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diff --git a/R/PEC_sw_sed.R b/R/PEC_sw_sed.R
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+# Copyright (C) 2015  Johannes Ranke
+# Contact: jranke@uni-bremen.de
+# This file is part of the R package pfm
+
+# This program is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free Software
+# Foundation, either version 3 of the License, or (at your option) any later
+# version.
+
+# This program is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
+# details.
+
+# You should have received a copy of the GNU General Public License along with
+# this program. If not, see <http://www.gnu.org/licenses/>
+
+#' Calculate predicted environmental concentrations in sediment from surface
+#' water concentrations
+#'
+#' The method 'percentage' is equivalent to what is used in the CRD spreadsheet
+#' PEC calculator
+#'
+#' @param PEC_sw Numeric vector or matrix of surface water concentrations in µg/L for
+#'   which the corresponding sediment concentration is to be estimated
+#' @param percentage The percentage in sediment, used for the percentage method
+#' @param method The method used for the calculation
+#' @param sediment_depth Depth of the sediment layer
+#' @param water_depth Depth of the water body in cm
+#' @param sediment_density The density of the sediment in L/kg (equivalent to
+#'   g/cm3)
+#' @param PEC_sed_units The units of the estimated sediment PEC value
+#' @return The predicted concentration in sediment
+#' @export
+#' @author Johannes Ranke
+#' @examples
+#' PEC_sw_sed(PEC_sw_drift(100, distances = 1), percentage = 50)
+PEC_sw_sed <- function(PEC_sw, percentage = 100, method = "percentage", 
+                       sediment_depth = 5, water_depth = 30,
+                       sediment_density = 1.3,
+                       PEC_sed_units = c("\u00B5g/kg", "mg/kg"))
+{
+  method = match.arg(method)
+  PEC_sed_units = match.arg(PEC_sed_units)
+  if (method == "percentage") {
+    PEC_sed = PEC_sw * (percentage/100) * (water_depth / sediment_depth) * (1 / sediment_density)
+    if (PEC_sed_units == "mg/kg") PEC_sed <- PEC_sed / 1000
+  }
+  return(PEC_sed)
+}