Aquifers and Geologic Deposits

The previous chapter illustrated the fundamentals of fluid dynamics and showed how they apply to groundwater, using idealized conditions including uniform porosity and permeability. Unfortunately, natural groundwater systems rarely closely resemble these conditions. We are faced with characterizing a very heterogeneous environment that consists of geological deposits with widely variable porosity and permeability. In order to explain groundwater observations and predict its behavior, some attempt is needed to understand this heterogeneity and frame it in a realistic manner.

Most groundwater is extracted from porous sediments or sedimentary rocks (Table 1). These materials form much of the outer skin of Earth’s continental crust and are either generated by continental erosion or are mineral precipitates, commonly created by marine life.

Table 1. Major freshwater aquifers around the world

Most sedimentary deposits form in the ocean near continental margins. This is where large rivers reach the sea and lose the energy that has conveyed the sediment load. As a result, suspended sediment settles on top of a large, accumulated, fan-shaped pile on the continental shelf (Figure 1). Commonly observed in these types of depositional environments is the general sorting of sediments by size. This results from differential transport energy of the water as it emerges into the open ocean. Consequently, deposits of coarser material will be abundant near shore while fine grained material will accumulate farther from the coast. This size sorting is preserved in the deposit and characterizes our naming schemes for different sedimentary rocks.

Figure 1. Google Earth image of the Amazon River terminus where it discharges into the Atlantic Ocean. Highlighted is t where a thick sedimentary wedge is built up over time from river sediments.

Over geologic time, and particularly in deltaic areas, this accumulated pile of sediment thickens and subsides because of its weight. Subsidence and the accumulated thickness of overlying material increases burial depth and triggers geochemical reactions in a complex process referred to as diagenesis. These reactions mobilize warm fluids that trigger dissolution and re-precipitation. Sedimentary grains will commonly become cemented, slowly converting the unconsolidated sediments to rock as the porosity and permeability are reduced (Figure 2).

Figure 2. Sandstone core from the Bakken Formation in the Williston Basin show how diagenesis and depressurization cause primary permeability as fractures rather than intergranular. From USGS Prof Paper 1653.

Over geologic time these now consolidated sediments can emerge as exposed bedrock in continental settings. As erosion removes the overlying material, the differential compressive stresses caused by burial are released, and fracturing commonly occurs. Because much of the original porosity was destroyed by compression and diagenesis, these secondary fractures can represent most of the porosity, so marine sedimentary rocks typically are not the most productive groundwater aquifers.

In contrast, carbonate rocks that originated as ancient reef complexes near continental margins form productive aquifers around the world. During the Paleozoic period (~540-250 million years ago), many continental margins were dominated by reefs, and their lithified remains are extensive. These reefs were built over tens of millions of years, and they became thick and subsided as they accumulated, as they also underwent diagenesis. This diagenesis largely increased overall density, and the carbonate material recrystallized into what is generally referred to as limestone.

These Paleozoic carbonate rocks have very low porosity as a result of the recrystallization. However, those that emerged near continental surfaces, due to tectonic processes and erosion, became susceptible to dissolution from recharging groundwater. Recharging groundwater is typically acidic, particularly in humid regions, and low pH promotes the dissolution of carbonate. Many ancient limestones develop intricate networks of solution channels that can become cavernous over time, forming what is known as karst terrain (Figure 3). As a result, overall porosity remains low, but permeability is among the highest of all aquifer types.

Figure 3. Photo of Maramec Spring discharge at 153 cubic feet per second on average from Paleozoic karst terrain encompassing recharge and groundwater flow in south-central Missouri. Phot credit ML Davisson.

Less abundant on Earth are rocks formed in subaerial continental environments. These rocks typically develop around topographic highs, forming proximal alluvial fans, or more distally are deposited as clastic wedges, or along large rivers or large lakes (Figure 4). One important example of this type is the Ogallala aquifer of the Great Plains, which represents a large wedge of clastic material that was eroded off the Rocky Mountains as they underwent great uplift during Tertiary time. All such deposits are generally referred to as fluvial sediments. Most groundwater in the world is produced from fluvial aquifers, because of their shallow depth and relatively high porosity and permeability that are typical of riverine sand and gravel layers or lenses. These rock types rarely are buried deeply, so cementation is weak and the porosity and permeability remain high.

Figure 4. Sand and gravel overbank deposit in Whittlesey Creek Wisconsin showing how high energy flow can transport and sort permeable bedload sediments that once buried become productive aquifers. Photo from USGS: http://wi.water.usgs.gov/surface-water/9ko41/index.html.

Much of the deglaciation of the North American continental at the end of the Pleistocene period left behind vast expanses of fluvial deposits. These include large sedimentary piles known as moraines that formed near the leading edge of the glacier. These moraines form productive aquifers in some areas of Canada and Scandinavia. Additional sources of productive groundwater in deposits formed on the continental surfaces are found in sand dunes formed along the coast during low sea-level stands, such as in the Atlantic Coast and Western Australia.

