How do I develop migration to groundwater remediation objectives for metals (inorganics)?
Three separate procedures for developing soil remediation objectives for the migration to groundwater pathway exist. For many inorganics, either a toxic characteristic leaching procedure (TCLP) test, a synthetic precipitation leaching procedure (SPLP), or a "totals" metal laboratory analysis may be used. To use the totals objectives, the pH of the soil that the metal is expected to migrate through must be obtained. It is useful to analyze for totals because the soil ingestion and inhalation remediation objectives are also measured in totals. For metals in groundwater, investigative samples should be unfiltered.
Can I use a background determination for metals?
Yes. A site specific background concentration can be calculated using an Illinois EPA approved statistical method. The background determination may then effectively be used as the remediation objective (
Fact Sheet 9). Also, Appendix A, Table G provides a look up table of values of concentrations of inorganic chemicals in background soils.
Also, in Appendix B, Table A, the footnote "t" is used for certain chemicals to indicate that the table value is likely to be less than the background concentration for the chemical. In those instances, screening or remediation concentrations using the procedures for determining area background may be more appropriate.
Any averaging and/or compositing of soil samples for a background determination must be consistent with the approach described in Section 742.225 (
Fact Sheet 10).
What are the remediation objectives for lead?
Appendix A, Table G provides background concentrations. The Tier 1 lead background objective is 20.9 mg/kg outside metropolitan areas and 36.0 mg/kg within metropolitan areas. The Tier 1 ingestion remediation objective for lead is 400 mg/kg. Concentrations exceeding 400 mg/kg generally require either: 1) remediation, or 2) engineered barriers and institutional controls.
Can I use the Tier 2 equations for metals?
Yes. The SSL equations may be used for metals. For example, to determine a migration to groundwater remediation objective you can use Appendix C, Table J, which provides Soil Water Sorption Coefficients (ks) values for eleven metals and nine ionizing organics. These values depend on the soil pH. Also, for all metals other than mercury, set Henry's Law Constant equal to zero.
How do I obtain the pH of the soil?
For Appendix B, Tables C and D and some Tier 2 groundwater equations, pH is a necessary component. Soil pH is a common laboratory test, and you can request it when submitting soil samples for analysis. Or, if using proper techniques, pH can be measured in the field. pH is based upon a logarithmic scale; therefore, pH sample results can not be averaged.
It is important to submit a sample for pH analysis that is representative of the soil that the contamination is expected to migrate through. Therefore, for evaluating the migration to groundwater pathway, the soil pH sample should be collected several feet below the surface in an uncontaminated area. For example, at a LUST site, the soil sample would be collected at a depth at or below the tank invert. For sites less than 0.5 acres, usually only one soil pH measurement is warranted.
For several metals, if the soil pH exceeds 8.0, a request can be made to the Bureau of Land to use the remediation objectives that correspond to a pH value of 7.75 to 8.0.
Metals, Example 1 - Application of the Industrial/Commercial Tables
|In this example, a site is requesting industrial/commercial soil remediation objectives, and the groundwater classification is Class II. Assume no routes are excluded from consideration, pursuant to Subpart C. The pH of the subsurface soil is 7.5. The barium and chromium soil remediation objectives for ingestion and inhalation, from Appendix B, Table B (Tier 1 Soil Remediation Objectives for Industrial /Commercial Properties) are:
| ||Ingestion |
The barium and chromium soil remediation objectives for migration to groundwater from Appendix B, Table B are:
| ||Soil Objective for the Migration |
to Class II Groundwater Route (mg/liter)
Also, the migration to groundwater objectives can be obtained from Appendix B, Table D, which gives pH specific objectives expressed in totals (mg/kg). This table shows 1,800 mg/kg for barium; no data is provided for chromium.
Therefore, the barium soil remediation objective is either:
1,800 mg/kg (which is protective for migration to groundwater, ingestion and inhalation), or
2.0 mg/l (which accounts for migration to groundwater) and 14,000 mg/kg (which accounts for ingestion and inhalation).
Also, for barium, the background soil concentration is 110 mg/kg for metropolitan areas, according to Appendix A, Table F.
Either 1, 2 or 3 above, may be used.
The chromium soil remediation objective is both 420 mg/kg and 1.0 mg/l. According to Appendix B, Tables C and D for chromium, a pH dependent value to determine the migration to groundwater remediation objective is not determined, so a TCLP or SPLP test must be used in addition to a totals test for ingestion and inhalation. However, the chromium background soil concentration is 16.2 mg/kg. Therefore, if all the on-site chromium soil sample results are less than 16.2 mg/kg, no further evaluation may be necessary, and you may not need to analyze by a TCLP or SPLP method.