The Reservoir is underground. It is the main source of water for the City. It comes from a subterranean river let in through thick, durable grating. The Wall (which does extend miles under the earth) is not any weaker here than it is anywhere else. The water is cleaned before it enters the Reservoir. Pipes take the water from the Reservoir to various locations around the City. Only special workers are allowed in, but incredibly sneaky people could also manage it. The Resistance would not dare poison the water because they too rely on it as a main source of water.
Roleplay!