A 19th century slide, involving an estimated 25 × 106 m3 of rock, devastated Rubble Creek Valley, 80 km north of Vancouver, B.C. Breaking away from a headwall composed of late glacial dacitic lava, the slide travelled as much as 4.6 km with a maximum drop of 1060 m, thus moving on an average slope of 8.5°. Velocities, estimated from the superelevation of the slide as it moved around curves in the valley, exceed 20 m/s (72 km/h) and sliding was probably completed within 10 min. Tree-ring data indicate that the slide occurred in the fall or winter of 1855–1856. The trigger mechanism has not been identified, but the presence of an exceedingly steep original slope of the lava front, attributed to ponding against latest Pleistocene ice occupying the valley below, was clearly a contributing factor. Both the precipitous headwall and a second ice-dammed lava front are considered to be potential sources for new slides. Some evidence suggests that previous slides have occurred here since the last glaciation, about 11 000 years ago. A court ruling barring residential development in the area devastated in 1855–1856 on the grounds of future hazard to life seems justified.

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Municipal wastewater collection and treatment systems are critical infrastructures, and they are also identified as major sources of anthropogenic CH4 emissions that contribute to climate change. The actual CH4 emissions at the plant- or regional level vary greatly due to site-specific conditions as well as high seasonal and diurnal variations. Here, we conducted the first quantitative analysis of CH4 emissions from different types of sewers and water resource recovery facilities (WRRFs). We examined variations in CH4 emissions associated with methods applied in different monitoring campaigns, and identified main CH4 sources and sinks to facilitate carbon emission reduction efforts in the wastewater sector. We found plant-wide CH4 emissions vary by orders of magnitude, from 0.01 to 110 g CH4/m3 with high emissions associated with plants equipped with anaerobic digestion or stabilization ponds. Rising mains show higher dissolved CH4 concentrations than gravity sewers when transporting similar raw sewage under similar environmental conditions, but the latter dominates most collection systems around the world. Using the updated data sets, we estimated annual CH4 emission from the U.S. centralized, municipal wastewater treatment to be approximately 10.9 ± 7.0 MMT CO2-eq/year, which is about twice as the IPCC (2019) Tier 2 estimates (4.3–6.1 MMT CO2-eq/year). Given CH4 emission control will play a crucial role in achieving net zero carbon goals by the midcentury, more studies are needed to profile and mitigate CH4 emissions from the wastewater sector.

An increasing percentage of US waste methane (CH 4 ) emissions come from wastewater treatment (10% in 1990 to 14% in 2019), although there are limited measurements across the sector, leading to large uncertainties in current inventories. We conducted the largest study of CH 4 emissions from US wastewater treatment, measuring 63 plants with average daily flows ranging from 4.2 × 10 −4 to 8.5 m 3 s −1 (<0.1 to 193 MGD), totaling 2% of the 62.5 billion gallons treated, nationally. We employed Bayesian inference to quantify facilityintegrated emission rates with a mobile laboratory approach (1165 crossplume transects). The median plant-averaged emission rate was 1.1 g CH 4 s −1 (0.1−21.6 g CH 4 s −1 ; 10th/90th percentiles; mean 7.9 g CH 4 s −1 ), and the median emission factor was 3.4 × 10 −2 g CH 4 (g influent 5 day biochemical oxygen demand; BOD 5 ) −1 [0.6−9.9 × 10 −2 g CH 4 (g BOD 5 ) −1 ; 10th/90th percentiles; mean 5.7 × 10 −2 g CH 4 (g BOD 5 ) −1 ]. Using a Monte Carlo-based scaling of measured emission factors, emissions from US centrally treated domestic wastewater are 1.9 (95% CI: 1.5−2.4) times greater than the current US EPA inventory (bias of 5.4 MMT CO 2 -eq). With increasing urbanization and centralized treatment, efforts to identify and mitigate CH 4 emissions are needed.

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