PUB00012

• Year: 1999
• Title: Sustainability of Ground-Water Resources: U.S. Geological Survey Circular 1186
• CWCB Section: Interstate & Federal
• Author: William M. Alley, Thomas E. Reilly, O. Lehn Franke
• Description: Illustration of the hydrologic, geologic, and ecological concepts to assure wise and sustainable use of ground-water resources
• Publications - Doc Type: Historical

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. The areal extent of ground-water-flow systems
varies from a few square miles or less to
tens of thousands of square miles. The
length of ground-water-flow paths ranges
from a few feet to tens, and sometimes
hundreds, of miles. A deep ground-water-
flow system with long flow paths between

areas of recharge and discharge may be over-
lain by, and in hydraulic connection with,
several shallow, more local, flow systems
(Figure 6). Thus, the definition of a ground-
water-flow system is to some extent subjec-
tive and depends in part on the scale of a
study.

Unsaturated zone

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EXPLANATION

D: High hydraullc-conductlvlty aquifer CD local ground-water subsystem

~ @ Subreg~al ground-water subsystem
~ low hydraullc-conductJvity confining unit
@ Regional ground-water subsystem
D Very low hydraulic-conductivlty bedrock

.......- Direction of ground-water flow

Figure 6. A regional ground-water-flow system that comprises subsystems at different scales and
a complex hydrogeologic framework. (Modified from Sun, 1986.)

Significant features of this depiction of part of a regional ground-water-flow system include (1) local
ground-water subsystems in the upper water-table aquifer that discharge to the nearest surface-water
bodies (lakes or streams) and are separated by ground-water divides beneath topographically high areas;
(2) a subregional ground-water subsystem in the water-table aquifer in which flow paths originating at
the water table do not discharge into the nearest surface-water body but into a nwre distant one; and
(3) a deep, regional ground-waterjlow subsystem that lies beneath the water-table subsystems and is
hydraulically connected to them. The hydrogeologic framework of the flow system exhibits a complicated
spatial arrangement of high hydraulic-conductivity aquifer units and low hydraulic-conductivity
confining units. The horizontal scale of the figure could range from tens to hundreds of miles.

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