TABLE 2.
Textural and related properties of soils.

Field Characteristics:

The field was a square, 10 acres in size, and had a 4% smooth slope. Channel flow and impoundment were not defined. The field was fallow, disked with 10% mulch cover.

Pesticide Parameters:

The pesticides parameters chosen were solubility, soil half-life, and the organic carbon partitioning coefficient (Koc). The values for half-life were 1, 2, 4 and 40 days. The solubility values were 0.1, 1, 10, 1,000, 10,000, and 100,000 ppm. The Koc was 100, 300, 500, 700, 10,000, and 100,000 ml/g. The pesticide was applied to the surface of a bare soil at a rate of 4 kg/ha.

Factors Not Included In Pesticide Loss Potential:

The meteorological components used in the rating process were for evaluating potentials independent of climate and were not intended to represent any climatic zone. The primary goal was to determine the capacity of a soil to retain a pesticide at the point of application, regardless of management or climatic inputs. The meteorological data used in the model to estimate pesticide losses was artificially generated to produce the most likely situations for pesticide loss (worst case scenario). A 3.5 inch precipitation event was generated every second day after application for five events, and then a 1.0 inch event every other day for at least four times the half-life of the pesticide.

The persistence (half-life) of a pesticide in a soil is partially dependent on soil moisture and temperature. The degradation of the pesticide is favored by warm and moist climates. The difference in half-life rates of the pesticide due to soil moisture and temperature has not been considered.

The type of crop and the method of pesticide application was not considered. The soil was assumed to be fallow and the method of pesticide application was to the soil surface. To consider each crop and method of application available for a pesticide is beyond the scope of this screening procedure.

Some soil parameters that are thought to influence pesticide half-life rates or solubility have not been considered. These factors include: soil pH, aluminum content, elements toxic to microbes, and total soil surface area.

PROCEDURE

Almost all possible combinations of the soil and pesticide parameters listed above were evaluated with GLEAMS (2). The total number of combinations examined was 40,896.

Development Of The Algorithms

A stepwise regression using SAS was used to select the soil or pesticide input parameters that weighted most heavily for estimating each category of pesticide loss from the GLEAMS runs. These parameters were then used with various algorithms or equations to group pesticide loss. Mostly a trial and error procedure was tried for setting algorithm break points to separate each group. Usually, three groups were separated, but for leaching loss a group fell out conspicuously and was established as a fourth group. The groups were assigned names. The groups for soil loss potentials are:

The groups for pesticide loss potentials are:

Relative pesticide loss values for each category group were combined into a matrix. The maximum values for each cell in the matrix were examined to see if they fell below the limits for a pre-established overall potential. The objective of the matrix was to establish each category of loss as one that does not contain pesticide loss greater than the established threshold for that category. The upper limit of loss for a VERY LOW rating was set at zero. The upper limits for a LOW rating were set so each category of loss potential did not contain pesticides that are expected to contaminate that water resource from a non-point source. The lower limit for the HIGH category was arbitrarily set at about three times the value of the upper limit of the LOW category.

The group ranking of soils and pesticides do not have an absolute definition relative to quantity. Pesticide losses from this model reflect only the relative ability of the soil to retain the pesticide at the point of application. The interplay of climate determines whether the leaching or surface loss potentials are reached in a given area. Tables 3 through 8 give the algorithms that resulted.

TABLE 3.
Soil Leaching Potential Algorithm. (SLP)

HIGH:

• ((hydrologic group = A) and ((Organic Matter times Horizon #1 Depth) <= 30)) or
  • ((hydrologic group = B) and ((Organic Matter times Horizon #1 Depth) <= 9) and (Soil K factor <= 0.48)) or
  • ((hydrologic group = B) and ((Organic Matter times Horizon #1 Depth) <= 15) and (Soil K factor <= 0.26))

LOW:

• ((hydrologic group = B) and ((Organic Matter times Horizon #1 Depth) >= 35) and (Soil K factor >= 0.40)) or
• ((hydrologic group = B) and ((Organic Matter times Horizon #1 Depth) >= 45) and (Soil K factor >= 0.20)) or
• ((hydrologic group = C) and (Organic Matter times Horizon #1 Depth) <= 10) and (Soil K factor >= 0.28)) or
• ((hydrologic group = C) and (Organic Matter times Horizon #1 Depth) >= 10)

VERY LOW:

• (hydrologic group = D)

INTERMEDIATE:

• Everything else

TABLE 4.

