TDS and selenium projections for the Las Vegas Wash, post completion of the Systems Conveyance and Operations Program (SCOP)

Abstract

The Las Vegas Wash (Wash) is the primary drainage channel for the Las Vegas Valley. Flows from the tributaries are a major source of contaminants of particular concern to the Wash, mainly total dissolved solids (TDS) and selenium (Se). Treated wastewater effluent discharged to the Wash currently provides enough dilution to lower the TDS and to maintain selenium concentrations below the current 5μg/L aquatic life criteria recommended by the Unites States Environmental Protection Agency (EPA; USEPA 1987); however, the Systems Conveyance Operations Program (SCOP) was intended to take most of this treated effluent through a pipeline directly into the Boulder Basin of Lake Mead. While construction of the pipeline may have had many water quality benefits for the Las Vegas Valley and Lake Mead, the dilution benefit to the Wash will be significantly reduced, primarily affecting TDS and Se concentrations in the Wash. The Wash enters Lake Mead at Las Vegas Bay, home to the endangered razorback sucker. The concern is the increased concentration of TDS and Se to the Las Vegas Bay, which potentially would impact the razorback sucker. Using data collected by the Southern Nevada Water Authority's (SNWA) environmental monitoring and management water quality team, projections have been made for expected TDS and Se concentrations post completion of the SCOP. These projections show 2 options for managing TDS and Se concentrations on the Wash: dilution and/or treatment of the tributaries. Discharging 2.08 × 10⁵ m³/d to the Wash provides the necessary dilution to lower Se to the target concentration of 5 μg/L but will still leave TDS levels higher than desired. Treating one or more of the major tributaries will bring both Se and TDS concentrations within targeted levels.

Key words:

• Las Vegas Wash
• SCOP
• selenium
• TDS

The Las Vegas Wash (Wash) is the primary drainage channel from the Las Vegas Valley to Lake Mead (Figs. 1 and 2). Flow in the Wash is composed of 4 components: treated wastewater, urban runoff, shallow groundwater and storm water. The treated wastewater is by far the largest flow component. Currently, an average of 6.05 × 10⁵ m³/d of effluent is discharged to the Wash by the valley's 3 wastewater treatment plants. This study focuses on the tributary flow component, urban run-off plus some shallow groundwater that discharges to and flows through the tributary channels, because it is the only component that is both a constant contributor to the Wash and one whose flow can be controlled, meaning there is an option to treat it in situ or by diversion to the treatment plants. The tributary flow contributes only 5.60 × 10⁴ m³/d of flow to the Wash but is the major contributor of most contaminants to the Wash. Currently, the discharge of the treated wastewater effluent provides enough dilution to keep most contaminant concentrations in the mainstream Wash below the maximum contaminant levels (MCL) regulated or recommended by the US Environmental Protection Agency (EPA) and the State of Nevada.

Figure 1 Aerial view of the Las Vegas Wash upstream of Lake Las Vegas, including locations of the 6 major tributaries and the 3 wastewater treatment plants.

Rapid population growth in the Las Vegas Valley over the past years has generated concern that the Wash may not be able to handle the amount of effluent discharge projected for the future. In November 2002, the Clean Water Coalition (CWC) was formed as a joint-powers authority made up of members from the Clark County Water Reclamation District, the City of Las Vegas, the City of Henderson and the City of North Las Vegas. These agencies are responsible for current and future wastewater treatment in the Las Vegas Valley. The Systems Conveyance and Operations Program (SCOP), currently suspended, was proposed by the CWC to deal with future wastewater treatment demands in the Las Vegas Valley (CWC 2009). Under SCOP, a pipeline will be built carrying the wastewater effluent from the treatment plants directly into the Boulder Basin of Lake Mead (Fig. 3). Once the SCOP pipeline is operational, flow rates in the Wash will potentially be decreased by up to 80%, significantly reducing the dilution of the contaminants from the tributary flows. The 2 contaminants most affected by the loss of dilution are selenium (Se) and total dissolved solids (TDS). Selenium is found naturally in the soils of the Las Vegas Valley and enters the Wash and its tributaries through direct groundwater discharge, dewatering pipes and direct run-off (Zhou et al. 2004). The Wash enters Lake Mead at Las Vegas Bay, home to the endangered razorback sucker (Xyrauchen texanus). The concern is that increased concentrations of TDS and Se to the Las Vegas Bay could potentially impact the razorback sucker. Selenium levels are also a concern for wildlife that inhabits the Wash's wetlands, mainly fish and birds (Cizdziel and Zhou 2005). The EPA recommended level for wildlife is 5μg/L. TDS levels are also a concern for the aquatic vegetation in the Wash. The loss of the vegetation would decrease food and habitat for the aquatic wildlife in the Wash. The Nevada Division of Environmental Protection (NDEP) set a Requirement to Maintain Existing Higher Quality (RMHQ) standard of <1900 mg/L for TDS in the Las Vegas Wash below the confluence of discharges from City of Las Vegas and Clark County wastewater treatment plants (NDEP 2008). The purpose of our study was to project concentrations of Se and TDS that can be expected in the Wash with the decreased flow, and to create a plan for keeping these concentrations at or below their targeted levels of 5μg/L for selenium and <1900 mg/l for TDS after the pipeline is operational.

