|
Scenarios and Model Describing Fate and Transport of Pesticides in Surface Water for Danish Conditions 1 Choice of Scenario Description1.1 Process considerations The aim of this report is to describe the scenarios to be used by the Danish EPA in their registration procedure when evaluating the risk of movement of pesticides to surface water. The project was initiated with an inception phase in which a review of existing knowledge on the subject of pesticide transport and occurrence was carried out. During this phase, an effort was made to describe the possible pathways, the scale of the processes, and the requirements of the scenarios. For each of the processes, relevant literature was reviewed and discussed in the inception report (DHI et al., 1998). Chapter 1 describes the main conclusions that led to the choice of the selected scenarios. In some cases, the text is updated with more recent knowledge. However, an attempt has been made to point out if new information has been added. 1.1 Process considerationsPesticides may arrive in a water body through:
The pesticide arriving in the drains and upper groundwater may have passed through the soil matrix or have travelled through macropores. 1.1.1 Spray driftOnly few studies exist, where the total drift is estimated as a percentage of the amount sprayed. Maybank (1978) states that 1-8% of the sprayed amount are deposited outside the sprayed area. In most studies, the drift is estimated in different distances from the sprayed area. In the European context, the study by Ganzelmeier et al (1995) was considered the best source of data concerning field spraying of annual crops under optimal conditions. It has since been superseeded by BBA (2000). Approximately 0.1% (0.03-0.3%) of the sprayed amount is registered in 10 m distance from the sprayed area. In the experiments, the sprayed area had a width of 24 m. As the drift declines exponentially, the contribution from areas further away is minute. The results of the study are used for determining drift values for spray techniques in Germany and several European countries, among others Denmark, for determination of safety distances for pesticides to surface water. The mentioned drift values are found on a flat field. Near streams, the sedimentation conditions will be different. In Holland, Porskamp et al (1995) measured 30% less pesticide sedimentation at the water level than at the field level. For fruit trees, the mean deposition is 1.8-5.7% depending on growth stage, in a distance of 10 m. Recent Dutch figures (vad de Zande et al, 2002), cited in FOCUS (2002), however, indicate that the Ganzelmeier values are considerably lower than Dutch measurements. The process is likely to have particular importance for ponds situated in agricultural land. For streams, the effect is more doubtful. Kreuger (1996) concludes that wind drift had little or no influence on stream water quality in the Vemmenhög catchment in Sweden. Only in one occasion during the four years of measurements could an increased concentration in the stream be related to spraying of adjacent fields, resulting in a stream concentration of 5 µg/l. This was, however, by far the highest concentration detected of this pesticide. For considerable periods every year, sampling was continuous. Similar results are found in the county of Funen, where few pesticides are recorded in stream flow during dry weather. One event, however, gave rise to a concentration of 9.8 µg/l (Rikke Clausen Schværter, pers.com. on data from Wiberg et al., 1997). Events could have been missed due to the sampling technique. However measurements carried out in this work (Iversen et al., 2003) supports the observation that spray drift appears less important than expected. In practice, the drift will depend on wind speed, direction of the wind, and presence of buffer zones near the water body. The duration of a peak occurring from spraying of 100-m field along a stream is in the order of one minute. The process is included in the scenarios and the parameterisation is described in Section 4.1.3. The process has been further investigated as part of this project, and is reported by Asman and Jensen (2003). 