Groundwater Supply Dr. Martin T. Auer Michigan Tech Department of Civil & Environmental Engineering

Approximately two-thirds of the population of the U.S. receives its supply from surface waters. However, the number of communities supplied by groundwater is four times that supplied by surface water. This is because large cities are typically supplied by surface waters and smaller communities use groundwater. Drinking Water Sources

Domestic Wells

water table An Aquifer water table well vadose zone capillary fringe unconfined aquifer water table impermeable layer

unconfined aquifer water table unconfined aquifer re-charge water table well Unconfined Aquifer water table – piezometric surface where water pressure equals atmospheric pressure Unconfined Aquifer manometer

Confined Aquifer piezometric surface aquiclude Confined Aquifer confined aquifer re-charge confined aquifer confining layer piezometric surface = f (K)

unconfined aquifer water table unconfined aquifer re-charge water table well Unconfined Aquifer confined aquifer re-charge piezometric surface = f (K) Confined Aquifer confined aquifer confining layer Comparison

Cone of Depression cone of depression aquaclude = impermeable layer

Effect of Pumping Rate drawdown radius of influence

Effect of Multiple Wells

Effect of Pumping Rate

Small soil particles pack together more closely than large particles, leaving many small pores. Large soil particles pack together less closely, leaving fewer, but larger, pores. Most soils are a mixture of particle sizes. Poorly sorted soils (greater range of particle sizes) will have a lower porosity, because the smaller particles fill in the "gaps“. A given volume of spherical solids will have the same porosity, regardless of the size of the particles. The significance of porosity lies in role of surface tension (higher for small pores) in retaining water and frictional losses in transmitting water. Porosity and Packing

Clays are small soil particles and thus one would expect tight packing. However, the net negative charge of clay particles separates them, resulting in a higher porosity than for a sphere of equivalent volume. Sands are large particles, more regular in shape than silts and thus having a porosity similar to that expected for spherical particles. Silts are intermediate in size between clays and sands and are irregular in shape. This irregularity leads to poorer packing than for spherical particles of similar volume and thus a higher than expected porosity. Porosity of Specific Soils

MaterialPorosity (%)Comment Clay55negative charge Loam (silts)35irregular shape Coarse sand30regular shape soil particles pores The net effect of the physicochemical properties of clay, silt and sand particles is that the porosity and thus water content tends to decrease as particle size increases. Porosity Values

This is the amount of water, expressed as a %, that will freely drain from an aquifer Specific Yield

Having a lot of water does not mean that an aquifer will yield water. Surface tension effects, most significant in soils with small pores, tend to retain water reducing the specific yield. MaterialPorosity (%)Sp. Yield (%) Clay553 Loam355 Coarse sand3025 A better expression of the water available for development in an aquifer is the ratio of specific yield to porosity. MaterialRatio of Specific Yield : Porosity Clay0.05 Loam0.14 Sand/Gravel0.83 Specific Yield

Hydraulic Gradient Darcy’s Law hydraulic conductivity

Hydraulic Conductivity (a.k.a. coefficient of permeability) K = m 3 ·m -2 ·d -1 = m·d -1

H M2M2 M1M1 E S1S1 S2S2 h1h1 h2h2 h = H - s r1r1 r2r2 An extraction well (E) is pumped at a constant rate (Q) and the drawdown (S) is observed in two monitoring wells (M) located at a distance (r) from the extraction well. Determination of Hydraulic Conductivity Hydraulic conductivity (m 3 ∙m -2 ∙d -1 ) is then calculated by solving Darcy’s Law to yield: Determining Hydraulic Conductivity

At maximum drawdown, conditions at r 1 (the well radius) are s 1 = H and h 1 = 0 and conditions at r 2 (the edge of the cone of depression) are s 2 = 0 and h 2 = H. H E S1S1 S2S2 h1h1 h2h2 h = H - s r1r1 r2r2 And the maximum pumping rate (m 3 ∙d -1 ) is calculated using the equation below: Estimating Well Production