This is a basic calculation of a contaminant concentration in bulk soil based on complete, instantaneous mixing. If an interval is given, an attempt is made at calculating a long term maximum concentration using the concepts layed out in the PPR panel opinion (EFSA PPR panel 2012 and in the EFSA guidance on PEC soil calculations (EFSA, 2015, 2017).

PEC_soil(
rate,
rate_units = "g/ha",
interception = 0,
mixing_depth = 5,
PEC_units = "mg/kg",
PEC_pw_units = "mg/L",
interval = NA,
n_periods = Inf,
tillage_depth = 20,
leaching_depth = tillage_depth,
crop = "annual",
cultivation = FALSE,
chent = NA,
DT50 = NA,
FOMC = NA,
Koc = NA,
Kom = Koc/1.724,
t_avg = 0,
t_act = NULL,
scenarios = c("default", "EFSA_2017", "EFSA_2015"),
leaching = scenarios == "EFSA_2017",
porewater = FALSE
)

## Arguments

rate Application rate in units specified below Defaults to g/ha The fraction of the application rate that does not reach the soil Mixing depth in cm Requested units for the calculated PEC. Only mg/kg currently supported Only mg/L currently supported Period of the deeper mixing. The default is NA, i.e. no deeper mixing. For annual deeper mixing, set this to 365 when degradation units are in days Number of periods to be considered for long term PEC calculations Periodic (see interval) deeper mixing in cm EFSA (2017) uses the mixing depth (ecotoxicological evaluation depth) to calculate leaching for annual crops where tillage takes place. By default, losses from the layer down to the tillage depth are taken into account in this implementation. Ignored for scenarios other than EFSA_2017. Only annual crops are supported when these scenarios are used. Only crops with a single cropping cycle per year are currently supported. Does mechanical cultivation in the sense of EFSA (2017) take place, i.e. twice a year to a depth of 5 cm? Ignored for scenarios other than EFSA_2017 An optional chent object holding substance specific information. Can also be a name for the substance as a character string If specified, overrides soil DT50 endpoints from a chent object If DT50 is not specified here and not available from the chent object, zero degradation is assumed If specified, it should be a named numeric vector containing the FOMC parameters alpha and beta. This overrides any other degradation endpoints, and the degradation during the interval and after the maximum PEC is calculated using these parameters without temperature correction If specified, overrides Koc endpoints from a chent object Calculated from Koc by default, but can explicitly be specified as Kom here Averaging times for time weighted average concentrations Time series for actual concentrations If this is 'default', the DT50 will be used without correction and soil properties as specified in the REACH guidance (R.16, Table R.16-9) are used for porewater PEC calculations. If this is "EFSA_2015", the DT50 is taken to be a modelling half-life at 20°C and pF2 (for when 'chents' is specified, the DegT50 with destination 'PECgw' will be used), and corrected using an Arrhenius activation energy of 65.4 kJ/mol. Also model and scenario adjustment factors from the EFSA guidance are used. Should leaching be taken into account? The default is FALSE, except when the EFSA_2017 scenarios are used. Should equilibrium porewater concentrations be estimated based on Kom and the organic carbon fraction of the soil instead of total soil concentrations? Based on equation (7) given in the PPR panel opinion (EFSA 2012, p. 24) and the scenarios specified in the EFSA guidance (2015, p. 13).

## Value

The predicted concentration in soil

## Details

This assumes that the complete load to soil during the time specified by 'interval' (typically 365 days) is dosed at once. As in the PPR panel opinion cited below (EFSA PPR panel 2012), only temperature correction using the Arrhenius equation is performed.

Total soil and porewater PEC values for the scenarios as defined in the EFSA guidance (2017, p. 14/15) can easily be calculated.

## Note

While time weighted average (TWA) concentrations given in the examples from the EFSA guidance from 2015 (p. 80) are be reproduced, this is not true for the TWA concentrations given for the same example in the EFSA guidance from 2017 (p. 92).

According to the EFSA guidance (EFSA, 2017, p. 43), leaching should be taken into account for the EFSA 2017 scenarios, using the evaluation depth (here mixing depth) as the depth of the layer from which leaching takes place. However, as the amount leaching below the evaluation depth (often 5 cm) will partly be mixed back during tillage, the default in this function is to use the tillage depth for the calculation of the leaching rate.

If temperature information is available in the selected scenarios, as e.g. in the EFSA scenarios, the DT50 for groundwater modelling (destination 'PECgw') is taken from the chent object, otherwise the DT50 with destination 'PECsoil'.

## References

EFSA Panel on Plant Protection Products and their Residues (2012) Scientific Opinion on the science behind the guidance for scenario selection and scenario parameterisation for predicting environmental concentrations of plant protection products in soil. EFSA Journal 10(2) 2562, doi:10.2903/j.efsa.2012.2562

EFSA (European Food Safety Authority) 2017) EFSA guidance document for predicting environmental concentrations of active substances of plant protection products and transformation products of these active substances in soil. EFSA Journal 15(10) 4982 doi:10.2903/j.efsa.2017.4982

EFSA (European Food Safety Authority) (2015) EFSA guidance document for predicting environmental concentrations of active substances of plant protection products and transformation products of these active substances in soil. EFSA Journal 13(4) 4093 doi:10.2903/j.efsa.2015.4093

## Examples

PEC_soil(100, interception = 0.25)#>      scenario
#> t_avg default
#>     0     0.1
# This is example 1 starting at p. 92 of the EFSA guidance (2017)
# Note that TWA concentrations differ from the ones given in the guidance
# for an unknown reason (the values from EFSA (2015) can be reproduced).
PEC_soil(1000, interval = 365, DT50 = 250, t_avg = c(0, 21),
Kom = 1000, scenarios = "EFSA_2017")#>      scenario
#> t_avg      CTN     CTC      CTS
#>    0  19.76834 13.8619 10.53795
#>    21 19.59345 13.7169 10.39882PEC_soil(1000, interval = 365, DT50 = 250, t_av = c(0, 21),
Kom = 1000, scenarios = "EFSA_2017", porewater = TRUE)#>      scenario
#> t_avg       CLN       CLC       CLS
#>    0  0.5541984 0.6779249 0.9816693
#>    21 0.5484576 0.6693125 0.9609119
# This is example 1 starting at p. 79 of the EFSA guidance (2015)
PEC_soil(1000, interval = 365, DT50 = 250, t_avg = c(0, 21),
scenarios = "EFSA_2015")#>      scenario
#> t_avg      CTN      CTC      CTS
#>    0  21.96827 11.53750 9.145259
#>    21 21.78517 11.40701 9.017370PEC_soil(1000, interval = 365, DT50 = 250, t_av = c(0, 21),
Kom = 1000, scenarios = "EFSA_2015", porewater = TRUE)#>      scenario
#> t_avg       CLN       CLC       CLS
#>    0  0.7589401 0.6674322 0.9147861
#>    21 0.7506036 0.6590345 0.8987279
# The following is from example 4 starting at p. 85 of the EFSA guidance (2015)
# Metabolite M2
# Calculate total and porewater soil concentrations for tier 1 scenarios
# Relative molar mass is 100/300, formation fraction is 0.7 * 1
results_pfm <- PEC_soil(100/300 * 0.7 * 1 * 1000, interval = 365, DT50 = 250, t_avg = c(0, 21),
scenarios = "EFSA_2015")
results_pfm_pw <- PEC_soil(100/300 * 0.7 * 1000, interval = 365, DT50 = 250, t_av = c(0, 21),
Kom = 100, scenarios = "EFSA_2015", porewater = TRUE)