Lecture 7/8: Groundwater Flashcards Preview

EHS 572: Environmental Impact Assessment > Lecture 7/8: Groundwater > Flashcards

Flashcards in Lecture 7/8: Groundwater Deck (30):
1

groundwater stats

provides a larger percentage of community and non-community public water systems, 80k Million gallons are withdrawn every day,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

2

sources of groundwater contamination. what is the largest?

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

3

characteristics of petroleum spills

- 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.

4

Top 5 contaminants at superfund sites

Trichloroethylene, lead, toluene, benzene, PCBs

5

What are the challenges associated with Groundwater contamination?

- 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

6

saturated vs unsaturated zones

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

7

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

A - organic layer, clay and sand
B and C - much less oxygen

8

soil size distribution

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

however different groups define this different ways

9

describe the hydrologic cycle

evapotranspiration
surface runoff
precipitation
infiltration

Precipitation = Surface Runoff + Evapotranspiration + Groundwater Recharge

10

Soil and Rock structure

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

11

filtration mechanisms

surface filtration, straining, physical and chemical adsorption

12

the vadose zone

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

13

Aquifer Recharge

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!

14

physical properties of soil

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

15

Aquifer

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

16

transport in rock?

fractured rock - diffusion into rock matrix vs flow along fractures

variability in permeability form lenses,

17

groundwater flow basics

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

18

aquifers / aquitards / windows

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

19

cone of depression

happens because of well draw-down

20

Contamination Mitigation and Fate (non-reactive)

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

21

Contamination Mitigation and Fate (reactive)

adsorption or desorption, biological activation, chemical reactions

22

describe the multiphase system present in the ground water media

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

23

general groundwater modeling approach

obtain field information, determine flows, simulate contaminant process,

24

key processes in GW flow

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

25

range of porosities

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

26

hydraulic conducivity

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

27

Darcy's law

relates flow rate to hydraulic pressure gradient
Q = K*A*dh/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

28

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

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

29

3 types of well clusters

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

30

Soil remediation strategies

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