The archipelago of Puerto Rico is divided into various geopolitical regions that have been established for legal or statistical purposes. These regions form a structured hierarchy, of which neighborhoods and municipalities constitute the two largest administrative divisions. Both are legal divisions, outlined by the Legislative Assembly of the Commonwealth of Puerto Rico (Estado Libre Asociado de Puerto Rico) (Puerto Rico Planning Board, 2018). Puerto Rico has 78 municipalities and 902 neighborhoods. The United States Census Bureau collects information on population, housing, businesses, and agriculture and is aggregated for these regions (Puerto Rico Planning Board, 2018).

The map shows the municipalities and neighborhoods that form the Estuario de la Bahía de San Juan, according to the Federal Census Bureau’s designations. (The borders are in TIGER/Line Shapefiles, which are a sample of geographic and cartographic information.) The Estuario de la Bahía de San Juan basin area is composed of 8 municipalities (Bayamón, Cataño, Carolina, Guaynabo, Loíza, San Juan, Toa Baja, and Trujillo Alto) and 36 neighborhoods. The region has a total area of ​​22,740.95 ha with San Juan occupying 48% of the estuary area with 10,925.62 ha. Following San Juan is Carolina with 24% and 5,448.27 ha and Loíza with 12% and 2,627.79 ha. 85% of San Juan’s territory is in the estuary zone, as is 82% of Cataño. The neighborhood that occupies the largest area within this region is Torrecilla Baja in Loíza with 8%, followed by Cupey in San Juan with 7%. The Estuary basin occupies 86% and 84% of the total area of ​​the municipalities of San Juan and Cataño, respectively. In addition, it occupies 54% of the total area of Loíza and 45% of Carolina. The most water quality samples are taken in Torrecilla Baja (Loíza) with 9% of the sampled areas, Cupey (San Juan) with 7%, Buena Vista (Carolina) with 7%, Sabana Abajo (Carolina) with 7%, Santurce (San Juan) with 7%, and Cangrejo Arriba (Carolina) with 6% of the sampled areas. If we look at the sample points added by municipality, San Juan is the municipality with the highest number of samples (48%), followed by Carolina (24%) and Loíza (12%).