What are the risks of running a floating ground through insulated wire underground?
500 Ohm is a pretty strong pull-up, which is fine. The current would be limited to 10mA, which sounds adequate. 

However, you have to consider voltage transients and potential EMI that come from "the outside" and could interfere with or even damage your digital logic (you probably have a controller connected to "PULSE_INPUT").

I assume the pulse from your meter are at least 20ms in length, so my advice would be:


Use some kind of low pass filtering (in the ballpark of 1kOhm and 1uF) on the PULSE_INPUT pin
Protect your PULSE_INPUT with a properly rated TVS diode or a Zener diode
Consider a small series resistance (100 Ohm) right before your micro controller input pin
Implement some simple software debouncing to prevent short voltage spikes to be detected, it comes for free.