To do or dye

As part of my masters degree I recently performed a dye trace to better understand how the groundwater flows in my study area. This is especially important in areas like mine known as karst topography, where the groundwater flows through caves and fractures in the bedrock along specific paths and in specific directions, almost like underground river systems. This I opposed to other types of ground water flow which usually involves water flowing slowly in all directions, which is usually easier to predict and monitor.

Sinkholes in these areas are often controlled by the way groundwater flows, so understanding groundwater flow can provide some insight into sinkhole formation and hazard, such as this sinkhole which formed (in a matter of hours) only a few miles south of my study site and destroyed a house.

Dye traces in theory are very simple. A type of fluorescent dye, either as a powder or a liquid, is dumped into a sinkhole or sinking stream, which act as inputs into the groundwater system. Springs nearby act as groundwater outputs and are monitored afterward to see if the dye begins to show up in the spring water. If the dye is found coming out of the springs, or when it doesn’t (which is also important), then connections can be made between the sinkhole and spring, giving insight into what the ground water is doing.

A dye trace can also be taken further. Measuring how much dye is recovered from can see if dye is being pirated away to other springs. Finding out exactly when the dye starts coming out of the springs relative to when it is dumped into the sinkhole will show how long it took the dye to travel underground. With the distance known between the sinkhole and spring the groundwater velocity can be calculated. Measuring dye concentration spikes in the spring can show if the dye is traveling as one “slug”. Multiple spikes would show dye splitting along different paths and rejoining before exiting through the spring.

My field site includes a number of sinkholes (numbers #1-3), springs (#5,6), and even a sinkhole in a stream (#4), which allow for a few dye tracing opportunities. Previous studies show that, generally, the groundwater flows southward. One of the sinkholes (#3) has just opened up in the last few months in the middle of a retention basin near a major highway.

Rhodamine WT dye was injected into the stream sink (#4) using a small pump. The idea behind this is it causes the dye to come out of the other springs at a constant, level rate. This getup was a special pump and reservoir system which we installed under a cart path.

Lissamine FF dye was dumped into the newly formed sinkhole and flushed in using a garden hose which was installed into a nearby fire hydrant. This was one of my more unique research experiences, rolling out 300+ feet of hose from a fire hydrant across a street and down into the sinkhole. Opening the fire hydrant was actually kind of frightening as it made a lot of low rumbling noises before finally opening up.

We flushed in the dye using about 250 gallons of water. This took about a half an hour, based on the rate of the hose. The whole process took about two hours to set up and complete. Unfortunately for myself and my field assistant we had no shade to rest in. As a side note, this was during some of the hottest days so far this summer in Missouri.

The dye from the newly formed sinkhole was detected at both of the springs to the south, which was expected but important to find out. One of the more important observations, though, relates to the dye injected at the stream sink. Although that dye shows up at both of the springs, it shows up at unequal concentrations. More specifically, it shows up as a higher concentration at spring #6 than spring #5. What this shows is that some of the dye is diverting to another path before it reaches spring #6 and mixing with another source of ground water which is diluting it before it reaches spring #5. Spring #6 is also farther away than spring #5 which explains why it shows up later.

Marco...Polo...Geologists in the Mist

I took 2-week field class to the Black Hills along with about 15 of my fellow geology majors. Some of us were graduating or recently graduated, confident in our learned skills, while others were at different stages in the program. It was by far one of my more enjoyable geology experiences, filled with a lot of stories I love to tell. Here is one I often told my geology labs when we get to the section about using maps, usually when a student will ask how accurate they need to be on an exam with map coordinates.

For a few days we were divided up into groups of 3 to map geology in sections in as much detail as we could. It was difficult at times to not take time and enjoy the amazing scenery, the tall hills thick with pine trees.

A competitive team mentality drove each team to want to map the most, as we were going to combine all of the teams mapped data into one larger geologic map of the area.

I considered my group the GPS-savvy group, and, along with each of us having a walky-talky, we thought it would only make sense to spread out a little to cover more ground while using the handheld GPS’s and radios to maintain contact and positional awareness. Besides, we were never really far off from each other, usually just beyond the adjacent hill, and only had to be accurate enough to get within shouting range. Without much practice we became very good at this divide and conquer method.

Our skills were tested one day when, as I was walking up a large hill, looking for some outcrop, I looked up and saw a cloud roll over and down the hill.

“Incoming fog”, I called on the radio.

“Really? It’s still sunny here,” one of my team members replied before the fog reached them. “Ah, there it is”

Everything got very quiet and muffled when that thick fog rolled in. I could only see about 50 feet, which was lucky, considering how dense the pine trees were. It seemed that my shouts couldn’t even travel that far. If someone wasn’t within 50 feet of me I’d never see them, let alone hear them. We only planned to go map this hill quickly and head back to camp, but first we had to find each other.

For the next half-hour we played 21^st century Marco Polo. This involved us agreeing on a prominent topographic feature to meet up at, a nice round hill, measuring its UTM coordinates off of our topographic map, then comparing those UTM’s to each of our current positions to calculate which direction and distance we should travel to reach the hill (using our Brunton compasses) and backing up with our handheld GPS’s. Once we were at the top of the hill, then, we need to meet in the same spot within a range of about 50 feet. I first saw one of my team members, due to the bright green shirt they were wearing at the time, and they couldn’t even hear me when I shouted at them. After more walky-talking and GPS-checking we eventually met up with the third member. Each of our maps was filled with check and re-check UTM coordinates, lines, and scribbles near that little hill.

So when a student asks, "How accurate do I need to be?", I tell them this story. I tell them to be as accurate as they can, because you never know when the fog will roll in and you'll have to play Marco Polo with GPS coordinates to get your team back together safely.