Water quality trends of streams in Puerto Rico: Evaluating 50 years of the Clean Water Act

Water quality regulations entail a substantial commitment of resources from governments and private entities. It is important to continually evaluate the effectiveness of these regulations to ensure they are having the intended impact. In this paper, we evaluated nutrient data as indicators of primary productivity and dissolved oxygen (DO) concentrations and pH as response variables to assess historical water quality trends from 55 stations of Puerto Rico. The stations were divided into impaired versus non-impaired categories based on their historical total phosphorus (TP) mean concentration. Mean TP and total nitrogen (TN) concentrations were significantly higher in the impaired stations relative to the non-impaired stations. In contrast, DO mean concentrations and mean pH values were significantly lower in the impaired stations. A generalized additive mixed model was used to demonstrate temporal trends. A significant decrease in TP and TN concentrations was observed with time at the impaired stations. This was accompanied by significant increases in DO concentrations and pH. The non-impaired stations showed a marginal (statistically nonsignificant) decreasing trend with time. The large reductions in nutrient concentrations observed at the impaired stations seem to be related to the closure of several primary wastewater treatment plants (WWTPs) across the island. The conversion of abandoned crop agricultural lands into secondary forest in recent decades has resulted in small but significant decreases in TN (not TP) in receiving streams. We conclude that the Clean Water Act has promoted improvements in water quality in Puerto Rico by advancing upgrades in sanitary infrastructure and the regulation of point sources of pollution.