Steps

Reference USGS's national Principal Aquifers of the United States dataset overview at usgs.gov/mission-areas/water-resources/science/principal-aquifers-united-states for background and downloadable GIS layers
For a live queryable REST service exposing this same USGS layer, use a state agency's ArcGIS re-hosting such as Nevada DNR's FeatureServer (https://arcgis.water.nv.gov/arcgis/rest/services/BaseLayers/USGS_Aquifers_Principal/FeatureServer/0), querying by point/bbox to identify the aquifer polygon covering a site
Cross-check the returned aquifer classification and national aquifer code against USGS's published aquifer documentation rather than treating the polygon label as self-explanatory
For state-specific hydrogeologic detail beyond the national principal-aquifer layer, use the relevant state geological survey's own well/aquifer web service (e.g. Kansas KGS, Illinois ISGS, already covered separately) rather than relying solely on the national dataset's coarser resolution
Note the principal aquifers dataset shows only the shallowest aquifer at a given location and was published in 2003 — it is not a substitute for site-specific test-hole or driller's log data

Known gotchas

The 'official' USGS REST endpoint for this exact layer is not consistently published on a usgs.gov domain — most live query access comes through state agencies re-hosting the same USGS data, so confirm data currency/version before relying on any particular mirror
The dataset only represents the shallowest principal aquifer at each location and dates to 2003 — deeper or locally significant aquifers may not be represented
This is a coarse national reconnaissance-scale dataset; do not use it as a substitute for a site-specific driller's log or state well database record

sgma.water.ca.gov / gis.water.ca.gov · 5 steps · unrated

Discover NGWMN groundwater web service capabilities via the SOS GetCapabilities request

cida.usgs.gov · 5 steps · unrated

Look up USGS monitoring site metadata (location, drainage area, available parameters) via the legacy NWIS Site Service

waterservices.usgs.gov · 5 steps · unrated

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Query a state or federal GIS service for the principal aquifer underlying a well site to support siting and hydrogeologic assessment

Steps

Known gotchas

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