def removal(self,columns):
        """
        total removal of nitrogen
        (1-(end_values/start_values))

        Parameters
        ----------
        columns : array of strings
        """
        inv=0
        outv=0
        indexes= self.time.values
        for column in columns:
            inv += self.data[column][indexes[0]]
        for column in columns:
            outv += self.data[column][indexes[-1]]
        removal = 1-(outv/inv)

        return removal