8.5 Examples of Flow Systems

Three groundwater potentiometric maps are presented in this section to illustrate basic principles of constructing groundwater flow lines based on contours of head and interpreting the factors influencing the head configurations.

High Plains Aquifer in Wyoming, USA

A potentiometric map of a portion of the High Plains Aquifer in Wyoming, USA is shown as Figure 85. Groundwater is obtained from the unconsolidated and consolidated sediments of the Ogallala Formation, Arikaree Formation and the White River Group that are underlain by lower permeability Cretaceous sedimentary rocks (Bartos and Hallberg, 2011). These formations outcrop east of the Laramie Mountains in the western portion of the region shown. Formations slope to the east forming the major regional High Plains Aquifer found in portions of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming.

Potentiometric map of a portion of the High Plains Aquifer located in Laramie County, Wyoming, USA.
Figure 85 – Potentiometric map of a portion of the High Plains Aquifer located in Laramie County, Wyoming, USA. Equipotential lines (thin blue-green lines) represent conditions from March to June 2009. Recharge areas are shaded in light blue. Local discharge areas associated with streams are shaded in light green. Flow lines (thick blue arrows) were constructed assuming isotropic and homogeneous conditions. The approximate location of cross section A-A’ shown in Figure 86 is indicated by the dashed line. This map covers only a small portion of the cross section shown in Figure 86 which extends far to the northeast (modified from Bartos and Hallberg, 2011).

The formations are considered hydraulically connected and water levels from wells penetrating these formations between March and June 2009 were used to create the generalized equipotential map. Flow lines were constructed assuming that flow behaves as if hydraulic conductivity is isotropic and homogeneous. Areas of recharge are inferred from the equipotential surface and are labeled and shaded light blue. These are areas of the highest heads and flow lines diverge with water flowing in multiple directions away from these elevated outcrop areas. Local flow systems discharge to local streams (labeled and shaded light green) where flow lines converge. The general regional groundwater flow is to the east (bottom of the map). It is useful to peruse the spacing of equipotential lines and the shape of flow lines and consider them in light of the information expressed in Figure 82, Figure 83, and Figure 84. A schematic geologic section along the dashed line A-A’ of Figure 85 is shown as Figure 86.

Schematic southwest to northeast cross section of the High Plains Aquifer
Figure 86 – Schematic southwest to northeast cross section of the High Plains Aquifer along A-A’ of Figure 85 extending over 200 km. The portion of the section represented on the map shown in Figure 85 is indicated (modified by Bartos and Hallberg, 2011; from Denson and Bergebdahl, 1961).

Memphis Sand Aquifer, Memphis Tennessee, USA

A contoured map of head data from the Memphis Sand aquifer representing generalized conditions in September-November 2010 is provided as Figure 87 (Kingsbury, 2018). The aquifer is confined and separated from the water table alluvial system by clays and fine-grained sediments. The aquifer supplies 100 % of the water to the city of Memphis, TN, USA. In the area shown, aquifer thickness varies from about 200 m in the east to over 250 m in the west. Prior to pumping, groundwater flow was from the east towards the Mississippi River on the western border of the map. The potentiometric surface and flow lines show the effect of long-term pumping on the confined aquifer flow system. Pumping has lowered the heads overall and formed closed lows in local areas around the well fields (shaded areas and hatched contours). The pumping captures groundwater flow coming from the east and draws water from the Mississippi River and western portions of the aquifer. Groundwater flow lines were drawn by assuming isotropic and homogeneous conditions. The flow lines converge and the gradient increases as they approach the pumping centers. Since 2010, water levels have risen in some areas in response to a slight decrease in water demand (Kingsbury, 2018). This can be observed by comparing the 1995 potentiometric map presented in Figure 65 with Figure 87. A cross section of the basin along the line shown in the southwest quandrant of Figure 87 is shown in Figure 88.

Map showing potentiometric surfacefor the Memphis Sand Aquifer
Figure 87 – Potentiometric surface based on September to November 2010 water level data for the Memphis Sand Aquifer located beneath Memphis, Tennessee, USA. Equipotential lines are solid green and dashed where inferred. Hatched contours indicate depressed areas of the potentiometric surface associated with pumping centers that have operated for an extended period-of-time. The curved black solid line in the eastern portion of the area represents the location of the transition from unconfined conditions in the east to confined conditions in the west. Flow lines are blue arrows. Pumping has captured groundwater flowing through the area as flow lines converge at well locations (modified from Kingsbury, 2018).
A schematic cross section of the aquifer and confining beds in the Memphis area
Figure 88 – A schematic cross section of the aquifer and confining beds in the Memphis area. The vertical dashed lines on the surface of the cross section represent where it crosses state boarders (AR Arkansas, TN Tennessee, and MS Mississippi). The approximate position of the schematic NW-SE cross section location is shown in Figure 87 as a dot-dashed line (modified from Taylor and Alley, 2001; Kingsbury, 1996 and 2018).

Unconfined Aquifer in East Helena, Montana, USA

Groundwater conditions in the unconfined aquifer system of East Helena, Montana, USA are shown in Figure 89. The southern portion of the area is overlain by a lead smelter that operated between 1888 and 2001. The aquifer is composed of sand and blends of sand and gravel that slope from higher ground in the south toward the Helena Valley in the north. The location of the cross section is shown as a dotted yellow line in Figure 89. The near-surface aquifer is contaminated with arsenic and selenium that originate from the smelter site. The groundwater flow system has transported plumes of contaminants northwest of the smelter site. The dissolved selenium plume originates at the smelter site and extends about 5 km to the northwest in the direction of groundwater flow. A stream (light blue line) flowing to the north passes through the site. It is losing stream water to the shallow groundwater system. This infiltrating stream water effects the groundwater flow paths forcing them to the northwest. The flow tubes originating from the smelter site narrow to the north as the aquifer thickens and stream water recharges the system.

Map showing unconfined groundwater conditions in the vicinity a closed lead smelter in East Helena, Montana, USA.
Figure 89 – Unconfined groundwater conditions in the vicinity a closed lead smelter (red circle) East Helena, Montana, USA. The region is underlain by a shallow sand and sand and gravel aquifer as shown in the cross section that is located on the map with a dashed yellow line. Groundwater flow is from the southeast to northwest though the industrial site. Equipotential lines (thin blue lines) originally labeled in feet, were converted to meters and represent water levels in June 2016. Flow lines (blue arrows) were constructed assuming isotropic and homogeneous conditions. A stream (light blue line) flowing to the north is losing water to the aquifer and this recharge is forcing groundwater flow from the smelter site to the northwest (with permission, modified from Hydrometrics, Inc., 2017).

Summary of Flow System Examples

In each of these examples head data were collected and contoured to create a potentiometric or water table map that represents horizontal flow conditions. Flow lines were constructed assuming conditions were locally isotropic and homogeneous. Evaluation of flow lines provides an opportunity to develop an understanding of site conditions. When field data indicating aquifer thickness and the distribution of hydraulic conductivity are available, then coupling observations of changing hydraulic gradients and flow line spacing with the principles stated in Darcy’s law can reveal additional information about the groundwater flow system. Building and analyzing flow nets allows groundwater scientists to investigate how boundary conditions influence groundwater flow, providing the foundation for developing a conceptual model of the groundwater system. Groundwater flow analysis encompasses all aspects of the physical and hydraulic conditions as described in this book. This section is intended as an overview to introduce the basic concepts applied during interpretation of groundwater flow.

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