3.8 Karst Aquifer Flow Characteristics Compared with Aquifers Composed of Other Rock Types

The main rock types for aquifers include sand and gravel; sandstone; sandstone and carbonate; carbonate; igneous; and metamorphic rocks. Sand and gravel and sandstone aquifers are considered granular porous media. Shallow aquifers composed of sand and gravel are considered unconsolidated sediments, whereas sandstone is considered an indurated sedimentary rock. Igneous and metamorphic rocks tend to form fractured rock aquifers. In igneous and metamorphic rock aquifers, the rock itself is not very permeable so water flows through cracks in the rock, but there are exceptions such as lava tubes in some basalts, which may behave as conduits. The uniqueness of karst aquifers is the solubility of the aquifer rock such that the aquifer continuously evolves over geologic time as a result surface and groundwater flowing through them and dissolving the rock.

Table 1 provides a list of flow characteristics for the three basic types of aquifers. Characteristics unique to karst aquifers are extreme heterogeneity, multilevel porosity and permeability, conduit-dominated groundwater flow, point-source recharge, and large temporal variability in flow and chemistry due to rapid recharge and to mixing of water from multiple recharge sources. Table 2 is a compilation of hydraulic conductivity data for aquifer and confining unit rock types or specific aquifer systems within a rock type. The enormous heterogeneity of karst aquifers is revealed by hydraulic conductivity, K, spanning eight orders of magnitude for some of the karst aquifers, whereas unconsolidated sediments have a range on the order of four orders of magnitude and fractured rock aquifers have roughly a six order of magnitude range in hydraulic conductivity. Additionally, for many of the karst aquifers the hydraulic conductivity of a conduit or preferential flow layer would be underestimated as aquifer tests estimate transmissivity and to estimate hydraulic conductivity, transmissivity is divided by either the length of the open interval of the well or total aquifer thickness which are larger than the actual thickness of the flow zone. The need for field investigations to account for the rapid flow through conduits resulting in large discharges from complex conduit networks cannot be overstated.

Table 1  Comparison of various hydrogeologic properties of granular porous media, fractured rock, and karst aquifers (Modified from ASTM, 2002).

Aquifer Characteristics Aquifer Type
Granular Porous Media Fractured Rock Karst
Effective porosity Mostly primary, through most of the intergranular pore space of the sediment matrix Mostly secondary, through joints, fractures, and bedding plane partings-not the rock matrix Mostly tertiary (secondary porosity modified by dissolution within large conduits) with primary if rock matrix permeable as with reefs or shell mixtures, through pores, bedding planes, and fractures
Isotropy Generally isotropic in a formation Often anisotropic related to fracture direction related to structure Frequently anisotropic as fractures form along joints related to calcite mineral or in formation units oriented along bedding planes in units that dissolve more readily than adjacent units
Homogeneity Generally homogeneous in a formation Often heterogeneous Extremely heterogeneous
Flow Generally slow and laminar; exception are large clean gravels with large pore diameters Slow and laminar when fracture apertures are less than 1 centimeter, but can be rapid under laminar and turbulent conditions if fracture aperture over 2 centimeters Often rapid flow under laminar and turbulent conditions in large pipe like conduits greater that 0.5 m wide
Storage Unconsolidated sediment has large specific storage. Indurated sedimentary rocks have smaller specific storage. Generally small specific storage as rocks has little elasticity and storage mainly related to the porosity and elasticity of water. Generally small specific storage as rocks has little elasticity and storage mainly related to the porosity and elasticity of water.
Temporal head and chemistry variations Generally, less variations than the other aquifer types Head variations can be large owing to small storage properties, but chemistry changes generally moderate variations Both head and chemistry can have moderate to large temporal variations.

Table 2  Hydraulic conductivity of rock types and karst aquifers (Modified from Halford and Kuniansky, 2002).

Rock Hydraulic Conductivity Ranges, values in meters per day
Aquifer Material Extreme
Unconsolidated Sedimentary Rock
Gravel 30 90 900 900 1,5
Sand and Gravel Mixes 0.3 9 90 90 1
Coarse Sand 10 20 90 90 1
Medium Sand 0.3 6 20 60 1,5
Fine Sand 0.01 0.9 6 6 1,5
Gulf Coast Aquifer Systems, USA 0.6 9 60 200 2
Stream Terrace Deposit, Texas, USA 0.003 0.3 30 90 3
Fine sand and silt, Florida, USA 0.003 0.03 9 10 4
Silt, Loess 9×10−5 0.0003 0.03 2 5
Till 9×10−8 0.0009 0.09 0.2 1,5
Clay soils (surface) 0.003 0.003 0.3 0.3 1
Clay 3×10−7 3×10−6 3×10−5 0.0003 5,7
Indurated Sedimentary Rock
Fine-Grained Sandstone 3×10−5 0.0003 0.3 2 1,6
Medium-Grained Sandstone 0.0003 0.3 3 20 6,9
Siltstone 3×10−7 3×10−6 0.001 0.01 6
Claystone 9×10−10 3×10−7 3×10−6 9×10−6 6,7,10
Anhydrite 3×10−8 3×10−8 0.002 0.002 5
Metamorphic or Volcanic Rock
Shale 3×10−9 3×10−8 3×10−5 0.3 7
Permeable Basalt 0.03 0.3 30 2000 5
Fractured Igneous/Metamorphic Rock 0.0003 0.02 3 30 1
Weathered Granite 0.03 0.3 3 6 6
Weathered Gabbro 0.03 0.03 0.3 0.3 6
Basalt 0.0003048 0.009 0.03 0.03 5
Unfractured Igneous/Metamorphic Rock 0.0304785 3×10−9 2×10−5 2×10−5 1,5
Carbonate Rocks
Unweathered Marine Clay 6×10−8 6×10−8 0.0002 0.0002 5
Karst 0.0002 3 300 10,000 4,5,8,11
Reef Limestone 0.09 3 300 300 5
Limestone, Dolomite 9×10−5 0.001 0.03 0.6 5
Upper/Unspecified Floridan Aquifer, USA 0.002 3 200 10000 11
Middle/Lower Floridan Aquifer, USA 0.0002 0.01 40 8,000 11
References: 1) Bouwer, 1978 (order of magnitude in m/d); 2) Prudic, 1991; 3) Sonia A. Jones, USGS, Written communication, 1998; 4) Kinnaman, 2002, Slug Test Results1998-2001, USGS, Orlando, Florida; 5) Domenico and Schwartz, 1990; 6) Morris and Johnson, 1967; 7) Wolff, 1982; 8) Reese and Cunningham, 2000; 9) Kuniansky and Hamrick, 1998; 10) Neuzil, 1994.

This book illustrates the underlying causes of complex flow fields in karst aquifers. The Minnesota Department of Agriculture developed an excellent animation of flow in karst systems to help the public understand how water moves through karst aquifers (Figure 32).

animation portraying groundwater movement in a karst landscape

Figure 32  This animation, portraying groundwater movement in a karst landscape, was created by the Minnesota Department of Agriculture as part of a series that highlights the geology and complex movement of groundwater in southeast Minnesota. The animation “brings to life” many of the concepts presented in this book and enhances one’s ability to conceptualize the hydrology that is typical of many karst aquifers.


Introduction to Karst Aquifers Copyright © 2022 by Eve L. Kuniansky, Charles J. Taylor, and Frederick Paillet. All Rights Reserved.