Locations
| Name | Sites | Samples |
|---|---|---|
| Columbia County | 2 | 18 |
| Columbus County | 1 | 5 |
| Concho County | 3 | 18 |
| Concordia Parish | 5 | 6 |
| Conway County | 3 | 16 |
| Cooke County | 5 | 19 |
| Cooper County | 3 | 12 |
| Coryell County | 116 | 362 |
| Covington County | 1 | 2 |
| Cowley County | 16 | 70 |
| Crawford County | 5 | 10 |
| Crawford County | 2 | 10 |
| Crisp County | 2 | 1 |
| Crittenden County | 1 | 6 |
| Croatia | 25 | 61 |
| Crockett County | 13 | 33 |
| Cross County | 6 | 26 |
| Crow Wing County | 7 | 19 |
| Culpeper County | 1 | 10 |
| Cumberland County | 3 | 12 |
| Cumberland County | 3 | 6 |
| Cumberland County | 1 | 1 |
| Currituck County | 3 | 7 |
| Custer County | 5 | 11 |
| Cyprus | 5 | 27 |
| Czech Republic | 123 | 420 |
| Czechia | 64 | 297 |
| Dade County | 1 | 3 |
| Daggett County | 58 | 206 |
| Dakota County | 2 | 3 |