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Consideration of monitored natural attenuation (MNA) as a remedy component for metals-contaminated sites can be achieved using a site-specific screening approach, followed by application of one or a series of sequential extraction measurements. Hazardous waste sites contaminated with metals can be screened for the implementation of monitored natural attenuation on the basis of contaminant-specific soil chemical characteristics (i.e., Kd's, solubilities, and nonexchangeable sorbed fraction). Field cases are used to demonstrate the screening approach and to outline the primary considerations involved in accurately applying sequential extraction procedures to support the of MNA for site remediation. The results of these case studies provide strong evidence that site-specific screening and the use of sequential extraction procedures are effective methods for evaluating natural attenuation for metals impacted sites.

Organic ligand-promoted dissolution of oxide minerals can be enhanced or inhibited in the presence of specifically adsorbed oxyanions. It has been proposed that an oxyanion inhibits or enhances dissolution depending on the type of surface complex formed and the strength of the bond. Mononuclear complexes (especially if they are bidentate) accelerate dissolution, while binuclear complexes inhibit dissolution. Recent spectroscopic evidence indicates that chromate and arsenate form different surface complexes depending on surface coverages. This study examined the influence of chromate and arsenate on the oxalate promoted dissolution of goethite. Based on a previous spectroscopic study, oxyanion surface coverages were varied to generate both mononudear and binuclear surface complexes. Chromate and arsenate inhibited the oxalate promoted dissolution of goethite at all surface coverages investigated except at pH 6 (Γ arsenate = 1.70 × 10 -6 and Γ chromate = 1.66 × 10 -6 mol m -2 ). It is proposed that chromate and arsenate inhibit goethite dissolution by decreasing oxalate adsorption. This is accomplished because arsenate and chromate are more effective competitors for goethite surface sites than oxalate and upon adsorption increase the negative charge of the goethite surface. At pH 6 the adsorption of chromate and arsenate increases the negative charge of the goethite surface which in turn increases proton adsorption. Since proton adsorption is a necessary step for oxalate-promoted dissolution of goethite, and since proton activity at pH 6 is low, an increase in the negative charge of goethite upon adsorption of the oxyanions accelerates dissolution.

Numerous studies have been conducted examining nitrate (NO 3 ) leaching losses from agricultural land. Simulation models have been developed that allow one to predict the potential of NO 3 to leach to groundwater. However, many of these models treat NO 3 as a conservative tracer and do not evaluate surface chemistry. This study evaluated the surface charge properties and NO 3 adsorption capacity of four acid southeastern subsoils. Significant anion exchange capacity and NO 3 retention was found for two of the soils. Point of zero net charge (PZNC) was determined using an ion exchange method. Values of 3.1 and 3.6 were determined for two of the soils while PZNC values were not quantifiable for the other two soils in the pH range of 3 to 7. Nitrate adsorption isotherms were measured on untreated and chloride-saturated soils. Nitrate adsorption maxima determined from the linearized form of the Langmuir equation ranged from 1.40 to 2.13 cmol c kg -1 . Coefficients of determination (R 2 ) and adsorption maxima increased after chloride saturation. This was attributed to competition from anions such as sulfate, fluoride, and phosphate. Net positive charge and NO 3 retention were found to depend on the type and quantity of both variable and permanent charged minerals present in the soil and the composition of the exchange complex. These results demonstrated that acid subsoils high in variable charge minerals may have the potential to retard NO 3 movement to groundwater. Therefore, simulation models may need to account for NO 3 adsorption when modeling NO 3 movement in acid soils dominated by variable charge minerals.