GROUNDWATER

  A.  INTRODUCTION

        Groundwater is an important resource, supplying drinking water
        for more than 50% of the U.S. population.  It is especially vital to
        people in rural areas, who typically have no public water supply.

        In terms of chronic risk, drinking groundwater exposes humans 
        to a variety of dissolved substances, some of which can have an
        effect on human health.  Our objectives here are to understand:
        (1) how groundwater occurs and moves, and (2) the effects that
        geology has on groundwater quality. 

B.  OCCURRENCE

       1.  SUBSURFACE VOID SPACE         

            Water cannot occur in the subsurface unless there is open (or
            void) space within the rock or sediment that make up the earth.
            The term porosity is used to describe void space, and this can 
            term be defined as the percentage of rock or sediment volume
            that consists of void space.  Three basic types of porosity are:

             a.  Intergranular:  typically between 5 and 50%

             b.  Rock Fractures:  typically less than 5%

             c.  Solution Cavities:  typically between 3 and 30%   

        2.  SUBSURFACE ZONES

             a.  Zone of Aeration

             b. Zone of Saturation
 

C.  MOVEMENT

       1.  DIRECTION OF FLOW

 

        2.  RATES OF FLOW

            The principal factor controlling the movement of fluids through
            the earth is called permeability.   Permeability depends on the
            size of the void spaces through which fluids move.

            a.  Highly permeable materials:  gravel, sand, sandstone,
                 and rocks with solution cavities (e.g., limestone)

                 Typical flow rates are between 0.5 and 3 feet/day
 

            b.  Lowly permeable materials:  clay, shale, granite and
                 any rock or sediment with low porosity.

                 Typical flow rates are between 3 and 50 feet/year
 

       3.  DISCHARGE POINTS

            a. Streams
 

            b. Lakes and Wetlands
 

            c. Springs

 D.  AQUIFERS

       1.  DEFINITION         

            An aquifer is a saturated geologic unit that is permeable enough
            to yield "significant" quantities of water to a well.
            
            a. Municipal Wells:  require highly permeable geologic units

            b. Homeowner Wells:  can utilize less permeable geologic units
 

       
       2.  TYPES OF AQUIFERS         

            a. Unconfined (Water Table)
 

            b. Confined (Artesian)

       3.  OVERPUMPING         

            a. Effect on Surface Water
 

            b. Aquifer Depletion

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