In 1969, the Caribbean District of the U.S. Geological Survey, in cooperation with the Commonwealth of Puerto Rico, initiated the operation of a network to monitor some parameters indicative of water-quality changes at selected stream sites. The initial network consisted of 18 stations, which were expanded to 20 in 1971, and to 30 in 1973 (Reed and others, 1971, 1972, Quinones, 1975).
In 1974, at the request of the Environmental Quality Board of Puerto Rico, the network was modified to conform with the Environmental Protection Agency National Water Quality Surveillance System. Under the revised concept, monitoring stations were established at sites near the headwaters and mouths of the principal rivers in Puerto Rico. Most of the stations in the p-evious network were located at sites downstream in the valleys in order that the overall water-quality conditions in the principal rivers could be monitored. The purpose of the present network is to monitor changes in water quality between the upstream and downstream stations. The expanded network consisted of 58 stations. During 1976, five had been discontinued. One other was added late in 1976. Progress reports have been published in 1976 (Quinones and others) and 1977 (Quinones and others). Most of the stations in the original network have been maintained, thus providing sore degree of continuity. The monitoring stations used in this report are shown in figure 1 and listed in table 2.
This report summarizes the results of the network operation from July 1976 to August 1977.
