Pesticide Translocation Control: Soil Erosion

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Soil Erosion: A Global Problem..............................................................

Water and Wind Erosion and Sediment Transport.............................

Water Erosion • Wind Erosion

Assessment of Pesticide Translocation..................................................

Monitoring and Modeling.......................................................................

Water Erosion Models • Wind Erosion Models

Monika Frielinghaus, Detlef Deumlich, and Roger Funk

Soil Erosion and Pesticide Translocation Control...............................

Erosion and Runoff Control: Part of the Environmental Legislation

Conclusion................................................................................................

References..................................................................................................

Soil Erosion: A Global Problem

Water and Wind Erosion and Sediment Transport

The processes of soil erosion involve detachment of material by two processes: raindrop impact and drag force traction. Material is then transported either by overland water flow or by saltation through the air.

Water Erosion

Runoff caused by heavy rainstorms (high intensity or long duration) is the most important direct driver of severe soil erosion by water (Figure 1). Mean losses of 20 to 40 tons ha 1 in individual storms may happen once every 2 or 3years. More than 100 tons ha-1 soil loss is measured after extreme storms.151

The water erosion rate is a complex result of high rainfall amount or intensity, soil erodibility (instability), slope steepness and length, and the type of land use. As to the latter, the main causes are inappropriate agricultural practices, deforestation, overgrazing, forest fires, and construction activities.

The formation of a water erosion system on agricultural areas without surface protection by crops or plant residues causes rainwater and sediment transport in rills or gullies. The first phase after ponding is a concentration of water at linear paths. The rill formation may be influenced by a variety of different factors like soil crusting, wheel tracks, and reduced infiltration by soil compaction. Rills and gullies often develop in close proximity to one another. The second phase is a heightened concentration of water at morphological deep lines (thalways). The hydrological power of transport water as well as the off-site risk for sediment and pesticide transport into the lakes or rivers is increased with the thalways’ catchment area (Figure 2).

Often, the interaction between runoff and sediment transport complicates the process’ description.

Wind Erosion

Wind erosion occurs when three conditions coincide: high wind velocity, a susceptible surface with loose particles, which can be picked up, and insufficient surface protection by plants or plant residues (Figure 3).

Wind erosion results from wind moving across a dry soil surface and dislodging soil particles by pressure and lifting forces. The process is self-perpetuating; blowing sediment disturbs additional particles that are then lifted into the airstream.

Water erosion with sediment translocation at a corn field. Source

FIGURE 1 Water erosion with sediment translocation at a corn field. Source: Photo courtesy of Deumlich.

Sediment and pesticide transport into a channel caused by water erosion. Source

FIGURE 2 Sediment and pesticide transport into a channel caused by water erosion. Source: Photo courtesy of Frielinghaus.

Wind erosion at a field after seedbed preparation. Source

FIGURE 3 Wind erosion at a field after seedbed preparation. Source: Photo courtesy of Schafer.

The potential wind erosion rate is influenced by high wind velocity, soil erodibility (size/weight of particles), the degree to which the landscape is wind exposed, and the uses of land. The real wind erosion risk depends on the actual soil moisture and the real soil cover rate.

The modes of soil particle motion are closely related to particle size, density, and shape. It is important on agriculturally used soils of each textural class with organic material and absorbed pesticides (Figure 4). Particles with a smaller terminal velocity than the turbulent motions become suspended. Particles smaller than 20 pm are subjected to long-term suspension whereby they can be carried across several hundred kilometres for several days. Particles with diameters between 20 and 70 pm remain suspended for only a few hours and cannot be transported very large distances. These kinds of small particles are an important factor regarding the adsorption of pesticides.161

Sediment and pesticide transport into a channel caused by wind erosion. Source

FIGURE 4 Sediment and pesticide transport into a channel caused by wind erosion. Source: Photo courtesy of Frielinghaus.

