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LAKE TAHOE: A CASE STUDY FOR ADDRESSING THE DECLINE OF CLARITY IN A SUB-ALPINE LAKE

JOHN REUTER and

GEOFF SCHLADOW

Lake Tahoe

UC Davis

San Francisco

Los Angeles

TAHOE ENVIRONMENTAL RESEARCH CENTER

TERC conducts research, education and public outreach on lakes and their surrounding watersheds and airsheds.

TERC provides objective scientific information for restoration and sustainable use of the Lake Tahoe Basin and inland waters in general.

terc.ucdavis.edu

11000 sq ft labs, offices1500 sq ft outreach3000 sq ft class rooms

LAKE TAHOE BASIC FACTS

11th deepest lake in the worldMaximum Depth = 501 mMean Depth = 330 mLake Surface Area = 500 sq. kmWatershed Area = 800 sq. kmShoreline length = 115 kmUltra-oligotrophicMonomictic63 Inflowing streams1 Outflowing streamMean residence time ~ 600 yrsAltitude = 1895 mLatitude = 39 °N

Cause for Concern30 m (100 ft target)

CONTRIBUTORS TO CLARITY DECLINE AT LAKE

TAHOE

POLLUTANTS OF CONCERN

Ultra-fine soil particles

Nutrients and algal growth

Land disturbance - soil erosion

Wetlands Removal

Before

After

Traffic Congestion

Photo – Larry Prosor

Road Construction

Atmospheric Deposition

Angora Fire- 3100 acres burned - 253 homes lost

Stream Runoff

Urban Runoff

ORIGINALLY

NOW

Lake Tahoe Water Quality Management Approach

What is the problem and what causes it?

What are sources and relative contributions of nutrients and fine-sediments?

How much of a reduction is needed to achieve the desired conditions?

How will this reduction be achieved?

TMDL Overview

Total Maximum Daily Load

TMDL = science based water quality restoration plan

Nutrient & Fine Sediment Budget

NitrogenTotal Nitrogen Estimates:

Percent Contribution per Source Category

Groundwater 12.6%

Atmospheric Deposition 54.9%

Non-urban Upland 15.6%

Urban Upland 15.9%

Stream Channel Erosion < 1%

Shoreline Erosion < 1%

Phosphorus

Total Phosphorus Estimates: Percent Contribution per Source Category

Urban Upland 39%

Shoreline Erosion 4%

Groundwater 15%

Atmospheric Deposition

15%

Stream Channel Erosion 2%

Non-urban Upland

26%

Particle Number <20 µmFines Estimates <20 µm:

Percent Contribution per Source Category

Shoreline Erosion < 1%

Stream Channel Erosion

4%

Urban Upland 72%

Non-urban Upland

9%

Atmospheric Deposition

15%

Lake Tahoe Clarity Model

DLM Hydrodynamic/Thermodynamic Model

Tributaries

Climate,Precipitation

Land Use Atmospheric Deposition

AlgalGrowth

LakeN, P, Si

Inorganic

Particles

LossLoss

SecchiDepth

LightScattering &Absorption

Groundwater

Tributaries

Climate,Precipitation

Land UseAtmospheric Deposition

WATER QUALITY INPUTSSTREAMS [DO, BOD, CHLa, P (PP, THP, POP, RP),

N (NO3, NH4, DON, PN, PON), # PART]G/WATER [P (THP, RP), N (NO3, NH4, DON), # PART]

ATMOS [P (THP, POP, RP), N (NO3, NH4, DON)]

WATER QUALITY INPUTSSTREAMS [DO, BOD, CHLa, P (PP, THP, POP, RP),

N (NO3, NH4, DON, PN, PON), # PART]G/WATER [P (THP, RP), N (NO3, NH4, DON), # PART]

ATMOS [P (THP, POP, RP), N (NO3, NH4, DON)]

PHYSICAL FORCING INPUTSMET [U, SW, LW, RH, T , PRECIP]

STREAMS [Q, T]GROUNDWATER [Q]

PHYSICAL FORCING INPUTSMET [U, SW, LW, RH, T , PRECIP]

STREAMS [Q, T]GROUNDWATER [Q]

LAKE MODEL

PHYSICSBIOLOGY

75% Reduction from Urban Runoff to Meet WQS of 30 m for Secchi Depth

y = 0.1363x + 19.773

R2 = 0.982

0

5

10

15

20

25

30

35

0 10 20 30 40 50 60 70 80 90 100

Urban Load Reduction Percentage

An

nu

al A

vera

ge

Sec

ch

i De

pth

(m

)

Application to Grand Lake

Science should be part of decision-making

Importance of Data

• Knowledge

• Nice existing data base

• Future collection to answer specific questions to guide solutions

(e.g. what is an appropriate target for water clarity?)

Solutions require:

Shared responsibility

Multi-year commitment

Stakeholder participation