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

Atrazine is a widely used herbicide for weed control and has been found in groundwater in many countries. The groundwater contamination potential of a trazine 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 mine ralisation in subsoil layers than in surface soils. Scenario 2 assumed a de creasing mineralisation rate with soil depth in proportion to changes in mi crobial biomass. Scenario 3 used a logistic model to describe atrazine mine ralisation rates compared with the first-order model used in Scenarios 1 an d 2. The results showed that serious errors could occur when modelling the groundwater contamination potential by assuming a decreasing degradation ra tes 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 pa st was shown to have a higher potential for groundwater contamination than a neighbouring site of the same soil which had been treated with atrazine i n 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 mineralisatio n half-lives in the Te Awa soil.