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

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.

Nevada Administrative Code Section 445a 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.

Chl-a Standard- RMHQ 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 mg P/L at LM3.

Enlarged area Lake Mead Sampling Stations

Las Vegas Bay Stations- Original Locations

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

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

Las Vegas Bay Stations LM2 LM3 LM4 LM5

Las Vegas Bay Stations LM2 LM3 LM5

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

LVW5 Total Phosphorus Loading TMDL- 434 lbs TP/day

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

Sample locations no longer represent intent of standards Distance from confluence of wash to monitoring sites does not allow proper dilution.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.

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

Las Vegas Bay Stations- New Method

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

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.

CR346.4 Total Dissolved Solids R 2 = 0.31

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).

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)

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.

Conductivity LVB1.85M January 1, 2000

Conductivity LVB1.85M March 13, 2000

Conductivity LVB1.85M March 21, 2000

Conductivity LVB1.85M March 27, 2000

Conductivity LVB1.85M April 4, 2000

Conductivity LVB1.85M April 11, 2000

Conductivity LVB1.85M May 2, 2000

Conductivity LVB1.85M May 8, 2000

Conductivity LVB1.85M May 30, 2000

Conductivity LVB1.85M June 6, 2000

Conductivity LVB1.85M June 26, 2000

Conductivity LVB1.85M July 10, 2000

Conductivity LVB1.85M July 17, 2000

LM2 Dilution Factors- conductivity based

LM5 Dilution Factors- conductivity based

LM2 Average Dilution Factors

LM5 Average Dilution Factors

LM2 Epi- Percent wash in sample- Chemical data

LM2 Hypo- Percent wash in sample- Chemical data

LM5 Epi- Percent wash in sample- Chemical data

LM5 Meta- Percent wash in sample- Chemical data

LM5 Hypo- Percent wash in sample- Chemical data

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.

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

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