PANEL DISCUSSIONS--ABSTRACTS OF COMMENTS

Michael E. Bialousz

The northern Shenandoah Valley can be used as a model for the entire Great Valley of the Appalachians in terms of its geology, land-use patterns, and their relationship. The valley is largely underlain by a carbonate lithology, while sandstone and shale underlie adjacent ridges. Urban and agricultural types of land use have traditionally occurred in the valley, carbonate area where the terrain is flat. This trend continues into the 21st century, fostering a need for increased geologic mapping.

Urban (residential, commercial, industrial) development today is taking place outward from the traditional city and town centers into the countryside. Due to the cost of infrastructure, these areas do not always have access to city or town water supplies, leading to the development of more wells. More wells increase the strain on ground water and result in a lowering of the water table. In a carbonate area, this increases the speed of karst processes and leads to sinkhole development and a further likelihood of ground-water contamination. As more ground water is used, the speed of its movement underground is increased, which leads to a faster solution of the carbonate rock. As this occurs, sinkhole collapse or subsidence is more likely, and the movement of pollutants becomes more rapid and may occur in more directions. Eventually, the ground water comes to the surface as springs, which then contaminate streams, the other major source of water in the area. Therefore, geologic mapping is needed to prevent these types of problems before they happen. Accurate mapping of existing and potential karst features is essential in order to restrict development in sensitive areas, lessening the chance of ground-water contamination and sinkhole development. It has been noted in previous studies that many karst areas have no surface expression, making these areas especially vital to be mapped as accurately as possible.

In areas where city and town water supplies are present or in agricultural areas, geologic mapping is still essential. More urban-type development increases the chances of water contamination through stormwater runoff, underground storage tanks, and industrial pollutants. Agricultural runoff containing fertilizers, pesticides, and nutrients from animal waste also contaminates water. In areas where karst processes are active, this leads to ground-water and surface-water contamination to the immediate and surrounding areas. In carbonate areas, it is not always clear exactly what direction the ground water is traveling, further complicating the problem.

As counties and localities develop geographic information system (GIS) capabilities, it would be extremely helpful if the geologic mapping data could be made available in a digital format. In addition, the most accurate mapping methods should be used, including global positioning systems (GPS's), if possible. With this accomplished, counties and localities will be more prepared to plan their future in a sustainable fashion.

Ann M. Samford

Virginia Geotechnical Services is an engineering consulting firm offering specialized services in geotechnical engineering and offers soil- and ground-water contamination, development, and permitting services. Geologic maps provide the fundamental data we use to understand site conditions. We use geologic maps to develop our first conceptual model of subsurface conditions of the site, and our subsequent investigations are based on this model.

We rely most heavily on geologic mapping for the following general types of projects:

• Water resource evaluations
• Water quality issues at landfill and hazmat sites
• Landfill permitting studies
• Wetland delineation studies
• Foundation design studies
• Geologic hazard projects
• Planning and siting studies

We need complete 7.5-minute geologic mapping coverage of developing areas, as follows:

• Geologic data support cost-effective land planning and development. With a limited budget, we should focus on areas with significant development plans, areas that are developing in spite of significant geologic constraints, and areas that current demographic data indicate will develop in the next 5 to 10 years.
• Existing 7.5-minute geologic mapping should be updated periodically.

Expanded data that could make future geologic maps more useful include the following:

• Background water quality information (ppb)
• Continued (increased?) focus on geologic hazards
• Updated data delivery methods (soft copies, GIS, Internet)
• Move to standardized terminology (ASTM)
• Geotechnical description of geologic formations (not conjecture about bearing capacity)
• Geotechnical implications of geologic history
• Updates of existing geologic maps

Future geologic mapping efforts should be part of a long-range plan for supporting development.

Keith Van Ness

The Montgomery County, Md., Department of Environmental Protection (DEP) has developed a comprehensive Long Term Stream Monitoring Program as part of its National Pollutant Discharge Elimination System (NPDES) municipal stormwater-management permit. The goals of the Long Term Stream Monitoring Program are--

• To assess the full range of biological, chemical, and physical stream conditions and
• To protect, maintain, and restore high quality conditions in Montgomery County waters.

An understanding of underlying geology and geologic processes is fundamental to this program. Geologic information is the foundation from which to understand the natural variability observed in the streams in the Piedmont ecoregion. Montgomery County is almost entirely within this ecoregion. Underlying geology and geologic processes influence stream base-flow recharge, ground-water flows, stormwater runoff, stream morphology, and composition of streambed materials. The Culpeper basin streams tend to have "droughty" base flow and "flashy" stormwater runoff. Channel morphology is shaped by these flow types. Areas of Montgomery County with underlying phyllite geology have stable base-flow patterns with "flashy" stormwater runoff. Channel morphology tends to have a wide storm runoff channel and a smaller base-flow channel. Bed materials here are larger cobbles and small boulders. Streams in the eastern part of the county with underlying schist geology have base flows that are not as stable as streams in the phyllite areas. Streams in schist areas with low imperviousness levels do not appear to have a "flashy" response to storm events, perhaps as a result of the deep loamy soils present in this area.

The application of geologic information is basic for many DEP programs. For example, stream reference conditions have been established for the three subecoregions within Montgomery County to account for the natural variability due in part to the three main geologic areas described above. These reference conditions serve as a "yardstick" to assess the water quality of all other Montgomery County streams. An understanding of underlying geology and geologic processes is also used in the recently developed Countywide Stream Protection Strategy to develop countywide resource conditions. The strategy also describes each watershed's unique stream hydrology, morphology, and other characteristics--characteristics that are better understood with the application of geologic data. Finally, recently enacted Montgomery County legislation has created Special Protection Areas that utilize geologic data, as well as data on regional landscape, hydrology, and stream morphology to develop and implement numeric and narrative performance goals that provide an extra level of protection to maintain existing high or sensitive water quality in watersheds planned for medium to high development.

Our current need for specific geologic map information is for the data to be at a large enough scale to be directly applicable to the scale of coverage used in Montgomery County mapping applications. We also need geologic features or processes that directly influence stream hydrology and morphology to be clearly and accurately mapped on large-scale geologic maps. These geologic processes or features could be provided through a map overlay. We also need to have geologic processes data available in digital format for application in a GIS environment. Many times, our need for geologic information is immediate and often in response to time-sensitive issues. Digital GIS data make it possible to compile mapped information in a quick and accurate fashion.

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Last modified 15 April 1998
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