With many urban gardeners growing vegetables, are the vegetables from these gardens safe to eat? Should we be concerned about the toxicity of trace metals in soil? Maybe, the soils are tested (link to the soil testing kits) for nutrients for determination of fertilizer needs, are the toxic metal levels in garden soil really get tested?
How is Oral Bioavailability Estimated for Toxic Metals in Soil
Unlike in water, not all toxic metals in soils (link to the EPA article Behavior of Metals in Soils) re ingested through food, because only a portion of these metals are taken up by the plants we eat thereby becoming bioavailable. The oral bioavailability (linked to the U.S. EPA guidance for evaluating the metal oral bioavailability in soils) can be estimated and calculated when the health effects of toxic metals from soils are evaluated.
In the U.S., the EPA sets the soil screening levels (SSLs) (link to the EPA soil screening guidance technical background document) for the contaminants’ levels (including metal levels) in the soil for the residential land use in future. The SSLs are used to determine if the contaminated soil needs to be further investigated, although the SSLs alone do not trigger the need for response actions. In other words, the SSLs are not a national clean-up standard.
The SSLs for metals are based on the total metal concentration in the soils being tested. The most common metal elements analyzed in soils include V, Cr, Mn, Co, Ni, Zn, Pb, and U, but other toxic metals or metalloid such as As, Cd, and Hg is also tested. You can read Guide for Soil Testing and Interpreting Results for a comprehensive understanding of soil testing.
Depending on the analytes, the speed of analysis, and the throughput required, different instrumentation is chosen. If the detection limit is not an issue, ICP-OES (link to the application note) is a common instrument used by many test laboratories due to its high matrix tolerance and fast speed for multiple metal element analysis.
The ICP-MS technique, with its high sensitivity, is often used for ultratrace metals detection, but the complex matrix and high total dissolved solids (TDS) in soil samples can be an issue for this technique because of the <0.2% (m/v) TDS limit (at the point the sample enters the plasma) imposed by the interface cones. However, there are ways to improve the TDS tolerance for ICP-MS, such as on-line autodilution or the gas dilution of the sample aerosols.
Soil Sample Preparation
Unlike water analysis, soil sample preparation is more complicated due to the inherent heterogeneity and variable mineral composition.
The traditional sample preparation method is EPA Method 3050B (link to method) and EPA Method 3051A (link to method). EPA 3050B is based on digestion of samples on a hot plate and provides two different procedures, depending on the elements tested and instruments used. EPA 3051A is a microwave digestion method for extraction of metal elements. However, neither of these methods are total digestion methods that can be used to meet the regulation for SSL.
EPA Method 3052 (link to method) uses microwave digestion and is an absolute total digestion method that removes immobile elements that bind strongly to silicate structure in soils.
If the total metal composition in soils is not required, quick methods, such as Mehlich-1 and -3 (link to a page on TRACE: Tennessee Research and Creative Exchange, the University of Tennessee), can be used. These methods do not need special sample preparation or extraction and are good for fast agriculture soil analysis of metals (link to the application note), but are not used in compliance with regulations.
There are no mandatory methods required for metal analysis in soils. EPA Method 200.7 (link to the application note) and EPA Method 200.8 (link to the application note) are often used as a reference guide for metal analysis method development in other countries.
In addition to the EPA methods, DIN 38406.E22 is used for Germany and ISO 11855 is used globally.
When running soil samples, consider the following factors that can improve analytical performance:
Increase radio frequency power to 1500 watts ( Thermo Scientific ICP-OES 7400) or 1600 watts (Thermo Scientific ICP-OES 7600) to generate a more robust plasma for high matrix soil.
Use an argon humidifier to increase the tolerance to complex matrix/high TDS.
Decontamination of Soils from Toxic Metals
While metal contamination can come from nature, human industrial activity is still the major cause of such contamination. Cleaning up contamination is expensive and time consuming; thus, preventing contamination is the best option.