Environmental and safety concerns mean that oil refineries are sometimes located some distance away from major urban areas. Nevertheless, there are many instances where refinery operations are close to populated areas and pose health risks. In California's Contra Costa County and Solano County , a shoreline necklace of refineries, built in the early 20th century before this area was populated, and associated chemical plants are adjacent to urban areas in Richmond , Martinez , Pacheco , Concord , Pittsburg , Vallejo and Benicia , with occasional accidental events that require " shelter in place " orders to the adjacent populations. A number of refineries are located in Sherwood Park, Alberta , directly adjacent to the City of Edmonton . The Edmonton metro area has a population of over 1,000,000 residents.