Methods

Sampling sites

This study used data from the urban tributary and mainstream Wash water quality monitoring programs conducted by the SNWA's environmental monitoring and management water quality team (SNWA 2008a, b). Under the urban tributary water quality monitoring program, the 6 main tributaries to the Wash were sampled at sites near the confluences of the tributaries with the Wash. These sites are SC_1 along Sloan Channel, FW_0 along the Flamingo Wash, LW12.1 along the Las Vegas Creek (upper Wash), MC_2 along Monson Channel, DC_1 along Duck Creek, and BS_1 along the Burns Street Channel. For the purpose of this study, the results from all 6 channels are combined to show the total flow volume and concentrations from the “Tributaries.” They basically represent the total contributions to the Wash from the tributaries (Fig. 1).

The sample sites under the mainstream Wash water quality monitoring program were chosen at strategic points along the Wash (Fig. 1). Site LW10.75 is the farthest upstream and is the only Wash site that does not contain any wastewater effluent. The flow at LW 10.75 is basically the combined flows of SC_1, FW_0 and LW12.1 and is located directly upstream of the discharge from the City of Las Vegas wastewater treatment plant. Site LW8.85 is directly downstream of the effluent discharge from Clark County's wastewater treatment plant and upstream of the confluence with Monson Channel. Site LW6.85 is downstream the confluences with both Monson Channel and Duck Creek. Site LW5.9 is downstream of Burns Street and the discharge from the City of Henderson wastewater treatment plant. LW5.9 is also the site that samples the combined flow from all urban tributaries and all 3 waste water treatment plants. The next 3 sites, LW5.5, LW4.95 and LW3.1, are positioned above or below major erosion control structures. Slight changes in water quality at these sites are due to the effects of these structures and also shallow groundwater discharge. The final sampling site LW 0.8, is located downstream of where the Wash passes under Lake Las Vegas and upstream of where the Wash flows into the Las Vegas Bay of Lake Mead (Fig. 2). For the purposes of this study, “Wash” values for both current and projected situations are referring to site LW 0.8 because it represents the final water quality of the Wash before it enters Lake Mead.

Figure 2 Aerial view of the Las Vegas Wash downstream of Lake Las Vegas as it flows into the Las Vegas Bay of Lake Mead.

Figure 3 Path of the SCOP pipeline from the 3 treatment plants into the Boulder Basin of Lake Mead.

Water quality sampling (Se and TDS)

Sampling methodology was identical at each location for each sampling event. Field staff recorded the following information at all sample locations for each sample event: sampling date, time, weather condition, air temperature, and a description of any and all factors that might influence the dataset from each site. At each site, a multi-parameter probe (Hydrolab Corporation Model Surveyor® 4) was used to measure field water quality parameters, including water temperature, DO concentration, pH value and electrical conductivity (EC). The probe was calibrated using standard solutions (pH = 10, pH = 7 and EC = 5000 or 2500 μS/cm) supplied by Southern Nevada Water System (SNWS). Field measurements at each site were entered into the SNWS Laboratory Information Management Software (LIMS) database.

Sample bottles not containing preservatives were rinsed 3 times with sample water before final sample collection. All samples were labeled specifying site and location, analysis requested, and date and time sampled. Sample bottles for Se and TDS were prepared and delivered for use in the field by Montgomery Watson Laboratories in Pasadena, California (2000–2005) and by Weck Laboratories in Monrovia, California (2006–2007). After collection, all samples were maintained in a cooler of ice at 4 C. Samples were distributed immediately after the sampling event to designated laboratories for analysis. All samples were accompanied by chain of custody record.

Samples for the urban tributary monitoring program have been taken quarterly since 2000. Samples taken as part of the mainstream Wash water quality monitoring program have been collected monthly since 2000 as well. All samples are collected under base flow conditions not during or within 48 h of storm events.

