The hydrogeologic framework of the shallow aquifer system at Virginia Beach was revised to provide a better understanding of the distribution of fresh ground water, its potential use, and its susceptibility to contamination. The revised conceptual framework is based primarily on analyses of continuous cores and downhole geophysical logs collected at 7 sites to depths of approximately 200 ft.
The shallow aquifer system at Virginia Beach is composed of the Columbia aquifer, the Yorktown confining unit, and the Yorktown-East-over aquifer. The shallow aquifer system is separated from deeper units by the continuous St. Marys confining unit.
The Columbia aquifer is defined as the predominantly sandy surficial deposits above the Yorktown confining unit. The Yorktown confining unit is composed of a series of very fine sandy to silty clay units of various colors at or near the top of the Yorktown Formation. The Yorktown confining unit varies in thickness and in composition, but on a regional scale is a leaky confining unit. The Yorktown-Eastover aquifer is defined as the predominantly sandy deposits of the Yorktown Formation and the upper part of the Eastover Formation above the confining clays of the St. Marys Formation. The limited areal extent of highly permeable deposits containing freshwater in the Yorktown-Eastover aquifer precludes the installation of highly productive freshwater wells over most of the city. Some deposits of biofragmental sand or shell hashes in the Yorktown-Eastover aquifer can support high-capacity wells.
A water sample was collected from each of 10 wells installed at 5 of the 7 core sites to determine the basic chemistry of the aquifer system. One shallow well and one deep well was installed at each site. Concentrations of chloride were higher in the water from the deeper well at each site. Concentrations of dissolved iron in all of the water samples were higher than the U.S. Environmental Protection Agency Secondary Drinking Water Regulations. Concentrations of manganese and chloride were higher than the Secondary Drinking Water Regulations in samples from some wells.
In the humid climate of Virginia Beach, the periodic recharge of freshwater through the sand units of the shallow aquifer system occurs often enough to create a dynamic equilibrium whereby freshwater flows continually down and away from the center of the ridges to mix with and sweep brackish water and saltwater back toward the tidal rivers, bays, salt marshes, and the Atlantic Ocean.
The aquifers and confining units of the
shallow aquifer system at Virginia Beach are heterogeneous, discontinuous, and
without exact marker beds, which makes correlations in the study area difficult.
Investigations using well cuttings, spot cores, or split-spoon samples with
geophysical logs are not as definitive as continuous cores for determining or
correlating hydrogeologic units. Future investigations of the shallow aquifer
system would benefit by collecting continuous cores.
TABLE OF CONTENTS:
|Purpose and scope|
|Description of the study area|
|Methods of field investigation|
|Collection of cores|
|Construction of observation wells|
|Collection of borehole geophysical logs|
|Collection and analyses of ground-water samples|
|Analyses of cores, geophysical logs, and ground-water samples|
|Old Pungo Ferry Road|
|Blackwater Neighborhood Park|
|Bellwood Estates Neighborhood Park|
|Bayside High School|
|Lynnhaven Pump Station|
|Creeds Elementary School|
|Conceptual hydrogeologic framework|
|Yorktown confining unit|
|General conception of ground-water flow|
|Future investigations of the shallow aquifer system|
|Summary and conclusions|
|1-3. Maps showing:|
|1. Location and selected features of Virginia Beach, Virginia (study area)|
|2. Land use of Virginia Beach|
|3. Well, core hole, and geoprobe sites, Virginia Beach|
|4. Photograph showing core samples from Old Pungo Ferry Road site, Virginia Beach|
|5-11. Diagrams showing core lithology and geophysical logs at:|
|5. Oceana II|
|6. Old Pungo Ferry Road|
|7. Blackwater Neighborhood Park|
|8. Bellwood Estates Neighborhood Park|
|9. Bayside High School|
|10. Lynnhaven Pump Station|
|11. Creeds Elementary School|
|12. Map showing surficial geology and geomorphic features at Virginia Beach|
|13. Sketch showing conceptual model of the shallow aquifer system at Virginia Beach|
|1. Land surface altitudes, well-screen depths, core-hole depths, and depths to water at core-hole sites in Virginia Beach, Virginia|
|2. Land surface altitudes and geoprobe depths|
|3. Concentrations of dissolved inorganic constituents in water from observation wells|
|4. Age and geologic and hydrogeologic units of the shallow aquifer system|
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|For more information, contact
U.S. Geological Survey
1730 E. Parham Road
Richmond, VA 23228
|To order printed copies, contact|
U.S. Geological Survey
Box 25286, Federal Center
Denver, CO 80225
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