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<br /> <br />. The age (time since recharge) of ground water <br />varies in different parts of ground-water- <br />flow systems. The age of ground water <br />increases steadily along a particular flow path <br />through the ground-water-flow system from <br />an area of recharge to an area of discharge. In <br />shallow, local-scale flow systems, ages of <br />ground water at areas of discharge can vary <br />from less than a day to a few hundred years. <br />In deep, regional flow systems with long flow <br />paths (tens of miles), ages of ground water <br />may reach thousands or tens of thousands of <br />years. <br /> <br />. Surface and subsurface earth materials are <br />highly variable in their degree of particle <br />consolidation, the size of particles, the size <br />and shape of pore or open spaces between <br />particles and between cracks in consolidated <br />rocks, and in the mineral and chemical <br />compo$ition of the particles. Ground water <br />occurs both in loosely aggregated and <br />unconsolidated materials, such as sand and <br />gravel, and in consolidated rocks, such as <br />sandstone, limestone, granite, and basalt. <br /> <br />. Earth materials vary widely in their ability to <br />transmit and store ground water. The ability <br />tif earth materials to transmit ground water <br />(quantified as hydraulic conductivity) varies <br />by orders of magnitude and is determined <br />by the size, shape, interconnectedness, and <br />volume of spaces behveen solids in the <br />different types of materials. For example, <br />the interconnected pore spaces in sand <br /> <br />l <br />8 <br />! <br />It <br /> <br />""" ,_&___w_~w. ,_ <br /> <br />and gravel are larger than tlwseinfiner <br />grained sediments, and the hydraulic <br />conductivity of sand and gravelis la~ger <br />than the hydraulic conductivity pflhe <br />fine~ grained materials. The ability of e'!rth <br />materials to store ground water also varie$ <br />among different types of materials. For <br />example, the volume of water stored in <br />cracks and fractures per unit volume of <br />granite is much smaller than the volume <br />stored per unit volume in theintergranular <br />spaces between particles of sand and gravel. <br /> <br />. Wells are the principal direct window'to <br />study the subsurface environmen~. Not <br />only are wells used to pump ground water' <br />for many purposes, they also provide^essen- <br />tial information about conditions in the <br />subsurface. For example, wells (1) allow <br />direct measurement of water levels in the <br />well, (2) allow sampling of ground water <br />for chemical analysis, (3) lliovideacc~ss fqr' <br />a large array of physical measurements in <br />the borehole (boreholegeophysic"Llogging) <br />that give indirect information on the ^proper~ <br />ties of the fluids and earth materials in the <br />neighborhood of the well, and (4) allow' <br />hydraulic testing (aquifer tests>" of the^earth <br />materials in the neighSqrhoodofthe wel!!o <br />determine local values oftheii transrnitt1J1g <br />and storage properties. In additior:, earth <br />materials can be sampled directly at ,!ny <br />depth during the drilling of the well. <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I' <br /> <br />~___ ...lii'!<l:-:Jl~'lI.;iji>>"ll:'lHbt,._~M4;'__ <br /> <br />10 <br />