Lecture 7/8: Groundwater Flashcards
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provides a larger percentage of community and non-community public water systems, 80k Million gallons are withdrawn every day,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.
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
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.
Top 5 contaminants at superfund sites
Trichloroethylene, lead, toluene, benzene, PCBs
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
saturated vs unsaturated zones
unsaturated / vadose zone > capillary fringe > saturated zone (i.e. the water table)
describe the different characteristics of the A,B, and C horizon
A - organic layer, clay and sand
B and C - much less oxygen
soil size distribution
cobbles (100mm), gravel(10mm), sand(0.1mm), clay (smaller)
however different groups define this different ways
describe the hydrologic cycle
evapotranspiration
surface runoff
precipitation
infiltration
Precipitation = Surface Runoff + Evapotranspiration + Groundwater Recharge
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
filtration mechanisms
surface filtration, straining, physical and chemical adsorption
the vadose zone
soil zone (organic rich A horizon), intermediate zone, capillary fringe, multiphase system, unsaturated,
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!
physical properties of soil
soil texture, infiltration rate, porosity, specific gravity, field capacity, crop extractable water,
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
transport in rock?
fractured rock - diffusion into rock matrix vs flow along fractures
variability in permeability form lenses,
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
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
cone of depression
happens because of well draw-down
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
Contamination Mitigation and Fate (reactive)
adsorption or desorption, biological activation, chemical reactions
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
general groundwater modeling approach
obtain field information, determine flows, simulate contaminant process,
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
range of porosities
gravel: 25-40%, limestone 0-20%
hydraulic conducivity
k, measure of friction losses incurred by groundwater in transit (anistropic, depends on direction of flow),
Darcy’s law
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
what is a way that wells are used to stop contamination?
interceptor drains downgradient of contamination plume, multi well system (extraction and injection)
3 types of well clusters
multiple port samplers, multiple wells single borehole, multiple wells multiple boreholes
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