Pesticide Leaching Potential Algorithm after Gustafson(3) (PLP)

HIGH:

• ((log10(Half-Life in days)*(4 - log10(Koc))) >= 2.8)

LOW:

• ((log10(Half-Life in days)*(4 - log10(Koc))) <= 1.8)

VERY LOW:

• ((log10(Half-Life in days)*(4 - log10(Koc))) < 0.0) or
• ((Solubility < 1 ppm) and (Half-Life <= 1 days))

INTERMEDIATE:

• Everything else

TABLE 5.

Soil Adsorbed Runoff Potential Algorithm (SARP)

HIGH:

• ((Hydrologic Group == "C") and (Soil K Factor >= 0.21)) or
• ((Hydrologic Group == "D") and (Soil K Factor >= 0.10))

LOW:

• (Hydrologic Group == "A") or
• ((Hydrologic Group == "B") and (Soil K Factor <= 0.10)) or
• ((Hydrologic Group == "C") and (Soil K Factor <= 0.07)) or
• ((Hydrologic Group == "D") and (Soil K Factor <= 0.02))

INTERMEDIATE:

• Everything else

TABLE 6

Pesticide Adsorbed Runoff Potential Algorithm.(PARP)

HIGH:

• ((Half-Life >= 40 days) and (Koc >= 1000)) or
• ((Half-Life >= 40 days) and (Koc >= 500) and (Solubility <= 0.5 ppm))

LOW:

• (Half-Life <= 1 days) or
• ((Half-Life <= 2 days) and (Koc <= 500)) or
• ((Half-Life <= 4 days) and (Koc <= 900) and (Solubility >= 0.5 ppm)) or
• ((Half-Life <= 40 days) and (Koc <= 500) and (Solubility >= 0.5 ppm)) or
• ((Half-Life <= 40 days) and (Koc <= 900) and (Solubility >= 2 ppm))

INTERMEDIATE::

• Everything else

TABLE 7

Soil Solution Runoff Potential Algorithm. (SSRP)

HIGH:

• ((hydrologic group =C) or (hydrologic group =D))

LOW:

• (hydrologic group = A)

INTERMEDIATE:

• (hydrologic group B)

TABLE 8

Pesticide Solution Runoff Potential Algorithm. (PSRP)

HIGH:

• ((Solubility >= 1 ppm) and (Half-Life > 35 days) and (Koc < 100000)) or
• ((Solubility >= 10 ppm) and (Solubility < 100 ppm) and (Koc <= 700))

LOW:

• (Koc >= 100000) or
• ((Koc >= 1000) and (Half-Life <= 1 days)) or
• ((Solubility < 0.5 ppm) and (Half-Life < 35 days))

INTERMEDIATE:

• Everything else

Tables 9 through 11 give the statistical parameters maximum, minimum, mean, and standard deviation for each overall potential within each category of loss. The purity of the groupings can be determined by comparing the statistical tables with the overall potential given in tables 12 through 14. An example would be comparing Table 9 with Table 12. These comparisons indicate the zero loss potential (VERY LOW -- Potential 4) is pure. That is, the VERY LOW category does not contain losses that are over 2. The LOW loss potential (Potential 3) does not contain occurrences of INTERMEDIATE or HIGH losses. The INTERMEDIATE loss potential does not contain occurrences of HIGH loss, but does contain many occurrences of LOW loss. The HIGH loss potential contains incidences of INTERMEDIATE and LOW loss.

Definition of Algorithms

Leaching algorithms:

The soil algorithm for grouping soils for potential loss to leaching are given in Table 3. The pesticide algorithm for grouping soils for potential loss to leaching are given in Table 4. The procedure used to establish the algorithms resulted in an algorithm very similar to the Groundwater Ubiquity Score (GUS) by Gustafson (3). The GUS was chosen as the algorithm to use since that work contained additional verification not planned for this procedure.

Surface algorithm:

The algorithms for surface losses are not as definite as the algorithms for leaching. The number of factors which correlate to surface losses were much greater than those to leaching losses. The stepwise regression procedure showed little difference in the weighting of the first five to seven parameters. Table 5 through 8 present the adsorbed and solution runoff algorithms for soils and pesticides.

TABLE 9
Potential Pesticide Loss To Leaching, Using GUS.

Table 10
Potential Pesticide Adsorbed Loss To Runoff.

Table 11
Potential Pesticide In Solution Loss To Runoff

TABLE 12
Potential Pesticide Loss To Leaching Screening Matrix (ILP)

TABLE 13

Definition of loss potential.

