UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

JulylO, 2015 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

MEMORANDUM 

PC Code: 105001 

DP Barcode: 428089 

Subject: 

From: 

Through: 

TO: 

Adjusted Time-Series Distribution of the Total Toxic Residues of Terbufos 

from Use on Sorghum to Account for the Theoretical Loading Reduction of 

Terbufos and its Degradates to Surface Water in the Presence of LabelPrescribed Vegetative Buffers _ 

Faruque Khan, Ph.D., Senior Fate Scientis~ { " /~- 7 /lo{ f;'J 

Environmental Risk Branch 1 

Environmental Fate and Effects Division (7507P) 

Sujatha Sankula, Ph.D., Branch Chief ~. ~ Environmental Risk Branch 1 / \ 

Environmental Fate and Effects Division (7507P) 

Mathew Manupella, Risk Manager Reviewer 

Tom Myers, Team Leader 

Kevin Costello, Branch Chief 

Risk anagement and Implementation Branch II 

Pesticide Re-Evaluation Division (7508P) 

Pete Savioa, HED Reviewer 

Michael Metzger, Branch Chief 

Risk Assessment Branch VII 

Health Effect Division (7509P) 

EXECUTIVE SUMMARY 

This memorandum provides adjusted (reduction of 50% and 90%) estimated drinking water 

concentrations (EDWCs) and time-series distributions for the total toxic residues (TTR) of 

terbufos (i.e. terbufos and two major oxidative degradates, terbufos sulfoxide and terbufos 

sulfone) in surface water based on label-prescribed vegetative buffers. Current labels of terbufos 

require certain setback distances and vegetative buffers between terbufos treated fields and 

surface waters. Vegetative buffers must be seeded with grass or other suitable cove and the 

following distances are to be maintained between terbufos treated areas and surface water: 

A 500 ft vegetative buffer between the treated area and surface water on neighboring 

land, 
2

• A 500 ft vegetative buffer between a standpipe drain outlet and surface water on 

neighboring land, 

• A 66 ft setback between the treated area and entry points to surface water bodies on nonhighly erodible soils and 300 ft on highly erodible soils,

• A 66 ft setback between the treated area and standpipes on terraced fields as well as a 66 

ft vegetative buffer between the tile outlet and surface water bodies, and

• Restricting loading, rinsing, and washing equipment within 300 ft from surface water 

bodies or within 50 ft from wells unless conducted on an impervious surface. 

A well maintained vegetative buffer could potentially intercept terbufos laden runoff (both 

soluble and sediment bound) prior to reaching surface waters. Currently, the Agency does not 

quantitatively assess the effectiveness of these practices in reducing pesticide concentrations in 

runoff. In addition, the current surface water model used by the Agency does not have the 

capability to account for prescribed setbacks or vegetative buffer distances. Therefore, the 

Environmental Fate and Effects Division (EFED) evaluated currently available scientific 

literature to quantify the reduction of pesticide loading in the presence of vegetative buffers. 

Many studies have been conducted to document the effectiveness of various types of 

vegetative buffers, commonly known as vegetative filter strips (VFS) and provided guidance in 

designing buffers to reduce agricultural pesticides and nutrients in runoff water and sediment 

into surface water bodies (USDA 2000). Soil is an important component of VFS design. The 

performance of a VFS obviously will be affected largely by the soil’s physical and chemical 

properties as well as supporting vegetation. The VFS are effective to provide resistance to runoff 

events and in trapping highly adsorbed and moderately adsorbed pesticides in presence of 

properly designed types of vegetation and the width of VFS. The soluble fraction of pesticide in 

runoff is generally removed through infiltration and the soil type plays an important role in 

pesticide transport reduction. In general, sedimentation is a predominant mechanism followed by 

infiltration of soluble fraction within VFS. However, excessive sediment load in runoff may not 

only change flow patterns caused by sediment accumulation within VFS, but can potentially 

reduce water infiltration, creating VFS less effective in trapping dissolved pesticides (USDA, 

2000).

Numerous research studies have shown that properly designed and maintained VFS are 

effective as a mitigation measure in reducing the volume of runoff and trapping highly sorbed 

pesticide laden eroded sediment. A recent review paper by Auroa et al. (2010) postulated that 

certain fate properties of pesticides such as soil sorption coefficient (Koc) can affect the transport 

of soluble and eroded sediment laden with pesticides into surface water bodies. Based on 

reviewed studies, they predicted the following overall pesticide retention by VFS as a function of 

soil sorption coefficient:

 For weak sorption coefficients (Koc < 100 L⁄ kg), pesticide retention ranged from 0-

100% with an average of 61%.

 For moderate sorption coefficients (100 < Koc < 1,000 L ⁄ kg) pesticide retention ranged 

from 0-100% with an average of 63%.

