Hydrogeologic settings for fractured rocks Fractured granite Florissant

Hydrogeologic settings for fractured rocks

Fractured granite Florissant, CO Sheeting joints in granite Yosemite, CA Unloading fractures in granite Scottsdale, AZ

granite, Albuquerque, NM granite batholith, NE WI Lineaments in granitic terrain (taken from plane)

vertical foliation, eastern US vertical foliation, Black Canyon of Gunnison

metavolcanics, central WI Precambrian quartzite central WI Fractured crystalline rock, Alps

Columnar jointed basalts Eastern, OR Devil’s Causeway Ireland Volcanic mudflow Clarno, OR

Fractured dolomite, Germany Canyonlands, Utah Dipping carbonates, Pyrennes

sandstone, Kansas sandstone, Arches sandstone, SE WI

clayey till, NW WI

Fractures n Fractures — mechanical breaks in rock • n n Joints — dilating fractures; formed under tension Shearing fractures/faults — formed under compression • n fracture is a term used for all types of generic discontinuities surfaces move predominantly parallel to each other Closing fractures/pressure solution surfaces

Fracture Network Geometry

Comparison of aquifer types Granular Fractured Effective Porosity Mostly primary intergranular pores Mostly secondary 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 usually applies Darcy's law may not apply Head Minimal variation Can be “flashy” Water Chem Minimal variation More variation

Characteristics of Fractured Aquifers n n n 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