The U.S. Geological Survey has studied water movement in the bedrock near Mirror Lake since 1990. Studying the movement of water in fractured rocks takes a great deal of time, money, and special equipment. To start, a well needs to be drilled into the rock using an air-hammer type of drill. Some of the bedrock wells near Mirror Lake have been drilled to depths as great as 300 meters (which puts their bottom about 80 meters below sea level). When drilling, the hole must first penetrate the glacial deposits overlying the bedrock. To prevent water in the glacial deposits from mixing with water in the bedrock along the bore hole, a steel casing is set through the glacial deposits and 3 meters into the bedrock. This casing is then cemented in place by forcing cement to fill the space between the outside of the casing and the wall of the drill hole. When the cement is dry, the hole is then drilled through the crystalline rock to the desired depth. Casing is not needed in the bedrock because the rock is hard and does not cave into the hole. Also, by having an open hole it is now possible to study water movement in the fractures that the hole intersects.

One of the first things that ground-water hydrologists do after the hole is drilled is run geophysical logs. This is done by using special probes and recording equipment that can record the physical characteristics of the rock, and the location of fractures. For example, probes are used to indicate the diameter of the hole, the type of rocks drilled through, and where the fractures are. Another very informative type of log for describing what rocks the well is drilled through is made using a video recorder. A borehole video log is made by lowering a specially designed video recorder down the hole and recording a continuous image of the borehole wall.

After knowing the characteristics of the rocks and the position of fractures in a borehole, the studies then move on to try to understand the movement of water in the fractures. The goals are to find out how interconnected they are and the amount of water and dissolved chemicals that move through them. This is done by placing packers, air lines, and water lines in the holes. The system is constructed so the air lines and water lines pass through the packers. The packers are spaced so they straddle a zone of fractures. The packers are then filled with air, by way of the air lines, so they expand and press tightly against the wall of the drill hole, thereby preventing water from moving up and down the drill hole. In this way, they “isolate” a particular fracture zone. It is necessary to have some air lines pass through the upper packers so the operators can pump and withdraw air from the deeper packers.

Packer air and water lines

And it is necessary to have some water lines pass through the upper packers so the operators can pump water from, and measure the water levels in, the deeper packed-off intervals. Once all of this equipment is in place, it is now possible to run tests of water production from each fracture zone, determine the interconnectedness of the fracture zones, and to withdraw water for chemical analyses of water in each fracture zone. The equipment you see here is used to continuously record the hydraulic heads, or water levels in each of the packed-off fracture intervals.

More information on the U.S. Geological Survey studies of “Fractured rock hydrology” can be found on the following web site:
http://toxics.usgs.gov/sites/mirror_page.html