 For strong sorption coefficients (Koc > 1,000 L ⁄ kg), retention ranged from 53- 100%

with an average of 76%. 
3

Reichenberger et al (2007) reviewed 180 publications and evaluated many aspects related to 

the effectiveness of VFS in reducing pesticide loads into adjacent water bodies. They concluded 

that the effectiveness of VFS to reduce pesticide loading into an adjacent surface water body 

depends on many factors, such as topography, field conditions, soil types and its antecedent 

moisture condition, rainfall intensity, properties of pesticide and application methods, width of 

the VFS and types of vegetation within the buffer strip etc. On average VFS efficiencies are 

roughly 50% reduction for 5m width and 90% for 10m width, and 97.5% for 20m width for all 

pesticides, runoff volume and sediment (Figure 1). However, VFS maintenance is critical for 

their continuing effectiveness in intercepting runoff loads to mitigate pesticide loadings from 

runoff into water bodies. Long-term effectiveness of VFS requires regular maintenance including 

excavation to remove overburdens of sediments, repairing vegetation damage, and removing 

over-mature vegetation or invasion of noxious weeds (USDA, 2000).

Figure 1. Pesticide load reduction efficiencies of edge-of-field buffer strips vs. various VBS 

width (Reichenberger et al., 2007). 

Reviews of above articles suggest that VFS could serve as a sink for terbufos and its two 

major degradates, terbufos sulfoxide and terbufos sulfone. Soil partition coefficient (Kd) values 

of terbufos and its degradates ranged from 0.4 to 14.6 mL/g, which suggest that they are 

moderately mobile to slightly mobile in soil (USEPA, 2014a [D418656]). However, terbufos 

sulfoxide and terbufos sulfone are more mobile (Kd ranged from 0.4 to 2.93 mL/g) than terbufos. 

Soil partition coefficients of terbufos and its degradates are also associated with inorganic 

constituents (i.e. clay) rather than organic carbon content of soils. Therefore, it is hypothesized 

that in presence of VFS, the reduction of total toxic residue of terbufos loading would be 

somewhere between 50 to 90%. 
4

In the previous drinking water assessments, the reported EDWCs and time-series were 

based on the maximum (USEPA, 2014a [D418656] and typical application rates of terbufos for 

sorghum (USEPA 2014b [D423572]). To estimate 50 and 90% reductions, previously reported 

EDWCs and time-series values were adjusted for TX sorghum scenario for maximum application 

rate. The reported results were simply multiplied with 0.50 and 0.10 for theoretical reduction of 

total toxic residue terbufos loading from runoff and eroded sediment into water bodies. Table 1

provides EDWCs based on maximum and typical used rates of terbufos for sorghum as well as 

hypothetical reduction of EDWCs for maximum application rate to account for the VFS. PRZM/

EXAMS generated amended “time series” files for the TTR of terbufos based on the TX 

sorghum OP scenario are also attached below. 

Table 1. EDWCs for Drinking water risk assessment for terbufos and its degradates 

based on maximum and typical rate as well as theoretical reduction of 50% and 90% for 

vegetative buffer strip. 

Source of Drinking 

Water

Estimated Drinking Water Concentration (μg/L)

1-in-10-year Peak 

Exposure

1-in-10-year 

Annual Mean 

Exposure 

30-year Mean 

Exposure 

Total Toxic Residues of Terbufos (Terbufos plus Terbufos Sulfoxide and Terbufos Sulfone)

Surface Water (max rate of 

1.68 lbs a.i for sorghum) 63.06 11.53 6.13

Surface water (typical rate of 

0.895 lbs a.i for sorghum1 33.42 6.11 3.25

Surface water (max rate of 

1.68 lbs a.i for sorghum and 

hypothetical 50% and 90% 

reduction of reported EDWCs 

due vegetative buffer

31.532 5.772 3.072

6.313 1.153 0.613

1 0.53 adjustment factor used in calculating EDWCs based on typical rate for sorghum

2 0.50 adjustment factors used in calculating EDWCs based on 50% reduction of terbufos and its major 

degradates, terbufos sulfoxide and terbufos sulfone loading due to vegetative buffer

3 0.10 adjustment factors used in calculating EDWCs based on 90% reduction of terbufos and its major 

degradates, terbufos sulfoxide and terbufos sulfone loading due to vegetative buffer

 

Sorghum_TTR_PCA 

and 50% reduction adjusted.csv

 

Sorghum_TTR_PCA 

and 90% reduction adjusted.csv
5

References

Arora K., S.K. Mickelson, M.J. Helmers. & Baker J.L. 2010. Review of pesticide retention 

processes in buffer strips receiving agricultural runoff. Journal of the American Water Resources 

Association, 46, 618–647.

Reichenberger, S. M. Bach, A. Skitschak, and H. Frede, 2007. Mitigation strategies to reduce 

pesticide inputs into ground- and surface water and their effectiveness; A Review. Science of the 

Total Environment 384:1-35.

US EPA. 2014a. Memorandum: Drinking Water Assessment for the Registration Review of

Terbufos. Environmental Fate and Effects Division, Office of Chemical Safety and

Pollution Prevention. U. S. Environmental Protection Agency (DP418656, 2014).

 USEPA. 2014b. Memorandum: Adjusted Time-Series Distribution of Total Toxic Residues for 

Estimated Drinking Water Concentrations Based on Typical Rate of Sorghum for Terbufos,

Environmental Fate and Effects Division, Office of Chemical Safety and Pollution Prevention. 

U. S. Environmental Protection Agency (DP423572, 2014).

USDA-NRCS, 2000. Conservation Buffers to Reduce Pesticide Losses. U.S. Department of 

Agriculture–Natural Resource Conservation Service. 

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs143_023819.pdf