UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

OFFICE OF 

PREVENTION, PESTICIDES ANO 

TOXIC SUBSTANCES 

PC Code: 09870 I 

DP Barcode: D425910 

MEMORANDUM 

DATE: March 10, 2015 

SUBJECT: Tier I Drinking Water Assessment for the Registration Review of Phenrnedipham 

and its Degradate, MHPC, for use in Human Health Risk Assessment. 

TO: Miguel Zavala, Chemical Review Manager 

Neil Anderson, Acting Branch Chief 

Risk Management and Implementation Branch 3 

Pesticide Re-evaluation Division (7508P) 

AND 

FROM: 

Sumitra Biswas, Risk Assessor 

Donna Davis, Branch Chief 

Risk Assessment Branch 6 

Health Effects Division (7509P) 

William P. Eckel, Ph.D. ___. ~,;.._y ~ 3 /I "/ 

Senior Science Advisor 

Environmental Risk Branch 6 

Environmental Fate and Effects Division (7507P) 

THROUGH: Mark Corbin, Chief ·~ l()/~sMonica Wait, RAPL ~ \,J ~ 

Environmental Risk Branch 6 

3 /\Q /\5 

Environmental Fate and Effects Division (7507P) 

The purpose of this memorandum is to provide the Estimated Drinking Water Concentrations 

(EDWCs) of phenmedipham (PC code 098701; CAS number 13684-63-4), and its transformation 

product MHPC (methyl-3-hydroxyphenylcarbamate), for use in HED's registration review 

human health risk assessment. 
The surface water EDWCs are based on the assessment of August 4, 2004, as cited in the new 

use risk assessment of October 30, 2006 (DP Barcode 327030). The results from the FIRST 

model as stated in the October 2006 document have been updated in this drinking water 

assessment (DWA) to use the corrected molecular weight of the degradate, MHPC (167.16 amu).

The ground water EDWCs are based on the new PRZM-GW model. In the previous drinking 

water assessment, EDWCs from the SCI-GROW model were reported. The new results show 

basically no exposure to parent phenmedipham, but up to 11 ppb peak exposure to MHPC in the 

Wisconsin sands scenario.

The values in Table 1 generally represent upper-bound estimates of the concentrations that might 

be found in surface water and groundwater due to the use of phenmedipham by aerial spraying. 

EFED emphasizes that this is a Tier 1 screening level analysis, and should there be a need by 

HED, additional refinements to this estimate could be attempted. The monitoring data search 

provided only limited detections in one site in Denmark; therefore, the modeling estimates are 

considered to be more appropriate and suitable for national estimation of drinking water values.

EXECUTIVE SUMMARY 

Table 1 provides a summary of the Tier I modeled drinking water concentrations (EDWC) for 

surface water sources, adapted from the October 30, 2006 DWA (DP Barcode 327030) and 

corrected for MHPC molecular weight.

The highest ground water EDWCs were obtained for the degradate MHPC in the Wisconsin 

sands scenario. Parent phenmedipham concentrations were essentially zero in all six PRZM-GW 

scenarios.

Table 1. Tier I Estimated Drinking Water Concentrations – FIRST Model

Chemical Acute (peak) Surface 

Water Concentration 

(ppb)

Annual Average Surface 

Water Concentration 

(ppb)

30-year mean (ppb)

Phenmedipham 29.1 10.8 Not calculated for Tier I

MHPC 5.6 1.8 Not calculated for Tier I

Total Residues 34.7 12.6 --

Tier I Estimated Drinking Water Concentrations – PRZM-GW Model (Wisconsin sands scenario)

Chemical Peak Concentration (ppb) Post-Breakthrough mean 

(ppb)

Entire Simulation Mean 

(ppb)

Phenmedipham < 1 < 1 < 1

MHPC 10.9 9.8 6.3

Phenmedipham [CAS name 3-[(methoxycarbonyl)amino]phenyl (3-methylphenyl)carbamate; 

IUPAC name methyl 3-(3-methylcarbaniloyloxy)carbanilate], is a carbanilate herbicide. Its PC 

code is 098701 and its CAS number is 13684-63-4. Phenmedipham is an herbicide of the phenyl 

carbamate family with a mode of action that interferes with the Hill Reaction during 
photosynthesis by decreasing the rate of carbon dioxide assimilation, at photosystem II. 