Assessment of Pesticide Translocation

TABLE 1 Total Amounts of Pesticide in Runoff Samples and Their Distribution between Sediment and Liquid Phases

Pesticide

Water Solubility

Adsorption, Кос

Total Amount

Distribution

(mg L-l)

(mL g-1)

(% of Applied)

On Sediment

In Solution

Atrazine

33

100

13

0.8

12.2

Simazine

6

130

9

0.6

8.4

Alachlor

240

170

14

0.8

13.2

Lindane

7

1100

17

6.0

11.0

Trifluralin

1

8000

14

12.6

1.4

The second rainfall simulation study on small field plots provided information about worst cases for pesticide translocation (rainfall intensity of about 70 mm hr1 and 100 mm accumulated rainfall).1101 The pesticides of the first group had water solubility concentrations of 65 and 700 mg L e.g., isoproturon and dichlorprop-p, respectively. The second group had solubility concentrations <1 mg L-1, e.g., bi-fenox. Two different soil cover conditions were tested.

The results demonstrate that the influence of soil cover characteristics on concentrations and total runoff losses of pesticides for the highly soluble group is restricted to the start of the rain event after pesticide application and the runoff rate. The sediment concentration of pesticides in the fairly soluble group was comparable to those of the first runoff event independent of the time lapse between the application and rainfall event. The fairly soluble pesticides are thus exiting the field sites only by being adsorbed to eroded sediment. The cumulative soil loss is the most reliable indicator that explains the decrease in sediment and pesticide concentrations during each rainstorm (Table 2).

It is not possible to compare these results with other data from literature, due to the great variety of parameters concerning the performance of experiments. However, the findings are consistent in that a rough estimation of pesticide concentration in runoff and total pesticide losses is possible when rainfall (duration, intensity) and soil erosion potential are known.

Pesticide transport by wind may occur through isolated pesticide displacement or in a sediment- bounded form. At present, only initial findings of the amount of translocated pesticides are available. It is assumed that the horizontal transport of particulate-bounded pesticides is more than 50% of the total loss resulting from extreme wind erosion events. The herbicide loss of about 1.5% of the total amount, which was applied and integrated into the upper humus horizon on steppe soils in Canada, was measured after 13 wind erosion events. Herbicide loss from a non-integrated application on soil surfaces was approximately 4.5%.|n|

Experiments in a wind tunnel with the pre-emergence herbicide flurochloridone showed <1% to <58% displacement dependent on soil erodibility and measuring height.161

Determination of pesticide content in the atmosphere is extremely important but very difficult. Additionally, pesticides enter the atmosphere via many different processes, particularly by volatilization (simulated with PEARL, PELMO), unconsidered in our context.1121

The investigation of atrazine, alachlor, and acetochlor concentrations on soil surface and dissipation rates of wind-erodible sediment and larger fractions from two soil types was important.1131 Undisturbed and incorporated (5 cm deep) soil surface were analyzed. The surface (1cm) of soil was removed by vacuum 1, 7, and 21 days after herbicide treatment. About 50% of the recovered material was classified as wind-erodible sediment. This erodible sediment contained about 65% (undisturbed soil surface) and 8% (incorporated soil) of the applied herbicides, respectively, after lday. The concentrations were similar after 7 and after 21 days. However, a 50% dissipation rate for each herbicide was found after 15 days for wind-erodible sediments compared with 30-55 days for greater fractions. These data indicate that wind-erodible size aggregates and particles could be a source of herbicide contamination, but there is currently no information about quantities.

TABLE 2 Average Pesticide Losses in Relation to Applied Pesticide Dependent on Time (Rainfall Intensity: 70 mm hr1, Accumulated Rainfall: 100 mm)

Name of Pesticide

Pesticide

Transporting

Medium

Time and Concentration Lapse between Pesticide Application and Rainfall Event

14 days

2 hr

lday

3 days

5-7 days

Application on Bare Soil Surface Sites (Pre-Emergence Application) % of Applied Amount

Isoproturon

Total

4.5-12.4

4.0-17.2

5.4-13.3

2.1-11.2

n.e.

In runoff water

3.8-9.8

2.9-13.9

4.0-9.8

1.4-9.0

n.e.

In eroded sediment

0.7-2.6

1.7-3.3

1.4-3.5

0.7-2.2

n.e.

Dichlorprop-P

Total

2.1-10.4

2.2-16.7

5.9-10.4

1.2-8.3

n.e.

In runoff water

1.9-9.8

1.8-15.3

5.3-9.3

0.9-7.0

n.e.

In eroded sediment

0.2-0.6

0.4-1.4

0.6-1.1

0.3-1.3

n.e.

Bifenox

Total

15.6-19.0

19.3-21.6

14.3-17.2

9.3-13.8

n.e

In runoff water

•cO.l-O.9

cO.1-0.3

<0.1-0.3

<0.1-0.2

n.e.

