This study examines sources of fecal coliform in Segment 2302 of the Rio Grande, located south of the International Falcon Reservoir in southern Texas. The watershed is unique because the contributing drainage areas lie in Texas and Mexico. Additionally, the watershed is mostly rural, with populated communities known as "colonias." The colonias lack sewered systems and discharge sanitary water directly to the ground surface, thus posing an increased health hazard from coliform bacteria. Monitoring data confirm that Segment 2302 is not safe for contact recreation due to elevated fecal coliform levels. The goal of the study was to simulate the observed exceedences in Segment 2302 and evaluate potential strategies for their elimination. Fecal coliform contributions from ranching and colonia discharges were modeled using the Hydrologic Simulation Program-Fortran (HSPF). Model results indicated that the regulatory 30-day geometric mean fecal coliform concentration of 200 colony forming units (cfu) per 100 milliliters is exceeded approximately three times per year for a total of 30 days. Ongoing initiatives to improve wastewater facilities will reduce this to approximately once per year for 14 days. Best management practices will be necessary to reduce cattle access to streams and eliminate all exceedences. The developed model was limited by the relatively sparse flow and fecal coliform data.

LaWare, Paul and Hanadi S. Rifai, 2006. Modeling Fecal Coliform Contamination in the Rio Grande. Journal of the American Water Resources Association (JAWRA) 42(2):337-356.

INTRODUCTION

Despite tremendous effort, advances in wastewater treatment, and point source reduction through National Pollutant Discharge Elimination System (NPDES) permitting, many streams still do not meet the water quality requirements of the Clean Water Act (CWA). Previously overshadowed by point source discharges, nonpoint sources are now recognized as significant contributors of pollution. Point source pollution results from discrete discharges that can be measured directly, such as municipal and industrial process or wastewater releases from a pipe or spillway. Point sources are continuous sources and have a greater impact during dry weather and low flow conditions.

Nonpoint source pollution, on the other hand, is a wet weather phenomenon that causes a rapid rise and fall of pollutant concentrations. Pollutants are not contributed as discrete, measurable quantities. Instead, they are contributed by runoff from large watershed areas carrying pollutants such as natural nutrients, minerals, metals, bacteria, acids, bases, and toxics. Some of the impacted water is carried by overland flow directly to surface water bodies, while some migrates along the shallow ground water table. Thus, the contribution is diffuse, difficult, and costly to evaluate and commonly results in violations of the local water quality standards.

This paper describes an analysis of point and nonpoint sources of fecal coliform bacteria to Segment 2302 of the Rio Grande River. The main goals of the study were to simulate the observed fecal coliform exceedences in Segment 2303 and to evaluate the impacts of proposed improvements to the sanitary infrastructure in the watershed on fecal coliform pollution. The Rio Grande forms the southern boundary of Texas and serves as a border between the United States and Mexico. As such, the contributing watershed to Segment 2302 lies on both sides of the border. The distinguishing aspects of this study lie in the transnational nature of the watershed and the characteristics of the contributing point and nonpoint sources of pollution.

The research described in this paper includes the development of a watershed model and nonpoint source analysis that can be used to establish a total maximum daily load (TMDL) for the segment. A TMDL is a calculation of the maximum amount of a pollutant that a water body can receive without exceeding water quality standards. Segment 2302 is 1 of 92 water bodies included in the 1999 CWA section 303(d) List of Impaired Waters in Texas because elevated bacteria levels do not support contact recreation. Elevated fecal coliform concentrations in Segment 2302 can be attributed to local nonpoint sources because there are no point source discharges within the segment. Also, the river segment immediately upgradient (Segment 2303 International Falcon Reservoir) has much lower fecal coliform concentrations that do not exceed the Texas water quality standard and therefore contribute very little in fecal coliforms to Segment 2302.

The development of TMDLs as mandated by the U.S. Environmental Protection Agency (USEPA) is carried out by state agencies and involves considerable stakeholder input. The Texas Commission on Environmental Quality (TCEQ) maintains the 303(d) list of impaired waters in the state and is charged with developing TMDLs. The water bodies included in the Texas 303(d) list are prioritized according to the Texas Surface Water Quality Standards and additional criteria relating to data quality, water use, consequence of the impairment, or other pertinent conditions. Once a water body is included on the list, the TCEQ assigns a priority that is used to schedule TMDL development. Segment 2302 is classified as Low priority, with TMDL development scheduled to begin in 2004 and be completed by 2010. It is important to note that all interstate/international water bodies were ranked Low because of the uncertainty in obtaining international collaboration on TMDL development, regardless of the risk to human health and the environment. Comparable data are often unavailable on both sides of the international border. This study takes a new approach of developing the international data required for nonpoint source analysis, and as such the study represents a starting point for TMDL development.