Department of Pesticide Regulation
Environmental Monitoring Branch1001 I Street, P.O. Box 4015
Sacramento, CA 95812-4015
Calculation of 1,3-D application factors based on HYDRUS flux estimates
Colin Brown
10/17/2019
Introduction
1,3-Dichloropropene (1,3-D) is a fumigant used to control nematodes, insects and disease organisms inthe soil. It is commonly used as a pre-plant treatment that is injected into soil. It may also be applied
through drip irrigation. Regardless of the application method, the possibility of offsite transport of thisfumigant due to volatilization may subsequently cause human exposure through inhalation. To mitigate
its potential cancer risk, the Department of Pesticide Regulation (DPR) limits the use of 1,3-D on aregional basis (township cap). The current township cap is 136,000 “adjusted” pounds during a calendar
year in any township (six by six mile area). Adjusted pounds refers to the amount of 1,3-D active
ingredient multiplied by an application factor (AF) to account for differences in air concentrations due toapplication method, region, and season of application.
AFs are multipliers that DPR originally intended to account for variation in cumulative flux density
between different application methods (e.g., deep versus shallow shank injection). The basis fordetermination of an AF is the emission ratio (ER, also referred to in some documents as an 'emission
rating'), an estimate of the emitted fraction of total applied mass over a given interval following
completion of an application (ER = cumulative flux / mass applied). DPR derived the ER values currently
used in AF calculations (Table A-1, appendix) from a small selection of field-estimated ER values
obtained from field flux studies. The first field flux studies were for an untarped, deep injection
application method, and DPR assigned an ER of 0.35 and an AF of 1.0 to this method (Johnson 2013a).
The AFs for other methods are relative to this application method and ER (Johnson 2014). Currently, DPR
applies additional mathematical adjustments to January AFs in an attempt to account for an increase in
1,3-D air concentrations due to seasonal and regional meteorological variation.
This document presents an alternate approach to generate AF estimates for each of the 17 approved
field fumigation methods ('FFM', summarized in Table 1) for 1,3-D based on the HYDRUS modeling ofERs performed by Brown (2019). AF for each method is taken as the mean ER at 21 days post-application(modeled cumulative flux) across 16 representative soil types, normalized by 0.35. Because HYDRUS
estimates ER for each method as a distribution of possible values, standard deviation (SD) of AF is also
calculated based on the SD associated with each HYDRUS-estimated ER. The AFs calculated here do not1

attempt to account for variation in seasonal or regional air concentrations resulting from meteorologicalfactors (i.e., only flux is considered).
Table 1. List of approved 1,3-D application methods.
Field Fumigation
Method (FFM) Code

Method Description

1201

1,3-D - Nontarpaulin/Shallow/Broadcast or Bed

1202

1,3-D - Tarpaulin/Shallow/Broadcast

1203

1,3-D - Tarpaulin/Shallow/Bed1204

1,3-D - Nontarpaulin/Shallow/Broadcast or Bed w/ 3x Irrigation

1205

1,3-D - Tarpaulin/Shallow/Bed w/ 3x Irrigation

1206

1,3-D - Nontarpaulin/Deep/Broadcast or Bed

1207

1,3-D - Tarpaulin/Deep/Broadcast

1209

1,3-D - Tarpaulin/Chemigation/Bed1210

1,3-D - Nontarpaulin/Deep/Strip1211

1,3-D – Nontarpaulin/Deep/GPS-targeted

1242

1,3-D - TIF/Shallow/Broadcast - 60% credit1

1243

1,3-D - TIF/Shallow/Bed - 60% credit1

1245

1,3-D - TIF/Shallow/Bed w/ 3x Irrigation - 60% credit1

1247

1,3-D - TIF/Deep/Broadcast - 60% credit1

1249

1,3-D – TIF/Deep/Strip – 60% credit1

12591,3-D - TIF/Chemigation/Bed - 60% credit1

1290

1-3,D – Other Label Method

1. '60% credit' refers to a form of buffer zone reduction credit. TIF stands for 'totally impermeable film'.

