ABSTRACT

A newly developed regional perspective of the hydrogeology of the Virginia Coastal Plain incorporates updated information on groundwater quality in the area. Local-scale groundwater-quality information is provided by a comprehensive dataset compiled from multiple Federal and State agency databases. Groundwater-sample chemical-constituent values and related data are presented in tables, summaries, location maps, and discussions of data quality and limitations.

Spatial trends in groundwater quality and related processes at the regional scale are determined from interpretive analyses of the sample data. Major ions that dominate the chemical composition of groundwater in the deep Piney Point, Aquia, and Potomac aquifers evolve eastward and with depth from (1) “hard” water, dominated by calcium and magnesium cations and bicarbonate and carbonate anions, to (2) "soft" water, dominated by sodium and potassium cations and bicarbonate and carbonate anions, and lastly to (3) "salty" water, dominated by sodium and potassium cations and chloride anions. Chemical weathering of subsurface sediments is followed by ion exchange by clay and glauconite, and subsequently by mixing with seawater along the saltwater-transition zone. The chemical composition of groundwater in the shallower surficial and Yorktown-Eastover aquifers, and in basement bedrock along the Fall Zone, is more variable as a result of short flow paths between closely located recharge and discharge areas and possibly some solutes originating from human sources.

The saltwater-transition zone is generally broad and landward-dipping, based on groundwater chloride concentrations that increase eastward and with depth. The configuration is convoluted across the Chesapeake Bay impact crater, however, where it is warped and mounded along zones having vertically inverted chloride concentrations that decrease with depth. Fresh groundwater has flushed seawater from subsurface sediments preferentially around the impact crater as a result of broad contrasts between sediment permeabilities. Paths of differential flushing are also focused along the inverted zones, which follow stratigraphic and structural trends southeastward into North Carolina and northeastward beneath the chloride mound across the outer impact crater. Brine within the inner impact crater has probably remained unflushed. Regional movement of the saltwater-transition zone takes place over geologic time scales. Localized movement has been induced by groundwater withdrawal, mostly along shallow parts of the saltwater-transition zone. Short-term episodic withdrawals result in repeated cycles of upconing and downconing of saltwater, which are superimposed on longer-term lateral saltwater intrusion. Effective monitoring for saltwater intrusion needs to address multiple and complexly distributed areas of potential intrusion that vary over time.

A broad belt of large groundwater fluoride concentrations underlies the city of Suffolk, and thins and tapers northward. Fluoride in groundwater probably originates by desorbtion from phosphatic sedimentary material. The high fluoride belt possibly was formed by initial adsorbtion of fluoride onto sediment oxyhydroxides, followed by desorbtion along the leading edge of the advancing saltwater-transition zone.

Large groundwater iron and manganese concentrations are most common to the west along the Fall Zone, across part of the saltwater-transition zone and eastward, and within shallow groundwater far to the east. Iron and manganese initially produced by mineral dissolution along the Fall Zone are adsorbed eastward and with depth by clay and glauconite, and subsequently desorbed along the leading edge of the advancing saltwater-transition zone. Iron and manganese in shallow groundwater far to the east are produced by reaction of sediment organic matter with oxyhydroxides.

Large groundwater nitrate and ammonium concentrations are mostly limited to shallow depths. Most nitrate and ammonium is recycled near the land surface, but some can also be produced by decomposition of sediment organic matter, by ion exchange along the leading edge of the advancing saltwater-transition zone, and possibly from site-specific human sources.

Groundwater is a heavily used water-supply resource throughout the study area, and its suitability for various uses is largely determined by its chemical quality. Whereas soft water in deep aquifers generally is suitable for a wide variety of uses, hard water can require ion-exchange treatment. Salty water in deep aquifers in the eastern part of the study area is undergoing increasingly widespread production for drinking-water supplies. Treatment operations use desalination and blending. Saltwater intrusion can potentially be mitigated by redirecting locations and operation of wells producing shallow freshwater and by conjunctive freshwater/saltwater groundwater development close to and within the saltwater-transition zone. Groundwater with large fluoride concentrations is blended with water from different sources in the city of Suffolk. An array of methods can treat large groundwater iron and manganese concentrations, and associated hydrogen sulfide concentrations, depending on the level of treatment needed. Large groundwater nitrate concentrations can potentially be mitigated by various treatment methods or management of groundwater withdrawals.

First posted August 18, 2010