Lecture 7/8: Groundwater

Question 1

groundwater stats

Answer 1

provides a larger percentage of community and non-community public water systems, 80k Million gallons are withdrawn every day,

supplies 1/2 the US pop, 1/3 the world’s pop. 0.6% of total freshwater.

Question 2

sources of groundwater contamination. what is the largest?

Answer 2

injection well, unlined landfill, septic tank, fertilizer, drinking water well, pumping well sewer leakage, salt-water intrusion, runoff (roads and agricultural),

Leaky underground storage tanks are the largest nation=wide (half of every underground storage tank leaks), followed by septic systems and landfills

Question 3

characteristics of petroleum spills

Answer 3

• Mixtures: hundreds of different compounds, typically find benzene,toluene, ethylbenzene, and xylenes.
• Petroleum-related pollution threaten ground water uses, e.g.,benzene is a carcinogen
• Added compounds to boost performance may be potent pollutants, e.g., methyl tert-butyl ether (MTBE) is highly water soluble
• the most significant impacts occur in the uppermost aquifer.

Question 4

Top 5 contaminants at superfund sites

Answer 4

Trichloroethylene, lead, toluene, benzene, PCBs

Question 5

What are the challenges associated with Groundwater contamination?

Answer 5

• Can’t direct see or often measure controlling parameters
• difficult and expensive to measure anything
• Groundwater system operates at many scales, from
  microscopic (pore) to regional water systems, to global
  hydrosphere.
• Dynamic system
• Complex physical, chemical, and biological processes
• Extremely limited validation of models

Question 6

saturated vs unsaturated zones

Answer 6

unsaturated / vadose zone > capillary fringe > saturated zone (i.e. the water table)

Question 7

describe the different characteristics of the A,B, and C horizon

Answer 7

A - organic layer, clay and sand

B and C - much less oxygen

Question 8

soil size distribution

Answer 8

cobbles (100mm), gravel(10mm), sand(0.1mm), clay (smaller)

however different groups define this different ways

Question 9

describe the hydrologic cycle

Answer 9

evapotranspiration
surface runoff
precipitation
infiltration

Precipitation = Surface Runoff + Evapotranspiration + Groundwater Recharge

Question 10

Soil and Rock structure

Answer 10

A - well sorted sediment with high porosity
B - poorly sorted sediment deposit with low porosity
E - rock rendered porous by solution
F - rock rendered porous by fracturing

Question 11

filtration mechanisms

Answer 11

surface filtration, straining, physical and chemical adsorption

Question 12

the vadose zone

Answer 12

soil zone (organic rich A horizon), intermediate zone, capillary fringe, multiphase system, unsaturated,

Question 13

Aquifer Recharge

Answer 13

areas that allow for recharge of groundwater through precipitation, arise because of permeability of soil, amount of rainfall, soil moisture content. These zones need protecting against contamination!

Question 14

physical properties of soil

Answer 14

soil texture, infiltration rate, porosity, specific gravity, field capacity, crop extractable water,

Question 15

Aquifer

Answer 15

flowing artisian, non-flowing artisian, water-table well

sand aquifer - water is found between pores, sand and gravel in alluvial valleys,

bedrock aquifers - sandstone, limestone. water within fissures and crevices

confined (artisian) vs unconfined (phreatic) - flowing means that well head or pressure is above surface, so water flows without pumping

Question 16

transport in rock?

Answer 16

fractured rock - diffusion into rock matrix vs flow along fractures

variability in permeability form lenses,

Question 17

groundwater flow basics

Answer 17

gw depth to water table is measured as m below ground surface or m above mean sea level

water flows perpendicular to head lines

GW contour maps, depth to water
Water table heights (unconfined aquifer)
Piezometric or potentiometric surface (confined aquifer)

Levels in wells may fluctuate:
Seasonal variation
Effect of pumping
Tides, atmospheric pressure, recharge
Others

Question 18

aquifers / aquitards / windows

Answer 18

aquitards (clay layers that restrict the movement of water, impermeable layer), windows are interuptions in that impermeable layer, discharge area is where water leaves the aquifer and flows to the surface (opposite of a recharge area) - happens at springs and streams

Question 19

cone of depression

Answer 19

happens because of well draw-down

Question 20

Contamination Mitigation and Fate (non-reactive)

Answer 20

non-reactive contaminants in saturated porous material are controlled by advection, mechanical dispersion (varying flow due to friction within pore channels,

longitudinal vs transverse dispersivity

hydrodynamic dispersion: homogeneous, spreading, fingering

geologic Stratification - Breakthrough & Tailing, retardation and monitoring

Question 21

Contamination Mitigation and Fate (reactive)

Answer 21

adsorption or desorption, biological activation, chemical reactions

Question 22

describe the multiphase system present in the ground water media

Answer 22

NAPLs, water, gas phase volatiles, solid (minerals, clay, organic material, colloidal material)

also flows are happening both in saturated and unsaturated zones, contaminants might change phase during the process

Question 23

general groundwater modeling approach

Answer 23

obtain field information, determine flows, simulate contaminant process,

Question 24

key processes in GW flow

Answer 24

Porosity
Hydraulic Conductivity
Pressure/Head gradient
Darcy’s Law
Specific discharge, velocity, travel time
Heterogeneity
Ground Water Flow Direction
Flow Nets (equipotential lines, flow lines)
Numerical Modeling
Particle Tracking

Question 25

range of porosities

Answer 25

gravel: 25-40%, limestone 0-20%

Question 26

hydraulic conducivity

Answer 26

k, measure of friction losses incurred by groundwater in transit (anistropic, depends on direction of flow),

Question 27

Darcy’s law

Answer 27

relates flow rate to hydraulic pressure gradient
Q = KAdh/dl, can be expressed as velocity v = K dh/hl
flow actually only happens in voids so you can divide by pore space to get flow (in absence of sorbtion)
works with laminar flow in saturated material

Question 28

what is a way that wells are used to stop contamination?

Answer 28

interceptor drains downgradient of contamination plume, multi well system (extraction and injection)

Question 29

3 types of well clusters

Answer 29

multiple port samplers, multiple wells single borehole, multiple wells multiple boreholes

Question 30

Soil remediation strategies

Answer 30

For organics: bioremediation (organisms), phytoremediation, incineration, soil vapor extraction, steam injection, flushing

for metals: bioimmobilization, reactive barriers, electrokinetic removal, excavate and store in hazardous waste landfill