The potential pesticide loss is relative, and explains no more than a relative expectation of pesticide loss. The HIGH Category has a higher expectation of contaminating the respective water source than INTERMEDIATE, which has a higher expectation than LOW. The VERY LOW category for leaching has essentially a zero expectation of having the pesticide leach below the root zone. Because HIGH and INTERMEDIATE contain occurrences of loss that are low, further examination of that soil-pesticide combination may be required. Possibly the GLEAMS (2) model should be run using the real pesticide-soil-crop-climate combinations to develop better estimates of the pesticide loss potentials. The screening procedure establishes an expectation of loss, not an absolute estimate of loss.

IMPLEMENTATION

The soil-pesticide interaction ratings are designed as screening tools. The ratings reflect current technology on a national scope. They are relative to each other and do not have absolute units of loss.

Background Soil And Pesticide Information

Pesticide Database:

The Pesticide Database (6) was primarily developed to provide data for development of this screening procedure. However, the database has found popularity for use by others in similar projects. A peer review group consisting of twenty-two NACA, ARS, SCS, Extension Service (ES), Environmental Protection Agency, Forest Service and industry members was formed to approve the screening procedure and pesticide database. The current screening procedure and pesticide database has been reviewed by this peer group. The pesticide database is being presented in another paper in this proceeding (6). This database is repeated in Table 17 and contains the estimated loss potentials using the algorithms developed in this paper.

Table 17
Pesticide Database

<Removed. Refer to WINPST for this data. --- MSH 6-4-98>

Soil Database:

Pesticide leaching and surface loss potentials for soils have been estimated using the algorithms developed in this paper. The potentials are available through queries of the Soil Interpretations Record (SCS-SOI-5) data set at Ames, Iowa. The soil ratings can be obtained for a state or for a survey area. If this service is not available, the local or state SCS Office may be of assistance in obtaining the soil potentials. The algorithms may also be used to estimate the soil potentials if the required soil information is available.

Adjusting soil leaching and surface loss potentials.

When evaluating the soil potentials, several factors should be considered that may cause an adjustment in the soil rating. Field soil scientists should review the soil leaching and surface loss potentials. Relative comparisons of potentials should be made to determine if they fall in logical ranks, and if the potential is in agreement with the soil scientist's information of soil series or map unit behavior. Soil scientists have the information about the hydrology of a particular soil series or map unit that is required to verify the calculated ratings. Table 15 lists factors that should be examined when a soil surface loss or leaching potential is suspect.

TABLE 15
Factors to consider when evaluating soil surface loss or leaching potentials:

• Slope Greater Than 15%.
• Surface Organic Matter Layers.
• Shallow and Perched Water Tables.
• Questionable Hydrologic Soil Group.
• Lithic Soil Subgroups

The potentials should always be checked if the potential has been calculated for slopes greater than 15 percent, organic soils, soils that have an organic surface layer, or water tables less than 6 feet from the surface. If the ratings are obtained from Ames or the SCS, these soils are flagged. The flags are:

 

• * -- Slopes > 15 percent.
• # -- Organic soils and soils that have an organic surface layer.
• & -- Water table less than 6 feet from the surface.

Slope greater than 15%:

Slopes greater than fifteen percent may have more pesticide surface loss than indicated by the ratings. This loss is due to higher erosion than estimated by the slope parameter used in GLEAMS. The surface adsorbed loss potentials should be adjusted upward one group.

Surface Organic Matter Layers:

Organic soils will rarely have pesticide loss from surface runoff or leaching. Pesticides with a Koc over 300 ml/g are strongly adsorbed to the organic matter (OM). Particles moved from these soils by wind or water would carry pesticides with them. Occasionally a map unit or soil series that has OM data for cultivation may be in range or forest land. In such cases, an organic layer may develop at the surface. If the organic layer is greater than one-half inch thick, considerable pesticide may be adsorbed in this layer, and not susceptible to leaching. These soils should have the soil surface loss and leaching potential estimated with the additional OM incorporated into the surface horizon.

Shallow And Perched Water Tables:

Soils that have water tables less than six feet below the surface and perched water tables are difficult to rate. These soils usually have a dual hydrologic soil group rating. The SCS uses dual hydrologic groups when water movement is impeded by water tables (for example A/D). The first group represents hydrologic characteristics after drainage, and the second group represents hydrologic characteristics before drainage.

The shallow water table has a greater expectation of contamination than deeper water tables, particularly if it occasionally rises into the root zone.

Perched water tables may flow into local surface waters rather than infiltrating to an aquifer. Pesticides leaching to perched water tables may have a greater potential to contaminate surface waters than solution pesticide in surface runoff. Use the greater loss potential between drained leaching potential and not drained soil solution runoff potential.

