There are 9 US Post Offices in Union County serving a total of 41,563 residents equating to 4,618 residents per post office. It's estimated that approximately 66,339 packages are handled across the 9 county post offices annually. Union County is a county located in the U.S. state of Arkansas. As of the 2010 census, the population was 41,639. The county seat is El Dorado. The county was formed on November 2, 1829, and named in recognition of the citizens' petition for a new county, stating that they were petitioning \"in the spirit of Union and Unity.\" The county is directly adjacent to Union Parish in the state of Louisiana. The El Dorado, AR Micropolitan Statistical Area includes all of Union County. Called once by boosters the \u201cQueen City of South Arkansas\u201d and, more recently, \u201cArkansas\u2019s Original Boomtown,\u201d the city was the heart of the 1920s oil boom in South Arkansas."