109:7 If a person sees that the water that stood overnight will not be enough for his needs, he may add more water to it so long as the majority of it is from the water that stood overnight. It’s preferable that two-thirds of it be from the water that stood, and it would be best if at all possible to draw the water to be added by pumping or from a covered well so that the sun doesn’t shine on the water.
109:8 When baking matzah on Sunday, the water must be drawn on Thursday evening. This is because it would be impossible to do so on Friday without running into Shabbos. In an emergency, if one didn’t draw the water on Thursday, he may do so on Friday after mincha, or on Shabbos by a non-Jew.