1.1.2 Wet and Dry DepositionConclusions of a Nordic seminar in 1994 (Helweg, 1995) state that the maximum concentrations of pesticides in rainwater were about 0.3-0.4 µg/l. The highest amount of one pesticide deposited on land with precipitation was about 300 mg/ha/ year. Most pesticide deposition comes from precipitation, whereas dry deposition accounts for below 20% of the total load. This is supported by newer findings (Felding and Helweg, 1998). For single pesticides, the total deposition measured in the presented studies from Denmark, Norway, Sweden and Germany did not exceed 250 mg/l . For seven pesticides measured in the Frankfurt area, the total deposition amounted to 560 mg/ha/year. Wet deposition does, in general, not occur as a function of local spraying. It is thus not relevant for the registration model. It may, however, be relevant to measure pesticide in rainwater with the aim of determining the background load of pesticide in the catchment. Felding and Helweg (1998) found maximum concentrations of 0.2-0.4 µg/l in the month of October at three different localities in Denmark. A single observation reached 0.6 µg/l. Direct rainfall input may thus produce a measurable effect in the stream. A rough assessment may be made as follows: With a detection limit of 0.01 µg/l, 0.2-0.6 µg/l require dilution by a factor 20 to 60 to become non-measurable. 10 mm of concentrated rainfall thus requires a flow of 50-150 l/s in the stream not to influence measurements. Felding and Helweg (1998) conclude that the total deposition reaches 50-500 mg/ ha/year (dry, wet, spray drift). In comparison to the total sprayed amount, it makes up approximately 0.01%. Dry deposition was studied in a separate sub-project with the specific aim of evaluating the importance of the process. The study concluded that at some distance from the field, dry deposition is more important than drift, and its effects may become measurable. The work is described in Asman and Jensen (2003). The process is included in the scenarios and the parameterisation described in Section 4.1.4. Dry deposition is not included in the FOCUS (2002) surface water scenarios. 1.1.3 Deposition onto the Soil Surface and PlantsDeposition onto the soil and plants is not a pathway for the stream, but constitutes the link between the air models and the description of the unsaturated and saturated soil. Deposition was investigated during the project and the work is described by Jensen and Spliid (2003). The results correspond quite well to the recommendations given in the FOCUS groundwater group (FOCUS 2000), but are rather different from what is used by the FOCUS surface water group (FOCUS 2002). The measurements and the model do not take into account wash-off from leaves as a pathway to the soil. Depending on the plant cover at the time of spraying, the retention on leaves may vary from almost 0 to almost 100%. 1.1.4 Dissolved in Surface Runoff or Transported with Soil ErosionSurface–related losses of 0.1-5% are reported by Wauchope (1978). This includes both dissolved and particulate surface transport. Overland flow amounts measured in plot studies in Denmark vary from negligible amounts, over 11-42 mm/year on the Ødum erosion plots to 41-163 mm/year on the Foulum erosion plots (Hansen and Nielsen, 1995). Only few Danish figures are available regarding transport of pesticides with surface runoff. Felding et al (1997) carried out an experiment in the catchment of Syv Bæk, resulting in the key figures presented in Table 1.1. Table 1.1 Pesticide losses recorded in surface water by Felding et al. (1997).
The runoff amounts during the trial period were 11 mm during the last three months of 1991, 34 mm during 1992, and 50 mm during the first eight months of 1993. The erosion plots were in use during 1987/88-89/90 for general sediment studies (Hasholt et al., 1990). During this period, a sediment balance was constructed for the catchment. In Table 1.2, the soil loss from the plots is compared with the sheet erosion estimated in the catchment based on a full sediment budget. The losses registered at the plots are multiplied with the area of the catchment to provide a comparable estimate. The losses registered at the plot are generally well below the average losses in the catchment.
A very rough calculation was carried out on data from Foulum and Ødum research station, assuming that pesticides could be compared to phosphorus. Assuming that
the losses in Foulum would be between 2 and 40 g of pesticide (of the 1 kg sprayed), and in Ødum between 0.5 and 5 g pesticide per ha per year, via the soil surface, or 0.05-4% of the sprayed amount. This equals a total concentration in the surface runoff of between 4 and 30 µg/l on both localities, but it varies with the year and the exact treatment of the soil surface. The calculations are illustrated in Table 1.3, and the figures represent absolute maximum amounts, as degradation is not taken into account. Table 1.3 Estimation of the maximum possible effect of erosion. Original erosion figures from Hansen and Nielsen (1995).
