Use of Elcom and Caedym for Water Quality Simulation in Boulder Basin

The growth of the Las Vegas Metropolitan area may eventually lead to increased wastewater discharges into Boulder Basin of Lake Mead (Figure 1). Boulder Basin has experienced several algal blooms over the last few years. As a result, alternate discharge locations and strategies are being investigated. Thus, studying the water quality in Boulder Basin becomes imminent in order to assist various agencies in making decisions on operations within Boulder Basin.

Due to its extremely irregular shoreline and large surface area, Lake Mead cannot be simulated adequately by one or two-dimensional models. Therefore, ELCOM (Estuary and Lake COmputer Model), an advanced three-dimensional hydrodynamic model coupled with CAEDYM (Computational Aquatic Ecosystem DYnamics Model) was chosen to simulate threedimensional transport and interactions of flow physics, biology, and chemistry in the reservoir.

ELCOM was designed for practical numerical simulation of hydrodynamics and thermodynamics for inland and coastal waters. The code links seamlessly with the CAEDYM model undergoing development at the University of Western Australia Centre for Water Research. The combination of the two codes provides three-dimensional simulation capability for examination of detailed changes in water quality. Figure 2 shows the three-dimensional ELCOM grid used for the Lake Mead simulation.

This work involves the set-up and application of the models for Boulder Basin. Comparisons between measurements and simulation results show that ELCOM can accurately simulate the temporal and spatial variations of physical (e.g., temperature and conductivity), biological (e.g., chlorophyll-a and total organic carbon), and chemical (e.g., nitrogen and phosphorus) parameters. This study indicates that the hydrodynamic patterns of Boulder Basin are mainly driven by the Colorado River inflow, the Hoover Dam outflow, and meteorological parameters (especially episodes of high wind speed). However, the water quality of Boulder Basin is also affected by the load of nutrients (mainly phosphorus) from the Las Vegas Wash, which carries the treated wastewater effluents from municipal wastewater treatment plants, surface runoff, and groundwater discharges into the basin. This presentation will show animations of the flow within the reservoir, as well as “movies” that depict tracer and contaminant concentrations and velocity vectors. For example, Figure 3 shows the model predictions for water velocity vectors at the level of the Hoover Dam Upper Intake (about 150 ft below the water surface) at different times.

Lake Mead is a large reservoir that spans approximately 300 million acre-ft with a maximum depth of approximately 500 ft. The Boulder Basin of Lake Mead comprises the downstream portions of the reservoir and covers approximately a quarter of the total reservoir volume (see aerial view of Boulder Basin in Figure 1). Flows enter Boulder Basin from the Colorado River (via The Narrows) and from the Las Vegas Wash. The Las Vegas Wash is the only drainage outlet from the Las Vegas Valley, an area of approximately 2,000 square miles.

In 1956, the Las Vegas Valley wastewater plants began discharging effluent into the Las Vegas Wash, and through the Wash, to the Inner Las Vegas Bay of Boulder Basin. By the early 1970’s, algal blooms began to occur on an annual basis in Las Vegas Bay. By the mid 1970’s, the water quality within the Las Vegas Bay had deteriorated enough to cause the Nevada Division of Environmental Protection (NDEP) to establish and enforce water quality standards for the Las Vegas Wash and Lake Mead. In 1981, the NDEP established a phosphorus limit of 1.0 mg/L for discharge into the Las Vegas Wash. Wastewater treatment facilities subsequently began tertiary treatment, including phosphorus removal in 1982. In 1994, the NDEP again reduced the phosphorus limit to 0.35 mg/L and set waste load allocations between the wastewater treatment facilities for phosphorus and ammonia. In 2002, the phosphorus limit was set at 0.20 mg/L.

The NDEP reached an agreement with the wastewater treatment facilities and implemented a Total Maximum Daily Load (TMDL) for phosphorus and ammonia. The NDEP issued NPDES permits to the treatment facilities establishing TMDL limits of 334 lbs/day of phosphorus and 994 lbs/day of ammonia. The treatment facilities responded with an aggressive program designed to meet all regulatory requirements, which resulted in the Las Vegas Wash and the Las Vegas Bay being removed from Nevada’s listing of impaired waters. However, the NDEP left the TMDL in place and in ensuing NPDES permits added other parameters to be removed from
the effluent.

The TMDL for Lake Mead for TP from the Las Vegas Wash is intended, in part, to limit algal blooms (i.e., chlorophyll). The bulk of the TP in the Las Vegas Wash comes from the effluent of the three municipal wastewater treatment plants: the City of Las Vegas, the City of Henderson, and the Clark County Water Reclamation District. Although there were violations of the chlorophyll standard because of severe algal blooms in 2001, those were the only violations and the water body is not currently impaired. For example, dissolved oxygen (DO) concentrations may decrease to near zero in a small area along the thermocline near the inner Las Vegas Bay. However, this is not defined as impairment and DO concentrations in the remainder of the reservoir typically stay above 4 mg/L. Fish response has not been studied.

The inner Las Vegas Bay of Boulder Basin has experienced several significant algal blooms over the last few years that have been partially attributed to the discharge of treated effluent into the lake. This has raised concerns regarding the possibility of future effluent discharges exceeding the current water quality standards and TMDLs established for Lake Mead. Of particular concern is the current TMDL for TP from the discharges of 334 lbs/day.

Due to these concerns and the high rates of development and population growth in the Las Vegas area, the three wastewater treatment plants that discharge to the Las Vegas Wash have joined together with other interested agencies to form the Clean Water Coalition (CWC). A major goal of the CWC is to evaluate the impacts of the WWTP discharges into Lake Mead given future projected wastewater flows in order to implement a solution that will protect the water quality in the lake 30 years from now. More specifically, several alternate wastewater discharge locations and strategies are being investigated to evaluate the impacts of TP loading on water quality within Boulder Basin and identify a solution to reduce algal levels in the future.

To investigate the various strategies for reducing algal growth, Boulder Basin of Lake Mead was modeled by coupling the ELCOM and CAEDYM three-dimensional models. The main purpose of this study is to evaluate the water quality impacts and algal growth in the inner Las Vegas Bay resulting from moving the treated effluent discharge to a new location. In the interest of maximizing flexibility for the WWTP discharges, the ELCOM/CAEDYM model is being used to evaluate various possible operating scenarios and alternative locations to quantify what the impact will be of exceeding a TP loading of 334 lbs/day, what will happen to lake water quality (especially chlorophyll) if the WWTP discharges are relocated, and whether a new TMDL will be required if the WWTP discharge is relocated.

Comparisons between measurements and simulation results show that ELCOM and CAEDYM can accurately simulate the temporal and spatial variations of physical (e.g., temperature and conductivity), biological (e.g., chlorophyll-a), and chemical (e.g., nitrogen and phosphorus) parameters. This paper describes the two models that are used and their methodology, a description of how the models were calibrated, and general modeling results and findings.

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