Using Core Samples to Measure Hydrogeologic Properties of Bedrock Aquifers

The Department of Earth Resources at Colorado State University is responsible for taking Kiowa core samples and measuring the hydrogeologic properties, such as porosity, specific yield, and hydraulic conductivity in the laboratory. The following describes how each laboratory analysis is performed.

1.) Porosity is defined as the amount of void space in a rock. The percent porosity can be expressed as

percent porosity = (volume of void space/total volume of rock) x 100

When the sample is saturated, this void space is filled with water. Porosity can be measured in the lab by completely saturating the core sample with water, weighing it, and then completely drying the sample and weighing it. The difference in weight is due to the water loss, which is equal to the volume of pore space. Typical porosity values for sedimentary rocks (of which the Denver Basin is composed) include:
24-36% Coarse Gravel
31-46% Coarse Sand
34-60% Clay

2.) Specific Yield is defined as the amount of water that can be removed from the saturated pores by draining due to gravity. We have chosen to use a pressure-plate apparatus to measure this property. It is very important to have a fully saturated sample. Think of the rock sample as a wet sponge. By applying pressure (squeezing) to the sponge, the water is being driven out of the sponge. With increasing pressure, more water is forced out of the sponge. Since is it relatively difficult to squeeze water from a rock, we use a pressure chamber (which resembles a pressure cooker). Pressure is increased stepwise until no more water can be drained from the core. The amount of water that will not drain from the pores is called the specific retention. The amount of water that is removed as a function of the amount of pressure applied to the rock can be used to calculate the specific yield.

3.) Hydraulic Conductivity is a measurement of the ease with which water moves through the pores of a rock. This is sometimes referred to as permeability. This is different than porosity. For example, clay materials hold a large amount of water (large porosity) but the water does not flow through the material easily (low hydraulic conductivity). Hydraulic conductivity can be measured using a permeameter. (See Figure No.1) A permeameter is typically a cylindrical tube containing the rock sample. Water is allowed to enter the tube, and the rate of water exiting the permeameter is measured. Using Darcy_s Law, a mathematical expression developed by a French engineer named Henry Darcy, the hydraulic conductivity can be calculated.

These tests are fairly easy to perform and provide important information pertaining to the amount of water stored in bedrock aquifers. Removing too much water from an unconfined aquifer will result in the eventual drawdown of an aquifer. Drawdown involves lowering the water table. If the water table becomes too low, water wells go dry. If a confined aquifer is partially drained, it becomes harder for water wells to pump the remaining water. (See Figure No. 2 for the difference between a confined and an unconfined aquifer) This has happened in some areas of the Denver Basin. Recharge is the water that the aquifer receives from rainwater or seepage from an overlying aquifer. Recharge rates can be very slow. Knowing more about the Denver Basin aquifers will allow our local decision-makers to make educated decisions regarding the amount of water that is pumped from the aquifers in order to minimize the effects of drawdown on the aquifers.

*Definitions and information were cited from C.W. Fetter's (1994) textbook Applied Hydrogeology.

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