WUD = winter wheat, sowed up and down the slope Measurements of erosion on different slope units in Denmark produced erosion figures from 0 to 25 t/ha lost to streams (Kronvang et al., 2000). Estimates provided on the basis of measurements in Syv Bæk (Hasholt and Styczen, 1993) result in rather low average erosion rates (max 65-kg sediment/ha, equal to 0.01% of the sprayed amount if subjected to the above calculation). However, the 76 t of soil generated by erosion in the catchment came from a small fraction of the area, resulting in much higher erosion rates in single fields. DMU estimates that about 3% of the Danish arable area are threatened by erosion. Serious events do not occur every year, but are mainly triggered by certain weather conditions, such as (Heidmann and Hansen, 1995):
However, erosion was not observed in the two selected catchments during the study period, and hardly any surface flow is calculated in the model. The process was therefore finally left out of the scenario calculations. In the FOCUS Surface water scenarios, erosion contributes little to the pesticide loads in water-bodies. 1.1.5 Unsaturated zoneFrom the soil surface to the saturated zone, the pesticide will be transported through the soil, either through the soil matrix or (in structured soils) through the macropores. Adsorption and degradation processes take place in this zone, particularly to pesticide transported through the matrix. The project has benefited from developments under SMP96 regarding process descriptions and modelling of these processes, and from the considerations made in the FOCUS groundwater group. General findings for the unsaturated zone in Danish soils show that sandy soils may be described reasonably well with the traditional flow theory (Høgh-Jensen, 1983, Høgh-Jensen & Refsgaard, 1991a,b). Solute transport follows the general convection/dispersion equations (Høgh-Jensen and Refsgaard, 1991c; Engesgaard and Høgh-Jensen, 1990a). For the sandy loam soils, however, macropore flow is an important pathway (eg Villholth, 1994; Styczen & Villholth, 1995, Engesgaard and Høgh-Jensen, 1990b, Thorsen et al, 1998). While the flow through the matrix still behaves according to the traditional flow theory, the macropores allow high fluxes of water and solute to move quickly through the profile when local saturation occur at the surface or in the profile (e.g. on a plough pan). The interaction between the solute and the soil is limited for the macropore flow. Both adsorption and degradation (mainly in the matrix) can limit the transport by close to 100%, and the two processes thus represent major loss factors. A study of pesticide in soil moisture (extracted with suction cups at a depth of 80-90 cm) was carried out in Bolbro Bæk and Højvads Rende by Spliid and Mogensen, (1995). The concentration range observed in the moraine soil around Højvads Rende was 0-0.29 µg/l and 0-1.36 µg/l in the sandy soil in Bolbro Bæk catchment. The frequency of pesticide observations was higher in the moraine soil than in the sandy soil. A total of 14 compounds were studied. (MCPA, 2,3-D, Mechlorprop, Dichlorprop and three of their metabolites, DNOC, Dinosep, Simazin, Atrazin, Bromoxynil, Ioxynil and Isoproturon). Moisture cups are expected to mirror the moisture in the soil matrix. A special study was, however, undertaken, investigating colloid transport, and attempting to model the process (Holm et al., 2003). The main conclusion is that for compounds with a high Kd-value, transport may take place in significant amounts on carriers such as organic molecules (or perhaps clay particles for other compounds) – this was clearly seen in the field data. The developed model, however, do not adequately describe the data. While the implemented process increases the concentrations moving through the unsaturated zone, it still severely underestimates the observed transport. It seems that the observed levels of transported pesticide can be obtained only if it is assumed that the particles are super-saturated with pesticide. The process has been included in the registration model of Lillebæk stream and pond. It was necessary also to change the macropore description of MIKE SHE to only allow water flow from the matrix to the pore in order to maximise the colloid transport. This is more or less in line with the description used in the DAISY-model, a Danish nitrate model). Even with the inclusion of the process, the observed pendimethaline levels were not obtained during calibration (Holm et al., 2003). The new process description had serious effects on the catchment model as the macropore flow became overestimated in general, and colloids moved along the surface with surface water in unrealistically high concentrations. 