Just in case you weren’t sure…groundwater flow around a fault zone is complex!

Just in case you weren’t sure…groundwater flow around a fault zone is complex!

By Erin Mundy – a plain language summary of part of her Masters thesis

Groundwater is the water that collects underground in pores and cracks in the rock. Understanding, protecting and sustaining groundwater flow is critical because over two billion people drink groundwater every day. The flow of groundwater can be impacted by geologic structures, such as fractures and faults. A fracture is a break in the rock; a fault is a break in the rock where the rocks move relative to each other (ie. one rock will move up, one rock will move down, as seen in Figure 1).


Figure 1. Diagram of a thrust fault

Faults can act as barriers slowing down groundwater flow, they can be a conduit speeding up groundwater flow, or amazingly they can act both slow it down and speed it up!

How groundwater moves through these rock structures is difficult to directly observe because it all happens underground and rarely exposed on the surface. The Champlain thrust fault at Lone Rock Point in Burlington, Vermont, provides a unique opportunity to study groundwater flow around a fault because approximately 1 km of the fault is exposed along the edge of Lake Champlain (Figure 2). Here, an older rock


Figure 2: Photograph of the Champlain Thrust fault at Lone Rock Point, Burlington, Vermont. Note the person at the bottom right for scale

(yellow) is thrust over a younger rock (black). No one has studied groundwater flow around this fault in detail, so we hoped to find out a basic understanding of the relationship between the fault and groundwater flow at this location.

To understand groundwater flow around this fault, we did three things: 1) we walked along the fault and made note of changes in the fault (ie. the width of the fault, the angle of the fault, the shape of the fault, etc.); (2) we looked for areas where groundwater was leaking from the rock surface (this is known as groundwater seepage – we wanted to see if there was a relationship between where groundwater was leaking out and the changes in the angle/width of the fault); and (3) we drilled three wells and then pumped water out of these wells. We pumped water out of one well and measured the water level in the other wells – this gives you an idea of how the groundwater moves. For example, if you pump water out of one well and the water level in a nearby well declines drastically, this suggests that the water is easily moving through the rock. So if you’re pumping water from the fault and that happens, then the fault is most likely channeling water along the fault. If the opposite happens, then the fault may be acting as a barrier to groundwater flow.

We found four main geologic structures at the Champlain thrust fault: (1) the main fault, (2) an area where the fault splayed into many smaller faults, (3) areas where the fault thickness increased to 3 m, and (4) areas where there are traces of older, cemented fault rock (Fig. 3).


Figure 3. Most important structures we observed at the Champlain thrust fault. the thin dashed white line follows the main fault, thicker black dashed line follows the main structural features. Hanging wall is the older rock; footwall is the younger rock. a) main fault; b) fault splay; c) increased fault thickness; and d) older abandoned fault rock

We found 19 areas along the rock where groundwater was leaking out of the cliff (Note: This was done in the winter so the groundwater was frozen into ice). We found that most of the groundwater seepage occurred in the younger (black) rock, with a few at the fault and where the fault splays out into smaller faults (Figure 4).


Figure 4. Seeps located a) at the intersection of a multi-stranded fault structure and b) at the fault core. Note measuring tape for scale (1 foot)

While drilling the two wells at the site, we had two unexpected problems. One, there was a large difference between the depth of the fault in the two wells. The fault depth in one well was 27.4m, while in the other well (which was 10 m away), the fault depth was 70 m. This suggests that there must be another fault in between these two wells that offsets the fault depth. The other unexpected complication was that we drilled into 1.8 m and 2.1 m caves beneath the ground. Caves are common features in limestone, but the rock at our site is a dolostone, which is usually more resistant, so caves are an interesting find! The pumping test revealed a complex system. Further testing is needed to better refine these results.

Combining the data from the surface and subsurface observations, we created a preliminary three-dimensional model of the Champlain thrust fault (Figure 5). Where the rock is exposed at the edge of Lake Champlain, the fault thickness varies, splaying out into smaller faults and showing traces of older fault rock. Groundwater is leaking out of the younger rock (footwall) and along the fault. At the well-site, the fault is offset by another fault and caves are present. The three approaches we used (geology, seepage, pumping tests) all revealed different aspects of the Champlain Thrust fault, and exposed the complexity of groundwater flow around faults.



Figure 5. Three-dimensional conceptual model of the Champlain thrust fault.


The home of our hearts day 1 – twenty-five strangers walk into a Mi’kmaq talking circle…

The home of our hearts day 1 – twenty-five strangers walk into a Mi’kmaq talking circle…

[part two of a special six-part blog series by Mark Ranjram, MEng student at McGill University. From June 8 to June 13 2014, Mark had the privilege of being a part of the Canadian Water Network’s (CWN) Student and Young Professionals (SYP) Workshop in Cape Breton Island, Nova Scotia. Here is the prologue to this series.]

When I first decided to attend this workshop, I made a commitment to myself that I would jump into it head first, and if the pool was shallow, well, at least I tried, and if the pool was deep, what a special experience it could be. Lucky for me, every single person at the workshop was ready to take a swim! Nothing better illustrates how committed everyone was to make something special than our first night together at the Membertou Heritage Centre in Sydney.

The Membertou First Nation is one of five Mi’kmaq groups in the tribal district of Unama’ki (Cape Breton Island). I won’t go into much detail here, but Membertou has an amazing success story (I encourage you to learn more about the community here). After an engaging few hours discussing the culture and history of the Mi’kmaq Nation and the Gaelic immigrants that settled the Island in Canada’s early colonial history, we organized our group into a large circle and were guided through a talking circle. A talking circle is a foundational communication ritual in Mi’kmaq culture in which a symbolic item is passed within the group, with the person holding the symbol (in our case, an eagle feather – an item of great significance in the culture) having sole authority to speak. Our guide, Jeff Ward, dressed in full regalia and lighting sage, led us through four rounds of the circle, with each round having a different theme connected to the four cardinal directions. Immediately after the first few people had spoken, it became clear that there was something special happening. The circle was at times serious, sombre, amusing, and painful, but underlying it all it was incredibly sincere.


Jeff Ward preparing us for our talking circle

A major theme in the circle was having hope when everything is hopeless, and the importance of reconnecting with the idealism and optimism that we all had before the pressures of the real world converted that idealism into cynicism. A major talking point was how difficult it is to stay inspired at work or with research, because of creative limits, financial limits, or just the daily grind of professional life, and how incredibly isolating it can be to be stuck without inspiration. The irony we all came to realize of course is that we had a community in that circle of people who felt isolated in the same way. And suddenly, a group of twenty-five strangers decided that we could be each other’s positive framework, that we could fight against cynicism to give us the chance to be idealistic once again. Whether it was the talking circle, or the strange consequence of being in a completely new place with completely new people, we decided in that moment to be vulnerable, open, and honest, and now, more than a week later, I can say that I consider each and every one of those people a friend who I will fight for to make sure they make their mark on the world. And after just five days with them, I am certain that they have the drive and talent to do just that! The energy in that circle when we realized we could be each other’s motivation was powerful, and when we as up and coming water researchers and professionals no longer have to worry about being bogged down by regressive thinking, the energy we have to help develop solutions is limitless. And if you are a young water professional or researcher, know that this is not an exclusive community! If you want to make a positive change, we want you with us. If you feel defeated and cynical, we want you with us. What a movement this could be!


A view around the circle. Photo Credit: Liana Kreamer

 Next post in series…