Chromium(VI) leaching from large undisturbed soil lysimeters following application of a simulated copper-chromium-arsenic (CCA) timber preservative

Abstract

Copper, chromium, and arsenic (CCA) solutions are commonly used in New Zealand as a means of preserving softwood timbers such as Pinus radiata. With stock working solutions of CCA salts in timber treatment plants frequently 10% w/v or more, there exists a potential for spillage and leaching of these compounds to groundwater.

High concentrations of Cr(VI) (up to 52 mg Cr/L) were found in the leachates of large undisturbed soil lysimeters where a Templeton sandy loam (Immature Pallic) had received surface applications of a simulated copper, chromium, and arsenic (CCA) timber preservative. Leaching was produced by using a combination of natural and imposed rainfall simulation over the lysimeters for a period of 102 days after CCA application.

An average of 26% of the applied chromium was collected in the leachates after 102 days. Of the mean 74% of Cr(VI) still retained within the soil profile after leaching ended, almost half was located in the top 100 mm of the profile. No copper or arsenic was detected in any of the lysimeter leachates, with soil analysis indicating that these elements had been retained within the soil profile.

In an incubation study, soil cores sampled from the same Templeton sandy loam and split into alternate 50-mm segments (to 450 mm) were stored at 10˚C for 102 days after addition of an identical CCA solution. These were periodically extracted for available chromium. Results showed that the reduction of dichromate/chromate anions (Cr₂O₇²–/CrO₄^2–) to the strongly sorbed chromic cation (Cr³⁺) was largely first-order and greatest in surface layers where soil organic matter contents were largest. After 102 days, <1% of the added Cr(VI) was still extractable in the 0–50 mm soil cores whilst ≈60% of Cr(VI) in the 400–450 mm cores (or deeper) was still extractable after the same period.

A linear systems model comprising a series of conceptual mixing cells was used to describe the individual and mean Cr(VI) leaching breakthrough curves (BTCs). This State-Space Mixing Cell model proved effective in simulating the Cr(VI) leaching using first-order kinetics to quantify rate-limited local solute adsorption coupled to advective-dispersive transport. The solute mass involved in the model process was ≈30%. The bulk of the remaining 70% of applied dichromate was assumed to have undergone reduction to the non-mobile chromium cation.

This study shows that there exists a significant potential for Cr(VI) to be a serious threat to groundwater in the event of a large uncontained spillage of a concentrated CCA solution. This potential can be significantly lessened if the Cr(VI) is reduced after retention in an organic matter rich layer.