Seems, in all likelihood, it is a bonding wire however my curiosity would drive me to measure it's resistance to a known bonded component using a multimeter. The reason you'd want to do that, is to be sure someone didn't just stick a wire in the ground without connecting it to anything, leaving you thinking you were protected when in fact you were not.
You'd need to find a known bonded metal component like an underwater light, disconnect it's ground wire (to separate the the grounding and bonding circuits) and measure the resistance between the green wire sticking out of the ground and light's shell. Something less than an ohm would likely indicate an intentional connection.
Never-the-less, and in the meantime I think it is safe to assume it is your pump bond connection. Good luck.