Water-quality assessment of the Trinity River Basin, Texas - Nutrients in two coastal prairie streams draining agricultural areas, 1994-95

Abstract

In 1991, the U.S. Geological Survey (USGS) began nationwide implementation of the National Water-Quality Assessment (NAWQA) Program. Long-term goals of NAWQA are to describe the status of and trends in the quality of a large, representative part of the Nation?s surface- and ground-water resources and to provide a sound, scientific understanding of the primary natural and human factors affecting the quality of these resources (Leahy and others, 1990). The Trinity River Basin in east-central Texas (fig. 1) was among the first 20 hydrologic areas, called study units, to be assessed by this program. The first intensive data-collection phase for the Trinity River Basin NAWQA began in March 1993 and ended in September 1995. Streams in the Trinity River Basin were assessed by sampling water, bed sediment, and tissue of biota and characterizing the aquatic communities and their habitat. Aquifers were assessed by sampling water from wells. The coastal prairie is a small part of the Trinity River Basin, but it is environmentally important because of its proximity to Galveston Bay and the extensive use of agricultural chemicals on many irrigated farms. Galveston Bay (fig. 1) was selected by Congress as an estuary of national significance and was included on a priority list for the National Estuary Program. The Trinity River is especially important because its watershed dominates the total Galveston Bay drainage area and because its flow contributes substantial amounts of freshwater and water-quality constituents to the bay. Historically, measurements of the quantity and quality of water entering Galveston Bay from the Trinity River Basin have been made using data from a station about 113 kilometers (70 miles) upstream from Trinity Bay, an inlet bay to Galveston Bay. With a focused objective of providing additional water-quality information in the intervening coastal prairie area and an overall objective of improving the understanding of the relations between farming practices and stream quality in the Trinity River Basin, a special study was conducted. This report provides a description of the occurrence and concentrations of nutrients in two streams in this intervening area. An earlier report by Brown (1996) describes the occurrence and concentrations of pesticides in these two streams. An overall analysis of nutrient data collected during 1974?91 in the Trinity River Basin is given by Van Metre and Reutter (1995).