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+#' Groundwater ubiquity score based on Gustafson (1989)
+#'
+#' The groundwater ubiquity score GUS is calculated according to
+#' the following equation
+#' \deqn{GUS = \log_10 DT50_{soil} (4 - \log_10 K_{oc}}{GUS = log10 DT50soil * (4 - log10 Koc)}
+#'
+#' @references Gustafson, David I. (1989) Groundwater ubiquity score: a simple
+#' method for assessing pesticide leachability. _Environmental
+#' toxicology and chemistry_ *8*(4) 339–57.
+#' @inheritParams endpoint
+#' @param chent If a chent is given with appropriate information present in its
+#' chyaml field, this information is used, with defaults specified below.
+#' @param DT50 Half-life of the chemical in soil. Should be a field
+#' half-life according to Gustafson (1989). However, leaching to the sub-soil
+#' can not completely be excluded in field dissipation experiments and Gustafson
+#' did not refer to any normalisation procedure, but says the field study should
+#' be conducted under use conditions.
+#' @param Koc The sorption constant normalised to organic carbon. Gustafson
+#' does not mention the nonlinearity of the sorption constant commonly
+#' found and usually described by Freundlich sorption, therefore it is
+#' unclear at which reference concentration the Koc should be observed
+#' (and if the reference concentration would be in soil or in porewater).
+#' @param lab_field Should laboratory or field half-lives be used? This
+#' defaults to lab in this implementation, in order to avoid
+#' double-accounting for mobility. If comparability with the original GUS
+#' values given by Gustafson (1989) is desired, non-normalised first-order
+#' field half-lives obtained under actual use conditions should be used.
+#' @param degradation_value Which of the available degradation values should
+#' be used?
+#' @param sorption_value Which of the available sorption values should be used?
+#' Defaults to Kfoc as this is what is generally available from the European
+#' pesticide peer review process. These values generally use a reference
+#' concentration of 1 mg/L in porewater, that means they would be expected to
+#' be Koc values at a concentration of 1 mg/L in the water phase.
+#' @param degradation_aggregator Function for aggregating half-lives
+#' @param sorption_aggregator Function for aggregation Koc values
+#' @return A list with the DT50 and Koc used as well as the resulting score
+#' of class GUS_result
+#' @author Johannes Ranke
+#' @export
+GUS <- function(...) UseMethod("GUS")
+
+#' @rdname GUS
+#' @export
+GUS.numeric <- function(DT50, Koc) {
+ score <- log10(DT50) * (4 - log10(Koc))
+ res <- list(DT50 = DT50, Koc = Koc, score = score)
+ class(res) <- "GUS_result"
+ return(res)
+}
+
+#' @rdname GUS
+#' @export
+GUS.chent <- function(chent, lab_field = "laboratory",
+ aerobic = TRUE,
+ degradation_value = "DT50ref",
+ sorption_value = "Kfoc",
+ degradation_aggregator = geomean,
+ sorption_aggregator = geomean,
+ digits = 1)
+{
+ DT50 = soil_DT50(chent, lab_field = lab_field, redox = aerobic,
+ value = degradation_value,
+ aggregator = degradation_aggregator, signif = 5)
+ Koc = soil_Kfoc(chent, value = sorption_value,
+ aggregator = sorption_aggregator, signif = 5)
+ GUS.numeric(DT50, Koc)
+}
+
+#' @export
+print.GUS_result = function(x, ..., digits = 1) {
+ cat("GUS: ", round(x$score, digits = 1), "\n")
+ cat("calculated from DT50 ", x$DT50, " and Koc ", x$Koc, "\n")
+}

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