Water Quality of Streams Draining Mixed-Used Agricultural Basins in Southwestern Puerto Rico.

The Eastern Lajas Valley Watershed in southwestern Puerto Rico is an important agricultural area. A combination of citizen science volunteers and professionals was used for land-use/land-cover analysis and partial physical, chemical and biological characterization of stream waters and at the drainage outlet of five pre-defined basins (Mondongo, La Plata, Bárbara, Maginas, Cristales). Unmanaged shrubland/forest-land was the most dominant land use class followed by agriculture and urban land use with 61, 30 and 9% of total land area of 14,668 ha; the population was estimated at 33,936. Mondongo basin had the greatest land-area under agriculture, urban footprint and population, numerous passive and active discharge conduits, plus had a waste-water treatment plant (WWTP) that discharged secondary/tertiary treated effluents for an estimated annual total P and total N load of 1,471 kg P and 6,034 kg N, respectively. In contrast, Cristales basins had the lowest land-area under agriculture, population, and urban footprint. The PR water quality standard and/or suggested levels of enrichment were exceeded in 30%, 43%, and 61% of the sampling stations, for total N, dissolved NO₃-N and total P concentrations, respectively. Overall, stream total N was positively correlated to the proportion of basin area in urban land (UD) (r= 0.49), while negatively correlated to grazed pastures (GP) (r= -0.42), row crops (RC) (r= -0.29) and pond (P) (r= -0.31). The best fit geographically weighted regression models showed positive relationships of total N with the proportion of UD and population. Stream total P was negatively correlated to unmanaged forest/shrubland (UF) (r= -0.40) while positively correlated to UD (r= 0.40), and hay and silage (HS) (r= 0.33). The main sources contributing to stream N and P concentrations are the Lajas WWTP and northern urban areas of the watershed and to a lesser extent agricultural areas consisting primarily of row crops and grazed pastures.