In eroded sediment

14.7-19.0

19.0-21.6

14.0-17.2

9.3-13.6

n.e.

Application on Small Covered Soil Surface (Barley with 3-5 Leaves) % of Applied Amount

Isoproturon

Total

2.8-13.3

1.8-16.4

4.1-11.2

1.5-6.0

i.6

In runoff water

2.2-11.4

1.5-13.5

3.3-8.6

1.0-4.6

i.i

In eroded sediment

0.4-1.9

0.3-2.9

0.8-2.6

0.5-1.4

0.5

Dichlorprop-P

Total

0.8-10.0

0.9-9.2

1.0-8.3

0.7-4.1

0.8

In runoff water

0.7-9.4

0.8-8.4

0.8-7.5

0.6-3.7

0.7

In eroded sediment

0.1-0.6

0.1-0.8

0.2-0.8

0.1-0.4

0.1

Bifenox

Total

11.2-15.0

7.8-15.5

7.8-15.9

3.4-9.8

5.2

In runoff water

<0.1-0.9

<0.1-0.2

<0.1-0.4

cO.1-0.3

<0.1

In eroded sediment

11.2-14.1

7.8-15.5

7.4-15.9

3.4-9.5

5.2

It is important to recognize that only one fraction of pesticides in use is very strongly soil bound as to be transported in the sediment phase of runoff principally today (insecticides paraquat and pyrethroid, and other non-ionic hydrophobic species). Such pollutants may be too insoluble or soil bound to be transported in runoff or wind stream, but erosion can mobilize them.

Monitoring and Modeling

Monitoring in landscapes with a high erosion risk (water and wind) is predicted upon the estimation of rain and wind erosivity, soil erodibility, and the morphological factors of the areas (slope steepness, field length, wind openness, thalways). In sites with an erosion risk, well-designed field studies are the best way to assess off-site transport paths. When these data are not available, estimation about modeling will be necessary.

Water Erosion Models

Pesticide transport models are rare. Therefore, currently, water and wind erosion models serve as a basis for risk assessment, as a means to an end (Table 3)JI41

Most of the models were developed to assess impacts of different agricultural management practices; they are not adapted to predict exact pesticide, nutrient, or sediment loading in an area. The curve number method is an event or field scale orientated model and limits the results of some other models (ANSWERS, GLEAMS, CREAMS, and SWAT). In most erosion models, runoff and sediment load are only computed for the catchment outlet. Most of the hydrological models predict total runoff better than

TABLE 3 Selected Erosion Models and Hydrological Models with Integrated Pesticide Transport

Erosion Models

Models with Pesticide Transport

ANSWERS

CREAMS

KINEROS

GLEAMS

EUROSEM

AnnAGNPS

LISEM

SWAT

EROSION 3D

WEPP

Note: Wind Erosion Models: Wind Erosion Prediction System (WEPS); Revised Wind Erosion Equation (RWEQ).

Source: http://wwrv.soilerosion.net/doc/models_menu.html.

sediment load. The models over-or underestimate empirical results for small erosion events especially. Also, these models do not consider any attenuation or partitioning during transport and therefore fail to predict loads of soluble pesticides to surface waters.

WEPP and EROSION 3D models simulate the erosion and sediment transport continuously. The WEPP model is based on fundamentals of erosion theory, soil and plant science, channel flow hydraulics, and rainfall-runoff relationship.

It is possible to extend erosion and sediment transport models to a transport model for pesticides as well as sediment particles. The pesticide transport behavior that includes interaction processes in solutions is unknown for watershed scale.

Wind Erosion Models

A physical-based process model is the wind erosion prediction system (WEPS). This is a continuous, daily time-step model for simulation of weather, field conditions, and wind erosion. It has the capability of simulating spatial and temporal variability or soil surface parameters and soil loss or deposition within a field. To aid in the evaluation of offsite impacts, the soil loss is subdivided into components and reported as saltation creep, total suspension, and fine particulate matter components (PM 10). The transport capacity for insoluble pesticides bounded in kind of suspension or PM 10 is appreciable.151

Further models are the revised wind erosion equation (RWEQ) and WHEELS, which can be used for single event simulation, long-term risk assessment, and assessment of changing management strategies.

 
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