The hydrogen sulfide record at Arbor Hills
Two numbers frame it. At the fence line nearest homes, hydrogen sulfide has reached 327 ppb, about eleven times the fence-line action level. In the gas at the wells, the operator's own survey found up to 30,000 ppm as recently as April 2026. The monitor at the school next door, meanwhile, has stayed below 1 ppb for 68 straight months.
GFL took ownership of Arbor Hills Landfill on October 30, 2020, per their regulatory submission to EGLE (Nov 4, 2020, SRN N2688, signed by David Seegert).
Hydrogen sulfide (H2S) is the gas that gives landfill emissions their rotten-egg smell, and a recognized respiratory irritant at higher concentrations. Two kinds of measurement in the public record describe it at Arbor Hills: what actually reached the air at the property boundary, and how strong the source is in the gas inside the wells. This page puts each one next to what its level means for health. Every figure is the operator's own EGLE filing or the fence-line and school monitors, unaltered.
What reached the air: the fence line
The site's perimeter network is a ring of six stations (MS-1 through MS-6) along the property boundary. Between May 2022 and mid-2025 it recorded 56 hourly readings above 30 ppb, in 25 distinct episodes. The most significant are below, with what each level means for health. The table runs through mid-2025; later readings, and the sensor problems that make the recent record incomplete, are noted under the table.
| Date (ET) | Station | Peak (ppb) | How long | What the level means for health |
|---|---|---|---|---|
| 2022-12-08 | 5 of 6 stations | 78 | same hour, site-wide | 2.6x the 30 ppb acute health reference level (the fence-line action level) |
| 2023-01-07 | MS-1 | 87 | single hour | 2.9x the reference level |
| 2023-09-22 | MS-2 | 79 | single hour | 2.6x the reference level |
| 2023-10-05 to 06 | MS-2 | 155 | ~10 hours | 5.2x the reference level |
| 2024-02-22 | MS-2 | 146 | ~19 hours | 4.9x the reference level |
| 2025-01-22 | MS-3 | 110 | ~3 hours | 3.7x the reference level |
| 2025-05-13 | MS-3 | 135 | ~2 hours | 4.5x the reference level |
| 2025-06-18* | MS-3 | 327 | single hour | 10.9x the reference level; the highest on record |
Each bar is scaled to the record reading (327 ppb), with the tick at the 30 ppb action level.
* The table covers May 2022 to mid-2025. On September 14, 2026, three stations again recorded hourly readings above 30 ppb: 153.4 ppb at MS-1, 67.9 ppb at MS-4 and 65.4 ppb at MS-3. Several stations have also stopped producing usable readings, so the recent record is incomplete (details below).
Reference points and sources. The 30 ppb level is the fence-line (perimeter) action level in the site's consent judgment, and equals the OEHHA 1-hour acute health reference level; it is a 15-minute rolling standard, so these hourly readings are a reasonable short-term approximation to it, though the raw 15-minute feed would be needed for an exact count. Odor detection begins around 5 to 20 ppb. A separate 24-hour ambient screening level (72 ppb, the EGLE Air Toxics value, also the school monitor's 24-hour action level) uses a longer averaging time, so a single hour above it is not by itself a 24-hour exceedance. The 750 ppb acute-discomfort level (EPA AEGL-1), the monitor's 15-minute precautionary tier, was never reached at the fence line. Studies of people living next to municipal landfills link ambient hydrogen sulfide to health effects: Heaney et al. (2011) associated low-parts-per-billion hydrogen sulfide with odor complaints and self-reported symptoms, and Mataloni et al. (2016) linked modeled landfill hydrogen sulfide exposure to elevated respiratory mortality and hospitalization in a 242,000-person cohort. These fence-line episodes reached levels well above the exposures in the Heaney study.
How strong the source is: in the gas at the wells
In April 2026 the operator surveyed hydrogen sulfide well by well and filed the results with EGLE. Of the 506 wells surveyed, 261 carried gas at or above 76 ppm and 91 were at or above 500 ppm. The strongest are below; the six shown are Drager-tube reads at the tube's range limit, so their true values are at or above the figures listed. These are concentrations inside the extracted gas, so the health column describes the hazard of that raw gas, not resident exposure.
| Well | H2S in gas (ppm) | What the raw gas is |
|---|---|---|
| AHWWTS01 (leachate sump) | ≥30,000 | The instrument's ceiling; raw gas far past every acute-hazard level |
| AHW250R3 | ≥10,000 | ~100x the 100 ppm IDLH (immediately dangerous to life) |
| AHW265R3, AHW235R4, second sump | ≥5,000 each | ~50x the IDLH (AHW265R3 is an EGLE-flagged odor-risk well) |
| AHWW402R | ≥2,000 | ~20x the IDLH |
| 91 of 506 wells | ≥500 | Five times the IDLH or more |
| 261 of 506 wells | ≥76 | Above the level considered immediately dangerous if breathed |
Across all 506 surveyed wells, hydrogen sulfide in the gas spreads like this:
0 ppm (117) under 76 ppm (128) 76 to 500 ppm (170) 500 ppm or more (91). The median well reads 80 ppm, just above the 76 ppm level considered dangerous if breathed, and 261 of the 506 wells (52%) are at or above it.
These are the strength of the source, not the air anyone breathes. The gas is captured and piped to the flares and the gas-to-energy plant, and the wellheads are tested as confined-space hazards for exactly this reason. What reaches residents is the fence-line ambient figure in the first table, in ppb. Elevated hydrogen sulfide here also predates GFL: EGLE inspectors in 2019, under the prior owner, recorded up to 23 ppm at gas holes with a handheld meter.
