Reducing current drain while asleep

I have an Arduino Uno R3 that I'm using as a datalogger to measure temperature and soil moisture every 30 minutes. I understand that when power consumption is an issue the Arduino is not the best choice, but I'd like to get something in the ground before I leave the state in a few days.

I started with putting the 328 into power down sleep mode, which reduced consumption only down to about the 40mA range... definitely not adequate.

So, I have started butchering :blush:. Because I am powering it from the 5V pin, I removed the voltage regulator, power LEDs, and atmega16u2 chip. The current draw is now only about 6mA in sleep, and datalogging functionality is preserved.

However, 6mA is still a little high for me. Ideally, I'd like to get power consumption while asleep down to about 2mA, which would allow me to run off of AAs for about a month. Are there any other extraneous components on the UNO board that are likely contributing to this 6mA draw?

Nick has some good hints:-

Thanks for the response and the resource--I think I'm using that same methodology to put the processor into powerdown mode. The 328 should only be drawing minimal current with everything powered down and ADC off--but something (or many things) elsewhere on the board are contributing to the 6mA draw while the 328 is powered down.

I'm curious about what hardware components could be responsible, since the voltage regulators, LEDs, and USB interface (16U2) have been removed. Could the op-amp or some other component on the board be generating this quiescent draw?

Yes it could. But you have not posted a schematic of the circuit so it is hard to say.