Can My General Manager "fire" An Employee For Refusing To Work In Sewage?

Hi! I'm currently a shift manager for a popular bakery/cafe chain located in Southern California, and we've been having a string of issues with our plumbing for the past few months. Occasionally, especially after it rains, the drains in varying parts of the store will back-flow, causing anywhere from 2 to 10 gallons of liquid to flow onto the floors before draining after about 10 to 20 minutes. During this time my manager has instructed us to contain the sewage and mop it up as best as we can while still serving customers.

At first this wasn't an issue as the water was mostly in the back of house and away from food and customers, but more recently it has grown more severe. Today I get a notification in our group chat from the current MiC (Manager in Charge) saying the drains are flooding again, but aren't stopping. She then follows up with pictures and it's quite severe, more than I've seen it do before, and it's even backflowing from the drains on the line where food is prepared and served to customers. There is a stench in the entire restaurant, and it's extremely unhygienic.

2 of our workers this morning walked out of the shift, saying that the store ought to be shut down and they aren't going to sit and mop up sewage while still serving food to unknowing customers, as it's unsafe. When the MiC reiterates this to the group chat, our GM (General Manager) responds with, "Just to be clear, if they walk out over this it is quitting. I just spoke with [District Manager], if they walk out it's quitting."

Is this even legal? As far as I'm aware, any restaurant with sewage backflow is required to cease operations until the issue is solved, but I may be wrong. Even so, does raw sewage backflowing into the kitchen and food preparation areas not constitute a public safety concern? At the very least is it not considered a hazardous working condition? I don't believe It's right to "legally" fire employees for refusing to work in conditions like that, or to knowingly serve potentially contaminated food to the public.

Is there any material or online sources anyone could reference so I can show this to him and help prevent some very much unnecessary potential lawsuits?