Guidance Manua pdf

 

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Table 4-4.  Timing of the Salt-to-Verde shift over the past five years. 

 

Year 

Timing of the Salt-to-Verde shift 

1996 

October 25 

1997 

October 25 

1998 

October 23 

1999 

October 2 

2000 

October 3 

2001 

October 17 

 

In summary, management implications of these patterns are: 

 

Lake Pleasant.  MIB is produced in the epilimnion, reaching concentrations of 15-45 ng/L 

during late summer.  However, CAWCD has altered its operations so that the high-MIB 

epilimnetic water never reaches the CAP canal.  CAP water is therefore a reliable source 

of low MIB water throughout the year.  The operational modifications used by the 

CAWCD to maintain low MIB water and the implications for blending are discussed in  

(Section 5).   

 

Bartlett Lake.  MIB is produced in the epilimnion during the summer when the reservoir is 

well-stratified. Concentrations in the hypolimnion are lower.  Because water is released 

the hypolimnion, water released to the Verde River during summer stratification has MIB 

< 10 ng/L.   MIB concentrations in the outlet of Bartlett Reservoir increase to 10-15 ng/L 

after fall turnover.  However, Phoenix’s WTPs receive very little Verde River water until 

the Salt-to-Verde switch occurs in early to mid-October.  By this time, much of the MIB in 

Bartlett Lake has been degraded. By November, MIB concentrations in the Verde River 

normally decline to < 10 ng/L.  In summary, the only time the Verde River contributes 

significantly to T&O problems is during the brief period in October just after the Salt to 

Verde switch. 

 

Saguaro Lake.  Among the three reservoirs, Saguaro Lake generally has the highest MIB 

levels.  The mid-summer MIB peak exceeded 100 ng/L in 2001 (Figure 4-4).  As with the 

other reservoirs, MIB levels are lower in the hypolimnion.  From August through October, 

when most of the water entering the Arizona and South Canals is coming from the Salt 

River, MIB levels in the Salt River below Saguaro Reservoir averaged ca. 15 ng/L, with 

little year-to-year variation (standard deviation = 6 ng/L).  In summary, among the three 

reservoirs, Saguaro is the most significant source of MIB. 

 

The very high levels in the Salt River below Saguaro River during the winter are 

misleading.  During this period, there is very little flow in the river.  MIB is probably 

produced by algae living on the bottom of the river and builds up with the low flow 

because there is no dilution.  This midwinter buildup of MIB is not a problem because 

very little water from the Salt River is delivered to Phoenix during this period. 

 

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4.4.3  MIB production in the Arizona Canal 

 

There is considerable MIB production within the Arizona Canal, particularly during late 

summer (Section 2).  The source of MIB -producing algae growing in the canals can be 

greatly reduced by treating the canals with copper or brushing (Section 5).   It would be 

prohibitively expensive to treat the entire Arizona Canal throughout the whole year.  To 

be cost-efficient, it is necessary to  target the treatments.  Targeting is based on two 

general observations: 

 

1. 

At any given time, most of the MIB produced in the Arizona Canal is produced 

within fairly short sections of the canal. 

 

2. 

Although small amounts of MIB are produced in the Arizona Canal throughout the 

year, peak production occurs at specific times of the year. 

 

It is impossible to predict  exactly where and when MIB pulses will occur, but some 

patterns can be seen.  First, temperature seems to be a critical factor.  The linear 

relationship between temperature and MIB production is weak, but there is a 

temperature threshold for high within-canal  MIB production.  This relationship is shown in 

Figures 4-6 and 4-7.The temperature threshold appears to be around 22 

o 

C.  When the 

temperature in the spring is below 22 

o 

C there is very little MIB production in the Arizona 

Canal.  MIB production starts when the temperature rises above 22 

o 

C; MIB episodes 

are often observed before the temperature reaches 25 

o 

C.  MIB production above the 

Squaw Peak WTP is almost always less than 10 ng/L when the temperature is less than 

22

o

 C.  MIB production above the Deer Valley WTP remains less than 20 ng/L until the 

temperature reaches 22 

o 

C.  This does not mean that there is always MIB production 

when the temperature is greater than 22

o

 C, but that MIB production will almost  always 

be relatively low until the temperature reaches 22

o

 C.  This point generally occurs in 

early June.  MIB production within the Arizona Canal typically remains significant through 

the end of the year (until canal shutdown).  

 

0

10

20

30

40

Aug-99

Oct-99

Dec-99

Feb-00

Apr-00

Jun-00

Aug-00

Oct-00

Dec-00

Feb-01

Apr-01

Jun-01

Aug-01

Oct-01

Dec-01

Feb-02

Apr-02

Temperature, oC

0

5

10

15

20

25

MIB production, ng/L

Temperature

MIB production

 

Figure 4-6.  Trend of within-canal MIB production and temperature at the Squaw Peak WTP.

 

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