Groundwater 40x larger than lakes+rivers

Groundwater 40x larger than lakes+rivers • Ocean Volume = 1.4x 109 km3 Oceanic Evaporation Rate = 3.2x105 km3/yr • Groundwater Volume = 8.4x106 km3 • River+Lake Volume = 2x105 km3 • Atmosphere Volume = 1.3x104 km3 Runoff Rate = 3.6x104 km3/yr

Surface rock typically contains several percent water within it

Permeability & Porosity Impermeable Permeable

Aquifer & Aquitard (Better flow) (Retards flow)

Factoid: Along the South Carolina Coast, half the water entering the ocean is entering by subsurface groundwater flow [a rare measurement]

Water Table & Vadose Zone Vadose Zone

Why a water table? (Discuss animation)

Regional Water Table

Water Flow Paths

Travel Times in Regional Aquifers

Hydraulic Gradient Hydraulic Gradient L L

Darcy’s Law Given a hydraulic gradient Dh/L Darcy Velocity V is a flow rate per unit cross-sectional area (has units of velocity) Hydraulic conductivity also has units of velocity since the hydraulic gradient is dimensionless

True Fluid Velocity larger than Darcy velocity Darcy velocity V = (volume fraction fluid ) x (fluid velocity v) so v = V /  = (true fluid velocity)/ (volume fraction fluid) e.g., for 10% porosity, the true fluid velocity is 10 times larger than the Darcy velocity

Hydraulic Head, Deviatoric Pressure, & Pressure Hydrostatic pressure Pw is the fluid pressure from the weight of the overlying column of water: Pw = wgz Deviatoric pressure p is the pressure at a given point that is in excess or deficit of the hydrostatic pressure, i.e. P = Pw + p Hydraulic head h is the elevation h of the water level in a hypothetical well drilled at that point. The total pressure P = wgh at that point: h = P/(wg) Likewise, the deviatoric pressure is similarly related to the change in hydraulic head:p = wgh or h = p/(wg)

Conservation of Mass Ingredients needed to solve for groundwater flow: Darcy’s Law (relates flow to hydraulic head h) Conservation of mass (i.e., balance between what goes in and out of a region) Put together, (1) and (2) lead to the equation (here assume constant K)

Using Mass Conservation to solve for flow into a well (1)Note that the problem is symmetric: Assume the flow Vr depends only on the distance r from the well. (2) By conservation of mass, the total flow (=extraction rate Q from the well) across each circle of radius r must be constant (the same amount flows across each surface of radius r and thickness b, or else mass would not be conserved) (3) Therefore Vr*b*2r = Q, or Vr = Q/(2r b) r b Vr

Darcy’s Law - Flow proportional to a ‘Potential Function’ (here assume constant K) Ingredients needed to solve for groundwater flow… Darcy’s Law (relates flow to hydraulic head h) Using a potential function simplifies the problem in that we only need to solve for one function h (or its generalization ) instead of two individual velocity components Vx and Vz. However, the equation we need to solve for the potential has higher order derivatives and is usually more complicated to solve.

Flow into or out of a well (2) Vr*b*2r = Q, or Vr = Q/(2r b) Can integrate this to find the Darcy flow potential The linearity of Darcy’s Law allows flow potentials to be added (e.g. can easily combine simple solutions to make a more complicated one…)

Flow from one well to another y r1 r2 x The linearity of Darcy’s Law allows flow potentials to be added …

Downdraw of the water table & cone of depression around a well… (Discuss animation)

Regional Groundwater Flow (1) (1)To solve for regional groundwater flow, in general need to use both conservation of mass and Darcy’s Law h is the elevation of the water table above the x-axis  Is the wavelength of the cosine curve representing topography (=2/k, where k is the wavenumber)

Regional Groundwater Flow (2) Darcy’s Law Mass Conservation Put together to create a relation for h

Regional Groundwater Flow (3) For constant K, this can be simplified to LaPlace’s equation. To solve this equation, we again assume (i.e.,hope for) a simplified form for the solution, that h = X(x)Z(z) (Separation of Variables — perhaps the most useful trick in PDEs)

Regional Groundwater Flow (4)

Groundwater 40x larger than lakes+rivers