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Water Quality in Carbonate Aquifers in the United States

The Report Summary of Findings Study Design

Study Design

Where are carbonate aquifers and how were they selected for study?

Locations of carbonate aquifers were determined from the USGS Principal Aquifer Map (see inset map); however, that map shows only carbonate aquifers that are designated as principal aquifers. Other carbonate aquifers can be determined by locations of areas underlain by carbonate bedrock. These areas may also be locally important aquifers, and have the same characteristics of the carbonate principal aquifers. In some cases, the principal aquifer may be predominantly non-carbonate, but includes areas of carbonate rock. The Cambrian-Ordovician aquifer is an example of this. The principal aquifer is designated as a Sandstone aquifer, but the Prairie du Chien aquifer is a carbonate aquifer within that system. The illustration shown is based upon figure 1 of SIR 2008-5240.

figure 1a

The 13 aquifers in this study include two aquifers (Silurian-Devonian and Upper carbonate) that were sampled as a single unit thus the report refers to 12 carbonate aquifers. The Prairie du Chien is the only aquifer in the study not designated as a Principal aquifer. In addition, 'subcrop areas' of the aquifer that is, areas where the aquifer may be overlain by other aquifers or geologic units - were included.

Locations of NAWQA study areas and sites sampled are shown above. Only five principal aquifers designated as carbonate-rock aquifers or sandstone and carbonate-rock aquifers did not have samples in this study. A fairly large area of the country is underlain by carbonate rocks that are not designated as principal aquifers. In some cases, the apparent differences between the principal aquifers and areas of carbonate geology are because of the resolution of the data sets, and in other areas, the units are located in areas that are not designated as principal aquifers or the carbonate-rocks are a minor part of an aquifer. Both the principal aquifer extent and the carbonate geology are shown to indicate the locations of potential sampling areas that would have been considered for the study. In a few cases, a small number of samples (less than 5) were collected from carbonate aquifers, but if these were not part of a larger sampling effort they were excluded from the study. The illustration shown is based upon figure 1 of SIR 2008-5240.

figure 1 inset

This figure illustrates the three-dimensional nature of these systems. The areas selected for the study included any well networks sampled by the NAWQA program that were located in a carbonate-rock principal aquifer or a sandstone and carbonate-rock aquifer if the lithology indicated that the well was completed in the carbonate part of the aquifer. Also, networks were included if wells were completed in a subcrop area of the aquifer.

What are the characteristics of the aquifers included in the study?

The characteristics of the aquifers are given in the report section on aquifer characteristics. Links to additional information about these aquifers are available at:http://water.usgs.gov/ogw/karst/karstaquifers.html

How many networks and wells were sampled?

NAWQA's sampling design targets groups of wells with similar characteristics called 'networks'. Networks can be designed to assess the aquifer overall (major aquifer studies), a specific land use within an aquifer (agricultural or urban land-use studies), background conditions (reference studies), water supply (source-water assessments), and springs (spring studies). Flow path studies and studies of transport of anthropogenic contaminants to public supply wells typically focus on a small area and were not included. One exception was a network of wells designated as a flow path study that represented a larger area and was included in the study. Wells with quality assurance problems were eliminated from the data set. In all 52 networks (some reference networks have only 1-3 wells) were included in the study. The overall data set included 1,042 wells and springs; however, the number of sites available for analysis of a given constituent was variable. Characteristics of wells and sites are available in table 3-5 of the report.

What is a Karst aquifer?

In addition to being an important source for water supply, carbonate aquifers are included in a group of aquifers termed 'karst aquifers'. Karst is a term used to describe the numerous geomorphic features that form in highly soluble rocks such as limestone or dolomite (carbonate aquifers) as well as rocks with evaporite minerals (such as gypsum and halite) and pseudokarst (basalt). Thus all aquifers in carbonate bedrock are considered karst aquifers. These highly soluble types of bedrock typically have solutionally enlarged fractures. Sinkholes often form in this terrain, providing a direct source of recharge to the aquifer. Also, conduit systems often discharge to the surface via large magnitude springs. These features of karst combine to make karst aquifers unique, in both production of water (wells completed in conduits can produce large volumes of water) and quality of water (direct recharge through sinkholes and transport through conduits bypass the filtration mechanisms typically associated with ground water). The combination of flow in the rock matrix, fractures, and conduits make the speed and direction of movement of contaminants more difficult to understand and predict than in other types of aquifers. More details about karst and karst aquifers are available at: http://water.usgs.gov/ogw/karst/

figure 4

Sinkholes at the land surface provide a direct pathway for water to enter an aquifer. Fractures in carbonate bedrock can become enlarged by dissolution of the rock by water as it travels through the aquifer.

(Photo: Pennsylvania Geological Survey)



figure 4

This is a horizontal fracture, enlarged by dissolution in a well in carbonate bedrock.

(Photo: Randy Conger, USGS)



figure 4

This photograph illustrates a vertical conduit through a six inch well bore. Conduits in the bedrock can transmit large amounts of water rapidly through the aquifer.

(Photo: Randy Conger, USGS)



figure 4

In karst aquifers, the conduit systems often discharge to the surface through large magnitude springs.

(Photo: Pennsylvania Geological Survey)

 

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