1.1.6 GroundwaterThe transport to surface water bodies via groundwater will, in most cases, take place through secondary groundwater. Concentrations reported in upper groundwater are generally in the order of 0.01-0.1 µg/l (Grant et al, 1997). Groundwater as such will not play an important role for small streams in the moraine clay areas as base flow amounts are negligible, but the drain flow is generated by grundwater at shallow depth, and this is an important parameter in moraine clay areas. Groundwater is important for the background concentration in streams in sandy areas, as the base flow amount is large (eg Miljøstyrelsen, 1992). Furthermore, sandy soils tend to have relatively fast flow rates in secondary groundwater and therefore limited time for degradation of the pesticide. During the calibration phase, it was observed that the model had problems simulating the first drain flows observed in the wet season. On the other hand, there was a tendency of over-simulating the drainflow later in the season. It is believed that the problem observed is due to the presence of layers of low permeability around drainage depth. In reality, the water forms a perched water table on these layers and runs out of the drains. In the model, drain flow is only activated when the saturated zone raises over drain depth. The error caused by this is tried quantified in Chapter 7. 1.1.7 Pesticide dissolved in drain flowStudies of pesticide concentrations in drainage water in Højvads Rende show concentrations of dissolved pesticide between 0 and 0,27 µg/l (Mogensen and Spliid, 1995; Spliid and Mogensen, 1995). These concentrations are considered low, and this may be due to that the sampling was done at 14-day intervals. Peak concentrations in the drains may thus not have been caught. However, at a later stage, concentrations up to about 3 µg/l were found (Spliid et al. 2001). Most of the samples were taken with 14-day interval. The common picture of drained moraine soils are high-concentration peaks of solutes of short duration (minutes or hours) caused by macropore flow (Flury, 1996; Villholth, 1994). A peak concentration of 24.0 µg/l for prochloraz was observed by Villholth et al (2000). A general estimate of losses through drains is given to be in the range 0.1-5% (Flury, 1996). The levels measured in the two study catchments in drains were low, as it appears from Table 1.4. Table 1.4 Occurrence of concentrations above 0.1 µg/l in drains in the two study catchments during the period of measurements.
1.1.8 Colloid-bound Pesticide in Drain FlowReported losses of particles through drains are between 15 and 3010 kg/ha/year (Øygarden et al, 1997; Brown et al, 1995; Kladivko et al, 1991; Bottcher et al, 1981; Schwab et al, 1977). The total losses of hydrophobic pesticides in two reported studies were between 0.001 and 0.2% of the applied pesticide (Brown et al, 1995; Villholth et al, 2000). Between 6 and 93% of this was sediment bound. In field experiments performed as part of the current study, total losses of applied doses of pendimethalin to drains was on average 0.0013 % for two sampling seasons (Holm et al., 2003). A quantification of the importance of drains for addition of fine particular material to the streams has shown that the drains on average contribute 29% of the transport, and in single intensive rainfall events up to 70% of the total load to a stream (Kronvang et al, 1997). The 6% loss in the sediment phase found in Villholth et al (2000) was associated with a load of sediment of only 50 g/ha/mm, which amounts to approximately 35 kg/ha/year. Laubel et al (1998) found a loss of 120-440 kg/ha/year on the same site during other periods. The pesticides used in Villholth et al (2000) (prochloraz) and in Brown et al (1995) (trifluralin) had similar sorption capacity (Koc of approximately 10000). The 93% recovery in the particle phase observed in Brown et al (1995), however, may be overestimated as trifluralin is relatively volatile and hence a significant fraction of the dissolved pesticide may have been lost. In the study by Holm et al. (2003), 67 drain water samples taken from the test area at Rørrendegaard had contents of pendimethalin above the detection limit. For these samples, between 0 and 30 % (on average 10-15 %) of the pendimethalin found in drain water samples was associated with particles larger than 0.7 µm (nominal filter size). Samples taken from the two model areas showed contents in the particulate phase, above app. 0.2 µm, of 66 % (one sample from Lillebæk) and 36-46 % (two samples from Odder Bæk). There was a strong correlation between particle content and pendimethalin concentration for the samples from Rørrendegaard, and modelling of the observations from the site, indicated that for strongly sorbing compounds, such as pendimethalin (Koc of 10000-18000), particle-facilitated transport would completely dominate the leaching through the unsaturated zone to the drains. Even for less hydrophobic compounds, particle-facilitated transport would still be a very important transport mechanism through the unsaturated zone (for conditions similar to those at Rørrendegaard). 1.1.9 Overview of PathwaysTable 1.5 summarises the information concerning pesticide pathways that formed the basis for the work on the registration model. Table 1.5 Main quantifying figures from Section 2.1.1-2.1.8. NB: Note that the figures given are not in all cases directly comparable, and that all processes do not have the same relevance for different pesticides.
