Nice find. It is a modern (1941-present) US Army Good Conduct Medal, Here is what I found:
The Army Good Conduct Medal (AGCM) was established by Executive Order 8809, 28 June 1941 and was ammended by Executive Order 9323, 1943 and by Executive Order 10444, 10 April 1953. It is awarded for exemplary behavior, efficiency, and fidelity in active Federal military service. It is awarded on a selective basis to each soldier who distinguishes himself or herself from among his or her fellow soldiers by their exemplary conduct, efficiency, and fidelity throughout a specified period of continuous enlisted active Federal military service, as outlined in this chapter. There is no right or entitlement to the medal until the immediate commander has approved the award and the award has been announced in permanent orders. See glossary for definition of "active Federal military service."
That's why this hobby is soooooooooo addicting. Never know what your gonna dig. Now that is a cool find. Don't see too many of those. Look up the history of it online, that's even more fun. It's the detective part of detecting. Sounds like you have a good spot there. Go back again with a fine tooth comb.
This isn't the only medal I have found. At a park in Long Beach I found a WWII medal. I will try to post it. Maybe you guys out there can tell me it's history. I am going detecting today to the park in Long Beach. It too is a big park, El Dorado Park. Let me know if anyone lives out that way and wants to meet up and hunt for a bit. Post finds later.