Lecture 19 Ground Water 2 Ground water storage

Lecture 19 Ground Water (2) • Ground water storage • Porosity • Water retention and yield • Storage change

Ground Water Storage • Aquifers are reservoirs for water storage • Water moves very slowly in aquifers acting as water pipelines • The age of aquifers ranges from a few weeks to several tens of thousand years, depending on the depth and size.

Porosity Figure 6. 10, WR

Terms to Remember Porosity: percentage of the total volume of a rock or soil which is occupied by interstices or voids Original interstices: pores which were created at the time of origin of the rock Secondary interstices: pores results from subsequent actions of geological, climatic and biotic forces or factors upon the original rock, such as faults and joints enlarged by weathering and solution Effective porosity: total porosity less the portion of interstices which are not interconnected with other and are hydrologically inert.

Porosity vs. soil/rock type Table 5. 1, WR Ground Water

Specific yield and specific retention Figure 5. 3, WR Ground Water

Terms to Remember Specific yield: the volume of water than can freely drain from a saturated rock or soil under the influence of gravity, usually expressed in percentage of the total aquifer volume Specific retention: the percentage of the total aquifer volume which is retained by the surface tension as films around the individual grains and in capillary openings. Ground Water

Ground Water Storage Change ΔЅ=Qr-Qd ΔЅ = storage change per unit time Qr = recharge per unit time Qd = discharge per unit time Recharge: • Infiltration of precipitation • Seepage from lakes, rivers, and even oceans • Inflow from adjacent aquitards and aquifers • Artificial recharge (irrigation, wells, pipelines, sewers, etc. ) Discharge: • Evaporation • Spring flow and seepage into surface water bodies • Leakage to aquitards and other aquifers • Artificial extraction

Self reading Characteristics of change in unconfined aquifers (WR Chapter 5. 4. 3) Characteristics of change in confined aquifers (WR Chapter 5. 4. 3)