Drilling on Earth
Most oil wells are aimed at 3-5 km depths. some rare ones go to 6 or even 7 but that's about it. Specialiased deep drilling on Earth has reached 12 km (Russia, Kola peninsula) but that took a purpose-built giant rig (tens of thousands of tonnes) and several year's drilling with a full support crew and plenty of spares.
In a seperate response you'll find my "why it's so deep" argument. If we accept here that it is around that deep, then I agree that the chance of contamination between the surface and the deep fluid aquifer are minimal. What is important and unknown is whether some mildly warm spots exist on Mars that could bring the depth from 7-10 km down to 2-3 km.