Volcanic deposits can also host valuable supplies of groundwater. These rocks have a wide range in composition and texture. Some of the most permeable volcanic rocks are laterally extensive basalt flows, as occur in Hawaii (Figure 5). Some eruptions form blocky “aa” flows that are highly permeable. Other flows develop interconnected fracture systems as the low viscosity lava cools and shrinks. In volcanic piles, sequentially deposited flows may be separated by permeable horizons of soil or tuff that readily convey groundwater. Finally, under certain condition lava tubes, a type of subterranean cave, are formed in thick lava flows as they erupt and cool; these tubes are well developed in parts of the Pacific Northwest of the USA, and can serve as excellent groundwater conduits.

Figure 5. Large and contiguous basalt flow make up a large part of the Hawaiian Islands, forming high permeability groundwater flow paths that supplies much of the water resources. Pictured are basalt cliffs at Makapuu, Oahu. Photo from USGS: http://hi.water.usgs.gov/studies/GWRP/hydrogeology.html.

Hydrogeology Framework of Geologic Deposits

Table 2 is a typical geological log of drill cuttings that emerge during the drilling of a groundwater well. This drill hole is located within a mile of a large river in the Sacramento Valley and is completed in young, unconsolidated fluvial sediments.

Table 2. Descriptive geological driller’s well log within one mile of the Sacramento River in the southern Sacramento Valley, CA

The log provides generalized descriptions of the geological material encountered as drilling proceeded. This is the type of qualitative information a hydrogeologist often encounters and must assess to characterize groundwater aquifers and their capacity to supply water. In order to better quantify the framework, a hydrogeologist will often simplify the geological log and define zones as either gravel, sand, or clay, or even more simply, as permeable (gravel and sand) or non-permeable zones (silt and clay).

To further enhance understanding of the water yielding capacity, geological logs can be gathered of nearby wells. If the different layers in Table 2 are contiguous over long distance, then these nearby logs should have similar descriptions with depth, although various geologists often come to differing conclusions. Nevertheless, simply using the permeable/non-permeable designation is an easy way to correlate geological logs. Sometimes the type of permeable zones can be more consistent between them so that gravel and sand or silt and clay can be distinguished. Consequently, a series of nearby logs provides a means to construct a hydrogeological cross-section or even a 3-D model of groundwater-yielding layers (Figure 6).

Figure 6. Hydrogeological cross-section constructed east-west from the Sacramento River one west using descriptive geological driller’s well logs simplified and lumped together as permeable and non-permeable aquifer zones.

Ultimately, a hydrogeologist will need to develop a more quantitative assessment of how much water can be drawn from subsurface permeable layers, and at what rate, in order to prevent a sustained depletion in the groundwater level. The rate and amount that can be withdrawn without sustained negative impact is often referred to as the “safe yield”. Safe yield assumes that there is no measurable negative impact, but relevant observations of the impact are commonly not made, such as the seepage of surface water into the groundwater, or decreases in the flow of up-gradient springs.

Groundwater pump tests are currently the main tools that are employed to understand groundwater yielding capacity from permeable layers. These tests induce groundwater level drawdown during an interval of pumping, then measure the recovery of the water level when pumping ceases. The time and shape of the water level curve is related to conductive properties of the permeable media.

All pump test methods are based on a single one “developed” by Theis in 1935. This transient model differs from its equilibrium predecessor, the Theim equation, and is:

The underlying mathematical solution to this general problem was developed more than 200 years ago by Euler. Definitions and analogies to heat flow are made in Table 3.

Table 3. Comparison of variables for the Theis equation to heat flow equation

In the present application, T is the transmissivity which represents the hydraulic conductivity from Darcy’s equation times the aquifer thickness. The hydraulic conductivity has been measured for many porous materials; ranges are provided in Table 4. Note the wide range in measured or computed values which illustrates the uncertainty that results when applying this parameter to an aquifer setting in the absence of any other observational data. Pump tests are designed to reduce this uncertainty and improve the understanding of the hydraulic properties of a particular aquifer.

Table 4. Ranges in hydraulic conductivity for typical geological material

As for heat flow, the geometry and conditions of a groundwater model are critical factors that affect its accuracy and utility. For groundwater pump tests, the most important criterion is the homogeneity of the permeable medium, which affects the isotropic flow properties. This condition is approximated in uniform sand deposits, but departures are greater in most aquifer settings. Accordingly, the Theis equation has been modified numerous times over the decades to adapt to non-ideal conditions that may include pseudo-equilibrium water level drawdowns, no flow boundaries, aquifer confinement, and leaking confined layer. All modified methods have strict, empirically-based rules to follow that are beyond the scope of this book. Those wishing to employ such methods can consult references provided at the end of this chapter. Nevertheless, derivations of transmissivity and specific yield provide a basis to assign hydraulic properties to an aquifer layer of interest for which a pump test was conducted. These parameters are typically used by hydrogeologists to build idealized models of groundwater flow, that are used to predict the impacts associated with future groundwater production scenarios.