Lead-sheathed telecom cables and historic leaded gasoline emissions substantially raise environmental lead levels in Portland, Oregon

Lead is a highly toxic metal with no safe exposure threshold, linked to hypertension, cardiovascular and renal disease, neurodegenerative disease, and neurodevelopmental harm in children. Leaded gasoline, introduced in the 1920s and banned for on-road vehicles in the U.S. in 1996 (globally phased out by 2021), was a major source of widespread contamination; residues persist via resuspended contaminated soil and dust. Recent reporting identified numerous aerial lead-sheathed telecommunication cables (installed from late 1800s to 1960s) remaining in U.S. cities, potentially near sensitive locations. Lead isotopes provide a fingerprint to trace sources because ore deposits have characteristic isotopic compositions preserved through industrial processing. Epiphytic mosses and lichens, which accumulate metals from air and deposition, have long been used as biomonitors and correlate with atmospheric lead deposition. Prior urban studies often used low spatial resolution, limiting detection of localized sources. The purpose of this study was to identify and characterize legacy lead sources in Portland, Oregon, including the impact of lead-sheathed telecommunication cables, using a high-resolution moss-based survey with lead concentration and isotope analyses and comparisons to rural background and archival samples.

The study builds on decades of research identifying leaded gasoline as a dominant historical source of environmental lead, supported by isotopic evidence from polar archives and urban environments. Biomonitoring with mosses and lichens has shown strong correlations with atmospheric deposition, though past urban surveys often lacked the spatial resolution to reveal localized hotspots. High-resolution approaches have previously uncovered hidden point sources (e.g., a glass manufacturer for cadmium in Portland). Numerous studies report persistent legacy lead from gasoline in urban soils and air via resuspension, and isotopic fingerprints have been used to trace historical emissions in environments such as the U.S. West Coast, European cities, and Australia. Recent investigative journalism highlighted the persistence of aerial lead-sheathed telecommunication cables in U.S. cities as a potential modern source, but systematic scientific evaluation at high spatial resolution has been limited.

Study area and design: Portland, Oregon, and nearby rural sites were surveyed for lead in Orthotrichum lyellii moss. Four datasets were analyzed: (1) a 2013 citywide high-resolution moss collection (n = 347) based on a 1 km² grid of randomly selected residential addresses, including 70 field pairs to assess small-scale variability; (2) targeted 2023 sampling (n = 38) near identified lead-sheathed telecommunication cables to evaluate local impacts; (3) archival moss and lichen (n = 6) from 1959–1974; and (4) rural background moss (n = 11) from regional protected areas.

Sampling protocol: O. lyellii was collected at least 1 m above ground from hardwood street trees or nearby trees, favoring recent growth (upper two-thirds of thalli) to represent recent accumulation (months to ~3 years). In 2013, field pairs included collocated duplicates and pairs separated by 10–300 m. In 2023, sampling locations were selected within 1–35 m of lead cable segments (often on the same or opposite side of the street), with intensive coverage in select neighborhoods (e.g., Kenton/Portsmouth).

Lead cable identification: 54% of 2013 sites (n = 186) were checked for lead-sheathed cables using Google Street View (typically 2019–2022 images), prioritizing older annexation years and outlier isotope ratios; suspected sites were verified in person when possible. Historical Street View images (as far back as 2007) were also reviewed to identify cables that may have been removed prior to recent imagery.

Sample preparation and analysis: Samples were cleaned of extraneous material, trimmed to retain recent growth, oven-dried, ground, homogenized, and digested using ultrapure acids and H2O2. Lead was isolated for isotopic analysis by ion exchange chromatography. Elemental Pb concentrations were measured by ICP-MS (ICP-RQ) via external calibration with In or Re as internal standards; isotopic ratios (masses 202–208) were acquired with sample–standard bracketing using NIST SRM 1949 for normalization and interference correction for 206Pb via 199Hg monitoring. Isotope ratios reported include 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 206Pb/207Pb, 206Pb/208Pb, and 207Pb/208Pb.

Quality assurance/quality control: Certified reference materials BCR-482 and IAEA-336 were digested with each batch, yielding Pb recoveries of ~86–97%. Additional reference materials (e.g., USGS glasses and rocks) were run for isotopic consistency. Replicate isotopic measurements were made for 33 samples. Analytical precision (2×MAD, 95% confidence) was approximately: ±0.01% for 206Pb/204Pb and 207Pb/204Pb; ±0.02% for 208Pb/204Pb; ±0.09% for 206Pb/207Pb and 206Pb/208Pb; ±0.04% for 207Pb/208Pb. Data analysis used log-transformed Pb concentrations due to lognormal distributions; statistical tests included one-way ANOVA with Tukey’s post hoc tests and two-sided t-tests. Spatial context included neighborhood annexation era derived in GIS.

