The California Aqueduct supplies water from Northern California to Southern California, dividing into the West and East branches above Pyramid Lake. In July and August 1990, elevated geosmin levels (10-48 ng/l) occurred in the East Branch of the aqueduct, which extends along the southern edge of the Mojave Desert. The geosmin episode was associated with attached algal growths on the sides of the aqueduct. A geosmin-producing cyanobacterium, possibly a Microcoleus sp., was isolated from both water and periphyton. In the summer of 1991, elevated levels of 2-methylisoborneol (MIB) occurred in the East Branch of the aqueduct (up to 78 ng/l), along with lower levels of geosmin. In July 1992, a recurrence of MIB production led to a severe off-flavor problem for a water agency that receives water directly from the aqueduct, resulting in numerous complaints from consumers. In both episodes, a Lyngbya sp. was isolated from periphyton and mud collected near the water's edge. These isolates were strong MIB producers in culture, yielding 240 and 260 μg/l, respectively. Beginning in 1992, a second, relatively weak MIB producer, a Hyella sp., was isolated from membrane-filter plates inoculated with aqueduct water. These off-flavor episodes - associated with low flows during a drought period - showed that previously untainted water sources can be affected by these problems when conditions change.

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