| Range (ppm) | Total | Percent |
| | 4 | 8 |
| >0 ≤ 1000 | 8 | 16 |
| >1000 ≤ 10,000 | 4 | 8 |
| >10,000 ≤ 20,000 | 11 | 22 |
| >20,000 ≤ 30,000 | 7 | 14 |
| >30,000 ≤ 40,000 | 5 | 10 |
| >40,000 ≤ 50,000 | 2 | 4 |
| >50,000 ≤ 60,000 | 2 | 4 |
| >60,000 ≤ 70,000 | 4 | 8 |
| >70,000 | 3 | 6 |
| Total | 50 | 100 |