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The Limnology of Lake Pleasant Arizona and its Effect on Water Quality in the Central Arizona Project Canal.
David Walker
University of Arizona Environmental Research Laboratory, 2601 E. Airport Drive Tucson, AZ. 85706-6985.
Kevin Fitzsimmons
University of Arizona Environmental Research Laboratory, 2601 E. Airport Drive Tucson, AZ. 85706-6985.
David Walker, 06/07/99, The Limnology of Lake Pleasant Arizona and it’s Effect on Water Quality in the Central Arizona Project Canal. Lake and Reserv. Manage. Vol. 11(0): 00-00
Abstract
Recent changes in the management strategy of water released from Lake Pleasant into the Central Arizona Project (CAP) canal have substantially reduced taste and odor complaints among water consumers. Most of the taste and odor complaints were probably caused by 2-methylisoborneol (MIB) and geosmin produced by periphytic cyanobacteria growing on canal surfaces. Except during flood events, Lake Pleasant consists almost exclusively of water brought in via the CAP canal. The location of the inlet towers and the Old Waddell Dam influence sedimentation of material brought in via the CAP canal. In-coming water was found to contain large amounts of periphyton of the type found growing on the sides of the CAP canal. Laboratory experiments with sediment from two different regions of the reservoir revealed that the region between the Old and New Waddell Dams contains sediments that have a higher potential for phosphorous release during periods of anoxia than those found in other areas. Withdrawal of hypolimnetic water early in the spring decreased the time that sediments were exposed to anaerobic conditions. This potentially decreased the amount of soluble nutrients released into the CAP canal and available for periphytic cyanobacteria. Utilizing this management regimen since 1997 has resulted in a substantial reduction (or elimination) of consumer complaints of earthy/musty tastes and odors.
Keywords: 2-methylisoborneol, geosmin, periphyton, sedimentation, eutrophication, allochthonous, autochthonous, stratification.
In the first few years after Lake Pleasant Arizona was used as a storage reservoir for CAP water supplied to several municipalities in the Phoenix Valley, many consumers complained of earthy/musty tastes and odors (T&O’s) in water delivered by utilities (pers.comm. Tom Curry, Central AZ. Water Conservation District). Powdered activated carbon (PAC) was used extensively to alleviate the earthy/musty T&O’s, often at great expense to utilities. Our project was initiated in an attempt to decrease the amount of T&O’s in water delivered to consumers. Anecdotal information suggested that T&O complaints decreased dramatically when the CAP canal contained raw Colorado River water as opposed to water that had been stored in Lake Pleasant (pers. comm. Matt Rexing, Mesa CAP Water Treatment Plant). Also, it appeared that T&O complaints increased among those utilities in the Phoenix Valley that were farthest from Lake Pleasant (pers. comm. William Vernon, Scottsdale CAP Water Treatment Plant; pers. comm. Gene Michael, Glendale CAP Water Treatment Plant). Earthy/musty T&O’s are often associated with certain species of cyanobacteria that are capable of producing 2-methylisoborneol (MIB) or geosmin (Izaguirre et al., 1983, Naes et al., 1988, Izaguirre & Taylor 1995). Previous studies that have dealt with MIB/geosmin production by cyanobacteria in source waters have tended to examine the role of reservoirs (Izaguirre et al., 1982; Berglind et al., 1983; McGuire et al., 1983; Slater & Blok 1983; Yagi et al., 1983; Negoro et al., 1988; Izaguirre 1992), or canals emanating from these reservoirs (Izaguirre & Taylor, 1995) as separate ecosystems. This study was initiated to determine whether conditions in Lake Pleasant might promote growth of known MIB/geosmin producing cyanobacteria within the CAP canal upon release of water from Lake Pleasant. This required understanding how the limnology of Lake Pleasant affects the water quality and aquatic biota in the CAP canal.
Materials and Methods
Study Site
Lake Pleasant is located about 48 km northwest of Phoenix, Arizona and is used as a storage reservoir for water transported from the Colorado River. Water is pumped into the lake during winter and released during summer when it is used for irrigation and drinking water. Prior to 19XX, Lake Pleasant was fed exclusively by the Agua Fria River entering from the north. The construction of the New Waddell Dam increased the size of Lake Pleasant from 1,497 to 4,168 hectares (AZ. Game and Fish Dept. unpublished report to U.S. Bureau of Reclamation, 1990). The Old Waddell Dam remains submerged within the reservoir immediately to the north of the new dam (Fig. 1). The primary water source for Lake Pleasant is now the CAP canal. At maximum capacity, Lake Pleasant contains approximately 811,000 acre feet of water (pers. comm. Tom Curry, Central AZ. Water Conservation District).
