Groundwater and Subsidence

Groundwater Subsidence Objectives: to use principles of groundwater flow and determine if various areas are affected by environmental pollution to correlate groundwater withdrawal and land subsidence with routine human practices

Land Subsidence: The sinking of land or “settling” due to the extraction materials In-proper soil compaction in the year 1173 led to the “leaning” tower of Pisa “air” in the soil void spaces forced out creating subsidence

Subsidence – removal of a coal bed

What if groundwater is removed from the subsurface Factors affecting groundwater subsidence Porosity: the amount of void spaces available to store water Expressed as a % 10% -20% 30%-50% Permeability: the ability for water to move (transmit) through the media

Rock Type Porosities High High “hard to Clay give up” Low Well sorted sandstone and gravel High High “hard to give up” Clay Poorly sorted sands Cemented Limestone Shale Low

Typical stratgraphic section of the San Joaquin Valley 1915 Sandstone Sandstone / siltstone 1960 Sandstone / siltstone / shale What will happen when groundwater is removed???

Example of Land Subsidence Pumping groundwater from 1934 - 1967 300 600 1200 1800 2400 Line of equal elevation of land subsidence Units of millimeters (mm) Example of Land Subsidence

A’ A Profile Construction A A’ Silver Creek Fault 300 Silver Creek Fault 600 San Francisco Bay 1,200 A’ 1,800 2,400 Line of equal long-term land subsidence (1934-67) (intervals of mm) 1,200 A 2,400 Vertical Scale Profile Construction 300 600 900 1,200 1,500 1,800 2,100 2,400 A A’

Groundwater: accumulation of water below the surface through infiltration processes Infiltration into the soil Groundwater

Groundwater Contamination groundwater is contaminated by outside sources that infiltrate through the subsurface petroleum (gasoline) agriculture products mine waste dump waste

Migration of groundwater (movement) using flow lines water table contour lines GW flow lines Flow lines always flow perpendicular to contour lines !!!!!!!