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Scenarios and Model Describing Fate and Transport of Pesticides in Surface Water for Danish Conditions 2 Selection of “Representative” Areas2.1 Soil types and hydrology The selection of model areas was based on several criteria:
The most obvious candidates as study catchments were the catchments belonging to the Danish monitoring programme, with a set of basic data and measurements of precipitation, streamflow and several other parameters since 1989. Lillebæk and Odder Bæk catchment were selected as candidates. Their key features are described below. 2.1 Soil types and hydrologyThe catchments were selected with the purpose of representing
Together, these soil types cover about 58% of the Danish arable area. A description of the Danish soil types and a comparison of soil types in the country and in the test areas is given in Appendix A. Presently, no data sets exist on which to base a third stream scenario, representing the soil types 3 and 4. These soils represent 28% of the arable area, and there are some indications of these soils being able to generate a higher amount of surface runoff (the Foulum plots mentioned in Section 1.1.4 belong to this group). 2.2 Overall Land UseThe catchments chosen are dominated by agriculture, and are therefore likely to represent risk areas from that point of view. In fact 98% of the Odder Bæk catchment and 89% of Lillebæk are used for agriculture. In the catchment of Odder Bæk, the last two per cent of the area are forested, and 12,9% of the agricultural area are covered by permanent grass. In Lillebæk, 2% are forest and 9% are villages and roads. Both cases are very realistic of intensive agricultural areas in Denmark, with the sandy areas being more sparsely populated. Although the permanent grass area appears large in the sandy catchment, similar permanent grass areas are found in other sandy areas of Jutland. For the counties of Sønderjylland, Ribe, Ringkøbing, Viborg and Nordjylland (dominated by sandy soils), the percentages of permanent grass are between 14,5 and 18,2. The 12,9% are therefore not unrealistic, in fact it is slightly less than average. Table 2.1 Existing Land Use in Odder Bæk and Lillebæk (figures from the County of Funen and NERI) at the time of selection of the test-areas.
Two features make Lillebæk ”low risk” with respect to wind drift: A considerable length of the stream is piped and along part of the open stream, trees provide a barrier between the agricultural land and the stream. 2.3 Hydrology of water bodiesThe stream flow characteristics correspond to the two soil types. Lillebæk is dominated by drain flow. Base flow is negligible, and the flows during summer are very small. Odder Bæk has much more baseflow, as expected in a sandy catchment. In agreement with this, drains were known to exist in Lillebæk, but only to a very limited extent in Odder bæk. A study regarding pond types was commissioned to Institute of Geography. Two types of ponds were described:
It was considered realistic to find the two types in the catchments selected. 2.4 ClimateThe climatic data available when the catchments were selected is shown in Table 2.2. The catchments lie within the range experienced. It was not considered obvious whether higher rainfall would lead to more leaching or a higher degree of dilution. Table 2.2 Precipitation (1.6-31.5) for 1989-1996, and average precipitation in the period 1961-1990 (Source: NERI, 1996).
Irrigation was considered for Odder Bæk, and the actual irrigation for the last few years was reviewed. The amounts used were, however, very small, and irrigation was only practiced by a few farmers. It was therefore decided to leave it out of the registration model. 2.5 Changes discovered during project implementationOdder Bæk was originally selected to represent the sandy areas of Denmark. According to the soil map from DJF, most of the area is classified as JB1 [1]. JB1 and JB2, both with 0-5% clay, are expected to make up 34% of the Danish agricultural area. However, when the data for the fields studied in the area arrived, it was discovered that only one topsoil was JB1, one was JB2, one was JB3 and three was JB4. While some of these matched more clayey patches along the boundary of the catchment, some doubt was raised regarding the actual soil types of the area. Table 2.3 Clay content and organic matter content for the A-horizons of the six investigated profiles in the catchment.
Data from bore holes in the area as well as a geological description was then matched with the above information. This indicates that a band of more clayey (at least JB 3-4) material runs across the area. However, it cannot be ruled out that JB1 is found in other parts of the area, where no detailed profile descriptions have been carried out. When it was realised that the area was more clayey than originally assumed, an attempt was made to obtain drain maps of the area. Hedeselskabet has records of 30-35 cases in their archive. The County has mapped drain outlets along the stream. The major drains identified are found in the middle of the catchment, along the stream in a rather short distance from the stream and along a piped stream branch. Some of the drain installations cannot be traced in the archive. Recent information from DMU and the County indicates that
All in all, the area is not expected to respond as a “pure”coarse sandy catchment as Karup or other areas of western Jutland. The expected distribution of the mentioned topsoils in Denmark are JB1: 24%, JB2: 10%, JB3: 7% and JB4: 21% of the agricultural area. The question is whether this violation of the original assumption means that Odder Bæk cannot be used as basis for a scenario. From the table above, it is obvious that the spread between JB1 and JB4-soils is only 2% clay. In spite of the difference in classification, the soils in the catchment are rather uniform, but, as mentioned with a slightly higher clay content than expected. For the station fields, the lowest JB-numbers have the highest organic matter content. This, however, is not a trend generally observed. But because of this, the plant available water is actually higher in the JB1 and 2-soils than for the other soils. Table 2.4 Plant available water (%) in the root zone for six soil profiles in Odder Bæk.
The main processes acting on the pesticide are degradation and sorption. Usually, sorption depends, in the model, on the amount of organic matter present. This is not strongly influenced by the slight change in texture, and the process is therefore not strongly influenced by the new discoveries. For pesticides sorbing to clay, the increased clay content will, however, matter, as a change from 4 to 6% clay increases the sorption with 50%. Degradation is indirectly influenced by how long time the solute will stay in the upper layers of the soil. As the retention capacity of the soil, at least in parts of the area, will be larger due to the more clayey textures (assuming similar organic-matter contents) the residence time of water (and solute) in the upper meter of the soil will be slightly longer than for the coarse sandy textures. Degradation will therefore be slightly higher. For comparison, the JB1 at Jyndevad experimental station has a plant available water amount of approximately 14% in the A-horizon (FC about 18% and WP about 4%). The figures of Table 2.4 are comparable to the FOCUS groundwater scenario “Hamburg”. On the other hand, the effect of drainage in the catchment will increase the speed with which pesticide in upper groundwater moves to the stream. It is difficult to weigh the two factors against each other. Degradation is by default set to 0 at one-meter depth, indicating that nothing should happen below this depth. Overall, it is expected that the mass flux to the stream over a year will be somewhat less than for a coarse sand, due to a slightly higher degradation. However, the drains will cause the solute to arrive in more narrow peaks than would have been the case if the solute had moved with groundwater all the way to the stream. The peak concentrations are thus not necessarily smaller, but the exposure over longer time could be smaller than in a more coarse-sandy catchment. In order to investigate the effect of soil texture, the pesticide leaching to the stream was simulated assuming that the soil texture over the whole catchment is replaces by a JB1 texture (coarse sandy soil), represented by soil profile data from the Jyndevad research station (see results in Section 7.3). It is rather more complicated to remove the drains in the simulation, because the groundwater will raise and part of the agricultural area will be too wet for agricultural use, and the simulation cannot be verified against measurements. It is therefore not realistic to change the texture and remove the drains, and still compare the agricultural area. The soil texture still represents 28% of the agricultural area, and it thus still representative of a considerable area of soils. The mixed geology is not atypical of northern Jutland. It was not realistic to change the site of the measurements, and and therefore of the scenario at the time where the problem was discovered. Footnotes[1] JB1: 0-5% clay, 0-50% fine sand, 75-100% sand in total.
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