1. The problem statement, all variables and given/known data I am working on a project where I have to design and size an aquifer. There are two which either store hot or cold water for 6 months which is then pumped up to be used in either the summer or winter. The aquifer is going to be 80 meters under the ground. The rest is basically variable. I made the assumption that it's a rectangular cube with (HxWxL) 10x40x20. The equation I am using is q=λ*A*ΔT/s (i will learn LaTeX eventually ;) ) where lambda is the thermal conductivity of the soil which is 3.44. The temperature difference is 4K where the soil is 11C (constant over time) and the water is 15C. "s" is the thickness.. well the top soil layer is 80m and assumed to be homogeneous with the same thermal conductivity throughout. But this obviously can't work because the temperature of the water decreases over time so I would need a differential equation but I have no idea how to set this up. So the question basically boils down to the following two: 1. How do I calculate the end temperature of the water after 6 months 2. When is temperature equilibrium reached Thing is that all this soil crap is not in my curriculum but my project group ended up with an aquifer to design so we have to do these calculations. EDIT: I found this: http://ec.pathways-news.com/Text-PDF/Part B-6.pdf And figured I could use equation 6.9 Is that correct? If so what are V and c in that equation?