Arroyo Nutrient Modeling
Problem/Need Statement
The Arroyo Colorado currently has low DO levels within the tidal segment, not meeting the aquatic life use designated by the State of Texas and described in the Water Quality Standards. This has been the case for every 303(d) list prepared by the state since 1996. In addition, bacteria has been a parameter of concern and as of 2006, the Arroyo became impaired due to high bacteria levels. There are many challenges associated with restoring water quality in the Arroyo Colorado Watershed. With the majority of the watershed being agricultural (cropland) and urban acreage, it is easy to attribute high nutrient contents to those land uses. In addition to this, the urban landscape is rapidly growing, increasing the threat for bacterial impairments.
The Arroyo Colorado Watershed Protection Plan: Phase I (ACWPP) currently in place was created to address these impairments, but primarily addresses low dissolved oxygen in the tidal segment (Segment 2201) of the Arroyo Colorado. The ACWPP was published in 2007, before the 2008 EPA Handbook for Developing Watershed Plans to Restore and Protect our Waters and generally follows the EPA’s 9 Key Elements for Watershed-Based Plans. There are missing components that are not outlined in the ACWPP such as loading reductions, which are required by EPA to accept the Plan. It is the goal of this project to quantify loading reductions, update the ACWPP, and provide a roadmap for achieving water quality standards.
General Project Description
This project will establish loading reductions (nutrients and bacteria), milestones, etc. for the tidal and non-tidal segments of the Arroyo Colorado Watershed Protection Plan as well as providing updated milestones and management measures. Load reduction benefits to DO in the Arroyo Colorado Tidal segment will be estimated through a state-of-the-art modeling system that links a watershed model to a hydrodynamic and water quality model (H/WQ). Predictions from the modeling system will be the basis for determining new milestones, timelines, and management measures needed to update the ACWPP. All of the following work will be building upon previous efforts to restore water quality in the Arroyo Colorado Watershed.
Through this project, a historical water quality analysis will be continued in an effort to calibrate the H/WQ model component. If it is determined that adequate data is not available, water quality monitoring will be conducted to meet calibration and validation needs that will include three synoptic events and deployment of one long-term water level recording site. Synoptic events are to occur between April and August, which is the time period of most frequent occurrences of depressed DO, with a minimum of three weeks between events; one synoptic event to attempt to capture conditions immediately following or during rainfall-runoff conditions and all other events preceded by two weeks without significant rainfall influences, to the degree weather allows. This historical data and, if needed, project monitoring data will be used to develop and calibrate/validate the H/WQ modeling component. SWAT, a watershed model that predicts watershed nutrient loadings and changes in loadings with different management measures and BMPs, will be utilized as well as the H/WQ component to predict DO in the tidal segment of the waterbody. The modeling system results will be used to determine the expected load reductions and resulting DO levels from management measures specified in the current WPP. Throughout all phases of the modeling process, results and predictions that are to be used in decision processes will be presented to local stakeholders to ensure participation, accuracy, and confidence in the data being used. It is important that all data is accurate since SWAT and H/WQ modeling results are essential components to the update of the ACWPP. Finally, the current WPP will be assessed to determine if additional BMPs are needed to achieve water quality standards in the Arroyo Colorado Tidal segment. A draft and final revised WPP will then be produced, by sections under a timeline established by project partners, to guide continued implementation efforts that improve water quality in the Arroyo Colorado.
Several components of the project will require environmental data operations. Specifically, these tasks will include: historical data review, water quality monitoring, data submittals, watershed and H/WQ modeling, water quality data analysis, and the development of Geographic Information System (GIS) maps.
The watershed and H/WQ modeling system to be developed for this project provides a state-of-the-art tool that will predict 1) pollutant load reductions in the watershed of the Arroyo Colorado resulting from implementation of specific suites of management practices and 2) improvements in water quality, especially DO, in the Arroyo Colorado Tidal segment resulting from the watershed load reductions. The tool will be applied to predict necessary load reductions and management practices to restore water quality and to meet applicable standards. Once necessary load reductions and management practices have been established, these findings will serve as a guide for stakeholders to utilize to improve water quality in the Arroyo Colorado Tidal segment. Through the Partnership sustainability efforts of another project and implementation of this updated ACWPP will continue beyond the project as the Plan will provide a roadmap of how the Partnership will move towards water quality restoration.
Funding Source
Funding provided by the Texas Commission on Environmental Quality as part of a Clean Water Act Section 319(h) Grant.