def rescol(rescol):
    """ Comfirms that a give value is a valid results column and returns
    the corresponding units column and results column.
    """
    if rescol.lower() == 'concentration':
        unitscol = 'units'
    elif rescol.lower() == 'load_outflow':
        unitscol = 'load_units'
    else:
        raise ValueError("`rescol` must be in ['concentration', 'load_outflow']")
    return rescol.lower(), unitscol