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Lessons from Rhodamine Dye Study in Minnesota

Feb 2019

UPL recently had the opportunity to facilitate a dye study for the Minnesota Department of Natural Resources on a 2500-acre-flowage lake. The study was performed after several small treatment plots for curly-leaf pondweed did not achieve an acceptable level of control in the year prior. This lake had been treating sub-seven-acre sites throughout the system with the goal of gaining control of a fairly new, and quickly growing curly-leaf pondweed infestation.  

The study results proved the importance of addressing local environmental conditions to ensure successful treatments.

Four four-acre sites studied 
UPL selected four sites, each approximately four acres in size, on which to apply Rhodamine WT dye.  The application was conducted by a professional applicator in the same manner that would be used to apply a liquid herbicide, using sub-surface injection hoses. Rhodamine WT concentrations were monitored using two types of fluorometers, a portable handheld unit for discrete sampling, and eight data loggers (sondes) that were buoyed in place, two inside each plot.

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Description generated with high confidence

Site A
Site A was the closest plot to the riverine section of the lake. The half-life calculated from the sonde furthest downstream was six hours. At 17 hours after treatment, a slight but steady increase was seen in the dye measurements, which was likely due to the dye from site B moving into site A. A similar trend was seen with the second sonde in site A with high dye concentrations passing through at approximately five hours after treatment. This influx of dye from site B resulted in a half-life of 14.25 hours on the eastern edge of site A. 

Site B
Site B was a relatively narrow but long plot located approximately 700 feet upstream from site A. The sonde furthest downstream had a half-life of six hours. Several sessions of high dye concentrations were noted, which could have been due to a band of dye that was applied on the eastern edge passing on its way downstream. The upstream sonde showed a low level of dye during the entire logging period, which likely meant the dye immediately moved downstream; thus, a half-life could not be calculated. 

Site C
Site C was the best site for dye retention as data from both sondes could not calculate a half-life because of little-to-no dilution. In addition, hand-held fluorometer data showed a significant level of dye retention through seven hours after treatment.

Site D
Site D was expected to be quite challenging due to the bottleneck the bridge would have on flow and the large expanse east of the bridge which could result in significant wind-fetch water movement. The data sonde in the cove on the western side of the plot had excellent dye retention, and a half-life could not be calculated, with numerous concentration peaks being noted, likely due to an eddy effect. A moderate east-to-west wind was noted when the dye was being applied, which likely resulted in the immediate pushing of the dye away from the bridge, as was seen in the hand-held fluorometer data as well as in several aerial drone pictures. After the treatment was completed, the wind died down and the eddy effect seemed to pull the dye into the cove and back around to near the bridge, which resulted in a 14-hour half-life for the sonde positioned near the bridge.

UPL was able to demonstrate that even with somewhat “ideal” small polygons, local environmental conditions need to be addressed to ensure successful treatment. A wide range of results was observed with four similar-sized polygons placed in four different locations on the lake. Had they been smaller sub-one-acre acre plots, many of them would have had little-to-no dye retention time inside the treatment plots. This dye treatment gave a visual and scientific representation to the lake board as well as the Minnesota Department of Natural Resources on the various ways water can move and affect treatments.