  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. ==history== union county was formed on november 2, 1828 from portions of clark county and hempstead county. ==geography== according to the u.s. census bureau, the county has a total area of , of which  is land and  (1.5%) is water. it is the largest county by area in arkansas. union county, along with columbia county, has the largest bromine reserve in the united states. the lowest point in the state of arkansas is located on the ouachita river in union county and ashley county, where it flows out of arkansas and into louisiana. ===adjacent counties=== *ouachita county (northwest) *calhoun county (north) *bradley county (northeast) *ashley county (east) *morehouse parish, louisiana (southeast) *union parish, louisiana (south) *claiborne parish, louisiana (southwest) *columbia county (west) union county in arkansas and union parish in louisiana are two of twenty-two counties
1829_establishments_in_Arkansas_Territory Arkansas_counties Populated_places_established_in_1829 Union_County,_Arkansas