Of the currently registered uses on table beets and spinach, phenmedipham’s maximum 

application rate is 0.975 lb a.i./A. The application rate for MHPC (0.076 lb/A) was calculated as 

its maximum production in the aerobic soil metabolism study (MRID 00142744) (14%) times the 

application rate of phenmedipham (0.975 lb/A), adjusted for the difference in molecular weight 

(167.16/300.34). 

This is a Tier I screening-level DWA. For surface water, the tier 1 aquatic model FIRST was 

used, and for ground water, the tier 1 aquatic model PRZM-GW was utilized. For the 

assessment, the maximum application rate and one application per year were used in the input 

parameters. 

The Metabolism Assessment Review Committee’s (MARC) decision document (July 26, 1994),

concluded that the transformation product MHPC (methyl-3-hydroxyphenylcarbamate) of 

phenmedipham is of potential concern for drinking water sources. Therefore, EDWCs of this

compound were also simulated using FIRST and PRZM-GW models. The approach used to 

calculate the transformation products was the separate degradate calculation, as opposed to the 

total residues approach. 

The aquatic models FIRST and PRZM-GW were utilized to obtain EDWCs in surface water and 

groundwater, respectively. The major data gaps are related to the transformation product MHPC. 

Modeling was used to obtain some of the parameters needed for the estimation of the drinking 

water concentrations of the above mentioned degradate, using EPISuite (EPIWIN).

Structure of Phenmedipham
Structure of MHPC

Background Information About Phenmedipham and its Degradate, MHPC:

Phenmedipham is a post-emergence herbicide of the chemical class bis-carbamate (noncholinesterase inhibitor). It is a strong inhibitor of photosynthesis. The major routes of 

dissipation in the environment for phenmedipham are hydrolysis, and biodegradation. The 

hydrolysis rate increases with the pH. The rate of biodegradation in aerobic soils and in the field 

appears to depend on the pH of the soil as well. It appears to be relatively immobile in various 

soils and remains in the top layers of aged and unaged soil columns. The major degradates were 

MHPC, and m-toluidine. Phenmedipham was stable to photolysis in pH 4 buffered solution 

(irradiated with a xenon arc lamp). The aerobic metabolism of phenmedipham in soils appears to 

depend on the soil pH and its organic matter. In a pH 5 soil with 0.85% organic matter, it 

degraded with a half-life of 120 days while in a pH 6.8 soil with 1.85% organic matter it 

degraded with a half-life of 20 days. The degradate MHPC formed at a maximum of 14.0% at 14 

days in the aerobic soil metabolism study. Several minor degradates were also formed. 

Phenmedipham does not photolyze in aqueous solutions, but appears to photolyze rapidly on 

soils, and it does not bioaccumulate substantially in fish due to rapid depuration.

Approach for Phenmedipham

According to the label, the maximum seasonal application rate for phenmedipham for all 

agricultural uses is 0.975 lb a.i./A. The worst-case scenario involving single application at the 

maximum seasonal rate was chosen for the conducting the modeling runs. Amongst the 

methods of application, the aerial spray is considered to yield the highest concentration in surface 

water modeling due to the potential for spray drift causing direct deposition in the water bodies. 

Since this chemical has been in use for several years, available surface and ground water 

monitoring data were searched as described later. 

Table 2 summarizes the input values used in the model runs for FIRST1.0 and PRZM-GW 1.07, 

respectively. The Koc value (1860 ml/g) was used in FIRST. The same Koc value was used in the 

ground water modeling using PRZM-GW. According to the modeling guidelines, the 90th
percentile upper confidence bound value from two available aerobic soil metabolism half-life 

values for phenmedipham was used in FIRST and PRZM-GW. Some of the physical and 

chemical properties of phenmedipham and its degradate, MHPC have been obtained from the 

TOXNET (peer reviewed) database and estimated by EPI suite modeling (see Appendix A of 

10/30/06 DWA). The modeling results obtained by using the upper bound scenarios are 

presented in Table 1. Output from the models (FIRST and PRZM-GW) are also attached 

(Appendix A).

