Comparison of biological and chemical indicators to evaluate soil remediation

Abstract

A large number of remediation technologies for polluted soils have been developed. Often the remediation goal is expressed as a maximum allowable total concentration of pollutant in the soil. In this case, the success of the remediation process can be assessed by simply measuring the concentration of the pollutant after the treatment. However, some remediation technologies, such as in situ fixation of metals using soil amendments, do not reduce the total concentration of the pollutant but modify its bioavailability and mobility. In this case, remediation can be assessed with techniques similar to those used in risk assessment procedures. We applied several chemical and biological indicators to assess the remediation efficiency of sixteen soil amendments applied to a total of thirteen contaminated soils. The ability of the soil amendments to reduce the mobility of the metals was chemically determined by measuring the concentration of metals in soil pore water, their extractability (DIN19730), and changes in the isotopically exchangeable fraction of metals (E-values). The potential bioavailability of As and Pb to humans was assessed using the Physiologically-Based Extraction Test (PBET). Plant biomass and metal concentration, soil microbial biomass, and N-mineralisation were the biological indicators employed. The results show that the different indicators tested responded differently to the remediation treatments. For instance E-values decreased less markedly than concentration of metals in soil pore water. However, these results have are complementary because give an indication of whether the reduction in metal toxicity is simply due to increase in partition coefficients of metals or to metal fixation. In some cases, microbial parameters such as measurement of soil microbial biomass and N-mineralisation did not agree. This is possibly due to differential responses of these tests to confounding factors such as changes in pH that occur as a result of the application of alkaline amendments. In conclusion, this study emphasises that the assessment of remediation procedures should be evaluated using a suite of biological and chemical tests that take into account both the nature of the remediation treatment, and the future land use of the site.