Document Type

Technical Report

Publication Date



Clark County Sanitation District, Nevada


A water quality modeling study of the Mohave Reach of the Lower Colorado River (from Davis Dam to the Nevada/California Stateline) was conducted to evaluate potential water quality impacts resulting from a proposed Laughlin, Nevada wastewater effluent discharge. The study included four major components: (1) review of the current regulatory framework; (2) a field data collection program to document existing water quality conditions in winter, summer, and fall; (3) development and verification of far-field and near-field (mixing zone) water quality models; and (4) application of the models to project future river quality conditions for several treatment-discharge alternatives as well as a no-discharge alternative.

Water quality criteria established by Arizona and California for the Colorado River are less stringent than those established by Nevada. The State of Nevada's "Requirements to Maintain Existing Higher Quality" for the Colorado River below Davis Dam (NAC 445.13495) are based on a strict interpretation of the Federal anti-degradation regulations, which have not yet been addressed by Arizona or California. For example, review of the Arizona and California regulatory framework showed that neither state's water quality criteria would dictate phosphorus removal and only California requires dechlorination of an effluent discharge.

A steady state water quality model of the Mohave Reach was developed using the EPA QUAL-2E program and verified against data from three water quality sampling programs conducted in 1987. The only direct wastewater discharge currently entering the Mohave Reach is the effluent from the River Bend wastewater treatment plant in Bullhead City, Arizona. The discharge from Bullhead City and the proposed discharge from Laughlin were the only point loadings simulated in the model. The major water user in the Reach is the Southern California Edison Mohave Generating Plant which has an average withdrawal rate of 18 cfs.

A mixing zone model was also developed to estimate dilutions downstream of the proposed outfall at Laughlin, for various diffuser configurations. The model was calibrated for lateral mixing conditions in the river using data collected during a dye diffusion test (1987). It was assumed that the location of the discharge would be on the Nevada side of the River, just below the Laughlin Bridge.

The study demonstrated that existing water quality in the Mohave Reach is usually in compliance with the State of Nevada and Federal regulatory requirements, and that the proposed discharge at Laughlin, with additional treatment beyond secondary, will not cause river quality to exceed these requirements for discharges up to 7 MGD. The treatment processes recommended are phosphorus removal and dechlorination. Under WP283A 1-1 868834 these conditions, adequate assimilative capacity is available for equivalent wastewater loadings from Arizona without measurably affecting compliance with water quality objectives.


Chlorine; Chlorophyll; Diffusion; Effluent quality; Fluid dynamics; Phosphorus; River water quality; Sedimentation analysis; Wastewater quality


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