Most of the Everglades, including the A.R.M. Loxahatchee National Wildlife Refuge (Refuge), developed as a rainfall-driven system with waters low in inorganic ions, but has been impacted by intrusion of water with high nutrient and solute concentrations. In June 2004, the Refuge began detailed water quality monitoring to provide information to aid in water management decisions. The Refuge was divided into one canal and three marsh zones for analyses. The perimeter zone was 0 to 2.5 km, the transition zone was from 2.5 to 4.5 km and the interior zone was greater than 4.5 km into the marsh from the canal. The perimeter zone, comprising approximately one third of the refuge, is characterized by elevated pH, alkalinity, hardness, dissolved organic carbon and concentrations of Ca, Cl, K, Mg, Si, and SO₄ compared with the Refuge interior. The transition zone, comprising approximately one fifth of the Refuge, is characterized by increased alkalinity, hardness, specific conductivity and increased SO₄ concentration compared in the refuge interior. The perimeter and transition zones were also characterized by lower dissolved oxygen than the Refuge interior. At the level of data aggregation examined, the interior of the Refuge was not affected by individual canal water intrusion events.