Sampling Sites
We established four sampling sites within Lake Pleasant (“A”, “B”, “C”, and “D”) (Fig 2), chosen according to an idealized model of reservoir zonation as proposed by Kimmel & Groeger(1984). Locations were determined with a Global Positioning System (GPS) unit (Magellan Model 2000XL). Site A (33o 50’ 57” N and 112o 16’ 18” W) is the closest to incoming CAP water. Site B (33o 51’ 04 N and 112o 17” W) lies between the New and Old Waddell Dams. Site C (33o 51’ 26” N and 112o 16’ 21” W) is to the north of the old dam and Site D (33o 52’ 20” N and 112o 16’ 11” W) is the farthest north from the CAP inlet. (Fig. 2)
Within the CAP canal, 5 sampling sites were established. The sites including approximate kilometers from Lake Pleasant were; Waddell Forebay (0 km), 99th Ave (6 km), Scottsdale WTP (45 km), Granite Reef (70 km), and Mesa WTP (78 km).
Field Data Collection
Lake Pleasant
Samples were collected at each site every 2 weeks when the reservoir was stratified (May – November) and monthly when it was de-stratified (December – April) from 05/1996 – 05/1998. A minimum of 3 samples was collected at varying depths at each site to obtain a profile of the water column. The number of samples collected at each site was based on the presence or absence of a thermocline. Large fluctuations in water levels also resulted in samples being collected at different depths over time. For example, if the depth of the water at Site C was 60 m in August and the thermocline depth was 10 m then samples were collected at 60, 10, and 0.5 m. However, if the depth at the same site in February was 80 m and no thermocline was evident then samples were collected at 80, 40 and 0.5 m
Water samples were collected in a 2.2l Van Dorn-style sample bottle (Wildlife Supply Company). Water collected was transferred to two 500ml, and one 100ml plastic bottles (Nalgene Corp). One of the 500ml bottles contained 2 ml’s of sulfuric acid for analyses of ammonia-N, nitrate-N, and total phosphorous. The other 500ml bottle was used for phytoplankton identification and enumeration and contained 50ml of formaldehyde. Samples collected for orthophosphorous were field filtered using a 0.45micron cellulose acetate sterile syringe filter and a sterile 100ml syringe and stored in a100ml bottle. All were kept on ice in coolers for transport to the laboratory.
CAP Canal
Samples were collected approximately every 14 days within each CAP site and were analyzed for MIB/geosmin and periphyton analysis during the period of time when water was being released from Lake Pleasant. Periphyton was collected from the sides of the canal at a depth of 0.5 m. The area scraped was measured and the sample diluted with 250 ml’s of distilled water and 15 ml’s of formaldehyde.
Geosmin and MIB samples were collected in 1-liter glass amber bottles and kept on ice for transport back to the University of Arizona.
Laboratory Methods
Water samples were analyzed for ammonia-N (Nesslerization), nitrate-N (Standard Method 4500-NO3-), orthophosphate (Standard Method 4500-P), total phosphorous (Standard Method 4500-P.5), ferrous iron (Standard Method 3500- Fe D), and total iron (Standard Method 3030 D followed by 3500-Fe D). Results were determined colorometrically using a Hach DR/890 colorimeter.
Phytoplankton and periphyton were enumerated using a Sedgwick-Rafter counting chamber and an ocular micrometer (Standard Method 10200 F) on a calibrated Olympus BH2 phase contrast light microscope (Olympus Corp.) at a magnification of 200X. Identification sometimes was made at higher magnifications (up to 400X), but all enumerations were performed at 200X. Identifications were made to genus level and all counts were natural unit counts and recorded as units/ml for phytoplankton and units/cm2 for periphyton (Standard Method 10200 F).
Statistical Analysis
Data were analyzed using univariate one-way analysis of variance (ANOVA) and principal component analysis (PCA) to determine which linear combination of X and Y variables had the highest correlation. These correlations were performed on an individual site basis and for the reservoir as a whole to determine what drives primary production within the reservoir and what contributes to tastes and odors within the CAP canal.
For Lake Pleasant data, the independent variables were location (sites A, B, C, and D) and depth. The depths were categorized based upon the presence or absence of stratification i.e. epilimnion, metalimnion, hypolimnion, or homogenous. The dependant variables were temperature, pH, specific conductivity, dissolved oxygen, turbidity, ammonia-N, nitrate-N, orthophosphate, total phosphorous, ferrous iron, total iron, phytoplankton taxa (Division or Genus), phytoplankton enumeration (units/ml), periphyton taxa (CAP canal only), and periphyton enumeration (units/cm2).
