PFAS sources nearby: not known at 121 of 124 sampling events, no at 3 of 124 sampling events; PFAS treatment in place: no PFAS treatment at 98 of 102 sampling events, granular activated carbon at 2 of 102 sampling events, another treatment at 2 of 102 sampling events.

Lead and copper at customer taps

In the latest completed monitoring period, ending Dec 31, 2025, 9 in 10 sampled homes had lead at or below 1.7 ppb, below the 15 ppb action level that triggers utility action. The action level has been reached in 1 of the 8 periods on record.

In the most recent completed lead monitoring period, ending Dec 31, 2025, the 90th-percentile lead result at customer taps was 1.7 ppb, below the action level of 15 ppb; 1 of the 8 monitoring periods on record reached the action level. 1 further record is excluded here as censored or implausible in EPA's database. The 90th percentile is the value that 9 in 10 sampled homes were at or below. Values are in parts per billion (ppb).

Drinking water violations since Sep 3, 2016

EPA lists no drinking-water violations for this utility since Sep 3, 2016.

EPA's SDWIS database lists no drinking water violations for this utility since Sep 3, 2016.

None on file.

Sanitary surveys and site visits

Most recent state inspection: Jul 3, 2024 (sanitary survey); 5 visits since Sep 3, 2021.

What you can do about these results

ZIP codes served

Sources

Data vintage: EPA UCMR 5 occurrence data, samples collected 2023-01-03 – 2026-06-17, final release, August 2026 (file dated Aug 28, 2026); EPA SDWIS via ECHO, 2026Q2 extract (file dated Jul 9, 2026); EPA service-area boundaries v3.0; Census 2020 ZCTAs. Page generated 2026-09-03.

• EPA ECHO detailed facility report
• EPA UCMR 5 Data Finder
• EPA service-area boundary: modeled, 78.1 sq mi