Lecture 10/4/04

I. Poroelasticity and Storage

Hydrogeologists generally consider storage only from the perspective of water availability, but there are also important mechanical implications of the coupled nature of pore pressure and effective stress. The couple processes are the focus of studies of "poroelasticity". Lab 5 provides an introduction to this. The concept of an "increment of fluid content" was introduced and the storage coefficient can be defined as the change in this parameter with changes in head.

II. Geology of GW Occurence

3 important characteristics were discussed: lithology, stratigraphy, structure

Aquifer properties, with an emphasis on the nature of heterogeneity and the expected degree of anisotropy were discussed, organized around depositional environments and rock forming processes outlined below.

Unconsolidated sediment

1. Water

i. Streams - braided and meandering

(Lake (lacustrine) deposits are also important, primarily as confining units unless the lake sediments are very permeable such as the Central WI Sand Plain)

ii. Marine deposits Atlantic coastal plain

2. Glaciers

3. Wind - sand (also loess, not discussed in lecture)

Semiconsolidated and consolidated sedimentary rock

1. Clastic sedimentary rock - Pores in sandstone

2. Chemical sedimentary rock - carbonates and evaporites

Volcanic rock

1. Basalt (also Tuff, not discussed in lecture)

Crytalline igneous and metamorphic rocks - Fractures

The text discusses simple models for estimating hydraulic conductivity as a function of fracture aperture (b) and fracture spacing (s). Note that K for a fractured system according to this model varies with the cube of the aperture, while porosity varies directly with aperture. Aperture is difficult to actually measure, so in practice, "effective" hydraulic apertures have been estimated on the basis of K measured with slug or pumping tests.