Background

In 2015, approximately 283 million people were served by public drinking water supplies across the United States, including more than 6 million in Tennessee (Dieter and others, 2018). Ensuring safe, clean drinking water not only is important for issues of human health and safety but also is a requirement of the Safe Drinking Water Act (SDWA) (42 U.S.C. 300(f)). The SDWA was established by the U.S. Environmental Protection Agency (EPA) in 1974 with amendments added in 1986 and 1996 that initiated programs to protect public-supply drinking water sources. States are required by the SDWA to establish Source Water Assessment Programs (SWAPs) that evaluate the vulnerability of public drinking water sources to contaminants that pose a threat to public health. The Tennessee Department of Environment and Conservation (TDEC) is the agency in the State of Tennessee with primacy that regulates the SWAP according to the SDWA requirements (TDEC, 2003). Tennessee is located in the Midsouth region of the contiguous United States and is made up of 95 counties divided into three separate regions called Grand Divisions (fig. 1). The Tennessee SWAP is intended to help communities in the State protect public-supply drinking water from contamination.

In 2021, TDEC and the U.S. Geological Survey (USGS) worked in cooperation to develop a geographic information system (GIS)-based methodology that systematically assesses the vulnerability of public-supply drinking water to potential contaminants consistent with the standards set forth in the Tennessee SWAP. The decision to update, improve, and automate the SWAP approach was made because the SWAP reports prepared by the public water systems that were developed using the initial assessment areas completed by TDEC in 2003 are inconsistent in format across different systems and are human resource intensive to review for inconsistencies. With an automated GIS approach, TDEC can consistently delineate source water assessment zones and evaluate source susceptibility on the basis of information such as the proximity of contaminant sources, land-use activities, geologic information, and additional environmental spatial data.

The newly developed methodology—referred to as “TN-SWAPyT”—is built as an Esri ArcGIS Pro toolbox (Esri, 2022) and uses the Python programming language (Van Rossum and Drake, 2022) and the LaTeX document preparation system (LaTeX Project Team, 2020) to generate summary SWAP reports for public water systems across the State. The TN-SWAPyT methodology is designed to delineate assessment zones around public-supply sources, collate and map the potential contaminant sources in assessment zones, utilize digital datasets to rate the susceptibility of the sources to potential contaminants, and generate consistent reports that summarize the findings. A major benefit of the TN-SWAPyT methodology is to provide TDEC with consistent reports that can be used as a starting point for assessing public supplies. Communities and public water systems can then build upon these reports by using local knowledge and site-specific information.

Drinking Water Supplies in Tennessee

In 2015, approximately 91 percent of the population in Tennessee relied on public supplies as their primary source of drinking water, with the remaining 9 percent relying on domestic self-supplied groundwater wells (fig. 2) (Dieter and others, 2018). Approximately 850 million gallons per day of water was used for public supply in Tennessee in 2015 (fig. 3), which provides drinking water in addition to delivering water used for commercial, thermoelectric, irrigation, and industrial purposes (Dieter and others, 2018). The 2022 water-use data were obtained from TDEC, which regulates self-supplied industrial and public water system withdrawals and usage in Tennessee. The data are not available publicly owing to restrictions on releasing information pertaining to public-supply water sources and systems protected under the Tennessee Office of Homeland Security Critical Infrastructure Protection Program.³ As of June 2022, public-supply water was provided by 643 active public water systems across Tennessee that withdrew water for public use from 1,378 individual water sources. One public water system may include one or multiple individual water sources; for example, multiple wells, multiple surface-water intakes, or systems that use wells and surface-water intakes.

In Tennessee, the types of sources for public supply include both groundwater (wells or springs) and surface water (surface-water intakes). Approximately 57 percent of the population is served by surface-water sources and the remaining 43 percent by groundwater sources, including groundwater from self-supplied domestic wells and from public supplies (fig. 2) (Dieter and others, 2018). The use of surface water versus groundwater varies across Tennessee, which is split into three Grand Divisions (eastern, middle, and western; fig. 1) that are formally defined by law and generally follow the geology of Tennessee (fig. 4) (Tennessee Code § 4–1–201; Tennessee Code, 2021). The predominant type of public-supply water source for people in western Tennessee is groundwater, and for middle Tennessee and eastern Tennessee it is surface water (figs. 5 and 6). This variation in public-supply water source is broadly controlled by the geology of Tennessee (fig. 4) and the resulting availability of water resources (Fenneman and Johnson, 1946; Bradley and Hollyday, 1985).

