13 July, 2000
July 13, 2000
Matanuska Glacier, Alaska
Over the past three days we have collected samples from two vents and from what we’ve seen so far we cannot conclusively determine that we’ve made a moulin/vent connection. In spite of doubling the amount of salt that we used we still don’t think we’re seeing a significant change in the conductivity of the samples. In addition we are starting to question what role the suspended parcticles (silt) in the samples is having on the conductivity meters. As a result we are allowing the samples collected today to settle out and in the morning we will check them all again. We also question whether or not the salt is getting dissolved very soon after we dump it into the moulin. If it continues to get flushed through the subglacial conduits then it should dissolve soon enough. However if it is getting washed into a subglacial lake or any somewhat calm body of water then it will sink to the bottom and take considerably longer to go into solution. We’ve already found it impractical to try to predissolve the salt out on the ice. We may find in the end that this method cannot be useful to determine connectivity.
Dye tracing on the other hand has met with some success by others in the past and is the method we will ultimately use regardless of the success of our current method. I mentioned in my July 7 journal that we would eventually use this method in order to be able to measure the flow through the glacier. In our study we will be using a special type of rhodamine dye that fluoresces, or gives off light, when subjected to ultraviolet radiation. This same thing occurs when you are in a room with a blacklight and certain objects seem to glow while the light is on. If we have a water sample containing some of this dye, we can put it into a an instrument called a fluorometer. When a sample is put into the fluorometer it is exposed to a certain wavelength of UV light. If there is some of this dye in the sample it will give off light or fluoresce. The more dye the sample contains, the more it will fluoresce. The instrument is designed to measure the amount of fluorescence and display a concentration based on that. How does the instrument know how to relate concentration to the fluorescence? How do you know how tall you are? You use a standard, usually a ruler, which measures in feet. Once a standard is established you can measure the height of any number of people or objects with it. It is also a linear measuring device. In other words if some object is twice as tall as another, you will end up with a measurement with twice the value. Similarly we use a standard solution that has been prepared with a known concentration of rhodamine dye. The instrument is allowed to measure the fluorescence of this standard and we then enter the value into the program. The fluorometer is also a linear measuring device, so if you put in a solution that’s half as concentrated as the standard it will actually be able to measure and display an output that’s half as high. We should be able to detect concentrations as low as 10 parts per trillion (or ten parts of dye for every trillion parts of water). Our dye is one part per ten thousand and so it can be diluted by a factor of 100 million and still be detected.
As I mention July 7, the dye is rather expensive and so we’re attempting to make our connections with salt first. Perhaps the salt experiment will fail to give conclusive results. But because the fluorometer can measure extremely small concentrations we are confident that it will be a very useful tool and will hopefully give clear results. In a later journal I’ll address how we can interpret those results to give a picture of the water flow under the glacier.
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