Hydrogeologic Sensitivity Assessment
by Kathleen Carson
Groundwater Technician at CRWA
Once the groundwater rule becomes effective, "hydrogeologic sensitivity assessment" is a term that will become quite familiar to most rural water systems. EPA describes the hydrogeologic sensitivity assessment as a simple, low-burden, cost effective approach to allow States to screen for drinking water supply wells that are sensitive to fecal contamination. EPA believes that a well obtaining water from a karst, fractured bedrock, or gravel hydrogeologic setting is sensitive to fecal contamination unless the well is protected by a hydrogeologic barrier.
Karst aquifers are aquifers formed in soluble materials such as limestone, dolomite, marble, and bedded gypsum. Some of the areas in Colorado having karst formations include the Roaring Fork River Corridor in Garfield, Eagle, and Pitkin counties, outcrops along the Front Range, and locations in Cheyenne County.
Fractured bedrock aquifers are generally comprised of igneous or metamorphic rock. Their size varies from tens to hundreds of square miles in area. Groundwater may travel as quickly as hundreds of feet per day in fractured bedrock, comparable to travel times in karst aquifers. Fractured bedrock aquifers are often present in mountainous regions.
Gravel aquifers are deposits of unconsolidated gravel, cobbles and boulders. Gravel aquifers are typically produced by catastrophic floods, physical weathering by glaciers, flashfloods at the periphery of mountainous terrain or at fault-basin boundaries. Gravel aquifers are generally not alluvial aquifers, which usually have high proportions of sand mixed with gravel. The sand in alluvial aquifers provides for filtering of microorganisms.
States may choose to use any of several means to evaluate wells to determine if they are located in one of the three sensitive hydrogeologic settings:
- Hydrogeologic data from published or unpublished maps, reports, and well logs.
- Regional and site specific information obtained from sources such as United States Geological Service (USGS), US Department of Agriculture's Natural Resource Conservation Service, USGS Earth Resources Observation System Data Center, USGS National Karst map, State geological survey, and universities.
- Information gathered through Source Water Assessment Programs and Wellhead Protection Programs.
- EPA-approved vulnerability assessment conducted for the purpose of obtaining waivers under the Phase II and Phase V.
- EPA map showing the distribution of fractured bedrock aquifers (USEPA 1991 c: "Delineation of Wellhead Protection Areas in Fractured Rocks"
If a drinking water well is located in a hydrogeologically sensitive area, the second part of the assessment is to determine the presence of a hydrogeologic barrier. EPA defines a hydrogeologic barrier as physical, chemical, and biological factors, which, alone or in combination, prevent the movement of viable pathogens from a contaminant source to a public water supply well. EPA, however, is leaving the specifc procedure for characterizing hydrogeologic barriers to each State. The most obvious example of a hydrogeologic barrier is an unbreached confining layer, such as a clay or shale layer, which prevents water movements from an overlying aquifer under low pressure to an underlying aquifer under high pressure.
EPA estimates that approximately 15 percent of un disinfected ground water systems nationally will be determined to be hydrogeologically sensitive. Reference: Federal Register, Wednesday, May 10,2000, pp. 30222-30226