One component of the surface-water part of the U.S. Geological Survey National Water-Quality Assessment Program is the use of continuous water-quality monitors to help characterize the spatial and temporal distribution of general water quality in relation to hydrologic conditions. During 1996–98, six continuous water-quality monitors in the South-Central Texas study unit collected water temperature, specific conductance, pH, and dissolved oxygen data. The data were compared among the six sites using boxplots of monthly mean values, summary statistics of monthly values, and hydrographs of daily mean values.
In 1991, the U.S. Geological Survey (USGS) implemented the National Water-Quality Assessment (NAWQA) Program to describe the status and trends in water quality of a large, representative part of the Nation’s surface- and ground-water resources. This program is based on a multidiscipline approach using standard protocols to collect data in more than 50 study units (Hirsch and others, 1988; Leahy and others, 1990). The South-Central Texas (SCTX) study unit (fig. 1) is one of the NAWQA units that began in 1994. One component of the surface-water part of the NAWQA program is the basic fixed-site assessment (Gilliom and others, 1995). The purpose of the basic fixed-site assessment is to characterize the spatial and temporal distribution of general water quality in relation to hydrologic conditions and contaminant sources. Nine sites for the basic fixed-site assessment were chosen to represent the range of habitat and water-quality conditions in the SCTX study unit. One way to characterize general water-quality conditions at the basic fixed sites is by continuous monitoring of selected properties (Gilliom and others, 1995). Therefore monitors were installed at six of the basic fixed sites (fig. 1) to collect data on water temperature, specific conductance, pH, and dissolved oxygen. These monitors were in operation to assess seasonal variability during the 1996–98 sampling period.
The main focus of the SCTX study unit is the Edwards aquifer in the San Antonio region. The Edwards aquifer is the source of water for about 1.5 million people in and near San Antonio and for ranchers and farmers in the region. Water from the aquifer also provides habitat for threatened and endangered species associated with major springs in the region. The Edwards aquifer is a sequence of extensively faulted, fractured, and dissolutioned limestone and dolostone that yields large quantities of water to wells and springs. The SCTX study unit extends beyond the Edwards aquifer to the Gulf Coast of Texas to include the complete watersheds of the Nueces, San Antonio, and Guadalupe River Basins (fig. 1); but to date (2003), the study has involved only the upper parts of the basins. The six basic fixed sites with continuous water-quality monitors are located in the upper parts of the basins.
The purpose of this report is to present the water temperature, specific conductance, pH, and dissolved oxygen data collected during 1996–98 at the six continuous water-quality monitors in the study unit. The data were compared among the sites using boxplots of monthly mean values, summary statistics of monthly values, and hydrographs of daily means values.
Six basic fixed sites (fig. 1) had data from continuous water-quality monitors during the period 1996–98. These sites were the Frio River at Concan (08195000), Blanco River at Wimberley (08171000), Comal River at New Braunfels (08169000), Medina River at La Coste (08180640), Salado Creek (lower station) at San Antonio (08178800), and San Antonio River near Elmendorf (08181800).
The drainage basin of the Frio River at Concan site (08195000) is 389 square miles (mi2) (Gandara and others, 1997). Land use in the watershed is 95-percent rangeland and forest with about 1 percent containing resorts along the Frio River. The majority of streamflow is lost to the Edwards aquifer below the Frio River at Concan site. This site is used as an indicator of rangeland with resorts for the SCTX study unit.
The drainage basin of the Blanco River at Wimberley site (08171000) is 355 mi2 (Gandara and others, 1997). Land use in the watershed is 87-percent rangeland and forest, 11-percent pasture, and 1-percent urban. This site is used as an indicator of rangeland with small town development for the SCTX study unit.
The drainage basin of the Comal River at New Braunfels site (08169000) is 130 mi2. Land use in the watershed is 69-percent rangeland and forest, 23-percent pasture, 5-percent urban, and 3-percent barren land. Except during rainfall, streamflow at the New Braunfels site is from Comal Springs, which emerge from the Edwards aquifer in the Balcones fault zone. Comal Springs is located about 1 mile (mi) upstream of the monitoring site. Streamflow at the Comal River at New Braunfels site can be affected by discharge from flood-detention pools of five floodwater-retarding structures within Landa Park in the city of New Braunfels (Gandara and others, 1997). This site is an integrator site of ground-water discharges, urban development, and rangeland for the SCTX study unit.
The drainage basin of the Medina River at La Coste site (08180640) is 805 mi2 of which 634 mi2 is above the dam forming Medina Lake located west of San Antonio. Therefore, streamflow at this monitoring site is controlled by the dam at Medina Lake (Gandara and others, 1997). The predominant land use in the watershed is rangeland with small towns; 10 percent of the land use in the watershed is agricultural. The Medina River at La Coste site is used as an agricultural indicator for the SCTX study unit because of the close proximity of predominantly cotton, corn, and sorghum fields. Some of the corn and sorghum fields are irrigated with Edwards aquifer water. Water quality at this site is affected by runoff from the agricultural fields into the stream.
