Why Some Counties Show No Data, and What a Negative Depth Means
"No data" means no qualifying well, not no aquifer
1,773 of 3,222 US counties have no USGS monitoring well that meets this site's bar (at least 15 years of record, a reading within the last 10 years, 6 or more usable measurements). That county's groundwater may well be moving, there is just no long-term well there to measure it. WaterTableCheck shows the nearest qualifying well's own trend as context on those pages, labeled as a neighbor's reading, never substituted for the county's own number. The exact bar is spelled out in the methodology guide.
A negative depth is a real reading, not an error
23 counties in this dataset, including Salt Lake County, UT, Honolulu County, HI, Polk County, FL, St. Tammany Parish, LA, currently show a negative depth to water. That is not a data bug: it means the well is flowing or artesian, water stands above land surface under its own pressure rather than sitting below it. Those wells still get a fitted trend like any other, the sign just flips which direction counts as "the water table dropping."
What to do if your own county has no data
A county with no qualifying well is not evidence either way about a specific property's well. A local well log, a driller's record, or a direct measurement from a licensed well contractor is the only way to know what is happening under one property, the same limit that applies even to counties this site does track, just without a county-level trend to use as a starting point. If your county does have a trend and it's declining, the well-pump risk guide covers what that means for a pump.