Verification of Large Scale Watershed Modeling Analysis Using Small Subwatershed Models

Lake Loiza and Lake La Plata are two impaired lakes located in Eastern Puerto Rico. Both lakes are impaired due to excessive phosphorus inputs from the surrounding watersheds with upstream river impairments due to fecal coliform. The goal of the Watershed Stewardship Program is to develop meaningful water quality improvement strategies for impaired waters in the Rio Grande de Loiza and Rio La Plata Watersheds tributary to and including Lakes Loiza and La Plata. Included in these location specific strategies is the identification of the water quality problems, identification of the sources of these problems, the development of water quality models to provide insight, and to develop remedies for these problems. During Phase 1 of the program, TMDLs were developed using the Loading Simulation Program in C++ (LSPC) model and existing data. The existing data were relatively sparse and unevenly distributed within the Loiza and La Plata watersheds. Phase 2 of the program involved identifying four small pilot subwatersheds that could be used to test the modeling assumptions of Phase 1 by collecting site specific data and developing Hydrologic Simulation Program — Fortran (HSPF) models for total phosphorus and fecal coliform. As LSPC and HSPF are "lumped parameter" models, they are useful for examining large scale watersheds and producing generalized results. Many of the large scale assumptions can break down when assessing smaller areas within the larger watershed. Overall the modeling analysis of the small subwatersheds verified the assumptions of the original TMDL analysis. However, certain loading assumptions, such as estimates of the percentage of failing septic systems, varied widely from one subwatershed to another. Management plans were also developed and implemented during Phase 2 of the Watershed Stewardship Program to improve water quality in each of the pilot subwatersheds based on the needs in each subwatershed. Post-implementation sampling is planned during 2010 to assess the effectiveness of the management plans. The intent is to use the pilot subwatershed management plans as a model for other watersheds within Puerto Rico.