• Spatial heterogeneity and neighborhood age: 2013 Portland moss had a mean Pb concentration of 5.42 ppm (n = 347). Rural background moss averaged 0.463 ppm (n = 11). The oldest neighborhoods (annexed ≤1915) had a geometric mean of 8.58 ppm (n = 152), 2.3× higher than post-1948 neighborhoods (4.13 ppm; n = 135) and higher than Portland outskirts (3.12 ppm; n = 60). ANOVA showed a significant effect of neighborhood age on Pb (F(2,344) = 107, P < 0.001), with all pairwise differences significant (Tukey P < 0.002). Across the city, small-scale variability was pronounced, with 2–3× differences over 100–300 m and even larger differences within tens of meters.
• Temporal persistence and maxima: 2023 resampling at selected sites found sustained or elevated Pb, with concentrations up to 271 ppm (~586× rural background), nearly double the maximum observed in archival Portland moss collected during peak leaded gasoline usage. The maximum 2013 concentration was 129 ppm.
• Isotopic signatures and sources: Excluding cable sites, Portland moss isotope compositions are consistent with historical leaded gasoline as the dominant source via resuspended contaminated soil and dust. In the oldest neighborhoods, isotope ratios span wide ranges, including extreme radiogenic and unradiogenic values; for example, 206Pb/207Pb ratios as high as ~3.1306 and as low as ~1.1316 were observed, and field pairs separated by 30 m differed by ~0.8 in 206Pb/207Pb, indicating localized sources.
• Lead-sheathed telecommunication cables: Re-examination identified multiple 2013 sites with nearby aerial lead-sheathed cables (within 0–100 m). Moss near cables had a geometric mean Pb of 16.7 ppm, over 2× older-neighborhood sites without cables (7.88 ppm) and ~38× rural background. A two-sided t-test showed significantly higher Pb at cable-adjacent sites versus similar-aged areas without cables (t(1,420) = 20.0, P < 0.001). Targeted 2023 sampling confirmed strong cable effects: moss at cable locations had a geometric mean of 15.3 ppm vs 2.74 ppm at non-cable locations (5.56× higher), with the highest concentrations and most extreme isotope ratios within <1 m of cables; contributions remained detectable 6–100 m away. Elevated Pb and distinct isotopic signatures persisted at sites even years after cable removal.
• Other local sources: Isolated elevations were associated with specific point sources (e.g., a gun range). Lead-based paint likely contributes in older residential areas but was deemphasized by street-side sampling strategy.

The findings indicate that resuspension of legacy contamination from historical leaded gasoline remains the dominant urban lead source across Portland, as reflected by a relatively coherent isotopic signature in most moss samples. However, superimposed upon this background are pronounced local inputs from lead-sheathed telecommunication cables, producing the highest concentrations and extreme isotopic compositions. These cable-derived signals persist spatially up to at least tens of meters and temporally even after cable removal, likely through contamination of proximate soils and dust that are subsequently resuspended and deposited on moss. The strong statistical association between cable presence and elevated moss Pb demonstrates that these relic infrastructures meaningfully elevate environmental lead burdens in adjacent neighborhoods, particularly in older areas where such cables are more common. While other point sources (e.g., shooting ranges) and lead-based paint can contribute locally, their signatures were secondary within the street-side sampling context. Overall, high-resolution biomonitoring proved critical for detecting these highly localized sources that would be missed by lower-resolution approaches, thereby improving source apportionment and risk identification in complex urban landscapes.

This study provides the first high-resolution assessment of lead concentrations and isotope ratios in an urban moss across Portland, Oregon, revealing two dominant contributors to current environmental lead burdens: widespread legacy emissions from historical leaded gasoline and localized, substantial inputs from relic lead-sheathed telecommunication cables. The highest moss lead concentrations and most extreme isotopic ratios occur within meters of these cables, and impacts persist even after cable removal due to contamination of nearby soils and dust. High-resolution biomonitoring is a cost-effective tool for uncovering such localized sources and informing mitigation. The environmental lead burden near cable sites will continue to accumulate until cables are removed and contaminated soils are remediated. Future research should quantify human exposure pathways and risks near cable corridors, evaluate the spatial extent and duration of contamination following cable removal, refine isotopic source apportionment for mixed sources, and expand high-resolution surveys to other cities with similar legacy infrastructure.

• Source identification for cables was constrained by availability and timing of Google Street View imagery; some cables may have been removed before the most recent images, leading to under-identification.
• The biomonitoring focused on moss collected primarily from street trees; contributions from lead-based paint and yard soils near building foundations were not directly assessed and may be underestimated.
• The study emphasizes moss as a bioindicator; while preliminary soil data indicate substantial contamination near cables, comprehensive soil surveys and direct exposure assessments are pending.
• Some inconsistencies or uncertainties inherent in archival sample metadata and spatial geolocation may affect historical comparisons.
• Analytical uncertainties are small but present; although QA/QC was robust, certified moss CRMs are unavailable, necessitating reliance on lichen CRMs and other standards.