3 Groundwater Occurrence in Earth Materials

Section 2 described the zones of subsurface water, defining groundwater as occurring in the zone of saturation where all openings are filled with water. This section defines terms hydrogeologists and engineers use to characterize how groundwater is stored and released from porous media.

Porous Media

Water occurs in the opening or spaces between solid particles of sediment and within fractures of rocks (Figure 3). These spaces are referred to as pore spaces, pores, openings, and voids as may occur in soil or sediments; fractures in sedimentary, igneous and metamorphic rocks; and caverns and cavities that are found in deposits of evaporates and carbonates (e.g., karst). These openings provide space to store water and, when the spaces are interconnected, pathways to transmit water through the vadose and saturated zones.

Earth materials containing pores are referred to as porous media. Hydrogeologists and engineers characterize porous media by quantifying the volumes occupied by pore space and solids, the degree and magnitude of pore space interconnections, and the response of the media to changes in loads and pressures.

Figure showing water filling void space in subsurface materials
Figure 3 – Water fills the spaces of subsurface materials in: a) pores between particles of sediments (from Heath, 1983); b) fractures of rocks (after Gale, 1982); and c) caverns and cavities of carbonate rocks known as karst (from Heath, 1983). As indicated on a), the volume of a sample is defined by all space within its exterior surface, the volume of voids is the non-solid space within that volume.

Representative Sample Scales

Hydrogeologic investigations are completed at various scales. For example: the exchange of groundwater with a small wetland; dewatering of a construction site; evaluation of the transport of contaminants from an industrial site to a municipal well field; and, regional investigations examining the sustainability of groundwater resources in large groundwater systems of arid basins require defining hydrogeologic properties of earth materials at a range of scales.

Hydrogeologic properties of earth material can be described at the microscopic scale where the behavior of water would be determined based on measurements of: the size and number of individual pore diameters, roughness of pore surfaces, pore channel shapes and the degree of pore interconnection. However, such small-scale measurements are neither practical nor useful because most hydrogeologic investigations focus on conditions encompassing volumes of earth material that are much larger than the pores. Thus, hydrogeologists use a macroscopic approach to represent properties of subsurface media.

The macroscopic approach uses a sample volume of porous media large enough so as to represent the average effects of pore character, sizes, and interconnectedness. “Sufficiently large” can be determined by thinking about how to establish a sample volume that reflects the average value of a hydrogeologic characteristic for the scale of the investigation (e.g., the storage or transmission capacity). One approach is to start with an extremely small sample volume, determine the characteristic value, and then progressively increase the sample volume until a stable characteristic value occurs. Bear (1972) described the minimum macroscopic volume where property characteristics stabilize as the representative elementary volume, REV (Figure 4). Any sample volume that provides these stable characteristic values can be used as a REV. Once a REV is identified, average hydrogeologic properties are assigned to the centroid of the REV. The minimum volume that yields Bear’s (1972) REV can also be referred to as the Min REV. The upper boundary of REVs occurs when progressively larger sample volumes yield either a higher or lower characteristic value (Max REV).

Figure showing conceptualization of a representative elementary volume
Figure 4 – Conceptualizing determination of a representative elementary volume, REV, of a porous medium: a) If a porous medium is sampled using extremely small volumes (V1) and then progressively larger volumes (volumes from V1 to V5) the macroscopic representative hydrogeologic characteristic value can be identified. In this schematic, the sample volume V3 establishes the REV. Measurement of larger sample volumes would yield the same average properties as V3. The Min REV corresponds to the definition of the REV (Bear, 1972). The Max REV marks the transition volume above which the average characteristics change (shown here as becoming higher). b) A sample dominated by zones of fine and coarse sand. The smallest circles (V1) represent the results of sampling a single pore or a single solid grain. Other numbers and red circles represent progressively larger sample volumes. At the highest volume (V5) additional material with characteristics that are different from the original sample material are included within the sample. The REV is defined at V3 and characteristics remain constant until V4 where samples of larger volumes begin to incorporate the coarse sand that, in this example, have a higher characteristic value.

The REV is not a standard volume (e.g., it is not always 1 cm3, 1 m3, 10 m3, 100 m3 or 1 km3), instead, it varies depending on the hydrogeologic character of the material and the proposed objectives of an investigation. The REV representing the groundwater storage properties of the sand at the site of the core shown in Figure 5 is most likely a smaller volume than the entire core, unless the formation includes multiple finer layers or cross bedding. In that case, the entire core or a larger sample may be required.

When conducting hydrogeological investigations, the properties of porous materials are used in equations and models for quantitative analyses. In some settings, using multiple laboratory scale measurements and then averaging them and assigning them to the entire site to represent the average field scale conditions is appropriate. However, often laboratory scale determinations are not sufficient to represent more complex geological conditions encountered at the field scale. As a consequence, field-scale hydrogeologic testing methods are used to provide average properties for larger volumes of earth material, because it is too expensive and time consuming to collect small sample volumes at thousands of locations, determine laboratory based hydrogeologic characteristics for each sample and then average the data to generate characteristics of volumes that represent many cubic kilometers of the subsurface. Instead, field testing methods designed to generate property values that incorporate field scale complexities are applied. At the field scale the REV may be conceptualized as being represented by much larger volumes than those used to represent laboratory scale samples (Figure 5).

Figure showing conceptual examples of the scale of REV’s obtained from laboratory and field scale testing
Figure 5 – Conceptual examples of the scale of REV’s obtained from laboratory and field scale testing. a) A 10 cm diameter core of beach sand. The REV based on lab testing is most likely represented by a volume of a few cubic centimeters. If the sample contains layering and other structures, like cross bedding, the complete core may better represent the sand REV of the field site (Demcheck, 2010). b) A sand rich environment being dewatered (flexible piping serves as point dewatering wells). The more complex nature of the sand deposit would most likely have a REV on the order of cubic meters. Field scale testing of characteristics in a number of boreholes and by a multiple well pumping test would be appropriate for this investigation. (photo from EVO, 2020). c) A regional study of a 30 m-thick sand and gravel water-bearing material (blue shaded area) where complexities of the geologic setting are represented by the average characteristics of large volumes of material. REV’s may be defined using large-scale field-testing methods and assigning values of characteristics measured in each of three major layers to the individual layers (coarse-fine-coarse) or average the characteristics into a single value for all three layers (e.g., one square kilometer by 30 m thick) (modified from Weight, with permission, 2019).

Both field- and laboratory-scale characteristics are used to qualitatively and quantitatively describe natural groundwater flow conditions, the transport of contaminants in earth materials, the consequences of extracting or injecting groundwater, and the environmental links between surface water and groundwater systems.

This book describes the characteristics of porous media that hydrogeologists use to describe groundwater systems; compute groundwater discharge, flux and velocity; develop general groundwater flow equations; and determine groundwater flow directions. A number of other Groundwater Project books provide specific details addressing the theoretical foundation of methods needed to characterize hydrogeologic conditions at the field scale and approaches to using that information to investigate groundwater systems. This book defines terms associated with hydrogeologic characteristics and explains laboratory methodologies for their measurement.

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Hydrogeologic Properties of Earth Materials and Principles of Groundwater Flow Copyright © 2020 by The Authors. All Rights Reserved.