The effect of mineralisation rates of atrazine in surface and subsurface soils on its groundwater contamination potential

Abstract

Atrazine is a widely used herbicide for weed control and has been found in groundwater in many countries. The groundwater contamination potential of atrazine in 2 soils on the east coast of the North Island of New Zealand was modelled under 3 scenarios. Scenario 1 used atrazine mineralisation rates as determined in an incubation experiment which showed higher rates of mineralisation in subsoil layers than in surface soils. Scenario 2 assumed a decreasing mineralisation rate with soil depth in proportion to changes in microbial biomass. Scenario 3 used a logistic model to describe atrazine mineralisation rates compared with the first-order model used in Scenarios 1 and 2. The results showed that serious errors could occur when modelling the groundwater contamination potential by assuming a decreasing degradation rates with soil depth, when in fact the degradation rates could be higher in some subsoil layers. A site that had not been exposed to atrazine in the past was shown to have a higher potential for groundwater contamination than a neighbouring site of the same soil which had been treated with atrazine in the past. The groundwater contamination potential was found to be higher in the Te Awa soil than in the Twyford soil due to the longer mineralisation half-lives in the Te Awa soil.