Pesticides Research, 62 – Calibration of Models Describing Pesticide Fate and Transport in Lillebæk and Odder Bæk Catchment

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

Summary and Conclusions

Pesticide transport and transformations are modelled in two Danish catchments. The two chosen catchments represent different landscape types in Denmark, that is moraine clay and sand. They are included in the national monitoring programme and a good database thus exists for each of them. The available data could be utilised for the work, and this was a necessary criterion for the selection.

The purpose of the modelling was thus to set up and validate models for pesticide transport and transformation in the two catchments, and later modify these to scenarios for use in registration.

Both catchments are small, the sandy loam catchment approximately 4.5 km² and the sandy catchment approximately 11 km². For both catchments, the groundwater catchment is different from the topographic catchment. In the sandy loam catchment model, the borders are open towards east and west, so water runs to the area from upstream and to the Great Belt via the groundwater. For the sandy catchment, the borders closed except towards the south where potential maps and simulations indicate that water drains to a stream to the south rather than towards the model stream.

Data are collected concerning climate, soil types, geology, hydrogeology, pumping, land use, pesticide use, pesticide properties, stream flow, groundwater levels and measured pesticide occurrences. Because most of the measurements are point measurements and the model is three-dimensional, the point measurements are given a spatial dimension. The model simulations are tested with respect to water and measured values for drain and streamflow and against groundwater levels, and for the pesticides against measurements in drains and streams.

To carry out the study it has been necessary to re-interpret the description of precipitation, the distribution of soil types and the geology in the Odder Bæk catchment, while less modifications of data were made for Lillebæk.

In the sandy loam catchment, the correspondence between measured and simulated values at the downstream station is very good and the groundwater fluctuations are reasonably well described, except in an area to the south where geological description is in disagreement with the observed hydrology.. The upstream station in sandy clay catchment is over simulated. As the total amount of flow downstream is correct this may be caused by the fact that the drains are connected to the river a slightly different in relation to the measurement station than it is in reality, or that water in the southern part runs south instead of towards Lillebæk, and that this loss is overcompensated through the open border towards the Great Belt.

With respect to the sandy catchment, all flow stations are somewhat oversimulated. With the uncertainties related to precipitation, the actual location of the borders and the drainage to the south, it is not possible to match the observations better. Special problems are found with respect to the drain in the sandy catchment. First, the actual catchment of the drain is much larger than it's geographical extent, and secondly the runoff during summer must be caused by inflow from the stream (or another unknown source). It does not appear to be groundwater, as the groundwater levels falls below drain depth. The drain is therefore not appropriate for pesticide validation.

With respect to pesticide, simulations are carried out on two drains, the up-and downstream stations in the sandy loam catchment and the downstream station in the sandy catchment.

The pesticide concentrations measured on the drains are generally very low, but for isoproturon, there is a factor 1000 difference between the measurements from the beginning of 1999 till the end of 2000. These variations are reproduced by the simulations.

For the catchments as a whole, the results are not quite as convincing. In the sandy catchment, the level of observed pesticide concentrations is simulated reasonably, but the simulated pesticide concentrations are much less variable than the measured. This is caused by two factors: It was not expected that preferential flow would play a large role in the sandy catchment – this was a pre-requisite for the selection of the catchment. The quick responses observed in the catchment imply that macropores play and important role. Furthermore the dilution in the groundwater appears to be too large. The result of this is that the background concentration increases over the simulation period and that the concentration peaks simulated in the stream become too wide. Direct drainage of the locally saturated layers in the unsaturated zone is expected to improve the description, because this would allow water and solute to leave the root zone without dilution. As a consequence, the peaks would be more condensed and the general background concentration would be less.

For the clayey catchment, where macropores are included in the description, the simulations of pesticide peaks are clearly linked to rainfall events, as in the observations. But as in the sandy catchment, the background concentration increases more than in the observed data. Furthermore, it became clear that the chosen description of colloid transport is not realistic at catchment scale. The description demands that macropores behaves as down pipes to ensure that the colloids reach the drains. The side effect of this is that too much pesticide is transported to groundwater. This is expected to be a major reason for the high background concentrations in the simulations.

Already early in the project is became clear that point sources could be a possible source of error in the measurement of pesticide in the streams and that this was a threat against the validation exercise. This became more serious than expected. An analysis of the three largest detections in the upstream station in 1999: Terbutylazin, Diuron and Simazin was carried out. In 1999, Terbutylazin was only used at two, Diuron at one and Simazin on no fields upstream of the measuring station. As the compounds are used on small areas only (if at all), and at the same time are found in high concentrations (for Terbutylazin, 4.2 microgram/l in May 1999), a simple calculation where only thinning is included, shows that the concentration under the root zone should have been in the order of milligram/l and not microgram/l which is the expected level. This indicates that these peaks are not due to field application but point sources created when pesticides are handled. More than 50% of the pesticide occurrences in the sandy loam catchment are the three mentioned compounds, identified as point sources, or their metabolites. Also the largest detection of Diuron downstream in the sandy loam catchment in 1999 must be classified as a point source, as there is no related use registered. The same suspicion exists for the largest detection in Odder Bæk, of 2.9 microgram/l of Bentazon. The fact that the simulations produce the levels of pesticide observed for e.g. Terbutylazin may therefore be misleading.

For compounds where the use is widespread, it is far more difficult to distinguish between point sources and field use. All else equal, the thinning will be less and the expected concentrations in the stream thus closer to the concentrations of the drains.