def join_field_w_lab(wq_single_gain, chl_lab_single_date, wq_site_fieldname, lab_site_fieldname):
	"""
	Joins lab data to a single gain for a single date
	:param chl_lab_single_date: all the lab values for a single sampling date
	:param wq_site_fieldname: the name of the field storing the siteid in the gain file to join on
	:param lab_site_fieldname: the name of the field storing the siteid in the lab file to join on
	:return: dataframe with lab values joined by site to the appropriate field measurement
	"""
	join_lab = wq_single_gain.merge(chl_lab_single_date, left_on=wq_site_fieldname,
	                                right_on=lab_site_fieldname, suffixes=('', '_y'))
	return join_lab