Aquifer Hydraulic Characteristics and Why They are Important to Understand

Sep 21

If you are dealing with groundwater as a water supply, whether it is for irrigation, municipal, commercial, industrial, or other purposes, understanding the hydraulics of the aquifer(s) being used is critical to being able to both manage the resource and optimize its use. So what are aquifer hydraulic characteristics? They are the parameters related to the magnitude and direction of groundwater flow. The primary characteristics to understand are the hydraulic conductivity and transmissivity, and some associated physical characteristics. Big words but let’s break down what each term means related to understanding groundwater flow.

Hydraulic conductivity is a measure of the rate of flow through a unit area in a porous media, typically measured as feet per day or centimeters per second. The concept of hydraulic conductivity is shown in Figure 1. The rate of flow is not only dependent on the hydraulic conductivity, but is also dependent on the area across which the groundwater is flowing and the hydraulic gradient of the groundwater. This results in the fundamental groundwater flow equation developed by Henry Darcy in 1856.

DARCY’S LAW

Q = k * l * A

where, Q = groundwater flow (L3/T)

K = hydraulic conductivity (L/T)

I = hydraulic gradient (L/L)

A = cross-sectional area (L2)

A schematic showing the concept of hydraulic conductivity is presented in Figure 1. While hydraulic conductivity is a measure of groundwater flow under a unit hydraulic gradient through a unit cross-sectional area, as shown in Figure 1, transmissivity is a measure of the total groundwater flow through a unit cross-sectional area for the entire saturated thickness of the aquifer (Figure 1). Therefore, transmissivity is calculated as the product of hydraulic conductivity and saturated thickness. When a well’s production capability is assessed, it is based on the transmissivity surrounding and in the near-vicinity of the well.

Now that we have gone over the concepts of hydraulic conductivity and transmissivity, how do we estimate these parameters? There are a number of methods to estimate the parameters and the ability of the aquifer to produce water. The needed accuracy related to understanding the hydraulic properties of an aquifer are related to what methods are applicable and can be successfully completed.

AQUIFER TEST METHODS

Constant-rate tests
Constant-head tests
Falling-head tests
Step tests
Estimations from specific capacity

The most accurate aquifer tests are constant-rate tests as long as the required parameters of the test are maintained, i.e., a constant rate throughout the test, water levels measured at frequent intervals, and the test being conducted for a long enough period of time for steady-state conditions to be developed in the water level drawdown versus time relationship. However, there are times when a constant-rate test can’t be completed, either because the well is not sized or equipped for the collection of the necessary data or the aquifer is simply too low-yielding to support a constant-rate test.

If you are dealing with small diameter wells, such as monitoring wells, where available pumping rates are limited due to the size of the casing, either constant-head or falling-head tests are typically used to estimate aquifer hydraulic characteristics. If the aquifer is estimated to have a high hydraulic conductivity, trying to pump a small diameter well is likely to result in little or no drawdown with time. Therefore, a constant-rate test can’t be accomplished. In this instance, a constant-head test would be the best method for estimating hydraulic conductivity and, from these data, the aquifer transmissivity. A constant-head test is just what it implies, typically a tank or some other small form of water storage has to be set up at the well and there needs to have the ability to refill the tank so a constant head is maintained during the test and the flow rate fluctuates, i.e., the amount of water being added per unit time to maintain the constant head.

Conversely, if the aquifer has a relatively low hydraulic conductivity the most appropriate means to conduct an aquifer test in a small-diameter well is a falling-head test, sometimes referred to as a slug test. This test is conducted by introducing a quantity of water to the well in a slug and then observing the decline in water level back to a static water level with time. An alternate method is to install an object in the well to displace water so the water level in the well increases and again, observing the decline in water level back to a static water level with time.

Step tests are conducted for a different reason, to assess the efficiency of the well and the aquifer, based on calculated well and aquifer losses. Typically, a step test is conducted with four pumping rates that are held constant during each step but are increased from step to step. During a step test water level data need to be collected at a frequent interval with a datalogger and downhole pressure transducer so a time-dependent water level relationship can be developed for each step. The water-level drawdown data from the step test can be analyzed to evaluate aquifer losses and well losses. These tests are important related to evaluating the condition of your well(s), as discussed below.

If you want to just get an idea of the aquifer transmissivity, a common means is to use the well specific capacity, which is the drawdown in the well per unit of discharge, typically in gallons per minute per foot of drawdown at a specific time in the test (as the specific capacity changes with time). A rule of thumb is that the transmissivity of the aquifer is approximately 1,500 times the specific capacity for an unconfined aquifer and 2,000 times the specific capacity for confined aquifers. While this rule of thumb can provide a rough estimate, it typically is not sufficient relative to understanding and managing your groundwater well system.

Next week LWS will discuss the importance of understanding and interpreting aquifer tests. In the coming weeks look for upcoming LWS blogs on the procedures for conducting aquifer testing to develop aquifer hydraulic characteristics, including the data collection requirements and interpretation of step test data. For more information about aquifer testing and what is required in your particular situation, please reach out to Lytle Water Solutions and we will be happy to provide you a proposal tailored to your needs, free of charge. We can be reached at 303-350-4090 or email any of our specialists below.

Bruce Lytle: bruce@lytlewater.com

Chris Fehn: chris@lytlewater.com

Anna Elgqvist: anna@lytlewater.com

Ben Bader: ben@lytlewater.com

Dan Rowe: dan@lytlewater.com