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Arroyo Colorado Agricultural Nonpoint Source Assessment

Project Goals/Objectives: The primary focus of this project is to better characterize agricultural runoff in the Arroyo Colorado, assess and demonstrate the effects of BMP implementation at the field and sub-watershed level, and measure progress towards meeting WPP goals. A secondary focus is to evaluate the natural phosphorus reduction capabilities of drainage ditches on runoff from irrigated cropland in the Arroyo Colorado watershed.

The Arroyo Colorado flows through Hidalgo, Cameron and Willacy Counties in the Lower Rio Grande Valley of Texas into the Laguna Madre (Figure 1). Flow in the Arroyo Colorado is sustained by wastewater discharges, agricultural irrigation return flows, urban runoff, and base flows from shallow groundwater. The Arroyo is the major source of fresh water to the lower Laguna Madre, an economically and ecologically important resource to the region. The Laguna Atascosa National Wildlife Refuge and several county and city parks are located within the Arroyo watershed. The mild climate, semi-tropical plants and animals, and many recreational opportunities draw large numbers of people to the Arroyo Colorado watershed. One third of the stream is also used for shipping from the Gulf Intracoastal Waterway to the Port of Harlingen.

Arroyo Colorado

As a result of low dissolved oxygen levels, the tidal segment of the Arroyo Colorado (2201), does not currently meet 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 1986. There have also been concerns for high nutrient levels in this river as documented on every 305(b) assessment prepared by the state since 1988. In order to meet the dissolved oxygen criteria (24-hour average of 4.0 mg/L and minimum of 3.0 mg/L) at least 90% of the time between the critical period of March through October, Texas Commission on Environmental Quality (TCEQ) (2003) estimates a 90% reduction in nitrogen, phosphorous, oxygen demanding substances and sediment will be necessary.

In response to this impairment, a local effort has been initiated to develop a watershed protection plan (WPP) to improve conditions in the Arroyo Colorado. Working with the TCEQ, the TSSWCB, and other agencies, a local steering committee will devise and implement strategies to increase dissolved oxygen in the Arroyo and improve its environmental condition.

The primary focus of this 319(h) project is to better characterize agricultural runoff in the Arroyo Colorado, assess and demonstrate the effects of best management practices (BMP) implementation at the field and sub-watershed level, and measure progress towards meeting WPP goals. A secondary focus is to evaluate the natural phosphorus reduction capabilities of drainage ditches on runoff from irrigated cropland in the Arroyo Colorado watershed.

This project will provide storm and routine monitoring of drainage ditches that contribute nonpoint source loadings to the Arroyo Colorado in order to better assess agricultural NPS loadings and reductions resulting from BMP implementation. Monitoring will primarily be directed at evaluating areas with significant irrigated cropland acreage to evaluate nonpoint source pollution (NPS) contributions and determine NPS reductions resulting from BMPs.
A final report will be developed assessing the effects of the conservation practices. Soil sampling and water quality monitoring will be utilized to gauge the impacts on water quality.
This project will be consistent with the WPP and highly coordinated with the Arroyo Partnership and Arroyo Ag Steering Committee as well as the educational and implementation projects already underway in the watershed. These groups and projects will provide for a great deal of public participation and many opportunities for public input.

This project will provide result demonstrations to landowners in the Arroyo Colorado watershed. This edge of field monitoring will represent both tiled and non-tiled irrigated cropland fields that drain to both drainage ditches and directly into the Arroyo. Surface runoff, along with outflow from the tile drainage system, will be monitored. Surface runoff and tile drain samples will be retrieved on an event basis and flow composited into a single sample. All water samples will be analyzed for various nutrient forms (i.e., total phosphorus, dissolved orthophosphate phosphorus [frequently referred to as soluble reactive phosphorus], total Kjeldahl nitrogen, dissolved ammonia, dissolved nitrite plus nitrate), and total suspended sediments (TSS). In addition, monthly grab samples will be analyzed for BOD5.

This project is closely linked to the CWA §319 funded FY05 Arroyo BMP Education Project being conducted by TWRI and Texas AgriLife Extension Service, the FY05 Arroyo WQMP Implementation Project being conducted by the TSSWCB and Hidalgo and Southmost SWCDs, and the Arroyo Watershed Coordination Project being conducted by TCEQ and Texas Sea Grant.

The results of this study will be used to support ongoing educational and implementation efforts and future modeling efforts planned for the watershed.

Project Location: Arroyo Colorado Watershed

Project Costs: Federal ($430,650); Non Federal Match ($287,502); Total Project: ($718,152)

Project Participant(s): TSSWCB, Texas Water Resources Institute, Texas AgriLife Extension Service, Texas AgriLife Research, Texas A&M Spatial Sciences Lab,Texas A&M Kingsville, Texas Department of Agriculture, Texas Sea Grant, and Texas Commission on Environmental Quality

Project Website: http://www.arroyocolorado.org/

Project Workplan: 06-10 (PDF, 294 KB)

Quality Assurance Project Plan: 06-10 (PDF, 2.28 MB)

Final Report: 06-10 Final Report (PDF, 8.22 MB)