In many shallow volcanic aquifers groundwater flow is so fast that the temperatures through the aquifer have little to no change in temperature from the top of the aquifer to the bottom. In this scenario the temperature curve through the aquifer is a vertical line. The rocks below the aquifer seem to treat it as if it were the ground surface. The water temperature tends to be very cold in these regions, allowing the denser water to sink and not thus temperatures do not warm with depth.
Just because there is fast moving groundwater does not necessarily mean that the bottom of the aquifer will be similar in temperature to the top, the last few examples are places where the lithology ignores the aquifer.