Virginia Eastern Shore Groundwater Resources

Science Center Objects

Informed management of groundwater resources for the Eastern Shore of Virginia depends on the availability of detailed and up-to-date scientific information. The USGS and the Virginia Department of Environmental Quality are conducting a long-term cooperative study to enhance the understanding of groundwater resources in the sole-source aquifer system beneath Accomack and Northampton counties through mapping of subsurface hydrogeology, monitoring of groundwater quality and water levels, and developing a groundwater-flow model to update the model currently in use.

Background

The Virginia Eastern Shore is entirely dependent on the underlying sole-source aquifer system, which provides at least 15 million gallons per day of groundwater withdrawn in Accomack and Northampton counties for public supply for about 45,000 residents and additional seasonal visitors, extensive agricultural irrigation, and a variety of industrial uses.

Fresh groundwater occurs in approximately the top few hundred feet of sediments on the Eastern Shore in several aquifers composed of relatively permeable sediments separated by less permeable fine-grained confining units. The Yorktown-Eastover aquifer system consists of upper, middle, and lower confined aquifers overlain by correspondingly named confining units and underlain by the Saint Marys confining unit. The Yorktown-Eastover aquifers are formed from medium- to coarse-grained sand and shells, while the confining units are formed from fine-grained sand, silt, and clay. An overlying, unconfined surficial aquifer is composed of sand and gravel with smaller amounts of cobbles and boulders. In addition, sediments that fill three buried paleochannels cutting across the Yorktown-Eastover system now have been explicitly delineated by McFarland and Beach (2019) as distinct hydrogeologic units. Two paleochannel aquifers are composed of medium- to coarse-grained fluvial sand and gravel, and an intervening paleochannel confining unit is composed of fine-grained estuarine sand, silt, clay, and organic material.

The confined Eastern Shore aquifers have long been important sources of fresh water because the confining units have isolated the aquifers from possible contaminants introduced into the surficial system, thereby protecting the groundwater quality. However, the confining units also limit recharge to the confined aquifers from modern precipitation and infiltration, limiting the quantity of this valuable, high-quality confined groundwater.

Groundwater withdrawals over approximately the past 100 years have lowered water levels, altered hydraulic gradients, and created concern about saltwater intrusion into fresh-water aquifers. Previous characterizations of groundwater conditions that are relied upon for management of groundwater resources have been limited by a lack of hydrogeologic information, particularly data on buried paleochannels that are critical to safeguarding the groundwater supply.

Concerns

Several years of research by USGS and Virginia DEQ have resulted in a substantially enhanced understanding of the hydrogeologic framework of the Virginia Eastern Shore (McFarland and Beach, 2019), including much more detailed information about the configuration of the buried paleochannels. These paleochannels are thought to be important hydrogeologic features because they intersect the confined Yorktown-Eastover aquifer system and likely affect groundwater flow through that system. However, these important framework revisions are not represented in the model (Sanford and others, 2009) currently in use.

It has been recognized that the rate of groundwater withdrawals from the confined Eastern Shore aquifers may be approaching the limit for long-term, sustainable use. As a result, groundwater withdrawals currently are monitored and regulated by the Virginia Department of Environmental Quality. As part of the planning and regulatory process, the unconfined surficial aquifer has been identified as a likely additional source of water, because it appears that only a small fraction of water that enters the ground recharges the confined aquifer system. However, groundwater flow through the unconfined system was not the focus of previous groundwater modeling efforts, and additional model detail is needed to better represent the surficial aquifer.

For the Virginia Eastern Shore, groundwater from the surficial aquifer is the primary source of fresh water in streams, lakes, and wetlands, and it is thought to be important in maintaining the saline balance of estuaries. However, the potential ecological effects of additional groundwater pumping from the surficial aquifer for human use are not well understood. Groundwater in the surficial aquifer also is vulnerable to contamination from human activities at the land surface, such as the application of agricultural fertilizers, but little is known about specific areas of vulnerability or the effects of pumping on groundwater quality in the surficial aquifer.

Approach

The approach for this investigation consists of four components: mapping and revision of the hydrogeolgic framework; logging and mapping to better understand and define the subsurface saltwater interface; ongoing monitoring of groundwater levels in all aquifers; and construction of an updated groundwater flow model as a tool for assessment of groundwater resources.

Borehole-geophysical logging techniques will be used to provide additional information on the hydrogeology as well as aquifer salinity as part of the saltwater-interface mapping component of the investigation. Surface geophysics also will be applied to enhance the understanding of hydrogeology, particularly in proximity to the buried paleochannels.

 

Physiographic provinces of Virginia and the location of the Virginia Eastern Shore study area.

Physiographic provinces of Virginia

From Sanford and others (2009)

 

Schematic cross-section of groundwater flow for the Eastern Shore of Virginia.

Schematic diagram of ground-water flow of the Eastern Shore of Virginia

From Sanford and others (2009)