In cooperation with the Lower Colorado River Authority
Evaluation of Water-Quality Data and Monitoring Program for Lake Travis, Near Austin, Texas
By Walter Rast1 and Raymond M. Slade, Jr.
U.S. Geological Survey
Water-Resources Investigations Report 97–4257
1 United Nations Environment Programme, formerly U.S. Geological Survey.
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Contents
Abstract
Introduction
Purpose and Scope
Description of the Study Area
Acknowledgments
Water-Quality Data
Collection
Methods of Analysis
Evaluation of Water-Quality Data
Field Measurements
Laboratory Measurements
Summary of Water-Quality Conditions
Evaluation of Monitoring Program
Statistical Comparisons Between Sampling Sites
Field Measurements
Laboratory Measurements
Statistical Comparisons Between In-Lake and Cove Sites
Limitations of Statistical Comparisons Between Sampling Sites
Addition of New Sampling Sites
Summary
Selected References
Figures
| 1. | Map showing location of study area | |
| 2. | Map showing location of sampling sites in Lake Travis for Lower Colorado River Authority monitoring program | |
| 3–14. | Boxplots showing range and distribution of: | |
| 3. | Specific conductance at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
| 4. | pH at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
| 5. | Temperature at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
| 6. | Dissolved oxygen at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
| 7. | Chlorophyll-a and Secchi-disk depth, surface samples only, at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
| 8. | Total alkalinity as CaCO3 at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
| 9. | Total suspended solids at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
| 10. | Nitrate nitrogen at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
| 11. | Ammonia nitrogen at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
| 12. | Total Kjeldahl nitrogen at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
| 13. | Total phosphorus at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
| 14. | Total organic carbon at selected sites in Lake Travis, near Austin, Texas, during (a) thermally stratified period and (b) mixed period | |
Tables
| 1. | Summary of required annual constituent removal prescribed by the Nonpoint-Source Control ordinance for Lake Travis, near Austin, Texas | |
| 2. | Sampling sites in Lake Travis, near Austin, Texas | |
| 3. | Summary of water-quality properties and constituents sampled in Lake Travis, near Austin, Texas | |
| 4. | Summary of analytical methods used for water-quality properties and constituents in Lake Travis, near Austin, Texas | |
| 5. | Summary of criteria for selected water-quality properties and constituents in Lake Travis, near Austin, Texas | |
| 6. | Sampling sites in Lake Travis, near Austin, Texas, with statistically similar concentrations of constituents or properties during thermally stratified period (May to November) and during mixed period (December to April) | |
| 7–9. | Statistically similar water-quality properties and constituents (95-percent confidence level) at: | |
| 7. | In-lake sampling sites in Lake Travis, near Austin, Texas | |
| 8. | Cove sampling sites in Lake Travis, near Austin, Texas | |
| 9. | All sampling sites in Lake Travis, near Austin, Texas | |
| 10. | Water-quality properties and constituents from surface samples that are significantly different statistically between adjacent sites in the main body of Lake Travis, near Austin, Texas | |
| 11. | Water-quality properties and constituents from bottom samples that are significantly different statistically between adjacent sites in the main body of Lake Travis, near Austin, Texas | |
Abstract
Statistical analyses were made of selected water-quality properties and constituents for Lake Travis, northwest of Austin in central Texas. Objectives for the evaluation were: (1) to provide information on levels of selected water-quality properties or constituents to use as reference values for assessing the future effectiveness of the Lake Travis Nonpoint-Source Control ordinance of the Lower Colorado River Authority; and (2) to determine whether water-quality constituents at any of the sampling sites are statistically redundant with other sites and, thus, can be discontinued without loss of information. The data were grouped into two periods—the thermally stratified period (May through November) and the mixed period (December through April).
Lake Travis is a biologically unproductive reservoir with acceptable water quality for virtually all current water uses. Nutrient (nitrogen, phosphorus) concentrations tend to be small in the reservoir throughout the year, indicating nutrient limitation of maximum phytoplankton biomass. On the basis of traditional limnological properties, Lake Travis exhibits small biological productivity and exceptional water transparency. However, dissolved oxygen concentrations for bottom samples often decrease to less than 5 milligrams per liter throughout the reservoir, especially during the thermally stratified period.
Statistical comparisons were made between data collected at the surface and at the bottom at each sampling site to determine statistical similarities. The available data were insufficient to perform the comparisons for nitrite nitrogen and dissolved orthophosphate phosphorus. In addition, no bottom data were available at the most upstream site because the shallow bottom was commonly above the thermocline.
The multiple-comparison tests indicate that, for some constituents, a single sampling site for a constituent or property might adequately characterize the water quality of Lake Travis for that constituent or property. However, multiple sampling sites are required to provide information of sufficient temporal and spatial resolution to accurately evaluate other water-quality constituents for the reservoir. For example, the water-quality data from surface samples and from bottom samples indicate that nutrients (nitrogen, phosphorus) might require additional sampling sites for a more accurate characterization of their in-lake dynamics.
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