Bedrock Wells: Are They Safe?

As with many things the answer is not straightforward, but U.S. Geological Survey (USGS) scientists are developing ways to answer such questions. Allen Shapiro, a USGS scientist funded by the U.S. Geological Survey's Toxic Substances Hydrology Program, presented an invited talk entitled "Sustaining Water Supplies in Fractured Bedrock Aquifers" in a USGS-sponsored special session on the "Sustainability of Ground Water" at the American Institute of Hydrology 2001 Annual Meeting, October 14-17, 2001, Minneapolis, Minnesota. Dr. Shapiro's talk discussed the challenges that are faced by water resource managers in sustaining drinking-water sources in fractured bedrock environments and the information they need to determine quantity and quality of water, and the risk of contamination in bedrock settings. The information presented at the meeting will help community water suppliers answer the question "Are bedrock wells safe?"

Abstract of the Presentation

Sustaining Water Supplies in Fractured Bedrock Aquifers, by Allen M. Shapiro, U.S. Geological Survey, Reston, VA

Many communities rely solely on bedrock aquifers for water supply or are turning to deeper formations because of pervasive contamination in near-surface, unconsolidated aquifers. Evaluating bedrock aquifers for water supply, however, is not only a question of developing a source of water of sufficient yield; it also requires the evaluation of the quality of and potential risks to that source of water.

A common misperception about water supplied from bedrock aquifers is that drilling deeper taps a new and unused source of water. Bedrock aquifers often are characterized by zones of high permeability with low porosity and low storativity. Sustaining sufficient well yields in such formations requires drawing water from either a huge volume of the bedrock, from fractures in direct contact with surface water bodies, or from formations with much higher porosity (such as overlying unconsolidated geologic materials). The ability to sustain water yield from a bedrock aquifer depends as much on the surficial geology as on the occurrence of highly permeable fractures or fracture zones in the bedrock.

Characterizing the water quality and potential migration of contaminants in fractured bedrock aquifers proves to be more problematic than characterizing the yield of bedrock wells. The travel time of dissolved constituents in ground water is particularly sensitive to spatially heterogeneous hydraulic properties. To accurately characterize chemical migration, one must delineate the connectivity of permeable fractures or fracture zones. Moreover, an estimate of the effective porosity is required to estimate the travel time of dissolved constituents. The effective porosity is the volume of void space in the formation that can transmit fluid per unit volume of the formation. Estimates of effective porosity from core samples, measurements of the mechanical fracture aperture, and hydraulic tests are unreliable. In general, the only means of inferring reliable estimates of effective porosity of fractured rock is through in situ tracer testing, in which a known mass or concentration of a traced fluid is injected into the formation, and its concentration is measured at one or more down-gradient locations.

Also of concern in the migration of dissolved constituents in fractured bedrock aquifers is the capacity of the primary porosity of the rock to retain chemical constituents through diffusion from fluid flowing in fractures. When concentration gradients are conducive, chemical constituents will diffuse out of the rock matrix and into the mobile fluid in fractures. Consequently, the rock has the capacity to retain and slowly release dissolved constituents over extended periods of time, which may adversely affect the water quality in bedrock aquifers.

Special Session Description

Sustainability is an important issue facing water managers throughout the world. A balance is needed between water supply and demand in the face of industrial growth and development. The objective of this special session, "Sustainability of Ground Water," was to highlight many different facets surrounding the issue of sustainability of ground-water resources including: social and economic implications of groundwater mining, sustaining water supplies in fractured bedrock aquifers, sustainability versus aquifer compressibility, effects of expanded development on water quality, sustainability relating to local water planning, and measuring environmental progress of sustainability.

Convener

• Geoffrey N. Delin, U.S. Geological Survey,

Meeting Information

• American Institute of Hydrology 2001 Annual Meeting, October 14-17, 2001, Minneapolis, Minnesota, Agenda and Abstract Information
  

Related Headlines

• What Controls the Migration of Chlorinated Solvents in Fractured Rock?

Related Meetings

• USGS co-sponsors Fractured-Rock Aquifers 2002, Denver, Colorado, March 13-15, 2002.
  
• USGS sponsored a special session on "Borehole Geophysical Analysis Techniques for the Definition of Aquifer Properties" at the Geologic Society of America's 2001 Annual Meeting, Boston, Massachusetts, November 5-8, 2001
  
• USGS Science on Fractured Rock Featured at the "Fractured Rock 2001" Conference in Toronto, Ontario, Canada on March 26-28, 2001
  
• USGS Technology Transferred at FRACDAY Workshop -- Technologies and field methods used to characterize fractured rock were demonstrated during the workshop.

More Information on Fractured Rock Hydrology

• Ground Water Flow and Transport in Fractured Rock
  
  • Geochemical and Microbiological Processes that Affect Migration and Natural Attenuation of Chlorinated Solvents in Fractured Sedimentary Rock -- Naval Air Warfare Center (NAWC) Research Site, West Trenton, NJ
    
  • Multidisciplinary Characterization of Fluid Flow and Solute Transport in Fractured Rock -- Mirror Lake, New Hampshire
• Branch of Geophysics Research with the Toxic Substances Hydrology Program - Contains information on the geophysical investigation of fractured rock
  
• Transport Phenomena in Fractured Rock
  
• Hydrology of Fractured Rocks
  
• Borehole Geophysics as Applied to Geohydrology
  
• Using Seismic Tomography to Characterize Fractured Bedrock