Water-quality units: Chemical concentrations, water temperature, and specific conductance are given in metric units. Chemical concentration is expressed in milligrams per liter (mg/L). Specific conductance is expressed in microsiemens per centimeter at 25 degrees Celsius (J..tS/cm). This unit is equivalent to micromhos per centimeter at 25 degrees Celsius (umho/cm), formerly used by the US Geological Survey.

No abstract

Transgression into adjacent uplands is an important global response of coastal wetlands to accelerated rates of sea level rise. “Ghost forests” mark a signature characteristic of marsh transgression on the landscape, as changes in tidal inundation and salinity cause bordering upland tree mortality, increase light availability, and the emergence of tidal marsh species due to reduced competition. To investigate these mechanisms of the marsh migration process, we conducted a field experiment to simulate a natural disturbance event (e.g., storm-induced flooding) by inducing the death of established trees (coastal loblolly pine, Pinus taeda) at the marsh-upland forest ecotone. After this simulated disturbance in 2014, we monitored changes in vegetation along an elevation gradient in control and treatment areas to determine if disturbance can lead to an ecosystem shift from forested upland to wetland vegetation. Light availability initially increased in the disturbed area, leading to an increase in biodiversity of vegetation with early successional grass and shrub species. However, over the course of this 5-year experiment, there was no increase in inundation in the disturbed areas relative to the control and pine trees recolonized becoming the dominant plant cover in the disturbed study areas. Thus, in the 5 years since the disturbance, there has been no overall shift in species composition toward more hydrophytic vegetation that would be indicative of marsh transgression with the removal of trees. These findings suggest that disturbance is necessary but not sufficient alone for transgression to occur. Unless hydrological characteristics suppress tree re-growth within a period of several years following disturbance, the regenerating trees will shade and outcompete any migrating wetland vegetation species. Our results suggest that complex interactions between disturbance, biotic resistance, and slope help determine the potential for marsh transgression.

No abstract

Ground water is an important water resource in Virginia. During 1990, an estimated 1,500 Mgal/d (million gallons per day) of freshwater was used in Virginia, in addition to 3,210 Mgal/d for hydroelectric power. An estimated 443 Mgal/d, or 30 percent of the total freshwater used, was withdrawn from ground-water sources. Approximately 1,060 Mgal/d was withdrawn from surface-water sources such as streams and lakes. The U.S. Geological Survey cooperates with State and local government agencies in Virginia, to describe hydrogeologic conditions that control the availability of ground water, and to define the potential for ground-water development. GROUND-WATER USE DURING 1990Ground-water users who withdraw 300,000 gallons per month (10,000 gallons per day) or more are required by the Virginia Department of Environmental Quality to report amounts of ground water that are withdrawn and the locations and uses of withdrawals. In addition, restrictions on groundwater pumpage are imposed on parts of southeastern Virginia because of water shortages. During 1990, a Statewide groundwater withdrawal rate of 197 Mgal/d was reported. The water was used primarily for public-water supplies and industrial and commercial purposes. Ground-water withdrawals reported