The Situation
This picturesque 8.1 square kilometer crater lake is the smallest in a chain of three lakes northeast of Lake Rotorua. There are no major streams flowing either into or out of Lake Rotoehu. Water flows into the lake from springs in the lake bed and also seeps into Rotoehu from nearby Lake Rotoma to the east. Water exits the lake from a sinkhole in the northwest corner of the lake.
Average water depth is 10 m. The deepest part of the lake is 13.5 m. Numerous fingers and sheltered bays run off the main lake and are extensively used for recreation.
The shallow nature of Rotoehu, combined with nutrient runoff and relatively little water transfer, causes the lake to suffer from poor water quality. Blue-green algae blooms make the lake unsafe for swimming at times.
Rotoehu Lake authorities approached AWT to discuss treatment options. PondPerfect was chosen as one of the treatment options. PondPerfect is a non-toxic, live bacterial product designed to restore a proper balance between algae, sludge, and bacteria in eutrophic lakes.
Test Site
To facilitate a test of the effectiveness of PondPerfect, a small finger-shaped cove in Omarupoto Bay was chosen as a test site. The test site has the advantage of relatively stagnant, non-flowing water. This allowed use of the finger cove as a test site, while the neighboring water could be used as control.
The volume of water in the test site was 25 000 cubic metres (about 6.6 million gallons). The actual test site cove is shown in the photo below:
Treatment began in December, 2010, and continued for 120 days. The following data were measured for the treated finger cove and for the untreated lake (100 meters beyond the treatment zone): BOD, Suspended Solids, Ammonia, Phosphate, Coliform Bacteria Count, and Secchi Disk Clarity Readings.
The data shows the treated and untreated portions of the lake were equivalent in most respects prior to the trial (see the first set of data in each chart).
During the trial period, the BOD decreased by about 50% in the treated portion of the lake while the untreated portion experienced a slight increase in BOD.
During the trial period, the Suspended Solids, similar to BOD, dropped dramatically in the test portion of the lake compared to the control portion.
The total nitrogen reading in the test zone decreased 70%, while there was no decrease in the nitrogen levels in the control section
Similar to the nitrogen data, orthophosphate (an available form of phosphorus) was reduced in excess of 70%. The untreated portion of the lake experienced a significant increase in orthophosphate levels during the trial period.
The decrease in coliform bacteria in the test section was very dramatic compared to the large increase in coliform bacteria in the control section of the lake. Coliform bacteria are standard measures of pollution and major criteria for halting of swimming and other recreational activities.
During the period of the trial, the clarity in the finger cove that was treated with PondPerfect improved from 3.5 to 7.8 feet. In contrast, the control section became less clear, going from 3.4 to 2.2 feet of visibility (typical for algae blooms, whereby clarity decreases during the warm periods and with algae growth)
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