How are setting and history significant in Therese Raquin?

A a realist, specifically, a naturalist, Emile Zola aims to bring to the reader a highly-defined visual of how life really is. Think of it as the 19th century "HD filter" of literature. That was the whole purpose of it. The language and choice of words used for works such as Thérèse Raquin are specifically designed to instill clear, vibrant, realistic, and fluff-free imagery of the story. Still, Zola uses the "naturalist" filter, which focuses on the negative aspects of reality. These ugly, clearly depicted images will be more salient in the novel than anything else. They will be the focus of the description, and there is a good reason for this: the story itself is quite crude and gruesome. No need for a pretty setting to tell a story that ends so tragically.
The setting of Thérèse Raquin is described right at the beginning of the novel. You will notice that Zola seems to go on and on about the details, but it is important that he does this. We can make many conclusions about the characters and their lifestyles with those pieces of information. We can even predict how everything might turn out, after all, what sort of events could possibly take place in the shadiest alleyways of Paris? This latter question is a great point of conversation regarding the setting of the novel and should be asked prior to starting the actual reading.

At the end of the rue Guénégaud, if you follow it away from the river, you find the Passage du Pont-Neuf, a sort of dark narrow corridor linking the Rue Mazarine to the Rue de Seine.

Notice how Zola speaks to the reader, preparing the narrative as if he were about to tell a piece of saucy gossip. He even uses real places that do exist, such as Pont-Neuf. This adds a dimension of reality to the narrative, which is why it matters.

This passageway is, at most, thirty paces long and two wide, paved with yellowish worn stones which have come loose and constantly give off an acrid dampness. The glass roof, sloping at a right angle, is black with grime.

This fragment is typical Zola. Just when you start getting comfortably into the narrative, he shakes things up with an added detail directed straight to the senses. The black grime will be followed by further descriptions of filthy panes, miserable views, foul winters, and slimy paving. The "eek" factor is definitely intended to cause disgust and dislike for the place. Yet, this the nest where our characters will develop and tell their stories. We can only imagine what sort of stories they will be.
Knowing that the setting is the ugly side of 19th Paris, let's look into the historical context. Some hear the words "19th century" and "Paris" together, and get false imagery of beauty and sophistication. That imagery quickly comes to an end by just taking a look at the work of Charles Marville, a world-class 19th century photographer of all things "Paris." In his Paris project, Marville photographed in clear detail every aspect of Parisian society during the historical time period in which Thérèse Raquinis set.
A few aspects of society at that time truly stand out. First, the sewage system was almost non existent in Paris. Second, urinals had been introduced in the streets and were widely used, so you can make your own conclusions as to the smell of the place. Essentially, what Marville did in photographs, Zola did with words. Both artists render a no-nonsese description of how things, nay, how truly horribly things were for some sectors of society.
Conclusively, the distasteful setting develops within an equally awful time period. Deep within, at the core of it, will be other gruesome things developing: the tragic fates of each of the characters of the novel. One could argue that the setting and the historical period work together against the characters, for misery permeates the atmosphere at all times.