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 non

highly erodible soils and 300 ft on highly erodible soils,

• A 66 ft setbac k 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 quantitativ ely 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 effec tiveness of various types of vegetative buffe r s , commonly known as vegetative filter strip s (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. T he 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 proper ly 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 importan t 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 accumulat ion 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 pesticide s 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

so il sorption coefficient:

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

- 100 % with an average of 61%.

 For moderate sorption coefficient s (100 < Koc < 1,000 L ⁄ kg) pesticide retention ranged from 0

- 100 % with an average of 63%.

 For strong sorption coefficient s (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

m oisture 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 mitigat e pesticide loadings from runoff into water bodies . Long

- term effectiv eness 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. S oil partition coefficient (K d ) values of terbufos and its degradates rang ed 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 (K d ra nged from 0.4 to 2.93 mL/g ) than terbufos. S oil partition coefficient s 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 in to water bodies

. Table 1 provides EDWCs based on maximum and typical used rates of terbufos f or sorghum as well as hypothetical reduction of EDWCs for maximum application rate to account for the VFS. P RZM / EXAMS generated amended “time series” file s for the TTR of terbufos based on the TX sorghum OP scenario are also attached below.

Table 1. EDW Cs 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 sorghum 1 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 31.53 2 5.77 2 3.07 2

6.31 3 1.15 3 0.61 3

1 0.53 adjustment factor used in calculating EDWCs based on typical rate for sorghum
2 0.50 adjustment factor s 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 factor s used in calculating EDWCs based on 90% reduction of terbufos and its major degradates, terbufos sulfoxide and terbufos sulfone loading due to vegetative buffer5 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 s trategies to r educe p esticide i nputs i nto g round

- and s urface w ater and t heir e ffectiveness; 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 (DP41865 6, 2014).

US US EPA. 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 (DP 423572 , 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