Methods
HYDRUS ER estimates were those calculated by Brown (2019) and summarized in Table 2. ER for eachmethod was estimated based on mean cumulative flux at 21 days post-application for HYDRUS
simulations performed across 16 soil types described by soil core data collected by DPR staff fromprepared fields prior to fumigation. The soil data obtained from those cores is intended to describe thedistribution of likely soil conditions (including soil texture and soil moisture) typical of fumigated fields inCalifornia (Johnson 2013b). Although there may be differences in the distribution of soil conditions byregion (with regional differences in soil texture having been previously evaluated by Johnson andSpurlock 2009), there is currently insufficient data to conclude whether these differences would
significantly affect mean ER by region. At this time, SD associated with mean ER describes a lower boundof expected variability for each method due to field-to-field variation in soil conditions including soil
texture and soil moisture, and does not account for other sources of variability such as changes in tarp
permeability due to tearing, stretching, or gluing.
AF for each method was calculated from HYDRUS-estimated ERs using equation 1, below:
AF = ER / 0.352(1)No ER estimates are available for FFM 1211 (Nontarpaulin/Deep/GPS-targeted) and FFM 1290 (OtherLabel Method). The AF for FFM 1211 is assumed equivalent to the AF for FFM 1206 (Nontarpaulin/Deep/Broadcast), the method to which it is most similar. The AF for FFM 1290 is assumed equivalent to thehighest estimated AF, here corresponding to FFM 1201 (Nontarpaulin/Shallow/Broadcast).
Table 2. HYDRUS-estimated ER by application method (from Brown 2019).
FFM
code

Method description

1201

ER @ 21 daysSD

1,3-D - Nontarpaulin/Shallow/Broadcast

0.58

0.09

1202

1,3-D - Tarpaulin/Shallow/Broadcast

0.46

0.12

1203

1,3-D - Tarpaulin/Shallow/Bed0.55

0.111204

1,3-D – Nontarpaulin/Shallow w/ 3x Irrigation

0.500.101205

1,3-D - Tarpaulin/Shallow/Bed w/ 3x Irrigation

0.52

0.11

1206

1,3-D - Nontarpaulin/Deep/Broadcast

0.38

0.13

1207

1,3-D - Tarpaulin/Deep/Broadcast

0.31

0.14

1209

1,3-D - Tarpaulin/Chemigation/Bed0.48

0.08

1210

1,3-D - Nontarpaulin/Deep/Strip

0.39

0.14

1242

1,3-D - TIF/Shallow/Broadcast - 60% credit

0.14

0.05

1243

1,3-D - TIF/Shallow/Bed - 60% credit

0.26

0.10

1245

1,3-D - TIF/Shallow/Bed w/ 3x irrigation - 60% credit

0.20

0.06

1247

1,3-D - TIF/Deep/Broadcast - 60% credit

0.110.05

1249

1,3-D - TIF/Deep/Strip - 60% credit

0.140.07

1259

1,3-D - TIF/Chemigation/Bed - 60% credit

0.20

0.06

Results and Discussion
AF estimates are presented in Table 3 and Figure 1 alongside the current regulatory values. As describedearlier, the current regulatory AF values were developed through a two-step process: first, a smallcollection of field flux studies was used to estimate ER for a given method; and second, a mathematicaladjustment was applied to account for increases in January air concentrations due to meteorologicalfactors unrelated to flux. With the exception of seasonal and regional temperature effects evaluated byBrown (2019), which were not found to be significant for the calculation of ER, the consideration of
meteorological factors goes beyond the scope of HYDRUS modeling. Therefore, this approach to AF
determination does not account for all of the parameters that have been used historically to calculate
the AFs and the HYDRUS AFs in Table 3 and Figure 1 are presented as ‘seasonally unadjusted’ values
which are best compared to the current regulatory AFs for February-November. A set of wintermultiplication factors (historically ranging from 1.0 to 2.0) would need to be applied to the seasonallyunadjusted HYDRUS AF values in Table 3 for the purposes of calculating January AFs as described byJohnson (2013a, 2014).

