def _flag_backfill_data(
    df, p_col="CTDPRS", flag_bool_col="interp_bool", flag_suffix="_FLAG_W"
):
    """Flag data columns which have been interpolated with flag 6."""
    for col in df.columns:
        if flag_suffix in col:
            df.loc[df[flag_bool_col], col] = 6

    return df