Locations
| Name | Sites | Samples |
|---|---|---|
| Marshall County | 1 | 1 |
| Marshall County | 1 | 9 |
| Martin County | 5 | 13 |
| Martin County | 1 | 24 |
| Mason County | 3 | 82 |
| Matagorda County | 1 | 8 |
| Mauritius | 1 | 11 |
| Maverick County | 5 | 28 |
| McClain County | 1 | 2 |
| McCulloch County | 3 | 17 |
| McDowell County | 2 | 7 |
| McIntosh County | 2 | 7 |
| McKean County | 1 | 2 |
| McLennan County | 4 | 87 |
| McMullen County | 7 | 25 |
| McPherson County | 2 | 5 |
| Meade County | 1 | 2 |
| Meade County | 7 | 15 |
| Meagher County | 1 | 5 |
| Medina County | 4 | 18 |
| Meeker County | 1 | 1 |
| Meigs County | 5 | 11 |
| Meigs County | 1 | 22 |
| Menard County | 2 | 14 |
| Menominee County | 2 | 4 |
| Mercer County | 1 | 1 |
| Mercer County | 1 | 7 |
| Merrick County | 2 | 5 |
| Mexico | 66 | 202 |
| Miami County | 2 | 9 |