3

Table 3. Current regulatory and HYDRUS-estimated seasonally unadjusted AF by method.

Current AF

HYDRUS AF

FFM code

JanA[Inside SJV]

JanA
[Outside SJV]

Feb-Nov[All locations]

Seasonally
Unadjusted[All locations]SD1201

Prohibited

2.3

1.9

1.670.26

1202

Prohibited

2.3

1.9

1.30

0.33

1203

Prohibited

2.3

1.9

1.57

0.32

1204

Prohibited

2.3

1.9

1.42

0.29

1205

Prohibited

2.3

1.9

1.49

0.30

1206

1.9

1.2

11.10

0.37

1207

1.9

1.2

1

0.89

0.39

1209

1.16

1.16

1.16

1.38

0.23

1210

1.9

1.2

1

1.11

0.39
0.37B

1211

1.9

1.2

11.10B

1242

0.6

0.6

0.3

0.41

0.14

1243

0.6

0.6

0.3

0.74

0.30

1245

0.6

0.6

0.3

0.57

0.19

1247

0.6

0.6

0.3

0.31

0.16

1249

1.2

1.2

0.6

0.390.211259

1.16

1.16

1.16

0.57

0.19

1.9

1.67C

0.26C

1290

AProhibited

2.3

All 1,3-D applications are prohibited during December.

B FFM 1211 ('1,3-D – Nontarpaulin/Deep/GPS-targeted') is assigned the value of FFM 1206.
CFFM 1290 ('1,3-D - Other Label Method') is by default assigned the highest calculated AF.

Figure 1. Summary of current regulatory AF (Feb-Nov values) and HYDRUS-estimated seasonally unadjusted AF bymethod. Error bars represent 1 SD around the mean due to field-to-field variation in soil conditions.

4References:Brown, C. (2019). HYDRUS-simulated flux estimates of 1,3-dichloropropene max period-averaged flux and emission
ratio for approved application methods. Sacramento, CA: Department of Pesticide Regulation, California
Environmental Protection Agency.
Johnson, B. (2013a). Calculation of use adjustment factors for 1,3-dichloropropene with the use of totallyimpermeable film for broadcast shank applications. Sacramento, CA: Department of Pesticide Regulation,
California Environmental Protection Agency.
Johnson, B. (2013b). Study #286: Selected physical properties of California soils prior to broadcast or bedded
fumigation treatments. Sacramento, CA: Department of Pesticide Regulation, California Environmental ProtectionAgency.
Johnson, B. (2014). How to calculate the application factor for new 1,3-dichloropropene application methods.
Sacramento, CA: Department of Pesticide Regulation, California Environmental Protection Agency.
Johnson, B. and Spurlock, F. (2009). Dominant soil types associated with fumigant applications in ozone
nonattainment areas. Sacramento, CA: Department of Pesticide Regulation, California Environmental Protection
Agency.

5Appendix ATable A-1. List of current application factors (AF) adapted from Pesticide Use Enforcement Program Standards
Compendium Volume 3, Appendix Section J.3 (Revision 41).
Location

Tarp Type

Months1

Fumigation
MethodApplication
FactorShallow
Deep
Drip
Shallow
Deep
Drip
Shallow
Deep
Strip
Drip
Shallow
Deep
Strip
Drip
Shallow
Deep
Drip
Shallow
Deep
Drip
Shallow
Deep
Strip
Drip
Shallow
Deep
Strip
Drip

Prohibited
1.9
1.16
1.9
1.0
1.16
0.6
0.6
1.2
1.16
0.3
0.3
0.6
1.16
2.3
1.2
1.16
1.9
1.0
1.16
0.6
0.6
1.2
1.16
0.3
0.3
0.61.16Jan
non-60%credit
Feb-Nov
Within SJV
Jan
60% credit
Feb-Nov

Jan
non-60%credit
Feb-Nov
Outside SJV
Jan
60% credit
Feb-Nov
1All applications prohibited during December.

6