Occurrence of Ground Water

Ground water reservoir

The ground water reservoir is the zone of saturated inter-granular pores and rock fractures below the land surface.   Ground water divides along the watershed boundary form the sides of the reservoir.  Its top is defined by the water table.  Its bottom lies at a depth where bedrock fractures are essentially closed due to elevated temperatures and pressures associated with deep burial or the depth at which saline ground water is encountered.  Saline water is denser than freshwater and acts as a density barrier to groundwater flow.  Well records for the watershed (see below) suggest that saline water is not a significant problem within at least 700 feet of the surface.  It is likely that the base of the ground water flow system in the watershed is at a depth where secondary porosity becomes negligible.  A reasonable estimate of the depth to the base of the flow system is about 800 feet.  However, the frequency of occurrence and aperture of fractures probably decrease rapidly below about 400 feet.

Storage of ground water

The volume of water stored in the groundwater reservoir depends upon the storage capacity of the saturated bedrock and unconsolidated deposits in the watershed.  Average specific yields  (the ratio of water that will drain from a volume of rock or soil under the force of gravity to the volume of rock or soil) for sandy till and medium sandstone of 0.02 and 0.12 (Morris and Johnson, 1967), respectively, probably approximate the storage values for the dominant bedrock and unconsolidated deposits in the watershed.  Using an assumed depth of 400 feet for the base of the flow system and an average depth to water in bedrock wells within the aquifer of 100 feet (Table 1), the approximate average ‘effective’ saturated thickness of bedrock is 300 feet.   Recent mapping by Braun (2002a-f; 2001b) showed that approximately 33% of the watershed is covered by unconsolidated deposits greater than 30 feet thick with an average thickness of  about 50 feet.  Assuming an average water level of 30 feet below the ground surface in unconsolidated deposits suggests an average saturated thickness of 20 feet.   The volume of water in storage per unit area can be calculated using the estimated specific yields and average saturated thickness; total storage is storage per unit area times the area of the watershed (412 mi²):
 

Thus, the average amount of water in storage for the entire watershed is about 3 trillion gallons (36.1 feet).

Based on the water budget analysis presented above, approximately 110 billion gallons of water (1.3 feet) pass through the reservoir each year.   Under steadystate conditions, this suggests an average residence time of 28 years for a water molecule within the reservoir.  Of course, individual particle paths and, thus, travel times will be highly variable, probably ranging from hours or days to centuries or millennia.  The fractured character of shallow bedrock, large volume of unconsolidated deposits and highly dissected topography of the watershed (Figures 16 and 17) suggest that much of the groundwater probably flows along shallow, short paths from the point of recharge to the point of discharge (one of the many streams in the watershed).  In the terminology of Toth (1963), this would be referred to as a local flow system, in which water moves from a recharge area to the next adjacent discharge area.   The flow would be quick relative to the regional flow system, in which water moves from the recharge area farthest from the main valley to the discharge area in the main valley, following a long, deep path through less fractured rock.  Intermediate flow systems lie between these two ends of a continuum of possible flow paths (Figure 18).
 

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