Document Type

Technical Report

Publication Date

10-28-1976

Abstract

1. The waters of Las Vegas Bay, a heavily utilized recreational resource, receive discharges from a variety of municipal and industrial waste sources. The U.S. Environmental Protection Agency has determined, on the basis of numerous studies, that the present water quality violates state and federal standards and constitutes a public nuisance. Consultants have advised the Sewage and Wastewater Advisory Committee that rapid abatement of the alleged pollution conditions can be achieved by an advanced wastewater treatment (AWT) plant.

2. The major problems in Las Vegas Bay are an objectionable water color, excessive turbidity, noxious odors, and oxygen depletion in certain portions of the water column. Previous studies indicate that these problems are due to the effect of Las Vegas Wash on the Bay, and that the problems decrease as distance increases from the Las Vegas Wash inflow. The Wash provides a source of nutrients for high levels of algal production, which, in turn, are responsible directly for the pollution perceived by the public. Conditions in Boulder Basin, as opposed to those in Las Vegas Bay, have not reached an objectionable eutrophic level.

3. Most of the previous studies point to industrial and municipal wastewater discharges as the ultimate source of nutrients entering the Bay. One advisor argues that the nutrients arise from soil eroded into the Bay.

4. Adherents of the first position suggest that removal of nutrients from wastewater will abate the problems in Las Vegas Bay. Adherents of the second position suggest that control of soil erosion is the only step that can mitigate these pollution problems.

5. The following deficiencies in these previous studies prevent the formulation of a dependable strategy for decreasing the excessive algal production in Las Vegas Bay to acceptable levels: (i.) There has been no detailed attempt to ascertain the fate of Las Vegas Wash inflow. The fact that the bulk of this inflow may leave the Bay in a well-defined current does not imply that partial mixing of the current with the Bay is not affecting algal production. The presence of enteric bacteria in the Bay suggests that partial mixing, in fact, is occurring. (ii.) A systematic series of algal bioessays has not been undertaken to ascertain directly which nutrients limit algal production. (iii.) Inputs of phosphorus other than via the Wash discharge have been neglected, particularly internal loading from the sediments to the water column. (iv.) The effect of changing water level on dilution of nutrients in Las Vegas Bay has been neglected, despite the fact that the water volume in the Bay has increased about 75% between 1967 and 1976. (v.) The standard of 0.5 mg 1-1 phosphorus established by the EPA for Las Vegas Wash water does not rest on adequate evidence that these levels are necessary or sufficient to abate the problems in Las Vegas Bay, but rather upon the predicted performance of AWT. (vi.) The studies have not been designed to predict changes in Las Vegas Bay following various alternative treatment strategies.

6. Ecological Research Associates initiated a field study of Las Vegas Bay and Boulder Basin (20-23 September 1976) to resolve certain discrepancies in previous studies and to provide supplemental information consistent with the short duration of the study.

7. Specific conductivity, temperature, pH, and oxygen profiles agreed with those collected by previous investigators. High conductivity values reflecting the presence of a well-defined current representing the Las Vegas Wash inflow were observed in Las Vegas Bay. However, this current was not detected at the mouth of the Bay in Boulder Basin. These results do not support the notion that the Wash inflow does not mix in the Bay, but the detailed investigation necessary to decide this issue could not be performed during the short time period of the study.

8. Virtually all dissolved phosphorus occurs in inorganic form and in very low concentrations, indicating that biologically-available phosphorus is cycling extremely rapidly and that the amount of analytically detectable soluble phosphorus may not be particularly relevant for determining the potential algal production.

9. Algal productivity measurements in September were lower than those of previous investigators by more than 90% in some cases. The discrepancy may represent differences in methodology or real decreases in algal production. Many hypotheses may be invoked to explain the results if the latter is the case, but insufficient evidence exists to distinguish between them. The results emphasize that important questions concerning the source and fate of inflowing nutrients remain to be answered, because previous studies cannot account unequivocally for this decrease in productivity. Severely eutrophic conditions were not observed in Las Vegas Bay during the course of this study.

10. Algal bioassays demonstrated that inner Las Vegas Bay algae are responsive to nitrogen because of the heavy phosphorus loading. Virgin Basin algae (and hence, presumably, Boulder Basin samples) are stimulated by addition of wastewater, nitrogen or phosphorus. Recalculated N:P ratios, based on previous studies, suggest that all Las Vegas Bay and Boulder Basin stations, except for the inner Bay, are most limited by phosphorus. The implication is that the middle and outer Bay and the Basin are sensitive to further loading of phosphorus. The uncertainty of decisions based upon nitrogen to phosphorus ratios is emphasized in this report.

11. Sediment analyses indicate that phosphorus is removed from the water by sedimentation. Proper standards for phosphorus concentrations in inflowing waters cannot be established without taking into account the role of phosphorus sedimentation in Las Vegas Bay.

12. The application of Vollenweider's relationship to Las Vegas Bay suggests that, even if EPA standards of 0.5 mg 1-1 phosphorus were met in Las Vegas Wash, the reduction in loading obtainable from present day AWT technology is not sufficient to produce the desired conditions in Las Vegas Bay. In any case, the Vollenweider relationship does not constitute an adequate basis on which to decide an abatement strategy for Lake Mead.

13. AWT treatment of Las Vegas Wash wastewater cannot guarantee the eradication of problems in Las Vegas Bay, both because the exact initial source of the nutrients is not established fully (i.e., wastewater or erosion) and because the effect of reducing phosphorus concentrations to 0.5 mg 1-1 in the Wash cannot be predicted at present. The predictive relationship used by other consultants to justify AWT has not been applied correctly. The correct application of this relationship to the condition in Las Vegas Bay demonstrates, on the contrary, that AWT technology will not be sufficient. The enormous expense and deleterious side effects of AWT technology are not justified on the basis of existing data.

14. Numerous alternatives to AWT exist. Upgraded secondary wastewater treatment combined with biological stripping of both nitrogen and phosphorus in an expanded Las Vegas Wash marsh is the alternative that deserves special consideration. Control of soil erosion and partial discharge of treated wastes directly to Boulder Basin also should be considered in this scheme.

15. Any further study should be addressed to the following specific points: (i.) The extent to which Las Vegas Wash inflow mixes with Las Vegas Bay water must be determined more precisely. (ii.) The stimulating effect of eroded soil washed into the Bay on algal productivity must be determined. (iii.) The level of algal growth that can be supported by water equivalent to that produced by various treatment strategies must be determined. A predictive model is recommended. (iv.) The magnitude of the potential internal phosphorus loading from the sediment upon the eventual reduction of external loading should be investigated. (v.) Downstream effects of alternative abatement strategies must be predicted.

Keywords

Aquatic pollution; Benthic organisms; Lake Mead (Ariz. and Nev.); Phytoplankton; Wastewater disposal

Disciplines

Biochemistry | Desert Ecology | Environmental Chemistry | Environmental Health and Protection | Environmental Indicators and Impact Assessment | Environmental Microbiology and Microbial Ecology | Environmental Monitoring | Fresh Water Studies | Natural Resources and Conservation | Natural Resources Management and Policy | Sustainability | Terrestrial and Aquatic Ecology

Language

English

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