Water-Quality Assessment of the Trinity River Basin, Texas Analysis of Available Information on Nutrients * and Suspended Sediments, 1974-91 By PETER C. VAN METRE AND DAVID C. REUTTER A Contribution of the National Water-Quality Assessment Program U.S. GEOLOGICAL SURVEY WATER-RESOURCES INVESTIGATIONS REPORT 94-4086 Austin, Texas 1995 U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY GORDON P. EATON, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government For additional information write to: Copies of this report can be purchased from: District Chief U.S. Geological Survey U.S. Geological Survey Earth Science Information Center 8011 Cameron Road, Bldg. A Open-File Reports Section Austin, Texas 78754-3898 Box 25286, MS 517 Denver Federal Center Denver, Colorado 80225-0286 Information regarding the National Water-Quality Assessment (NAWQA) Program is available on the Internet via the World Wide Web. You may connect to the NAWQA Hone Page using the Universal Resource Locator (URL) at: <URL:http://wwwrvares.er.usgs.gov/nawqa/nawqa_home.html> FOREWORD The mission of the U.S. Geological Survey Improve understanding of the primary natural (USGS) is to assess the quantity and quality of the and human factors that affect water-quality earth resources of the Nation and to provide informa­ conditions. tion that will assist resource managers and policymak- This information will help support the development ers at Federal, State, and local levels in making sound and evaluation of management, regulatory, and moni­ decisions. Assessment of water-quality conditions and toring decisions by other Federal, State, and local trends is an important part of this overall mission. agencies to protect, use, and enhance water resources. One of the greatest challenges faced by water- The goals of the NAWQA Program are being resources scientists is acquiring reliable information achieved through ongoing and proposed investigations that will guide the use and protection of the Nation's of 60 of the Nation's most important river basins and water resources. That challenge is being addressed by aquifer systems, which are referred to as study units. Federal, State, interstate, and local water-resource These study units are distributed throughout the agencies and by many academic institutions. These Nation and cover a diversity of hydrogeologic set­ organizations are collecting water-quality data for a tings. More than two-thirds of the Nation's freshwater host of purposes that include: compliance with permits use occurs within the 60 study units and more than and water-supply standards; development of remedia­ two-thirds of the people served by public water-supply tion plans for a specific contamination problem; oper­ systems live within their boundaries. ational decisions on industrial, wastewater, or water- National synthesis of data analysis, based on supply facilities; and research on factors that affect aggregation of comparable information obtained from water quality. An additional need for water-quality the study units, is a major component of the program. information is to provide a basis on which regional This effort focuses on selected water-quality topics and national-level policy decisions can be based. Wise using nationally consistent information. Comparative decisions must be based on sound information. As a studies will explain differences and similarities in society we need to know whether certain types of observed water-quality conditions among study areas water-quality problems are isolated or ubiquitous, and will identify changes and trends and their causes. whether there are significant differences in conditions The first topics addressed by the national synthesis are among regions, whether the conditions are changing pesticides, nutrients, volatile organic compounds, and over time, and why these conditions change from aquatic biology. Discussions on these and other water- place to place and over time. The information can be quality topics will be published in periodic summaries used to help determine the efficacy of existing water- of the quality of the Nation's ground and surface water quality policies and to help analysts determine the as the information becomes available. need for and likely consequences of new policies. This report is an element of the comprehensive To address these needs, the Congress appropri­ body of information developed as part of the NAWQA ated funds in 1986 for the USGS to begin a pilot pro­ Program. The program depends heavily on the advice, gram in seven project areas to develop and refine the cooperation, and information from many Federal, National Water-Quality Assessment (NAWQA) Pro­ State, interstate, Tribal, and local agencies and the gram. In 1991, the USGS began full implementation public. The assistance and suggestions of all are of the program. The NAWQA Program builds upon an greatly appreciated. existing base of water-quality studies of the USGS, as well as those of other Federal, State, and local agen­ cies. The objectives of the NAWQA Program are to: Describe current water-quality conditions for a large part of the Nation's freshwater streams, Robert M. Hirsch rivers, and aquifers. Chief Hydrologist Describe how water quality is changing over time. CONTENTS CONTENTS Abstract ............................................................... 1 Introduction............................................................................................................... 2 The National Water-Quality Assessment Program .......................................... 2 Purpose and scope ............................................................................................ 2 Nutrients in natural waters ............................................................................... 3 Acknowledgments............................................................................................ 4 Description of study unit........................................................................................... 4 Climate ............................................................................................................. 5 Geology............................................................................................................ 5 Population and land use ................................................................................... 5 Water use, diversions, wastewater treatment and returns ................................ 7 Streams and reservoirs ..................................................................................... 11 Aquifers............................................................................................................ 13 Assessment approach ................................................................................................ 13 Selection of constituents for analysis ............................................................... 15 Methods of analysis ......................................................................................... 15 Spatial distribution ............................................................................... 15 Relations to categorical variables ........................................................ 15 Relations to continuous variables ........................................................ 15 Loads and yields .................................................................................. 18 Available data ............................................................................................................ 18 U.S. Geological Survey .................................................................................... 18 Texas Water Commission ................................................................................. 20 Trinity River Authority .................................................................................... 20 City of Arlington .............................................................................................. 20 City of Fort Worth ............................................................................................ 22 Texas Water Development Board .................................................................... 22 National Atmospheric Deposition Program ..................................................... 22 Analysis of available data for nutrients .................................................................... 22 Atmospheric deposition ................................................................................... 22 Surface water ................................................................................................... 24 Spatial distribution ............................................................................... 24 Relations to point sources and reservoirs ............................................. 25 Relations to agricultural land use ......................................................... 27 Relations to streamflow ........................................................................ 31 Seasonal variations ............................................................................... 38 Temporal trends ................................................................................... 41 Loads and yields .................................................................................. 52 Relations to Livingston Reservoir .............................................. 62 Comparison of