text: #' Calculate the longitudinal dispersivity value based on the approach developed by Xu and Eckstein (1995)
  #' 
  #' This function takes plume length (PLUME_L) and length units (UNITS) as arguments to calculate 
  #' longitudinal dispersivity (α_L). Length units can be meters or feet. The function will return the 
  #' longitudinal dispersivity in the same length units as plume length.
  #' 
  #' disp(PLUME_L, UNITS)
  #' @param PLUME_L is the plume length
  #' @param UNITS is the length unit for plume length and longitudinal dispersivity
  #' @export
  #' @examples
  #' disp(PLUME_L = 880, UNITS = "feet")
  #' [1] 23.18827
  
  disp <- function(PLUME_L = 1, UNITS = "feet"){
      CONV    <- ifelse(toupper(UNITS) %in% c("FEET", "FT"), 1 * 12 * 2.54 / 100, 
                 ifelse(toupper(UNITS) %in% c("METER", "METERS", "M"), 1, NA))
      ALPHA   <- 0.83 * log10(PLUME_L * CONV)^2.414 / CONV
      return(ALPHA)
  }