Heat and hydrogen sulfide, in the same wells
Arbor Hills has a documented elevated-temperature problem, tracked on this site's thermal map and wellfield explorer and visible in the 2022 subsurface event. In the operator's April survey, the two problems show up in the same wells: the hotter a well, the more hydrogen sulfide its gas tends to carry.
This is an association in the operator's own data, a rank correlation of 0.60, not a proven cause. As far as we can find, no study has directly tested whether higher landfill temperature drives more hydrogen sulfide, so this is best read as a real but unexplained pattern, one worth putting in front of EGLE and the operator rather than a settled mechanism. It is not simply an artifact of a few unusual wells, a point checked by well type.1 And these are in-gas source readings, not resident exposure.
The fuller picture
The school next door has stayed clean. Ridge Wood Elementary, the most sensitive receptor near the landfill, has the longest clean record on the site. Under an EPA agreement, a monitor at the school has reported a value every month for 68 consecutive months, December 2020 through July 2026, and every one is below 1 ppb. This is a different instrument from the fence-line stations, a 24-hour sampler that reports down to a 1 ppb floor, so a reading below 1 ppb is a genuine low value rather than a frozen zero of the kind described next. On the 24-hour-average basis that monitor uses, the school has never approached the 72 ppb screening level.
Data-quality note (updated October 9, 2026; corrects the September 25 note): for long stretches, several fence-line stations' hydrogen-sulfide (H2S) channels have reported a single constant value while the station kept reporting and its methane channel kept varying. A sensor in clean air can legitimately read 0.0 for weeks or even months, but a constant non-zero value for months, or a constant 0.0 for more than a year while neighboring stations vary, is a standard data-quality flag, not evidence of clean air. The feed marks every reading below 7 ppb as "BDL" (below detection limit), apart from a small number with no label, and that includes the stuck 3.0–3.2 and 5.4 ppb values described here. So the BDL label cannot tell a stuck sensor from a working one, and a "BDL" reading is not on its own evidence that the air was clean. GFL's public dashboard shows only these labels (for both hydrogen sulfide and methane), not the numbers, so a stuck sensor and a working one look the same there; the numeric values are available in the feed's underlying data service and in this project's dataset. Now: MS-3 and MS-4 are reporting varying readings. MS-5 and MS-2 have resumed varying readings only recently, so their records are short: MS-5 since September 26, 2026 (up to 11.0 ppb), after reading 0.0 from May 28, 2025 (apart from one 0.1 ppb reading) and sending no data from September 16 to 26; MS-2 since September 24, 2026 (up to 2.3 ppb), after reading 0.0 from June 30, 2025; for its last week of that stretch, from 9 p.m. on September 16 to 7 p.m. on September 24, MS-2's methane channel also read a constant 0.0 ppm. Since 11 a.m. on October 6, MS-5 and MS-2 have each read 0.0 ppb in all but one hour. MS-6 read 3.0–3.2 ppb from May 28, 2025 (apart from two readings on June 11, 2026) until 11 a.m. on September 29, 2026, when it dropped to 0.0 ppb; it then sent no data after 5 p.m. that day until it resumed at 11 a.m. on October 5, read 0.0 ppb H2S and 0–2 ppm methane every hour through 5 p.m. on October 6, and has sent no data since. MS-1 has sent no data since noon on September 17, 2026, after reading exactly 0.0 for both gases from 9 p.m. on September 16. Earlier: MS-1 read a constant 3.0 or 3.1 ppb from May 28, 2025 to May 28, 2026, starting the same hour MS-5 and MS-6 went flat; MS-3 read a constant 5.4 ppb from July 15 to October 16, 2025, and 3.1 ppb from November 24 to December 1, 2025, the same value MS-1 and MS-6 read in those hours. A quiet record at a station during these periods is not evidence of clean air. The stations then reporting varying readings recorded hourly values above 30 ppb (the 2022 Consent Judgment's fence-line action level, which is defined on a 15-minute rolling average; an hourly value from the public feed is a screening signal, not a confirmed crossing), most recently on September 14, 2026, up to 153.4 ppb at MS-1 (which has sent no data since September 17). None of the stations reporting varying readings has recorded an hourly value at or above 30 ppb since. These are observations from GFL's public perimeter feed, hosted by Barr Engineering; the cause has not been publicly confirmed by GFL, EGLE or Barr Engineering. Status as of October 9, 2026, 12:00 p.m. ET.
1 The obvious question is whether the correlation is just a well-type effect, since leachate wells run both hot and high in hydrogen sulfide. It is not. Overall Spearman rank correlation 0.60 (505 wells); within the vertical gas wells alone 0.58 (295 wells); excluding every leachate, sump, and riser well 0.59 (only 6 wells excluded); controlling for well type across all categories, a moderate 0.35 remains. Computed from the operator's filed April 2026 survey and wellfield temperatures (in the data releases below). On the science: the abiotic elevated-temperature reaction pathway described in the landfill literature tracks other gases, not hydrogen sulfide, and we could not find a study that directly tests temperature against in-gas hydrogen sulfide, so this is an untested question, not a resolved one. Return.
The documents
- The operator's 2026 annual wellhead hydrogen-sulfide survey, all 506 wells, with the source PDF and a reproducible parser: wellfield hydrogen-sulfide survey release.
- The perimeter air-monitoring dataset, May 2022 to September 2026, behind the fence-line episode table: perimeter air data release.
- The wellfield gas and temperature dataset, 2021 to 2026, the temperatures paired against hydrogen sulfide in the figure: wellfield gas and temperature release.
- The school monitor's monthly reports: arborhillsmonitoring.com/Home/Ridgewood.