Groundwater

Movement of Groundwater

- water flows downhill and follows the landscape topography - Flow happens perpendicular to topographic lines

Problems with groundwater withdrawals

-Saltwater Intrusion (freshwater generally flows out to the ocean and they mix in a zone of dispersion however over pumping can reduce the amount of fresh water going out and draw in saltwater that contaminates fresh groundwater wells) - Sinkholes (karst geology, removing water can cause cavities to collapse

Factors that influence velocity of groundwater movement

1) hydraulic conductivity of medium through which groundwater moves, 2) gravity, and 3) pressure.

Reasons why groundwater is important to Michigan's aquatic ecosystems

1. Supply for inland lakes, streams, and Great Lakes 2.Stable flows/temps create world-class trout fisheries on rivers with large inputs of groundwater, also important to cool water fisheries (i.e., smallmouth bass, walleye Tourism is a $2 B industry in Michigan •Groundwater is the source of drinking water for ~half of Michigan residents (~5 M people) •Michigan industries, municipalities, farms pumped ~730 MGD in 2000 -approximately 2.6% of water entering underground aquifers

zone of aeration

Between the soil surface and the zone of saturation. Can become saturated (temporarily) The water table marks the upper edge of the zone of saturation - Soil water zone (extends through rooting zone) - Vadose zone (end of soil water zone through capillary fringe) - Capillary fringe (water from saturated zone gets drawn upwards in this region, negative water potential)

Components of Michigan geology important to groundwater:

Glacial and bedrock aquifers

Porosity vs. hydraulic conductivity

Greater porosity is partially related to greater hydraulic conductivity •However, relationship does not work with small-sized particles (water clings to soil particles, such soils may have high porosity and low hydraulic conductivity) -Clay can store large amounts of water, but water doesn't move through clay easily -Small pore sizes -Properties of clay •Clay has fairly low hydraulic conductivity

Storage of Groundwater

Groundwater is stored in many different kinds of soils and rocks - pore spaces of soil and rock particles - rock fractures - solution openings Water stored and released depends on - number of pore spaces - Size of pore spaces - connection among pores

Local vs. regional groundwater flow

Groundwater that's part of a "local" flow systems travels relatively short distances before discharging into streams, lakes, or wetlands •Amount of groundwater moving through these systems is not well quantified; historically, water-supply studies have focused on deeper regional flow systems •Regional groundwater flow tends to occur deeper than local flow; water tends to flow longer distances •While confining layers tend to separate local vs. regional groundwater flow, thick, unconfined aquifers can have regional flow

Hydraulic conductivity(property of aquifer material, capacity to yield water)

How readily water moves through soil (or rock) -Porosity (capacity of material to hold water) -Pore size -Pore shape -Connections among pores -Type of soil (i.e., clay) •Expressed as velocity(distance over time

Porosity influences storage and permeability

Porosity for well-sorted materials is higher than poorly sorted - Pores must be connected -Void space between grains and in cracks - Porosity Vv/Vt*100 Vv: volume of void space in a rock Vt: total volume of rock

Transmissivity of an aquifer

Product of hydraulic conductivity and aquifer thickness (most commonly used to characterize movement in confined aquifers) •Estimate of an aquifer's ability to transmit groundwater horizontally through its entire saturated

Darcys law

Q= kv *A*change in H/L Q: flow (cm3/sec) Kv: hydraulic conductivity A: cross sectional area (cm2) change in H: hydraulic gradient (change in head or elevation) L: length of soil column

Aquitard

Strata that are minimally permeable and slow groundwater (silts, bedrock)

How to Mgmt Groundwater

To successfully and sustainably manage a resource, it's important to know how much and what the consequences of the use of a resource may be •Characterizing 1) the amount of groundwater available in Michigan (and other places), 2) recharge rates, and 3) connections among aquifers is challenging•Affects the ability to regulate its use and manage pollutants •These challenges extend beyond the realm of science; have major economic consequences

More on groundwater movement

Water moves from locations of higher pressure to lower pressure - typically movement is slow <1 ft/day but can be faster under pressure

Unconfined aquifer

aquifer that contains water in direct contact with the atmosphere

Confined aquifer

contains water separated from the atmosphere by a (relatively) impermeable layer (aquiclude)-Confining layer can result in the creation of perched water table or artesian well due to pressure

Hydraulic gradient, unconfined aquifer/confined

h1-h2/L h1 +hp1 - h2 + hp2/ L

Permeability

property reflecting how connections among pores of sufficient size encourage (or discourage) water movement

Aquiclude

soil or rock strata that are relatively impermeable (shales, slates, clay)

Hydraulic Head

water pressure surface and water table elevation. - measure of energy causing groundwater to flow between 2 points. - a change in hydraulic head between two locations is a "gradient" - Groundwater movement is always in downward direction of the hydraulic head gradient - If there is no hydraulic head gradient there is no flow

Aquifer

water-bearing porous soil or rock strata that yield significant amounts of water to wells