Table 1. Datasets used in this study.

Dataset	Location	Year(s)	Description	Sampling method	Dataset completeness and potential biases	Ref.
American Housing Survey	US excluding Island areas	2013, 2019, 2021	This was a longitudinal survey of housing, demographic, and economic characteristics of approximately 60,000–80,000 representative households.	The sampling method was a representative sample of all U.S. households at the time of sample selection (most recently in 2015 and before that, in 1985). New housing units were added each survey cycle.	The survey excludes group quarters, businesses, hotels, and motels. Additionally, the survey was more likely to classify rural seasonal homes as vacant units, counted large cluster septic systems as public sewer, and was thought to lose new households built in rural areas due to its longitudinal design.	[12, 22–26]
American Community Survey 5-year estimates	US excluding Island areas	2021	This was a monthly survey of characteristics of populations and households of a representative subsample of addresses in the US. The survey reports the number of households lacking complete plumbing facilities by geographic subdivision.	The survey was sent to a random sample of addresses in the U.S. every month. Each address was selected no more than once every 5 years.	The surveys were completed by self-response (although a subset of non-responses were followed up with personal visits). Responses were reviewed for completeness and questionnaires needing clarification were followed up by phone calls. The data was averaged over 5 years to reduce statistical fluctuations.	[27]
Florida Department of Health septic tank inspections	Florida	Up to 2012	This dataset includes the onsite sewage treatment and disposal systems locations inspected by the Florida Department of Health reported in June 2012.	The sample included onsite sewage treatment and disposal system permits recorded in the Florida Department of Health system.	As of 2022, a permit was required for construction and inspection was recommended every 3–5 years after. From this dataset, an estimated 7% of households were connected to septic tanks across the state. This was substantially less than the Florida Department of Environmental Protection’s estimate that approximately one third of Florida’s population used septic tanks [28].	[29]
EPA Clean Watersheds Needs Survey	US	2012	This survey included voluntary submissions of the locations of publicly owned wastewater collection and treatment facilities and their estimated population served for the purposes to assess funding needs for treatment works projects.	The responses were coordinated by states.	This survey had voluntary responses and variable effort and resources that each state put into the survey. States that had the most comprehensive responses were New York, California, Florida, New Jersey, Maryland, Iowa, Minnesota and Michigan. Missing facilities included those in South Carolina, the Northern Mariana Islands, and American Samoa; facilities whose projects did not have documented solutions or cost estimates; privately owned wastewater facilities; facilities on tribal lands and Alaskan Native Villages; and facilities whose projects received funding from other sources. Small community facilities are thought to be underrepresented due to having less resources for completing the survey, but this was not quantified.	[30, 31]
US Census Island Areas Decennial Survey	US Island Areas	2020	This was a survey of housing, social, and economic information of all housing units.	The survey was conducted in person and via phone interviews.	The COVID-19 pandemic impacted collection of data on group quarters [32] and the survey had higher nonresponse rates than previous years’ surveys, particularly for Guam. Quality control checks were performed on responses.	[33]

The datasets are further described in the S1 Appendix and the dataset completeness and potential biases are further assessed in the S2 Appendix. Additional datasets that are briefly described in the main text but primarily analyzed in the Supplementary information are summarized in S1 Table.