Ecological Soil Screening Level Organic Contaminants
Of the six organic contaminants (DDT and Metabolites, Dieldrin, PCP, PAHs, RDX and TNT), DDT and metabolites, and dieldrin are very hydrophobic, highly lipophilic, and persistent nonionic organic contaminants. These contaminants are highly sorbed to soil surfaces and organic matter domains, thus persistent in soil, and tend to bioaccumulate and biomagnify in the food chain. The structure and degree of chlorination of these contaminants and associated congeners for each directly impacts their behavior, persistence, and bioavailability (. Solubility decreases, sorption increases, and thus bioavailability generally decreases with increasing chlorination. However, uptake, degradability, and toxicity are also impacted by placement of the chlorines in the biphenyl structure. The remaining nonionic organic contaminants, polyaromatic hydrocarbons (PAHs) and explosives (TNT and RDX) are generally considered less persistent and therefore, are more bioavailable than pesticides under identical soil conditions. PAHs are compounds with two or more aromatic rings in their structure and consist of only C and H. PAHs can be highly retained by soil in a similar manner as pesticides, but are considered less persistent due to their higher affinity to be degraded microbially. TNT and RDX, a trinitro aromatic and trinitro nitrogen-heterocylcic respectively, are explosive materials and are more polar than PAHs. The only ionizable organic contaminant being considered at this time in the development of Eco-SSLs, is the organic acid pentachlorophenol (PCP). Organic acids can exist as either a nonionic species or as an organic anion, which is dependent on the acid dissociation constant (pKa) and pH. In the pH range relevant to most environmental scenarios, PCP can exist as both a neutral species and as an anionic species; however, the majority will exist as the organic anion.
For all nonionic organic compounds (NOC) and the neutral form of PCP, sorption by soil is primarily related to their hydrophobicity and the amount of organic matter present in the soil , with the exception of the more polar, nitro-substituted organic contaminants (i.e., the explosives). Differences in the distribution of several NOCs in diverse soil-water and sediment-water systems have been minimized by normalization to organic matter or more specifically organic carbon (OC) with OC-normalized distribution coefficients, referred to as Koc values . The greater the affinity of a contaminant for organic matter, the larger the Koc, and a soil with higher amounts of organic matter has a higher propensity to sorb NOCs. The hydrophobicity of organic compounds, thus the Koc, increases with the size of the compound and with increasing chlorine content, in the case of chlorinated organics. Therefore, sorption by soils of PAHs increases with the number of aromatic rings. Increasing compound hydrophobicity also reflects increasing lipophilicity, which will result in a greater propensity to bioaccumulate in the lipid fraction of biota. For PCP, an ionic contaminant, the anionic species has a greater tendency relative to the neutral PCP to remain in the pore-water similar to metal anions. Therefore, pH-dependent speciation drastically modifies the solubility, sorption, transport, and bioavailability of PCP. Although organic matter is the primary sorption domain in soils, all contaminants have some affinity to be associated with any surface through weak physical forces . In addition, the nitro-substituted NOCs are known to have specific interactions with clay surfaces that are impacted by the inorganic cations present and clay charge density, and less so by the amount of organic matter present.
A common contaminant index representing the degree of hydrophobicity and lipophilicity of an organic contaminant is the octanol-water partition coefficient (Kow), which is the contaminant distribution between octanol and water phases. Kow values are positively correlated to both Koc values and bioconcentration factors.
For additional information, see the Ecological Soil Screening Level site.