Using soil-specific partition coefficients to improve accuracy of the new South African guideline for contaminated land

Abstract

Contaminated land in South Africa is regulated through the National Environmental Management Waste Act (Act 59 of 2008) (NEMWA) and the National Norms and Standards for the Remediation of Contaminated Land and Soil Quality (NSCLA) (GN R.331 of 2014). These standards were obtained from the Framework for the Management of Contaminated Land. A soil screening value (SSV1) for the protection of groundwater resources is proposed which is based on a 2-phase (stage) equilibrium partitioning and dilution model which includes a dilution factor and partitioning coefficient (Kd), converting the water quality guideline to a total soil screening value. The appropriateness of the screening values has been questioned because of the uncertainties surrounding the Kd values used by the Framework. This paper investigates the Kd values of Cu, Pb, and V for selected South African diagnostic soil horizons to evaluate the reliability of the current Kd values used by the Framework during Phase 1 screening. The Kd values of Cu for the 10 horizons ranged between 13 and 19 044 ℓ∙kg^-1, all exceeding the value of 10 ℓ∙kg^-1 provided by the Framework. For Pb the values ranged from 25 to >252 294 ℓ∙kg^-1 as compared to the Framework’s 100 ℓ∙kg^-1. Similarly, the Kd value of 200 ℓ∙kg^-1 for V recommended by the Framework is higher than the measured Kd value of 15 to 173 ℓ∙kg^-1 for all 10 diagnostic horizons. This study demonstrated that the observed wide Kd value ranges for each element were related to the variation in basic soil properties such as soil pH, organic carbon, clay, Fe, and Al content. Therefore, the Kd values for Cu, Pb, and V currently used by the Framework are not representative of typical South African diagnostic soil horizons. Linear regression models were developed for the prediction of Cu, Pb, and V Kd values from measured soil properties, which could be used to generate soil-specific Kd values.

Keywords: Kd value, contaminated land, NEMWA, screening value, risk, soil