1.1.10 Measurements in StreamsThe study conducted by Spliid and Mogensen (1995), which included a sandy loam catchment (Højvads Rende) and a sandy catchment (Bolbro Bæk), also included measurements in the stream. The conclusions were that the number of positive samples and the concentration levels were highest in the stream in the sandy loam area. This difference may, however, not solely be caused by the soil types. Furthermore, for the sandy loam catchment, measured concentrations in the stream were higher than in the drainage water and the soil water. In the sandy catchment, the concentrations in soil water were generally higher than in the stream. The fact that there is a discrepancy between soil moisture (suction cups) and stream water content of pesticide in the sandy loam catchment was attributed to preferential flow paths, which often are of great importance on these moraine soils. The highest concentration measured in the stream was 7.3 µg/l in the sandy loam area, and 0.66 µg/l in the sandy area. Measurements in streams have been carried out in the two streams mentioned above, but also in Lillebæk (also sandy loam) and Odense Å. Table 1.6 Maximum concentrations of pesticide recorded in streams in some Danish studies before mid-1998.
The timing of the events in Lillebæk and Odense Å (Wiberg-Larsen et al., 1997) shows a clear connection between the occurrence of high concentrations in the stream and rain events during the spraying season. These observations indicate a close link to the macropore and drain flow on sandy loam soils. In other Nordic studies maximum concentrations reported are generally between 1 and 10 µg/l, with some extremes, however, up to about 50 µg/l (Kreuger, 1996, Høysæter, 1995). The findings of this project fall within the range of the above measurements. The interpretation of the high concentration has, however, changed. High concentrations were found for pesticides used on very limited areas in the catchments. Calculations were done for the two solutes found in highest concentrations in the stream in order to determine what the concentration in drain water under sprayed fields should have been to reach the concentrations observed at the measuring station. These concentrations were in mg/l, and thus much higher than what is normally observed. Drift could be ruled out. These observations have to be attributed to point sources or access by overland flow directly to the drain system through wells. These processes are not accounted for in the model (Styczen et al., 2004a). 1.1.11 Measurements in PondsFour ponds were sampled 5-9 times between November 1989 and December 1990. Most analyses were negative. The highest concentration recorded was 1.1 µg/l (Spliid and Mogensen, 1995). The concentrations are not higher than what has been found in the streams. In a period from November 1990 until mid May 1991, VKI has carried out analyses for pesticides in biota and sediment in selected ponds. For most of the samples and pesticides, a content below detection limit was found (0.5 -50 µg/kg for sediment; 1-100 g/kg for biota). Pesticides detected in sediment and biota were: propiconazol (3.2 µg/kg in sediment), metsulforonmethyl (56 – 170 µg/kg in sediment) and tribenuron (11 µg/kg in biota) (VKI, 1990, 1991, 1992). 1.2 Considerations of ScaleStarting with the scale of the processes, Table 1.7 highlights the main processes and the scale at which they are considered important. From the scale of the processes alone, one could argue that if the only important processes are wind drift (deposition) and drain flow, the source calculation could be limited to a 200-500m long field draining into, and providing all the water for, a stream. The width of the field will then depend on an accepted relation between catchment size and stream length. The key issue, however, is that if the field generates all the water to the stream, it is, in fact, a catchment. In case of interactions between the secondary groundwater and the stream, the natural scale of the process is the catchment. A dynamical calculation of groundwater levels is possible only through a catchment simulation. This also goes for erosion events, which to a large extent will depend on local saturation under Danish conditions. Table 1.7 Main pathways and relevant scale for description of the process.
* Surface runoff and erosion will usually take place where drains are not present or during events where they do not function. It is more or less an “either /or“ situation. The choice of a catchment as the base for the simulation rather than an ”edge of field” scenario was not in line with the initial approach of the FOCUS surface water group. However, the presently proposed surface water scenarios attempts to represent catchments. For the stream calculations, project participants recommended a stream length of minimum 1-km. It was decided to use two small 1st order stream catchments as the unit for modelling, and to parameterise them on the basis of existing catchments. It was therefore necessary to find two catchments that would adequately represent Danish conditions.
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||