Western Tennessee is dominated by unconsolidated and productive sandy aquifers of the Mississippi Alluvial Plain and East Gulf Coastal Plain physiographic sections (fig. 4) (Lloyd and Lyke, 1995). These sections include the regional aquifers of the Mississippi River Valley alluvial aquifer, the Mississippi embayment Tertiary sand aquifer, and the Cretaceous sand aquifer. Groundwater use is relatively high in western Tennessee compared to middle and eastern Tennessee (figs. 5B, C and 6A, B) (Dieter and others, 2018). Much of this groundwater use occurs in Shelby County, which includes the city of Memphis, in southwestern Tennessee (figs. 1 and 5B).

Middle Tennessee includes consolidated rock and karst aquifers—which are characterized by fractures, fissures, and sinkholes in carbonate rocks—of the Highland Rim and Nashville Basin physiographic sections (fig. 4) (Lloyd and Lyke, 1995). These sections include the Mississippian carbonate and Ordovician carbonate regional aquifers, although much of the area provides limited groundwater supplies to domestic wells, springs, and small public water systems. Groundwater use from both wells and springs does occur (fig. 5B), but a greater proportion of water for public supply is sourced from surface water in middle Tennessee than in western and eastern Tennessee (fig. 6B) (Dieter and others, 2018). Many of the public-supply surface-water withdrawals in middle Tennessee occur in Davidson County, which includes the city of Nashville and surrounding communities (figs. 1 and 5A).

Eastern Tennessee includes consolidated rock aquifers of the Cumberland Plateau, Tennessee Valley and Ridge, Sequatchie Valley, and Southern Blue Ridge physiographic sections (fig. 4), which are dominated by complex geology caused by folding and faulting (Lloyd and Lyke, 1995). The regional aquifers in these sections include the Pennsylvanian sandstone aquifer of the Cumberland Plateau, the Cambrian-Ordovician carbonate aquifer of the Tennessee Valley and Ridge and Sequatchie Valley, and the crystalline rock aquifer of the Southern Blue Ridge (fig. 4). In some areas, the complex geologic structure results in small well yields sufficient for only domestic supply; therefore, the proportion of self-supplied groundwater is greater in eastern Tennessee than in western and middle Tennessee (fig. 6A). Both surface-water and groundwater sources provide public-supply water in eastern Tennessee (fig. 5A, B), but most (87 percent) of the public supply is sourced from surface water (fig. 6B), which supplies much of the water to the cities of Chattanooga in Hamilton County and Knoxville in Knox County (figs. 1 and 5A) (Dieter and others, 2018).

Assessment Zones

An assessment zone is the area around a public-supply water source that is inventoried for contaminant sources and evaluated for susceptibility to potential contamination (TDEC, 2003). These areas are delineated by different methods in the Tennessee SWAP depending on whether the source is surface water, groundwater, or groundwater under the influence of surface water.

Source Water Assessment Zones for Surface Water

Two zones—zone 1 and zone 2—are designated for each surface-water intake (fig. 7) (TDEC, 2003). Zone 1 is designated the “Critical Source Water Protection Zone” and encompasses an area 5 miles upstream and one-half mile downstream from the surface-water intake. Zone 1 includes tributaries to the stream with the surface-water intake and a survey corridor of 1,000 feet on either side of the stream and tributaries. Zone 2 is designated the “Source Water Management Zone” and includes the remainder of the area upstream, tributaries to the upstream area, and the 1,000-foot survey corridor within the watershed. Zone 2 is further divided into two zones, zone 2A and zone 2B. Zone 2A is defined as the area from 5 to 15 miles upstream from the intake, and zone 2B is defined as the area from 15 miles upstream from the intake to the upper end of the watershed. A few surface-water systems use multiple intakes along a water body and are classified as a consolidated assessment zone. For surface-water sources with very small watersheds, wherein the contributing stream may not reach 5 miles upstream, the streams and tributaries and the 1,000-foot survey corridor within the watershed are included in assessment zone 1. Figure 8A shows the cumulative (zones 1 and 2) delineated assessment zones originally created by TDEC in 2003 for surface-water intakes.

