Lake Pleasant Limnology and Down-Canal Water Quality

Implications

Spatial Variation in Reservoirs

Thermal stratification (latitudinal variation)Longitudinal variation from incoming river to the dam

A priori Information

Taste and odor complaints decreased dramatically when the CAP canal contained water directly from the Colorado River as opposed to water that had been stored in Lake Pleasant.Taste and odor complaints increased among utilities in the Phoenix Valley that were the farthest from Lake Pleasant.

Thermal Stratification

Nutrient Loading

Allochthonous (from canal into reservoir) during periods of annual refilling of reservoir.Autochthonous (from reservoir into canal) during periods of release into the canal.

0

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Tota

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-34.5 -32.2 -30.8 -22 -16 -15.8 -15.6 -10.2 -0.1Depth (meters)

Lake Pleasant Total Algae Count 12/04/96

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Algal Divisions with Depth

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A lg a l D iv is io n s with D e p th 1 2 /0 4 /9 6S um of Chlorophy taS um of Cy anophy taS um of Chry s ophy ta

Algal Speciation During Filling With Water From CAP Canal

Between dams at depthmostly periphytic species that are usually found growing along the side of the CAP canal

Sites to the north of the old dammore planktonic (true lacustrine) species

Preliminary data from 1996 suggested an increase in dissolved and/or reduced forms of algal nutrients within the hypolimnion

Hypolimnion

Metalimnion

Epilimnion

.0 .1 .2mg/L

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Mean(Total P)

Mean(Nitrate/nitrite-N)

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Mean(Ammonia-N)

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Sediment Mesocosm Results for Sites B and D Non-Aerated

Mean Ferrous Iron (mg/L) 10.03 0.6

Mean Ortho P (mg/L) 5.45 0.65

Mean Ammonia-Nitrogen (mg/L) 5.56 0.63

B D

Recommendation

Prior to 1996, water was released from the top gate (epilimnion) into the canal.This left the hypolimnion intact to become anoxic and accumulate nutrients.These nutrients were slowly released into the canal when stratified and in a large pulse during turn over.

Hypolimnetic Withdrawal

Recommended for the Spring/Summer of 1997 to try and siphon off the hyplominion as early in the year as possible. Done to increase dissolved oxygen over the sediments especially in the area between the old and new Waddell dams.

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1997

Mean Hypolimnetic Nutrient Levels in Lake Pleasant by Year

0.06 mg/L0.14 mg/L0.01 mg/L1997

0.18 mg/L0.21 mg/L0.06 mg/L1996

Ortho PTotal PAmmoniaYear

Numbers of Periphytic Algae by Distance from Lake Pleasant During 1996

Cyanophyta

Chrysophyta

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Pyrrophyta

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Chrysophyta

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Pyrrophyta

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Numbers of Periphytic Algae in the CAP Canal by Year.

Cyanophyta

Chrysophyta

Chlorophyta

Pyrrophyta

Cyanophyta

Chrysophyta

Chlorophyta

Pyrrophyta

1996

1997

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Mean Levels of MIB by Distance from Lake Pleasant by Year

0 - 45

70 - 78

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70 - 78

1996

1997

0 1 2 3 4 5 6 7 8 9 10 11Mean MIB (ng/l)

Mean Levels of Geosmin by Distance from Lake Pleasant and Year

0 - 45

70 - 78

0 - 45

70 - 78

1996

1997

.0 .5 1.0 1.5 2.0 2.5 Mean Geosmin (ng/l)

Generalized Model of MIB and GeosminProduction in the CAP Canal1) Increased sedimentation of material between the old and new Waddell dams during re-filling of Lake Pleasant with CAP water. 2) This sedimentation may lead to increased oxygen demand and anoxia within the hypolimnion during thermal stratification.

3) Under anoxic (and reducing) conditions, this sediment may release nutrients at a faster rate than other areas of the reservoir. 4) These nutrients accumulate within the hypolimnion. If water is released from the top gate, the hypolimnion remains undisturbed for long periods and this may lead to further nutrient accumulation.5) Geosmin or MIB may be quickly degraded in the turbulent release water

6) Release of nutrient-rich water from the hypolimnion into the CAP canal may lead to the proliferation of taste and odor causing organisms in the canal, especially in areas 70 km or more away from Lake Pleasant.

Current Problems

Increased biomass of periphyton growing alongside the CAP canal (not this summer).Hydrogen sulfide emissions upon release during mid-late summer.Iron and manganese in canal water from Lake Pleasant. Spikes in tastes and odors during late summer/early fall 2004.

Hypolimnetic DO Levels by Year

DO

_mg_

per_

L

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Summer 02 Summer 03 Summer 04

Sampling_Period

Sampling_PeriodErrorC. Total

Source 2 57 59

DF 99.05619 63.94067 162.99686

Sum of Squares 49.5281 1.1218

Mean Square 44.1519

F Ratio <.0001Prob > F

Analysis of Variance

Summer 02Summer 03Summer 04

Level 15 26 19

Number 3.64400 0.45577 1.19526

Mean0.273470.207710.24298

Std Error 3.0964 0.0398 0.7087

Lower 95% 4.1916 0.8717 1.6818

Upper 95%

Std Error uses a pooled estimate of error variance

Means for Oneway Anova

Oneway Anova

Oneway Analysis of DO_mg_per_L By Sampling_Period

Taste and Odor Increases During Late October of 2004.

Probably attributable to earlier-than-normal de-stratification.Huge pulses of mib and/or geosmin have occurred in the past when water was switched from the lower to the upper gate and occurred even when there was little or no mib/geosmin production within the lake itself.

The majority of the mib spike downstream of Pleasant is probably due to lysing of periphyton growing alongside the canal.Lysing of periphytic cells occurs when there are large, sudden changes in temperature, or other parameters, in the canal. Tastes and odors greatly diminished when release was once again switched back to the lower gates.

Numbers of periphytic species capable of miband geosmin production growing periphytically alongside the CAP are less than what we find growing in the Salt and Verde Rivers below the reservoirs.However, if large amounts are suddenly lysed, it will result in large mib or geosminhits.

Why Earlier-Than-Normal De-stratification?

Profile data on 9/24/04 indicated strong stratification (>24o C at the surface to <14o C at the bottom of the hypolimnion). Samples taken by Steve Rottas and Al Grochowski of CAP on 10/19, showed that almost the entire reservoir was de-stratified. They also noticed a strong smell of hydrogen sulfide in the open water, an indication of recent turnover.

USGS data from the Agua Fria River near Rock Springs, showed a significant flooding event on 9/19 and 9/20/04. The Agua Fria approached 2000 cfs and this pulse of water could have aided in an earlier-than-normal de-stratification.We have no data on other drainages into Pleasant such as Humbug Creek, Castle Creek, or Cole’s Wash.

However, it appears that the lake level actually rose slightly while water was being released from Waddell Dam.

Summary

Most of the water quality problems down-canal of Lake Pleasant are dependant upon dissolved oxygen and ORP levels within the hypolimnion which are, in turn, dependent upon other limnological processes.

Operational changes in the release of water from Lake Pleasant will address several of

these water quality issues.