Discharge measurements

Discharges at each of the tributary sites were measured monthly using one of 3 devices: the USGS Parshall Flume, the Price pygmy current meter or Sontek's Acoustic Doppler Velocimeter (ADV), depending on the hydrological conditions at the site. When water depth was too shallow and flow velocity was too low for the pygmy meter or the ADV, the USGS Parshall flume was used. The flume has been used at Sloan Channel (SC-1); flow at the remaining sites was sufficient to use the current meter or ADV. The methods for measurement and computation of stream flow developed by the USGS (Rantz et al. 1982) were followed. The discharge rates for the Wash are from the USGS gauge site 09419800, Las Vegas Wash below Lake Las Vegas near Boulder City, Nevada, located near site LW 0.8 (USGS NWIS). Discharge rates for the treated effluent were provided by each of the wastewater treatment plants.

Mass balance analysis

Using results from the water quality sampling and the measured discharge at each of the sites, mass balances for Se and TDS were computed for all 6 tributaries, for the total effluent flow and for the entire Wash at LW 0.8 (Table 1). The projected TDS and Se concentrations were computed using these mass balance values and adjusting the discharge rates for the effluent flow component to the Wash based on the proposed rates from the SCOP.

Table 1 Selenuim (Se), total dissolved solids (TDS) and flow rates for the 6 tributaries and for Wash site LW 0.8.

	SC_1	FW_0	LW12.1	MC_2	DC_1	BS_1	LW 0.8
Average Flow Rate (m^3/s)	0.01	0.20	0.10	0.03	0.28	0.02	7.88
Average Se Concentration (ug/L)	6.4	11.3	8.0	19.6	16.1	11.2	3.3
Average TDS Concentrations (mg/L)	2100	2900	3000	4500	5100	4500	1700
Yearly Se Loading Rates (kg/yr)	2.7	70.8	24.0	15.9	143.0	8.2	859
Daily TDS Loading Rates (×10^4 kg/day)	0.25	5.00	2.50	1.00	12.33	0.89	120

Results

Current status

Under dry weather conditions, the average flow from the Wash to Lake Mead is around 6.80 × 10⁵ m³/d, including, 6.05 × 10⁵ m³/d from wastewater effluent, 5.6 × 10⁴ m³/d from tributaries and the remaining 1.9 × 10⁴ m³/d from shallow groundwater (SNWA 2008a). In the tributaries, selenium concentrations averaged 11.0 μg/L and TDS averaged 3400 mg/L. Selenium concentrations in the effluent averaged 2.5 μg/L and TDS averaged 1100 μg/L. At LW 0.8, Se averaged 3.3 μg/L and TDS averaged 1700 mg/L (SNWA 2008b).

The tributaries make up about 8% of the total flow in the Wash. Total yearly Se loading from the 6 tributaries to the Wash was computed as 265 kg/yr (Fig. 4). The tributaries that contribute the greatest amount of Se to the Wash are Duck Creek (54%) and Flamingo Wash (27%). The total yearly Se loading from the Wash to Lake Mead was 859 kg/yr, 31% of which was from the tributaries. In other words, 31% of the Se is coming from only 8% of the flow. All the tributaries have high TDS concentrations. but Duck Creek has the highest TDS values (>5000 mg/L). The tributaries contribute 22 × 10⁴ kg/d of TDS to the Wash, and the Wash contributes 120×10⁴ kg/d of TDS to Lake Mead.

Figure 4 Total yearly Se loading from 6 tributaries to the Las Vegas Wash = 265 kg/yr.

The average Se and TDS concentrations along the Wash for 2007 (Figs. 5 and 6) demonstrate how Se and TDS levels from the tributary flows are greatly reduced by the treated effluent discharge (between sites LW10.75 and LW 8.85) and the impact Duck Creek has as it enters the Wash between sites LW8.85 and LW6.85.

Figure 5 Average selenium concentrations (μg/L) in the Wash for 2007.

Figure 6 Average total dissolved solids (TDS; mg/L) in the Wash for 2007.

Projections

Case 1 (no action)

Case 1 shows the projected concentrations for Se and TDS based on the original proposal from SCOP to continue to discharge 1.14 × 10⁵ m³/d of effluent to the Wash (Fig. 7). At site LW 0.8 the Se concentration would increase to ∼7.7 μg/L, and TDS levels would increase to ∼2100 mg/L (EC ∼3000 μS/cm). The effluent flow in Case 1 is not adequate to dilute Se and TDS concentrations to their desired levels in the Wash. This leaves 2 options for managing Se and TDS in the Wash post completion of the SCOP: discharge more effluent to bring the concentrations down solely by dilution, or treat the urban tributaries.

Figure 7 Projected Se and TDS concentrations in the Wash.

