Central Valley Aquifer, Chapters A and B - MAE Class Websites
Central Valley Aquifer, Chapters A and B - MAE Class Websites
Central Valley Aquifer, Chapters A and B - MAE Class Websites
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When long-term pumping<br />
lowers ground water levels<br />
<strong>and</strong> stresses on the aquitards<br />
beyond the preconsolidationstress<br />
thresholds, the<br />
aquitards compact <strong>and</strong> the<br />
l<strong>and</strong> surface subsides<br />
permanently.<br />
CH 1 Critical head at time, t 1<br />
CH 2 Critical head at time, t 2<br />
h 1<br />
h 2<br />
h<br />
t2 greater than t 1<br />
Head in well at t1 Head in well at t2 b Total l<strong>and</strong> subsidence,<br />
which equals compressibility<br />
times h<br />
Difference in head,<br />
(h2 minus h1) Water table<br />
Monitoring the Hydrologic System<br />
h<br />
<br />
<br />
<br />
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<br />
<br />
<br />
<br />
-<br />
-<br />
<br />
<br />
<br />
-<br />
<br />
<br />
t 1<br />
L<strong>and</strong> surface<br />
Clay <strong>and</strong> silt<br />
(aquitards)<br />
Granular aquitard<br />
skeleton defining<br />
fluid-filled pore spaces<br />
storing groundwater<br />
h 1<br />
CH 1<br />
L<strong>and</strong><br />
surface<br />
b<br />
h2=CH2 Rearranged, compacted granular<br />
aquitard skeleton with reduced<br />
porosity <strong>and</strong> groundwater<br />
storage capacity<br />
Monitoring the Hydrologic System 109<br />
Depth<br />
to<br />
water<br />
When h 1 is greater than CH 1 , sediments are<br />
overconsolidated with compression, <strong>and</strong> l<strong>and</strong><br />
subsidence is predominately elastic <strong>and</strong> recoverable<br />
When h 2 is equal to CH 2 , sediments are<br />
normally consolidated with additional head<br />
decline causing inelastic <strong>and</strong> nonrecoverable<br />
l<strong>and</strong> subsidence<br />
Compaction of the aquifer system<br />
is concentrated in the aquitards<br />
Figure B18. Relationship of water-level changes <strong>and</strong> critical heads to subsidence <strong>and</strong> inelastic compaction.<br />
t 2<br />
Time<br />
Long-term decline in water level<br />
modulated by the seasonal cycles<br />
of groundwater pumping<br />
<br />
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