20.07.2013 Views

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

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

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 />

<br />

<br />

<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 />

-

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!