Hydrogeologic settings for fractured rocks

Sheeting joints in graniteYosemite,CA

Unloading fractures in graniteScottsdale, AZ

Fractured graniteFlorissant, CO

granite batholith, NE WI

Lineaments in granitic terrain(taken from plane)

granite, Albuquerque, NM

vertical foliation, eastern US

vertical foliation, Black Canyon of Gunnison

metavolcanics, central WI

Fractured crystalline rock, Alps

Precambrian quartzitecentral WI

Columnar jointed basaltsEastern, OR

Devil’s CausewayIreland

Volcanic mudflowClarno, OR

Dipping carbonates, Pyrennes

Fractured dolomite, Germany

Canyonlands, Utah

sandstone, SE WI

sandstone, Arches

sandstone, Kansas

clayey till, NW WI

Fractures Fractures — mechanical breaks in rock

• fracture is a term used for all types of generic discontinuities

Joints — dilating fractures; formed under tension Shearing fractures/faults — formed under

compression• surfaces move predominantly parallel to each other

Closing fractures/pressure solution surfaces

Fracture Network Geometry

Comparison of aquifer types Granular Fractured

Effective Mostly primary Mostly secondaryPorosity intergranular pores concentration of joints, fractures

bedding plane partings

Flow Slow, laminar Possibly fast & turbulent

Isotropy More isotropic Probably anisotropic

Homogeneity More homogeneous Highly heterogeneous

Flow Darcy's law Darcy's lawusually applies may not apply

Head Minimal variation Can be “flashy”

Water Chem Minimal variation More variation

Characteristics of Fractured Aquifers porosity is primarily secondary hydraulic conductivity distribution is very heterogeneous

• ranging over several orders of magnitude• may be bimodal• frequently anisotropic

flow paths rapid flow in fractures, slower paths in matrix blocks

flow rates may be rapid and difficult to predict recharge ranges from focused (point) to dispersed

(diffuse) storage is primarily from matrix blocks