Table 2. Environmental Fate and Other Input Parameters for Phenmedipham and MHPC 

(degradate) used in FIRST and PRZM-GW

Parameter Phenmedipham MHPC Source

Water Solubility (25°C) 4.7 ppm

46,960 ppm

Phenmedipham - product 

chemistry and 

[Worthing, C.R. and S.B. 

Walker (eds 1987).]

MHPC - EPI suite.

Hydrolysis Half-Life (pH 7) 14.5 hrs. N/A MRID 40502708

Aerobic Soil Metabolism Half-Life (upper 

confidence bound value, 90th percentile, 

from 2 values - 20 and 120 days).

224 days

104 days

Phenmedipham -

MRID #00142744, 

Supplemental;

MHPC half-life derived by 

EFED (same study): 2.3 

Hatzenbuhl 72.2 days; 2.2 

Neuhofen 41.5 days; 90th

percentile upperbound 

value 104 days.

Aerobic Soil Metabolism Half-Life 

(average median/mean for SCIGROW)

70 days 57 days

Aerobic Aquatic Metabolism Half-life

(2X aerobic soil metabolism input value)

448 days 208 days Used EFED modeling 

guidance to estimate.

Aqueous Photolysis Half-Life 0 (stable) N/A MRID# 42429901

default input value is 0 

for stable products 

(modeling guidance)

Adsorption/Desorption Coefficient 

(Modeled KOC)

1860 ml/g

138 ml/g 

Phenmedipham - Lyman, 

Reehl, and Rosenblatt 

(1990)

MHPC - MRID# 

46286303, Supplemental

Application Date (PRZM-GW) 7 days after 

emergence 

7 days after 

emergence

postemergence 

application

PCA (worst case scenario) 0.87 0.87 Default value for other 

crops

Application Method (for max. application 

rate)

Aerial Spray Aerial Spray Label
Table 2. Environmental Fate and Other Input Parameters for Phenmedipham and MHPC 

(degradate) used in FIRST and PRZM-GW

Parameter Phenmedipham MHPC Source

Application Rate (lb a.i./A) 0.975 0.076 (1) Label

Applications Permitted per Year 1 1 Label

Application Interval (minimum days) NA NA Label

Depth of Incorporation NA NA Label

N/A - Not available; NA - Not applicable

(1). Maximum concentration observed in the aerobic soil metabolism study x application rate x mole ratio = 

 (0.14)(0.975)(167.16/300.34)=0.076

Monitoring Data

Surface Water: The USGS National Water Quality Assessment (NAWQA) database was 

searched for phenmedipham and MHPC separately. No detections of either of these compounds 

were reported in this monitoring database. Also, the state of California (Cal EPA) database was 

searched, and no detections were reported there. Amongst all of the pesticides used, including 

phenmedipham, the database listed the pesticides which were detected during the monitoring 

effort. 

Ground Water: Based upon the US Geological Survey National Water Quality Assessment 

(NAWQA) data search, no specific detections of phenmedipham have been reported. However, 

results from a Danish (Denmark) Pesticide Leaching Assessment Program reported detection of 

phenmedipham (0.01-0.02 μg/l) and MHPC (0.03-0.19 μg/l) in two samples at the Faardrup site 

containing predominantly clay till soil (Pesticide leaching at Faardrup, Chapter 6, PLAP 

monitoring results, May 1999 - June 2002, Denmark EPA). This information is available on the 

internet at: http://pesticidvarsling.dk/monitor_uk/2002_uk/vap-results-99-02-6-uk.html

These concentrations are substantially lower than those calculated by the PRZM-GW model for 

MHPC (up to 11 ug/L). Parent phenmedipham detections in the Danish monitoring data are less 

than 1 ug/L as would be expected based on the PRZM-GW modeling.
Appendix A

Results from FIRST modeling for parent phenmedipham - Phenmedipham Aerial Spray

1. Aerial Spray One Application (upper bound scenario)

July 20, 2004

 RUN No. 1 FOR Phenmedipham ON Sugar Beet * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP

 ONE(MULT) INTERVAL Koc (PPM ) (%DRIFT) AREA (IN)

 --------------------------------------------------------------------

 .975( .975) 1 1 1860.0 4.7 AERIAL(16.0) 87.0 .0

 FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (RESERVOIR) (RES.-EFF) (RESER.) (RESER.) 