Shallow water tables that represent ground water levels in an aquifer, or perched water tables that leak to an aquifer, require special concern. These areas are typically not suitable for pesticide application. When evaluating these conditions the soil leaching loss potential should be used for the hydrologic soil group with the highest infiltration.

Questionable Hydrologic Soil Group:

The National Soils Handbook (5), pages (603-49) to (603-51), explains hydrologic soil groups and suggestions for rating soils into hydrologic soil groups. (This information is now found on pages (618-20) to (618-21) in section (618-35) of the NSSH Volume 1, 1996. – MSH 6-5-98) The hydrologic soil group assigned to a soil series may adequately express runoff potentials but not reflect infiltration as it affects leaching. This most commonly occurs for hydrologic soil group B soils. If the hydrologic soil group does not reflect infiltration, then the estimated leaching loss potential is incorrect. A lower leaching potential may be estimated using a hydrologic soil group indicating lower infiltration rates.

Relative comparison between mapping units of soil leaching and surface loss potentials is a good method to determine if the rank of each soil or mapping units is logical. This can be misleading when two soils have a HIGH leaching loss potential, but you intuitively know one soil should be considerably less than the other. This is usually not an inaccurate rank. The modeling technique to rank the soil loss potentials selected a certain loss potential level to divide the HIGH and INTERMEDIATE ranks. A coarse sand may have an effective saturated hydraulic conductivity of 4.0 in/hr although a sandy loam may have a value of 0.4 in/hr. The pesticide lost from the coarse sand would be several times that of the sandy loam, but the amount lost from the sandy loam was sufficient to rank it as HIGH. Table 16 lists the range of minimum infiltration rates of soils by hydrologic group. The modeling technique equated minimum infiltration rates to effective saturated conductivity. These values may assist in placing soils into hydrologic groups.

TABLE 16
Ranges of minimum infiltration rates of soils by hydrologic group (Musgrave, (4)).

Lithic soil subgroups:

Lithic soil subgroups are also difficult to rate. In the soil classification system a lithic subgroup is used to indicate coherent material between 7 in. and 20 in. The problems here are similar to shallow water tables. According to SCS procedures a lithic subgroup should have a soil hydrologic group of C or D. A soil that would be rated soil hydrologic group A or B without the lithic contact may have significant water infiltration and movement along the lithic contact. This water may be discharged into and contaminate surface waters. Use the greater loss potential between the hydrologic group without lithic contact leaching potential and hydrologic group with lithic contact solution surface loss potential for the solution surface loss potential.

The lithic material should be examined for potential leakage to the aquifer. Rapid percolation to the aquifer may occur if the lithic material is fractured. If fracturing exists the soil leaching potential should be calculated from tables for soil leaching potentials using the more permeable hydrologic soil group.

USING SOIL-PESTICIDE INFORMATION

Determining the Potential

Figure 1 is an example Soil-Pesticide Work Sheet for evaluating pesticide use in a specific environment. The parameters needed are soil map unit or soil series, crop, target pest, and the recommended pesticides. The recommended pesticide is obtained from the Cooperative Extension Service (CES), a pesticide bulletin provided by the CES, or a state agency responsible for pesticide recommendations. The pesticide adsorbed or solution surface loss or leaching potential is obtained from Table 17 or calculated from algorithms. The soil adsorbed surface, solution surface or leaching loss potentials are obtained from the SCS database in Ames, Iowa, local or state SCS offices, or calculated from algorithms. The soil potentials should be adjusted as previously discussed. The soil and pesticide ratings are combined in a matrix to obtain the overall pesticide loss potential. Table 12 is a matrix for potential leaching loss. Table 13 is a matrix for potential surface adsorbed loss. Table 14 is a matrix for potential surface solution loss.

Several pesticides from the Pesticide Database in Table 17 have estimated or guessed values for these parameters. The estimated values are flagged with an 'E'. The guessed values are flagged with a 'G'. These flags should be carried through the screening procedure.

FIGURE 1

PESTICIDE WORKSHEET

Rating Classes:

• H – HIGH
• I – INTERMEDIATE
• L – LOW
• V – VERY LOW (Leaching only)

Ratings:

• SLP – Soil Leaching Potential
  • The vulnerability of a soil, based on it’s physical properties, to pesticide leaching below the rootzone.
• SSRP – Soil Solution Runoff Potential
  • The vulnerability of a soil to pesticide loss as pesticide dissolved in surface water that leaves the edge of the field.
• SARP – Soil Adsorbed Runoff Potential
  • The vulnerability of a soil to pesticide loss adsorbed to sediment and organic matter that leaves the edge of the field.