For the CAP canal sites,
All statistical analyses were performed using JMP version 4.0.3 statistical software (SAS Institute Inc.).
Results
Lake Pleasant
Physical Data
Temperatures ranged from 11.05o C on 2/13/97 (40m) to 29.76o C on 8/29/96 (surface sample). There was no significant difference among sites for temperature (F = 0.50, p = 0.68). Thermal stratification was evident at all sites beginning in late spring and lasting until mid– to late fall. There was a large difference in temperature among the epi-, hypo-, and metalimnion (Figs. 3&4) (ANOVA Temp X Layer). Depth of the thermocline increased throughout the summer at all sites.
Depth and dissolved oxygen levels were correlated among all sites during the time of peak stratification (Aug-Oct) both years (r = 0.80) (Fig. 5). Dissolved oxygen levels were not significantly different among sites (F = 0.416, df = 3, p = 0.7416). Dissolved oxygen levels between the epi-, hypo-, and metalimnion however, showed significant differences (F = 263.13, df. = 2, p = <.0001) (Fig. 6) with the hypolimnion becoming completely anoxic during late summer and early fall of 1996 (Fig. 7). Differences also existed between years with 1996 having much lower levels (x = 5.1) than 1997 (x = 7.73) (F = 266.62, df = 1, p = <.0001) (Fig 8). A comparison of summer hypolimnetic dissolved oxygen levels between 1996 and 1997 (x = 4.74) reveals even larger differences (x = 0.20 and 3.26 respectively) (F = 254.521, df = 1, p = <.0001) (Fig 9).
Significant differences also existed among layers for pH levels when the reservoir was stratified (F = 938.93, df = 2, p = <.0001). The hypolimnion had the lowest mean (x = 7.94), which would indicate that reducing conditions might occur within this layer seasonally. Differences between sites for pH values were not significant in either the homogenous or stratified condition (F = 0.145, p = 0.9327 and F = 0.268, p = 0.8487 respectively). There were, however, large differences in hypolimnetic pH values between 1996 and 1997 (F = 257.801, df = 1, p = <.0001) (Fig 10).
Turbidity by layer revealed that levels were significantly higher when the reservoir was in the homogenous condition than the epi-, meta-, or hypolimnion when the reservoir was thermally stratified (F = 5.5903, df = 3, p = 0.0008). When the reservoir was homogenous, turbidity levels increased with depth (F = 56.45, df = 1, p = <.0001). During the period of homogeneity, turbidity levels also differed between sites (F = 11.3483, df = 3, p = <.0001). Site B had the highest levels (x = 10.9 NTU’s) followed by site C (x = 9.12 NTU’s), site A (x = 5.85 NTU’s) and site D (x = 4.17 NTU’s). The time of homogeneity within the reservoir is also the time of annual re-filling via the CAP canal.
Specific conductance levels differed between sites when the reservoir was homogenous (F = 3.8053, df = 3, p = 0.0100) but not when the reservoir was stratified (F = 1.3924, df = 3, p = 0.2437). When the reservoir was homogenous, specific conductance levels increased with increasing distance from the CAP inlet towers. Since most of the re-filling of Lake Pleasant occurs during the period of homogeneity, this would suggest that the infusion of “fresher” CAP canal water plays a more significant role in the differences between sites than does increased evaporation during the summer. Specific conductance levels increased with increasing depth during periods of stratification (F = 98.5862, df = 1, p = <.0001) but exhibited no significant depth-related change during periods of relative homogeneity (F = 1.6608, df = 1, p = 0.1978).
During 1996, de-stratification occurred in November. In the summer of 1997, water levels were lower than they were during the same period in 1996. However, dissolved oxygen levels at similar depths still revealed a significant increase during 1997 compared to 1996. As early as October of 1997, the reservoir was homogenous in terms of temperature and dissolved oxygen levels with isolated pockets of anoxia occurring only in areas deeper than 32 m. (Fig. 11). The mean hypolimnetic dissolved oxygen level at site B during 1996 and 1997 were 1.19 and 5.28 mg/l respectively. Also, at site B in 1996 the hypolimnion was at times completely anoxic from 16.5 meters to the bottom (35 meters). At this site in 1997, the lowest dissolved oxygen level recorded over the sediment was 2.53 mg/l.