 --------------------------------------------------------------------

 224.00 2 N/A .00- .00 448.00 448.00

 UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.0 AUG 1, 2001

 --------------------------------------------------------------------

 PEAK DAY (ACUTE) ANNUAL AVERAGE (CHRONIC) 

 CONCENTRATION CONCENTRATION 

 --------------------------------------------------------------------

 29.090 10.820

Results from FIRST modeling for MHPC July 21, 2004

Reported results are corrected for actual molecular weight (167) versus previous value used (159)

1. One Application of Parent

 RUN No. 1 FOR MHPC ON Sugarbeet * INPUT VALUES * 

 --------------------------------------------------------------------

 RATE (#/AC) No.APPS & SOIL SOLUBIL APPL TYPE %CROPPED INCORP

 ONE(MULT) INTERVAL Koc (PPM ) (%DRIFT) AREA (IN)

 --------------------------------------------------------------------

 .072( .072) 1 1 138.046960.0 AERIAL(16.0) 87.0 .0

 FIELD AND RESERVOIR HALFLIFE VALUES (DAYS) 

 --------------------------------------------------------------------

 METABOLIC DAYS UNTIL HYDROLYSIS PHOTOLYSIS METABOLIC COMBINED

 (FIELD) RAIN/RUNOFF (RESERVOIR) (RES.-EFF) (RESER.) (RESER.) 

 --------------------------------------------------------------------

 104.00 2 N/A .00- .00 208.00 208.00

 UNTREATED WATER CONC (MICROGRAMS/LITER (PPB)) Ver 1.0 AUG 1, 2001

 --------------------------------------------------------------------
 PEAK DAY (ACUTE) ANNUAL AVERAGE (CHRONIC) 

 CONCENTRATION CONCENTRATION 

 --------------------------------------------------------------------

 5.340 1.726

Modeling Results (PRZM-GW):

Groundwater Analysis for MHPC and the Wisconsin Corn - WI Central Sands Scenario

Estimated groundwater concentrations and breakthrough times for MHPC are presented in Table 

1 for the Wisconsin Corn - WI Central Sands groundwater scenario. A graphical presentation of 

the daily concentrations in the aquifer is presented in Figure 1. These values were generated with 

the PRZM-GW (Version 1.07). Critical input values for the model are summarized in Tables 2 

and 3.

Table 1. Groundwater Results for MHPC and the Wisconsin Corn - WI Central Sands 

Scenario.

Peak Concentration (ppb) 10.9

Post-Breakthrough Mean 

Concentration (ppb)

 9.82

Entire Simulation Mean 

Concentration (ppb)

 6.34

Average Breakthrough 

Time (days)

4219.351

Throughputs 2.597556

Table 2. Chemical Properties for Groundwater Modeling of MHPC.

Koc (ml/g) 138

Surface Soil Half Life (days) 104

Hydrolysis Half Life (days) 0

Diffusion Coefficient Air 

(cm2/day)

0.0

Henry's Constant 0.0

Enthalpy (kcal/mol) 0.0

Table 3. Pesticide application scheme used for MHPC. This application scheme was 
applied every year of the simulation.

Application Days 

Relative to 

Emergence Date 

(05/01)

Application Method Application Rate

(kg/ha)

7 Ground application with mass 

distribution increasing 

proportionally with depth to 4 cm

0.076

Figure 1. Aquifer Breakthrough Curve for MHPC and the Wisconsin Corn - WI Central 

Sands Scenario