1. Effects of changes in monitoring station location on reported Lake Mead water quality Dr. David James (UNLV) Randy Hadland (CLV) Dan Fischer (CLV)

2. Outline Water Quality Standards and how they dictate monitoring. What changed and the concerns in the monitoring. Determining new locations. Observed and expected (summertime) changes in water quality at the new locations.

3. Nevada Administrative Code Section 445a 193-201 Covers Lake Mead, Las Vegas Wash and Colorado River below Hoover Dam Beneficial Use Standards- determine water quality necessary for determined use. RMHQ’s- Anti-degradation criteria.

4. Chl-a Standard- RMHQ 1987- WQS changed from a P standard to a Chl-a standard. New standard set the LV Bay monitoring locations. Lake was not meeting the new Chl-a standard. Phosphorus TMDL 434 lbs/day- 334 lbs/day for point source discharges. TMDL is based on dilution calculations that targeted a TP concentration of 0.051 mg P/L at LM3.

5. Enlarged area Lake Mead Sampling Stations

6. Las Vegas Bay Stations- Original Locations

7. Background LM2 and LM3 fluctuate with lake levels. LM4 and LM5 stationary. 2000-2001 beginning of drought and level declines. 7/02 LM3 moves past LM4, 9/04 LM3 100 yards from LM5. LM5 exceeding WQS.

8. Lake Mead Elevation History Glen Canyon Dam finished- 1963 Hoover Dam finished-1935 Current elevation- 1141ft

9. Las Vegas Bay Stations- 1996 LM2 LM3 LM4 LM5

10. Las Vegas Bay Stations- 2003 LM2 LM3 LM5

11. LM5 Chl-a Growing Season (April - September) Average

12. LVW5 Total Phosphorus Loading TMDL- 434 lbs TP/day

13. Effluent Total Phosphorus Loading Point Source TMDL- 334 lbs TP/day TMDL does not apply Nov- Feb

14. Sample locations no longer represent intent of standards Distance from confluence of wash to monitoring sites does not allow proper dilution. Distance does not allow proper assimilation before LM5. Historical perspective of sites no longer represented. NAC contact boundary is now on dry land.

15. Proposed solutions NDEP recognized that a new system is necessary. Variety of methods: Make all 4 stations depth dependent LM2 still close to confluence, LM2 and LM3 right next to each other Sample based on volume of sections Difficult to calculate, Plume might push right through section Make stations distance dependent Possible volume issue when lake level rises

16. Las Vegas Bay Stations- New Method

18. Site Volume Comparison Old Name New Name Original Volume (ac-ft) at 1220’ New Volume (ac-ft) at 1140’ LM2LWLVB1.21,9235,004 LM3LWLVB1.855,70010,451 LM4LWLVB2.721,31821,695 LM5LWLVB3.568,44146,074

19. Need to estimate dilution Wash plume dilution estimates help gauge effects of moving stations. Why not just raw conservative constituent data? Lake TDS rises and falls with water balance in lake. Rising lake levels tend to reduce TDS Dropping lake levels tend to raise TDS Must “normalize” data for changes in background.

20. CR346.4 Total Dissolved Solids R 2 = 0.31

21. Dilution factor, D Historically used by NDEP to estimate assimilation of effluent in Lake Mead D = volume of lake water / volume of wash in a sample Best calculated using conservative tracers, eg, TDS, chloride, or conductivity Formula: D = [Cwash – Cmix] / [Cmix-Cback] Range of possible D values: 0 to infinity If D = 1, Vlake = Vwash in sample (50% dilution).

22. How dilution, D, calculated Data used: Matched closest dates for Wash, LV Bay, and background data. LVW5- Wash data LVB1.8 and LVB3.5 for LVB data CR346 and CR350 for background data. Period of record, 2000-present (Feb 2005)

23. Conductivity Dilution Density shifts in winter and summer make looking at only epilimnion, metalimnion, or hypolimnion difficult. Even combining all three can miss peak concentrations. To compensate for issue we averaged the conductivity profile data for the whole water column.

24. Conductivity LVB1.85M January 1, 2000

25. Conductivity LVB1.85M March 13, 2000

26. Conductivity LVB1.85M March 21, 2000

27. Conductivity LVB1.85M March 27, 2000

28. Conductivity LVB1.85M April 4, 2000

29. Conductivity LVB1.85M April 11, 2000

30. Conductivity LVB1.85M May 2, 2000

31. Conductivity LVB1.85M May 8, 2000

32. Conductivity LVB1.85M May 30, 2000

33. Conductivity LVB1.85M June 6, 2000

34. Conductivity LVB1.85M June 26, 2000

35. Conductivity LVB1.85M July 10, 2000

36. Conductivity LVB1.85M July 17, 2000

37. LM2 Dilution Factors- conductivity based

38. LM5 Dilution Factors- conductivity based

39. LM2 Average Dilution Factors

40. LM5 Average Dilution Factors

41. LM2 Epi- Percent wash in sample- Chemical data

42. LM2 Hypo- Percent wash in sample- Chemical data

43. LM5 Epi- Percent wash in sample- Chemical data

44. LM5 Meta- Percent wash in sample- Chemical data

45. LM5 Hypo- Percent wash in sample- Chemical data

46. Conclusions New monitoring system was necessary. Plots show declining dilution with time with highest dilutions occurring in late winter. A moving contact boundary is necessary. New system does not perfectly mimic original system. Chemical and physical data trends agree. Jan- Feb 2005 dilution values at new stations seem to agree with NDEP’s original intent in 1987.

47. Additional Information Necessary Summer time Chl-a values will be a big indicator. Periodicity of dilution factors difficult to explain. Our ideas: Relative Thermal Resistance to Mixing at minimum in mid winter and at maximum in summer, plus Larger portion of water column available for mixing in winter

48. Acknowledgements SNWA- Art Ehrenberg Dischargers – Water quality data Clark County Water Reclamation District City of Henderson City of Las Vegas US Geological Survey – Wash flow US Bureau Reclamation – Lake levels NDEP – water quality rationales