Pesticides Research, 62 – Calibration of Models Describing Pesticide Fate and Transport in Lillebæk and Odder Bæk Catchment – 8 Evaluation of the Appropriateness of the Chosen Model for Description of Fate and Transport of Pesticides in Lillebæk and Odder Bæk Catchment

Calibration of Models Describing Pesticide Fate and Transport in Lillebæk and Odder Bæk Catchment

8 Evaluation of the Appropriateness of the Chosen Model for Description of Fate and Transport of Pesticides in Lillebæk and Odder Bæk Catchment

It is believed that the approach chosen is sound and that the combined simulation of all the involved processes is the most correct way to handle the pesticide movements at catchment scale.

It is however clear that some process descriptions and part of the parameterisation require further work. The two processes of particular concern are the colloid transport in the macropores and drain from the unsaturated zone. Presently it must be expected that the transport of strongly sorbing pesticides is distorted in the model and that peaks are lower than what may be the case in reality.

On the other hand, the simulation of Isoproturon on drain 2 in Lillebæk shows that the model description in some cases catches the observations surprisingly well and even over decades in concentration levels.

It must be concluded that the description of colloid transport and the parameterisation provided by Holm et al. (2003) is not adequate for use at catchment scale. The generated colloids moved around along the surface in huge amounts in the model and the parameterisation used at the end produced considerably less colloids. On the other hand, the colloid module requires the macropore description to be parameterised in a manner where exchange with the matrix is underestimated. All in all, macropore transport is considered being overestimated. It is thought that a better correspondence between observation and simulations could be obtained without the colloid description applied, and using a parameterisation of the macropores based on the profile descriptions.

On the other hand, macropores obviously should be included in the Odder Bæk description. However, then the area does not live up to the requirements specified in the beginning of the project, where the area was supposed to represent a very sandy site. With respect to Odder Bæk. the Danish Environmental Protection Agency has to consider whether it can be considered a reasonable catchment of registration purposes or whether a sandier site is required.

The third problem mentioned in the report is the discretisation at the border between the saturated and unsaturated zone, leading to thinning of peaks and probably too much transport to the groundwater. Furthermore, it is thought that the possibility of draining intermittent groundwater would improve the present description.

The model is not considered sufficiently well calibrated to be fully reliable. The levels of pesticide observed in the stream were in several cases reasonable, particularly at the top end of Lillebæk. However, the loads of pesticide moved through groundwater appear too large. The peakiness of the observations is much better described in Lillebæk than in Odder Bæk, due to the macropores, but this is to some extent overshadowed by the overestimated groundwater transport.