Monitoring temperature and other water-quality parameters ensures habitat and recreational uses in two artificial lakes.

By Paul Jensen, Gregg Tieken, Andrew Labay, and Ka-Leung Lee

A water-saving innovation designed decades ago has produced unanticipated benefits for San Antonio in the 21^st century. In the 1960s, City Public Service (CPS) of San Antonio, TX, the nation's largest publicly owned combined electric and gas utility, built Braunig and Calaveras Lakes to provide cooling water for new electric generating units. Today the cooling lakes continue to work as designed while providing important fish and wildlife habitats and recreation near a major metropolitan area.

CPS provides electricity to almost 600,000 customers and natural gas to more than 300,000 customers in the San Antonio metropolitan area, the second-largest municipality in Texas with more than 1.1 million residents.

Before CPS built the two lakes, the Edwards Aquifer, immediately to the north, supplied 100% of the water for all the utility's generating units. The aquifer, an immense underground reservoir, served—and continues to serve—all of San Antonio's drinking-water needs. In addition, the aquifer is a habitat to seven endangered and one threatened species.

In 1957, CPS made the commitment to use San Antonio River water rather than rely on the Edwards Aquifer. This decision was made after a seven-year drought caused a drop in aquifer levels to an all-time low and the cessation of spring flows throughout the region.

Today the aquifer water level remains high and natural springs continue to flow, partly as a result of CPS's decision.

Water for the Lakes

Braunig Lake was completed in December 1962 and covers 1,350 ac. Its capacity is 26,500 ac.-ft., and its average depth is 19.6 ft. The successful use of Braunig Lake for cooling led to construction of Calaveras Lake, completed in August 1969. It has a surface area of 3,550 ac., a capacity of 63,200 ac.-ft., and an average depth of 17.8 ft.

The lake levels are maintained by pumping water from the San Antonio River. Except during runoff events, the river downstream of San Antonio is largely wastewater effluent that has undergone primary and advanced secondary treatment in San Antonio Water System (SAWS) sewage treatment plants. CPS was one of the first utilities in the nation to use treated wastewater effluent for power-plant cooling.

Key improvements in wastewater treatment technology and source control have improved water quality over the years. In 1987, San Antonio's Dos Rios wastewater treatment plant came on-line, leading to a dramatic drop in coliform bacteria and total Kjeldahl nitrogen concentration. A major reduction in total phosphorus levels came about in the early 1990s as manufacturers of such products as laundry detergent switched to low-phosphate formulations.

Recreation Benefits

The two lakes provide 4,900 ac. of aquatic habitat and associated recreational opportunities. In fact, the parks surrounding the lakes provide the only significant source of water-based recreation in the county. In fiscal year 1999-2000, more than 278,000 people visited Braunig and Calaveras Lakes. The San Antonio River Authority has maintained the lakes' recreational activities for more than 30 years through an agreement with CPS.

At one time, swimming in the lakes was discouraged because the water source was treated wastewater effluent. Water quality improved as expected once the Dos Rios wastewater treatment plant was constructed and better treatment technologies were employed. The lakes are now up to state standards for contact recreation.

A Habitat and Resource

The lakes provide wetlands and nesting areas for migratory and wading birds, and the area serves as prime habitat for other wildlife, including deer, small mammals, and songbirds. The nutrient-rich waters of these lakes support an abundance of game fish, such as largemouth bass, hybrid striped bass, and channel catfish. Because of the warm water the power plants provide during winter months, the lakes also support such species as red drum and tilapia, which would not normally survive in Texas lakes.

CPS awarded a $100,000 grant to the Texas Parks and Wildlife Department (TPWD) Inland Fisheries Division to maintain and promote the fisheries at the two reservoirs. TPWD has increased the frequency of monitoring fish populations, fish habitat, and environmental quality of the lakes since the grant was awarded in 1999.

In addition to the support for TPWD, CPS retained PBS&J, a national environmental engineering firm, to conduct a detailed study of the reservoirs. Data on nutrients, indicator bacteria, and basic water quality were collected through monthly monitoring. During fall turnover and summer stratification, particular metals in fish tissue and sediment were monitored, as well as a range of metals in water. Although wastewater treatment technology and source control have improved over the years to reduce nitrogen and phosphorus concentrations entering the reservoirs, nutrient concentrations have remained high.