• PLP – Pesticide Leaching Potential

• The predicted movement potential of a pesticide to move below the rootzone in leachate.
• PSRP – Pesticide Solution Runoff Potential
  • The movement potential of pesticide dissolved in surface water that leaves the edge of the field.
• PARP – Pesticide Adsorbed Runoff Potential
  • The movement potential of pesticide adsorbed to sediment and organic matter that leaves the edge of the field.
• ILP – Soil/Pesticide Interaction Leaching Potential
  • The leaching potential based on consideration of the SLP and PLP.
• ISRP – Soil/Pesticide Interaction Solution Runoff Potential
  • The Solution Runoff Potential based on consideration of the SSRP and PSRP.
• IARP – Soil/Pesticide Interaction Adsorbed Runoff Potential
  • The adsorbed runoff potential based on consideration of the SARP and the PARP.

Interpreting the Potential

General considerations:

The method of application should be considered when interpreting the potential. Keep in mind that:

• Foliar applications can result in only a small portion of a pesticide reaching the soil surface where it can be subject to loss.
• Pesticides applied in a band below the surface or incorporated into the soil may have a lower loss potential for surface runoff, but a higher loss potential for leaching than estimated by this technique.
• Application rates may be near or below health advisory limits.

Consult locally developed guidelines or the manufacturer when these conditions exist

The probability of rainfall soon after pesticide application should be considered in most climates. The loss estimates used in this procedure assume considerable precipitation immediately after application. If little or no precipitation occurs, a significant loss may not occur. Considerations in this area depend largely on the half-life of the pesticide. After the elapse of one half-life, half of the original pesticide concentration has been degraded, thus one-half remains. A pesticide with a half-life of four days will be at 25% of the original concentration in 8 days (two half-life periods). Thus, if a rainfall event is not expected for a time equal or greater to three times the half-life of the pesticide, little pesticide loss would be expected.

The purpose of this method is a first tier screening procedure. If a HIGH or INTERMEDIATE potential is the result of the screening procedure, do not reject the use of that chemical on that soil. Further evaluation is warranted. However, if a LOW or VERY LOW Potential is the result, the use of that chemical can be considered safe.

Potential 1, HIGH:

This potential has a high probability of being lost from the field. Before deciding to use HIGH Potential pesticides, they should be evaluated for their health hazard to humans, animals, and plants. If a pesticide is a potential danger to health, an alternative pesticide, or other pest management techniques should be considered. A second tier evaluation may prove this combination not a hazard.

Potential 2, INTERMEDIATE:

This potential is a gray area. INTERMEDIATE guidelines differ from HIGH in that:

1. The Pesticide Solution Runoff Potential (PSRP not ISRP) may be reduced one rank (HIGH to INTERMEDIATE) if the pesticide is foliarly applied, soil incorporated, or banded under the surface.
1. The pesticide leaching potential (PLP) could be reduced one rank if foliarly applied.
1. The use of this pesticide on this soil could be considered LOW if rainfall probability is low.

Potential 3, LOW:

This pesticide applied on this soil has very low probability of being lost to surface runoff or leaching. This pesticide could be used according to the label with little hazard to the respective water resource.

Potential 4, VERY LOW:

This potential is used only for leaching. The probability of leaching loss is essentially zero unless the soil contains cracks or macropores to depths greater than 30 inches.

Example --

The Pesticide Work Sheet (Fig. 1) is an example from Minnesota. The corn rootworm example provides some interesting comparisons.

For the soil/pesticide interaction leaching loss potential (ILP) four pesticides have a LOW rating, and one has a HIGH rating (the most likely to be lost). If ground water is a potential contamination hazard, the pesticides with a LOW rating would be an environmental "best choice" without further evaluation.

For the soil/pesticide interaction adsorbed to sediment or organic matter loss potential (IARP), one pesticide has a HIGH rating (the most likely to be in runoff), two have an INTERMEDIATE rating, and two have a LOW rating (the least likely). For the soil/pesticide interaction solution runoff potential, one pesticide has a HIGH rating and four have a LOW rating. If surface water is a potential contamination hazard the pesticides Terbufos, or Chlorpyrifos would be an environmental "best choice" without further evaluation. If conservation practices could be employed that reduced sediment loss then Chlorpyrifos would be the environmental "best choice" since the influence of the adsorbed pesticide loss would be reduced.

REFERENCES