Dilution

How much treated effluent would need to be discharged to the Wash to reach the desired levels by dilution alone? Based on the average Se concentrations of the tributaries and of the effluent from the treatment plants, 2.08 × 10⁵ m³/d (Fig. 8) is the minimum flow needed to achieve a Se concentration of 5μg/L in the Wash; however, a minimum of 2.27 × 10⁵ to 3.03 × 10⁵ m³/d would be needed to maintain TDS levels in the Wash □ 1900 mg/L (Fig. 9).

Figure 8 Projected Se concentrations in the Wash with different flow rates from wastewater preatment Plants (without treatment options).

Figure 9 Flow from the wastewater treatment plants vs. conductivity in the Wash. 1700 and 1900 uS/cm are assumed as average electrical conductivities from the wastewater treatment plants.

Based on these findings, 4 additional options (Cases 2–5) were considered. These cases assume a total flow of 1.89 ×

10⁵ m³/d in the Wash, including flows from the treated wastewater effluent and the tributaries (Fig. 7). For the purposes of this study, we assumed that the treatment would be done by routing the tributary(s) through one of the existing wastewater treatment plants.

Treatment of the tributaries

Case 2 projects concentrations at LW 0.8 by taking Duck Creek to the treatment plants and keeping a total of 1.89 ×

10⁵ m³/d of flow in the Wash. By treating only Duck Creek, Se concentrations in the Wash would be ∼4.9 μg/L and TDS ∼1600 mg/L (EC ∼2300 μS/cm). This puts the Wash slightly below the recommended level for Se and well below the recommended level for TDS.

In Case 3, both Duck Creek and Flamingo Wash would be routed through the treatment plants, and 1.89 × 10⁵ m³/d of flow would be discharged to the Wash. In this case, Se levels in the Wash would be ∼4.2 μg/L and TDS ∼1450 mg/L (EC ∼2100 μS/cm) and achieve the desired concentrations for both Se and TDS in the Wash.

In Case 4, Duck Creek, Flamingo Wash and Monson Channel would be routed through the treatment plants, and 1.89 × 10⁵ m³/d of flow would be discharged to the Wash. Case 4 would yield Se levels of ∼4.0 μg/L and TDS ∼1400 mg/L (EC 2065 μS/cm) in the Wash.

In Case 5, all tributary flows would be routed through the treatment plants, and 1.89 × 10⁵ m³/d of flow is discharged to the Wash. Se concentrations at LW 0.8 would be ∼3.8 μg/L and TDS concentrations would be ∼1360 mg/L (EC ∼1975 μS/cm).

Discussion

We made several assumptions in this study with regard to projected TDS and Se concentrations in the Wash. First, the TDS and Se concentrations of the effluent discharged from the treatment plants remain at or near their current levels. New technologies or changes in the operations of the treatment plants could potentially affect these concentrations. If the plants are able to remove more Se, for example, less volume would be required for dilution. Second, the current conditions of the tributaries remain consistent. The water quality in the tributaries could improve or degrade as management practices, population or other factors change. Tributary flow rates could increase due to additional growth and development or possibly decrease due to conservation measures. Any of these factors could affect the projected TDS and Se concentrations.

Based on the projections from this study, with the above assumptions, if insufficient effluent is discharged in the Wash to lower Se and TDS concentrations by dilution, treatment of the tributaries will be necessary to keep concentrations below their recommended levels. Three possible options for treating the tributaries have been studied (Brown and Caldwell 2009): diversion of the tributaries to existing wastewater treatment plants from the base of watershed, diversion of the individual upstream dewatering sources to existing treatment plants, or construction and operation of an in situ selenium treatment system for upstream dewatering sources. Preliminary results show that the most feasible treatment option is to route Flamingo Wash, Monson Channel and Duck Creek to either the City of Las Vegas or the Clark County wastewater treatment plants from their base of watershed. Both the City of Las Vegas and Clark County treatment plants have sufficient capacity to take on and provide treatment to these 3 tributaries.

The implementation of any plan should be executed in phases while continually monitoring the system to see how it responds to the changes. The next phase should be modified or implemented based on how the system responded in the prior phase. The CWC along with SNWA and the member agencies of the Las Vegas Wash Coordination Committee are working together to address these issues and finalize a management plan for the Wash post completion of the SCOP.

Acknowledgments

We would like to acknowledge the members of SNWA's Environmental Monitoring and Management Divisions Water Quality team who participated in the collection of the Water Quality Samples used for this study. Special thanks to Keiba Crear, Seth Shanahan and Robert Huening who since the beginning have been involved with and helped coordinate each of the 3 sampling programs from which the data for this study were obtained, and to the United States Bureau of Reclamation for funding the programs.