Groundwater Review Aquifers and Groundwater Porosity Hydraulic Conductivity and Storage Hydrologic Balance Heads and Driving Forces Darcy’s Law

Aquifers An aquifer is a geologic formation, or a stratum, that (1) contains water, and (2) transmits significant amounts of water under normal field conditions. A sand-gravel layer is an example. An aquitard is a geologic formation that transmits water at a very slow rate as compared to an aquifer. A silty-sand layer is an example.

Other Formations An aquiclude is a formation that may contain water, but is incapable of transmitting significant quantities under ordinary field conditions. A clay layer is an example. An aquifuge is an impervious formation that neither contains nor transmits water. Unweathered granite is an example.

Groundwater Groundwater is a term used to denote all waters found beneath the Earth's surface. However, the groundwater hydrologist, who is usually concerned with the water contained in the zone of saturation, uses the term groundwater to denote water in this zone. The portion of the subsurface not occupied by solid material is the void space. This space contains water and/or air (or other fluids). Only connected voids serve as conduits for flow.

Porosity The ratio of void volume to bulk volume is called the porosity. A one cubic meter sample that is completely saturated (all the voids are filled with water) with a porosity of 0.30 would contain 0.30 cubic meters of water and 0.70 cubic meters of solids.

Water Content The water content of a sample (hydrologic definition) is the ratio of water volume to total volume in a sample.

Saturation The degree of saturation Is the ratio of water volume to void volume in the sample; Slightly different definitions are common in the literature, but they should be obvious from the context - the concepts are identical.

Moisture Zones The saturated zone is that portion of the subsurface where the saturation is equal to or very near one. Sw > say 0.9 => Zone of saturation The zone of aeration (or vadose zone) is that portion fo the subsurface where the saturation is less than one. Sw< say 0.9 => Zone of aeration

Other Degrees of Saturation The field capacity of a soil is the water content (or saturation) of a soil that been drained by gravity - all remaining water is held in the soil by capillary and surface tension forces. The wilting point of a soil is the water content of a soil below which plants are permanently wilted. The wilting point is the dryest a soil can become under typical field conditions yet still support plant life.

Aquifer Classifications A confined aquifer is an aquifer bounded above and below by impervious (relative to the aquifer itself) formations. The upper and lower boundary to flow are geologic foemations. An unconfined aquifer is an aquifer whose upper boundary to flow is a phreatic surface (water table aquifer). An aquifer that gains or loses water to adjacent aquifers through vertical components of flow is refered to as a leaky aquifer.

Hydraulic Conductivity The hydraulic conductivity of an aquifer is a measure of the ability of the aquifer to conduct water through it under hydraulic gradients. It is a combined property of the porous medium and the fluid flowing through it. When the flow is essentially horizontal, the aquifer transmissivity indicates the ability of the aquifer to transmit water through its entire thickness. The common symbol for hydraulic conductivity is K

Storativity The storativity of an aquifer (storage coefficient) indicates the relationship between changes in the quantity of water stored in an aquifer and the corresponding changes in the elevations of the piezometric surface (or water table). It is defined as the amount of water added to or released from storage per unit change in head per unit area of aquifer;

Storage Mechanisms The storativity in a confined aquifer is due to the compressibility of the water and the elastic properties of the aquifer. The storativity of an unconfined aquifer is due to drainage and filling of the pore spaces. The storativity of an unconfined aquifer is smaller than the porosity by an amount called the specific retention. The difference between the porosity and the specific retention is called the specific yield, and is equal to the storativity in an unconfined aquifer.

Hydrologic Balance The hydrologic balance is simply the expression of the conservation of mass in hydrologic terms. Generally it is expressed as a rate (or volume) balance as Rate of inflow - Rate of outflow = Rate of change of storage + Rate of internal mass transferred.

Hydrologic Balance (s)

Groundwater Hydrologic Balance Recharge + Inflow - Discharge - Outflow = Storage + Generation Recharge Leakage from streams, lakes, ponds, landfills, and injection. Injection and spreading basins are sometimes referred to as artifical recharge. Discharge Leakage to streams, lakes, ponds, landfills, and pumping. Discharge (in our picture) would also include losses due to plant uptake of water, and direct evaporation in some basins.

Hydraulic Head Hydraulic head is a measure of energy per unit weight of water - in hydrology it represents the driving energy for fluid motion. Energy = Potential Energy + Internal Energy + Kinetic Energy + Chemical Energy + Electrical Energy + ... Typically the chemical and electrical energy are ignored - Energy = Potential Energy + Internal Energy + Kinetic Energy expressed as energy per unit weight becomes the familiar hydraulics terms: Total Head = Elevation Head + Pressure Head + Velocity Head The correspondence is: Potential Energy = Elevation Head Internal Energy = Pressure Head Kinetic Energy = Velocity Head

Energy Conservation Conservation of energy requires that the total head along a flow path be conserved - this requirement is called Bernoulli's equation. With friction this energy relationship is written as

Head Loss In terms of head the energy equation is Darcy's law can be interpreted as the empirical relationship for head loss: In groundwater flows the velocity is so small that its square is negligible Total Head = Pressure Head + Elevation Head.

Darcy’s Law Darcy's law was determined experimentally when Henri Darcy was studying the flow of water in sand filters. He discovered that the discharge, Q, through a column of sand of length, L, is proportional to the cross sectional area of the column (perpindicular to the flow), the head loss through the column, and inversely proportional to the length of the column.

Darcy’s Experiment The constant of proportionality is called the hydraulic conductivity, K, or the coefficient of permeability (archaic). Darcy's law is expressed as;

Specific Discharge The specific discharge in a porous medium is simply the total discharge, Q, divdided by the cross sectional area, A. U = Q/A = specific discharge.

Average Linear Velocity The average linear velocity in a porous medium is expressed as the ratio of specific discharge and porosity.

3D Extensions The 3D expression for Darcy’s Law is The 3D expression for specific discharge is The 3D expression for fluid velocity (in porous flow) is