FINAL EXAM - Groundwater

Budget for hydrologic cycle

Includes river and groundwater flow

Groundwater zones

• GWT separates 2 zones:

  • Zone of aeration (vadose) → pore space partly filled with water and partly filled with air

  • Zone of saturation (phreatic) → pore space entirely filled with water

• Capillary Fringe → water drawn upward between grains (above water table)

• Level of saturation

  • Dry season: water content in surface is low

  • Wet season: water content in surface is high

Aquifers

• Aquifer → layer that can store or transmit groundwater (fer=transfer)

  • Unconfined aquifer → porus surface, recharge anywhere

  • Confined aquifer → limited recharge

• Aquitard → layer that allows weak groundwater flow (tard = retard)

• Aquiclude → layer that is total barrier to groundwater flow (clude=exclude)

Patterns of GW Flow

• Homogeneous material (sands)

  • Regional discharge

  • Parallel flow lines

  • Convex water table

• Inhomogeneous material (fractured rock)

  • Point discharge

  • Convergent flow

  • Concave water table

• Flow is horizontal once water table is reached

• Local GWT merges with bodies of water

• Discharge points tend to be swampy (Mudspring lake, Canada) → high salts = quicksand

• Liquefaction → occurs when weight of sediment is balanced by upward flow of water.

• GWT rises in wet season, falls in dry season

Local vs Regional Flow Systems

Local systems are shallower & contain younger GW

Age of GW

• Depends on rock properties, depth, distance, and GW flow patterns

• Deeper = older (generally)

• Depends on presence of aquicludes

• Date water by figuring out half time of isotopes in that water

Confined Aquifers

• Bounded by aquiclude layers above and below

• Potentiometric surface → elevation to which pressurized water in a confined would rise if not confined.

  • Slopes downward away from recharge zone

  • Long-term changes = urbanization

  • Short-term = climate

Unconfined and Confined Aquifer Relationship

• Unconfined aquifer can transition to a confined aquifer in the downdip direction

Darcy’s Law

• Velocity of GW flow is related to:

  • Hydraulic gradient → slope of GWT

  • Hydraulic conductivity → rock through which water flows aka coefficient of permeability (K)

Hydraulic Head

• Measurement of liquid pressure relative to vertical datum

  • Greater in confined aquifers

  • Hydraulic conductivity is lower in confined aquifers

• Measured by piezometer

Hydraulic Gradient

• Steepness of slope of GWT

• Measured by difference in hydraulic head between two wells divided by difference in distance

• 0.0001-0.1 ft/ft

Hydraulic Conductivity (K)

• Measure of ease with which water can flow through an aquifer

• High conductivity = more water flows through at given gradient

• sand/gravel →1-100 ft/day, clay →0.001 ft/day

• K increases with permeability and density and inversely with viscosity

Testing of core plugs

• Test characteristics of aquifers

• 1cm cylinders from drill cores

• Measures porosity and permeability

Pressure Test

• Testing well or aquifer response

• Test hydraulic conductivity and permeability

Water use in the US

• Thermoelectric Power → Uses the most but 98% returned

• Agriculture → Consumes the most water

• Western US uses a lot more water resources → NW more reliant on surface water, TX and Nebraska more reliant of GW

• Withdrawals of GW across the country:

  • Snake River Valley, Idaho → Agriculture

  • Central Valley,CA → agriculture

  • Ogallala aquifer, Great plains → agriculture

  • Chicago → urban supply

  • Mississippi Delta → agriculture

  • South TX → agriculture

Local effects of GW withdrawal

• Normally → horizontal upper surface

• Localized removal → cone depression

  • Funnel shape that only exists while the GW is being pumped out

• Regional overdraft → removal over larger area can result in drop in elevation of regional GWT

• Fossil GW → water that filled aquifer and can’t be replenished

• GW mining → unsustainable extractions

  • Chicago → water supply from GW which led to regional overdraft (800’ decline of GWT). Shift to lake MI led to recovery (rise by 100’)

GW Drawdown

• Can be predicted using Darcy’s Law

  • Discharge increases with hydraulic conductivity and hydraulic gradient

Gulf Coastal Plain Aquifer

• Tx to Fl

• Water withdraw from aquifer caused:

  • lowering of hydraulic heads at pumping centers

  • reduces discharge to streams

  • induced movement of saltwater

  • land subsidence

Sinkholes

Form in areas with abundant water and soluble bedrock

Stalagmites

Underground cave

• Dripstone and flowstone form from GW that degasses entering cave

1. Acidic water flows along fractures, enlarging them

2. Carbonate GW infiltrates cave, causing formation of flowstone

Water resources in Cincinnati

• Miller water plant

  • Treatment: settling, sand & gravel infiltration, granular activated carbon filtration, pH adjustment, addition of chlorine

  • GAC (granular activated carbon) → best way to remove organic material from water

• Bolton Plant on the Great Miami Aquifer

• US Department of Energy’s Fernald Uranium Processing Plant

Hydrological Basins

• GW systems are integrated at the scale of hydrological basins

• Nested hierarchy → large river basins contain multiple smaller river basins (dendritic drainage system)

GW pollution

• Plumes → GW pollutants that expand contamination volumes

  • Surface plume → on surface channels

• Ultimate fate of GW contaminants is discharged into surface water

• Soluble substances dissolve into groundwater

• Insoluble subtances:

  • Lighter than water → float on top of GWT

  • Heavier than GWT → sink to base of aquifer

Remediation of Contaminated soils

• Geomembrane → synthetic material that is used to contain leaching

• Pump Recovery → contaminated GW pumped to surface for treatment

• Surfactant Flushing → injecting surfactants (fuels) contaminated aquifers or soils to release contaminants into GW

• Chemical Treatments → reactants pumped down to react with contaminants (oxidizers are common)

• Oxidation Processes → UV light facilitates reaction that breaks down pollutants

• Air stripping → fluids pumped to top of tower and contaminants flow down while air pumps up (separates them)

• Bioreactors → degrade contaminants using microorganisms

• Ion exchange → strips out contaminants by using resent

• Constructed wetlans → capture heavy metals

• Growth of vegetation

• Monitored natural attenuation → just monitoring

Edwards Aquifer

• bad water line

  • surface = low salinity

  • deeper = higher salinity

  • deeper than bad water line = too saline to consume

Air & Water pollution in US

• Pitt & LA

• Effects of sulfate

  • acid rain

  • destruction of ozone

• Clean Air Act of 1970