By Damien Knight
Ben Tobin gave this seminar over the karst regions of the Grand Canyon. I arrived late to the seminar, but it was very interesting. When I entered Ben was talking about the morphology of the caves. They are following the fractures into the canyons itself. They mapped the caves from faults and fractures traced in the groundwater. To do that, they had to do dye traces of the different cave entrances.
To do dye injections, you put dye into the ground with water. One must put water first then dye then more water. For their experiments they found doing this during winter worked better because of an unsaturated zone of 3-6000 feet to fill. They used the snow to inject dye. The dye trace injections were then snow pack dependent.
Ben mentions how difficult working in Grand Canyon is. They had to monitor 45 springs with 35 sites. They had to visit these sites once a month. This takes a lot of hiking, 25 hours of driving, and Ben said the elevation it is equal to hiking sea level to Everest and back down. He says this “It takes 62 days to do all of it every month.” They have a staff of fit people who work 8 days at a time and take breaks out every 5 days then going back in.
When it came to collecting data, they were starting pretty much fresh. There was work done on the canyon back in the 70’s. They were trying to understand the groundwater systems, and they came up with a map that showed how the systems might be connected. Not having anything else this is what Ben Tobin’s group started with in terms of the karst study. They tested if the blue basin on the map existed. They injected dye to see if it would show up at Roaring Springs.
The results for the experiment came out. The dye showed up to the west side of the canyon at Thunder River Spring and others. It also showed up at a spring on the far eastern side at Vasey’s Paradise. So that dye split east and west meaning the original basin map didn’t work. They then looked at the faults and fractures and it made sense to them. The water flowed through the one fault system and into another fault. The dyes all seemed to follow the monoclines and faults except perhaps the last trace which followed the monocline but was different.
Ben Tobin talked about the flow directions in the aquifers. There approximately two flow paths for the aquifers. It had vertical heterogeneity which could be seen in the data. They had the upper perched aquifer, then a large place with little connection, then the pathways to the large regional aquifer beneath. This was driving how water moved from one layer to the next.
They were able to collect 3 of the 5 dyes from the trace. The dye from injection to the springs was almost 2000 meters deep and travelled almost 23 miles. Water had a variability in how it showed up. Some in less than a month, the southward flow took 2 to 3 months to show. To understand this, they looked at hydrographs, with the equation volume of water per unit time of cubic meters per second.
They were able to measure the storm response. They recorded a big storm event June 10th, 2010. The Roaring Springs hydrograph showed that the spring and the temperature had a response within three days. The temperature dropped, and water arrived at the spring. It took six days for the peak temp to get there. The systems typical residence time is normally seven years according to Millett’s equation. This is slow compared to most karst systems which is three months. This means it does not behave as a typical karst system. They believe they have an upper unit behaving one way and a lower behaving another. Their next goal is trying to understand the upper part of the system. They hope collecting data on that will help understand the lower part. They are still studying the system and making discoveries.
Source:
November 16 – Dr. Ben Tobin, Karst Hydrogeologist, Kentucky Geological Survey, presents: Grand Canyon Karst: Understanding Flow Dynamics in a Critical Groundwater System. EST 260 @ 3 pm
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