Locations
| Name | Sites | Samples |
|---|---|---|
| Carver County | 6 | 30 |
| Cass County | 1 | 19 |
| Cass County | 5 | 12 |
| Catahoula Parish | 21 | 92 |
| Cedar County | 2 | 4 |
| Chad | 1 | 1 |
| Chambers County | 25 | 113 |
| Charles City County | 5 | 32 |
| Charles County | 10 | 22 |
| Charlotte County | 1 | 3 |
| Chase County | 1 | 4 |
| Chatham County | 21 | 53 |
| Chatham County | 5 | 14 |
| Chautauqua County | 1 | 4 |
| Chautauqua County | 2 | 19 |
| Cheatham County | 7 | 53 |
| Cherokee County | 4 | 4 |
| Cherokee County | 1 | 1 |
| Cherokee County | 2 | 4 |
| Cheshire County | 4 | 18 |
| Chesterfield County | 3 | 5 |
| Chesterfield County | 1 | 1 |
| Chickasaw County | 3 | 11 |
| Chile | 27 | 50 |
| China | 2 | 0 |
| Chippewa County | 3 | 14 |
| Chippewa County | 3 | 5 |
| Chisago County | 1 | 1 |
| Choctaw County | 1 | 1 |
| Choctaw County | 3 | 17 |