Source Water Assessment Zones for Groundwater

Contaminant transport is different in groundwater than in surface water and is controlled by hydrogeologic factors (such as aquifer transmissivity and anisotropy) and withdrawal rates (Lloyd and Lyke, 1995). In areas where the delineation between surface water and groundwater is less distinct, such as karst aquifers or other aquifers wherein groundwater is under the influence of surface water, the effects of withdrawal rates on contaminant transport are less clear. However, quantifying hydrogeologic properties can provide detailed information for developing appropriately protective assessment zones for groundwater sources.

aquifer transmissivity The capacity of an aquifer to transmit water is characterized by the rate at which water of prevailing kinematic viscosity is transmitted through a unit width of aquifer under a unit hydraulic gradient. The transmissivity of an aquifer is equal to the hydraulic conductivity of the aquifer multiplied by the saturated thickness of the aquifer (Lohman, 1972; Heath, 1983).

aquifer anisotropy In anisotropic aquifers, the hydraulic conductivity is different for flow in the horizontal and vertical directions, as opposed to isotropic aquifers, wherein hydraulic conductivity is the same in all directions (Heath, 1983).

In Tennessee, assessment zones are delineated for both types of groundwater sources, wells and springs, by using the location of the wellhead, which is the point near the surface where groundwater exits from the well or spring and is collected for use. Source water assessment zones, often referred to as “wellhead assessment zones,” are established for two zones of protection—zone 1, the “Critical Source Water Protection Zone,” and zone 2, the “Source Water Management Zone,” also referred to as the “Wellhead Management Zone” (fig. 9). Zone 1 is proximal to the source (well or spring), is intended to protect the immediate area from spills and other point sources of contaminants, and is a fixed radius around the wellhead or spring source. The radius, ranging from 250 to 750 feet, is dependent on the number of service connections and the average daily withdrawal rate but may be further refined by public water systems to reflect the hydrogeologic conditions of each individual source. Zone 2 is intended to provide for long-term management of the well, wellfield, or spring considering the wide variety of geologic conditions across Tennessee. The size and shape of zone 2 are developed by the public water system or the State through a variety of hydrogeologic assessment approaches including dye tracing, analysis of topography, and flow modeling to determine the recharge area of the well or spring. Additional work to delineate zone 2 is performed for the public water systems using groundwater under the influence of surface water. Figure 8B shows cumulative assessment zones delineated for public water systems reliant on groundwater.

TN-SWAPyT Methodology

The TN-SWAPyT methodology allows TDEC to consistently and automatically generate summary assessment reports for the 643 active public water systems across Tennessee. To protect the security of the public-supply source data, the TN-SWAPyT toolbox and inputs are required by the State to remain proprietary and will not be publicly available. The methodology fits into the TDEC SWAP cycle (fig. 10) and follows a multistep process to meet the TDEC SWAP requirements by leveraging a custom Esri ArcGIS Pro Toolbox (Esri, 2022) created using Python open-source scripting language (Van Rossum and Drake, 2022), which allows TDEC to follow logical steps in a GIS. The methodology also leverages Python with LaTeX (LaTeX Project Team, 2020) to query information from various sources and prepare a report that is consistent for all public water systems in the State. The methodology is intended to be robust, reproducible, and capable of handling errors from missing data or incorrect information in input datasets. The methodology can also incorporate data from adjoining States as needed for assessment zones that cross State lines (for example, fig. 8). Additionally, the input datasets can be updated as new information is made available. Key components of the development of the TN-SWAPyT methodology include designing, creating, and analyzing the following databases or datasets:

The TN-SWAPyT methodology supports stakeholders, water managers, and TDEC because it increases the ability to (1) delineate and inventory source water assessment zones through consistent and reproducible methods, (2) identify potential contaminants relevant to public water systems across the State, and (3) understand the influence of environmental characteristics on the vulnerability of public drinking water sources to potential contamination. TN-SWAPyT provides the public with a consistent methodology for how requirements of the SWAP are implemented to create public-supply source water assessments in Tennessee.

Acknowledgments

The initial work by Rebecca K. Ransom for this information product was done while serving as a student contractor with the U.S. Geological Survey.