The drainage basin of the Salado Creek (lower station) at San Antonio site (08178800) is 189 mi2. Land use in the watershed is 80-percent urban and 20-percent undeveloped. The urban land use consists mostly of residential and commercial and only a small percentage of industrial. Streamflow at this site might be affected during storms by 11 floodwater-retarding structures located in the upper part of the watershed (Gandara and others, 1997). Closer to the site, streamflow is maintained by precipitation and ground-water discharges. The Salado Creek site is used as an indicator of urban development for the SCTX study unit.
The drainage basin of the San Antonio River near Elmendorf site (08181800) is 1,743 mi2. Land use in the watershed is 14-percent urban, 22-percent agricultural and pasture land, and 61-percent rangeland and forest. Streamflow at this site is somewhat regulated by Medina Lake and by Olmos Reservoir on Olmos Creek 4 mi north of downtown San Antonio. Some water is diverted above the site for irrigation purposes. The city of San Antonio periodically discharges wastewater effluent into the San Antonio River from the Leon Creek, Salado Creek, and Dos Rios plants, which affects water quality at this site (Gandara and others, 1997). This monitoring site is used as an integrator site of all types of land use in the SCTX study unit but is primarily affected by urban land use and wastewater effluent in the city of San Antonio.
Each of the six basic fixed sites selected for this study contained a four-parameter monitor that recorded water temperature, specific conductance, pH, and dissolved oxygen at 15-minute intervals. Boxplots (Helsel and Hirsch, 1992) of monthly means for water temperature, specific conductance, pH, and dissolved oxygen are shown in figures 2–5. Sites are grouped on the basis of the predominant land use in each watershed. Summary statistics (minimum, maximum, and mean values) by month are listed for the period of record for the four properties at each site in tables 1–6 (at end of report). Hydrographs of the daily mean values for each property monitored at each site are shown in figures 6–11 (at end of report). Median values were computed from those used for boxplots.
The medians for monthly mean water temperature at the six selected sites ranged from 18.6 degrees Celsius (°C) at the Blanco River at Wimberley site to 24.1 °C at the San Antonio River near Elmendorf site (fig. 2). The smallest range of temperature was at the Comal River at New Braunfels site, which is a result of the ground-water influence at this site. As shown by the hydrographs of each site (figs. 6–11), water temperature fluctuates at all sites during the year with lows in winter and highs in the summer and daily means ranging from about 5 to 30 °C.
The medians for monthly mean specific conductance ranged from 415 microsiemens per centimeter at 25 °C (µS/cm) at the Frio River at Concan site to 901 µS/cm at the San Antonio River near Elmendorf site (fig. 3). The highest values and largest range in specific conductance were at the two urban classified sites, Salado Creek (lower station) at San Antonio and San Antonio River near Elmendorf. Specific conductance at all sites indicates a correlation with discharge; as discharge increases, specific conductance decreases (figs. 6–11).
The medians for monthly mean pH ranged from 7.4 standard units at the Comal River at New Braunfels site to 8.0 standard units at the Frio River at Concan (fig. 4). pH at all six sites appears to be fairly uniform most likely because of the limestone terrain in the study unit.
The medians for monthly mean dissolved oxygen ranged from 6.8 milligrams per liter (mg/L) at the Salado Creek (lower station) at San Antonio site to 9.8 mg/L at the Blanco River at Wimberley site (fig. 5). Dissolved oxygen varied from site to site with hydrographs for some sites indicating a correlation with water temperature. Dissolved oxygen concentrations tend to be higher in the winter and lower in the summer because colder water can hold more oxygen (Hem, 1992).
Ranges in water temperature, specific conductance, pH, and dissolved oxygen were similar for all sites except for the Comal River at New Braunfels site and the urban sites, Salado Creek (lower station) at San Antonio and San Antonio River at Elmendorf. The Comal River at New Braunfels site had the smallest range in water temperature, specific conductance, and dissolved oxygen. The urban sites, Salado Creek (lower station) at San Antonio and San Antonio River at Elmendorf, had the largest range in specific conductance.
One component of the surface-water part of the NAWQA Program is the use of continuous water-quality monitors to help characterize the spatial and temporal distribution of general water quality in relation to hydrologic conditions. Six continuous water-quality monitors in the SCTX study unit were used to obtain water temperature, specific conductance, pH, and dissolved oxygen data during 1996–98.
Boxplots of mean monthly values for the four properties were plotted at each of the six sites. The boxplots (sites) were grouped on the basis of predominant land use in each watershed. Summary statistics of monthly values and hydrographs of daily mean values for each of the four properties are given for the six sites. The medians for monthly mean temperatures ranged from 18.6 to 24.1 °C, and the medians for monthly mean specific conductance ranged from 415 to 901 µS/cm for the six sites. The medians for monthly mean pH ranged from 7.4 to 8.0 standard units, and the medians for monthly mean dissolved oxygen ranged from 6.8 to 9.8 mg/L. Ranges in water temperature, specific conductance, pH, and dissolved oxygen were similar for all sites except for the Comal River at New Braunfels site and the urban sites, Salado Creek (lower station) at San Antonio and San Antonio River at Elmendorf. The Comal River at New Braunfels site had the smallest range in water temperature, specific conductance, and dissolved oxygen. The urban sites, Salado Creek (lower station) at San Antonio and San Antonio River at Elmendorf, had the largest range in specific conductance.
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