Pesticides Research No. 104 2006 – Establishment of a basis for administrative use of PestSurf

Establishment of a basis for administrative use of PestSurf

6 Conclusions

PestSurf consequently generates higher concentrations than FOCUS SW. This is due to a number of differences between the scenarios.

The PestSurf sandy stream resembles the D3-ditch. The simulation results are almost linearly correlated, and the cause of maximum concentrations is similar. However, the presently used assumption regarding drift (spraying of the whole length of stream within 30 minutes) exaggerates the exposure and cause unrealistic concentration levels in the stream. The concentrations are about a factor of 4.3 higher than the concentrations in the D3-ditch. The water depth at the point of maximum concentration seldom reaches 30 cm.

The PestSurf sandy pond is strongly influenced by rather superficial groundwater, particularly in wet years. The mechanism is therefore completely different from the mechanism causing high concentrations in the D3 ditch. Generally the pond concentrations are lower than the concentrations in the D3-ditch.

The PestSurf sandy loam pond generates higher concentrations than the D4-pond when the cause is drift. This is explainable by a difference in pond size (250 m² compared to 900 m²) and therefore greater exposure. However, particularly for autumn applications, a low water level contributes to create higher concentrations.

For the PestSurf sandy loam stream, the concentrations are strongly influenced by the assumptions due to drift, causing a build-up at certain stretches. The water level in the stream is generally (much) below the 30 cm of the D4-stream, particularly for the autumn applications. This means that drift concentrations always will be higher in the PestSurf simulations than in the FOCUS SW-D4-stream simulations. It is recommended that the assumptions regarding drift be changed to represent field conditions more realistically.

The different causes of pesticide in the stream generate different patterns. With a more realistic drift simulation, the highest concentrations are most likely to occur in the upper end of the stream, where the water depth is small. The severe drainage event in the sandy loam catchment covered the whole catchment, except the very top end. The contributions from groundwater are only significant in the lower end of the sandy loam catchment. For two of the event types, the top end of the stream is thus not affected, and re-colonisation of downstream areas may occur. However, some small drainage events are observed, where very high concentrations occur in very small amounts of water in a single tributary.

It has already been argued that the contribution to the stream from the groundwater is exaggerated in the sandy loam catchment of PestSurf. The very high concentrations generated by some compounds in the lower end of this catchment are therefore likely to be unrealistically high, and should not be used as the only reason to discard a compound. On the other hand, the feature is closely linked to the long degradation times, and indicates that there is a danger of a build-up of concentrations in lower layers of the soil. This also indicates that the FOCUS approach assuming that the groundwater contribution is zero, is an underestimation. At the same time, the stretch appears to suffer from an instability problem. It is therefore recommended to avoid locations on the lower part of the stream, when data are extracted for administrative purposes.

It is recommended that stream results are used for the main stream only, and for the sandy loam stream, only from 500 m from the top end to avoid the stretches, which are dry during summer periods every year. This will also avoid the smallest drainage events, which are connected to very small amounts of water.

The high concentrations generated for some compounds by the 20-year event in the sandy loam catchment require consideration. A decision by the EPA is required with respect to the weight that should be given to this event in administrative decisions.

A major, still outstanding, issue is the lack of a method to interpret the spatial variation in results. How much of the stream should be affected before it is important? The very conservative method used in the report is to analyse the maximum values occurring in the catchment without any weight on the spatial occurrence may be too conservative.

It would be possible to generate a 3-D-graph showing length of stream sections on the x-axis, time on the y-axis and concentrations (greater than or equal to) on the the z-axis. The resulting graph would be a “mountain-shaped” and for each cross section of the z-axis it would show the time that a concentration level was “equal to or exceeded” for a given length of stream.

It is recommended that the Danish EPA at least decides on a “time criteria” like what is used for groundwater. If very high concentrations only occur during the 20-year-event, it must be considered equal to a 95 %-fractile according to the present climate. A similar consideration could be made for the very dry year (1996), but the simulations show that the summer period of the “normal year” was just as dry, and this event does therefore not really justify as an exception.

To avoid that a single point with a very high concentration becomes decisive for the simulation, a similar criterion could be implemented for the spatial distribution. The models contain 133 and 72 calculation points, of which 96 and 40 are present in the main streams, and 15 of the 40 lie between the 500-m-point and 1700 m point. A 95 %-criteria would be equal to 4-5 points in the sandy catchment and <1 point (60 m) in the sandy loam catchment.

It should be noticed that neither conditions such as the extreme drainage event, the groundwater contribution (which may be exaggerated, but nevertheless is relevant), the varying water level in the stream, the varying water level in the ponds nor the pond sizes identified as relevant under Danish conditions are represented in FOCUS SW. Water depths tend to be considerably lower than the 30 cm during summer and autumn in Danish 1. order streams, and the water level in ponds also drop during dry periods. FOCUS drift values will thus tend to underestimate the actual risk. One average year cannot catch all the different climatic conditions that may be relevant.

If the Danish EPA decides to continue with the PestSurf system, it is recommende that it is upgraded as described in Chapter 6.