New Technologies, New Studies

With the help of PBS&J, CPS has also evaluated the thermal effects of the cooling-water effluent. This new information helps the utility better understand how daily and future plant operations might affect this valued resource. This should result in the reservoirs being used more efficiently for their primary function and providing a better resource for the public.

Researchers investigated past and existing water-quality conditions and temperatures in the lakes to identify trends. Vertical temperature profiles at a number of fixed monitoring stations within different areas of the reservoirs were collected.

These data were used in a two-dimensional (longitudinal and vertical) model known as CE-QUAL-W2. This model is effective in studies of not only temperature but also water quality of rivers, estuaries, lakes, reservoirs, and river basin systems. It models longitudinal-vertical hydrodynamics, water quality in stratified and nonstratified systems, and the effects of submerged hydraulic structures, among other parameters.

First, the model was used to re-create the temperature structure in the reservoirs. Data collected by TPWD and intake temperatures produced by the model at the same location were compared. The model was shown to capture the characteristics of the temperature data well. The model also calculated water temperatures at 1-meter-depth intervals to compare with the vertical temperature structure around each lake. As expected, the areas near the plant discharges were the warmest.

At Calaveras Lake, there are two separate discharges at the upper end of each arm of the lake. The intake for the plant is near the downstream end of the lake. An inverted weir has been placed over the intake so that only water deeper than 26 ft. is drawn into the heat exchangers. Withdrawing water from the lower part of the reservoir results in good vertical mixing and ensures that the coolest water is used in the heat exchangers.

Fishery Management

CPS is using results of the thermal modeling study, in conjunction with the fishery study, to find ways to improve fishery management. The fishery study evaluated habitat use and thermal tolerances of selected fish, including red drum and hybrid white (x) striped bass, found in open water, and largemouth bass, found in shoreline areas. The impact of fish-eating birds, particularly double-crested cormorants and white pelicans, was also assessed.

Forty fish were implanted with ultrasonic transmitters, each transmitter emitting a unique signal. Movement of each fish was tracked over the course of a year using a receiver that detects the ultrasonic signal. In conjunction with fish location, water quality and habitat were observed. Once the data analysis is complete, CPS staff will have a better understanding of the thermal preferences and tolerances of selected species as well as use of the habitat.

Another component of the fishery study included an evaluation of the effect of fish-eating birds on the fisheries in these lakes. Numerous double-crested cormorants and white pelicans overwinter on Braunig and Calaveras Lakes. Fisheries biologists have been concerned about the birds' depredation of fishery resources. This study enumerated the populations of double-crested cormorants and white pelicans that overwinter on these lakes. In addition, the gut content of double-crested cormorants was studied to determine prey selection, the species of fish most impacted.

Future Innovations

CPS is dedicated to finding water sources beyond the Edwards Aquifer and has purchased millions of dollars in property rights for lignite mining. The utility has transferred water rights from these properties to SAWS to be developed as another water source for Bexar County.

CPS pays SAWS $2 million per year to contractually reserve 40,000 ac.-ft./yr. of SAWS sewage effluent. SAWS uses this money for the development of other markets for wastewater effluent in an effort to further reduce demands on the Edwards Aquifer. Currently SAWS also pipes treated effluent upstream of the city to serve irrigation needs and to augment flow in the river.

CPS strives to maintain a good relationship with state agencies. The utility works closely with TPWD on fisheries management and with the San Antonio River Authority on public access and recreation. Although state environmental groups are not highly involved with such special-purpose, manmade systems as the reservoirs, CPS does report reservoir discharges, discharge temperatures, and flow limits for once-through cooling flows.

CPS's efforts earned the utility the 2001 National Association of Environmental Excellence Award in the conservation category from the National Association of Environmental Professionals. But the best reward for CPS is that its lakes preserve a key water source while providing effective cooling for its electric generating units, public recreation, and a thriving wildlife habitat.

Paul Jensen, Ph.D., P.E., is a vice president with PBS&J and heads its Water Quality Department. Gregg Tieken is an environmental analyst with City Public Service of San Antonio, supervising a number of projects and environmental compliance activities. Andrew Labay is a fisheries biologist with PBS&J. Ka-Leung Lee, Ph.D. P.E., is a water-quality analyst and modeler with PBS&J.

SW - July/August 2003

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