Naturally-Occurring Metals
There are several methods for determining if metals concentrations represent natural background under RCSA 134tt-2(f) reflected in the table below. The options are listed from simplest to most complex.Single sample direct comparison to Connecticut baseline low values
If the analytical result is less than or equal to the Low Value in the table, it is a naturally occurring concentration.
Three sample comparison to Connecticut baseline high values
Collect at least three soil samples from:
- area(s) not impacted by site activity
- from the same geologic unit as the detection in question
If all three metal results are all less than or equal to the High Value in the table, the highest of those three detections can be considered a naturally occurring concentration.
Five sample evaluation below Residential Direct Exposure Criteria
Collect at least five soil samples from:
- area(s) not impacted by site activity
- in the same geologic unit as the detection in question
- from which outliers have been removed
If all metal results are less than the Residential Direct Exposure Criteria, the highest of those 5 detections can be considered a naturally occurring concentration.
Seven sample evaluation above Residential Direct Exposure Criteria with Commissioner approval
Collect at least seven soil samples from:
- area(s) not impacted by site activity
- in the same geologic unit as the detection in question
- from which outlier sample data has been removed
If any metal result is greater than the Residential Direct Exposure Criteria, the Commissioner's approval may be sought to consider that detection as naturally occurring.
Note: Outlier analyses may be done in ProUCL.
Any other method requires Commissioner written approval
Other methods may be used to establish a metal concentration as naturally occurring, however, an explanation of the method used and all associated documentation must submitted for the Commissioner's review and written approval.
| Inorganic Substance | Option 1 Default Values in mg/kg (ppm) | Option 2 Upper Limit in mg/kg (ppm) |
|---|---|---|
| Aluminum | 58,800 | 85,040 |
| Antimony | 0.1 | 0.3 |
| Arsenic | 3 | 6 |
| Barium | 385 | 756 |
| Beryllium | 2 |
2 |
| Bismuth | 0.2 | 0.5 |
| Cadmium | ND < 0.1 | 0.3 |
| Calcium | 5,610 | 20,870 |
| Cerium | 73 | 138 |
| Cesium | ND < 5 | 8 |
| Chromium | 31 | 60 |
| Cobalt | 9 | 20 |
| Copper | 17 | 45 |
| Gallium | 15 | 24 |
| Indium | 0.05 | 0.1 |
| Iron | 26,080 | 51,940 |
| Lanthanum | 32 | 66 |
| Lead | 18 | 27 |
| Lithium | 20 | 60 |
| Magnesium | 5,840 | 15,320 |
| Manganese | 544 | 1,195 |
| Mercury | 0.03 | 0.1 |
| Molybdenum | 0.7 | 4 |
| Nickel | 13 | 36 |
| Niobium | 10 | 17 |
| Phosphorus | 339 | 817 |
| Potassium | 16,930 | 29,620 |
| Rubidium | 80 | 117 |
| Scandium | 12 | 18 |
| Selenium | ND < 0.2 | 0.8 |
| Silver | ND < 1 | ND < 1 |
| Sodium | 9,930 | 18,500 |
| Strontium | 125 | 283 |
| Sulfur | ND < 100 | 304 |
| Thallium | 0.4 | 0.8 |
| Thorium | 11 | 15 |
| Tin | 2 | 3 |
| Titanium | 2,880 | 4,660 |
| Tungsten | 0.9 | 2 |
| Uranium | 3 | 7 |
| Vanadium | 69 | 102 |
| Yttrium | 19 | 31 |
| Zinc | 44 | 104 |
Content last edited February 27, 2026