USE OF SCANNING ELECTRON
MICROSCOPY TO EXPLAIN CHANGE IN
HYDRAULIC CONDUCTIVITY OF
HYDROCARBON-CONTAMINATED
AQUIFER SAND
Qays J. Saud1,2 and Syed E. Hasan2
Department of Geosciences
1 Ministry
of Higher Education and Scientific Research –Iraq
2 University
of Missouri-Kansas City
October 30, 2013
qjsfgd@mail.umkc.edu
HasanS@umkc.edu
1
Research objective
Utilize a scanning electron microscope (SEM) to
study reduction of hydraulic conductivity of aquifer
sand upon exposure to common organic
contaminants
2
Test material
• Sand deposits are common
geological formation for most
aquifers
• Missouri River alluvial aquifer
is a major source of drinking
water supply in Midwestern
states.
• Mid America Sand and Gravel
Co. quarry, in Jackson County,
Missouri was selected for bulk
sampling of the test material
3
Missouri River alluvial aquifer
N
Modified after (Kelly, 2003)
http://mo.water.usgs.gov/indep/kelly/mo-alluvial-gw/geology/images/excplot.gif
4
Selected chemicals
Three commonly occurring
chemical pollutants were selected
for this study:
 Trichloroethylene (TCE)- known
human carcinogen
 BTEX, one of 15 hazardous
chemicals occurring in gasoline
 Gasoline – isooctane
 All occur at numerous NPL sites
5
ATSDR priority list of hazardous
substances
2011 Rank
(out of 275)
Substance
6
Benzene
16
62
74
Toxicological effects
• Nervous system changes
• Liver, lungs, kidney damage,
TCE
and leukemia
Xylenes, total • EPA listed TCE as a known
human carcinogen
Toluene
ATSDR. Agency for Toxic Substances and Disease Registry; 2011 http://www.atsdr.cdc.gov/SPL/index.html
6
Methodology
• Fifty-four sand samples, from aquifer and the overlying unsaturated
zone comprising -2.36 mm and -0.425 mm fractions were
contaminated with TCE and gasoline at various levels of saturation
(v/v %): 25, 50 and 100% for 2, 4, 8, 16, and 32 weeks
• Aquifer sand samples were fully saturated for eight weeks with BTEX
and isooctane, the main constituents of gasoline, to compare the
results of their hydraulic conductivity with that of sand contaminated
with gasoline
• Ten aquifer sand samples were exposed to aqueous solution
contaminated with 1,2,3….10% gasoline for eight weeks, as well as
with TCE (fully saturated) to find out how the chemicals, when mixed
or dissolved in water, affect hydraulic conductivity.
7
Methodology (2)
• Geotechnical properties (void ratio, porosity, dry and
saturated unit weights) were determined before and after
exposing sand samples to the chemicals
• Hydraulic conductivity was determined before and after
contaminating all sand samples, following the ASTM test
method D2434–68
• SEM - secondary electron method (SE) was used to
examine the grain shape and size of aquifer sand samples.
SE provides better resolution than the equivalent
backscattered electron image (BSE)
8
Hydraulic conductivity of aquifer sand samples
contaminated with TCE, BTEX and isooctane,
fully saturated
Solubility at
(20–25° C)
(mg/L)
1100
1770
530
Safety data sheet, https://irmm.jrc.ec.europa.eu/refmat_pdf/IRMM-442_msds.pdf
Chemical Properties Database http://www.gsi-net.com/en/publications/gsi-chemical-database/list.html
169
160
Insoluble in water
(< 0.001 g/100 ml)
9
Hydraulic conductivity of aquifer sand
samples contaminated with gasoline (v/v %)
10
Hydraulic conductivity of aquifer sand
samples contaminated with TCE (v/v %)
11
Hydraulic conductivity of sand samples from
vadose zone contaminated with gasoline (v/v %)
12
Hydraulic conductivity of sand samples from
vadose zone contaminated with TCE (v/v %)
13
Hydraulic conductivity of aquifer sand samples
contaminated with varying concentration of TCE
14
Hydraulic conductivity of aquifer sand samples
contaminated with varying concentration
of gasoline
15
Electron micrograph of aquifer
sand grains
16
K feldspar grain in aquifer sand sample
contaminated with gasoline
17
K feldspar grains
Submicroscopic features on sand grains
Small secondary crystals in sample
contaminated with TCE & isooctane
18
Microtopography of contaminated sand
grains
Find submicroscopic features on the sand grains contaminated with TCE & gasoline
19
Conclusion (1)
SEM of contaminated sand grains revealed highly
irregular surfaces with presence of grooves along
with presence of prominent granules and fragmented
grains. All contribute to increase in surface area with
attendant increase in surface tension. This explains
the reduction in hydraulic conductivity of sand after
contamination.
20
Conclusion (2)
Organic hydrocarbons have different chemical
structures, which cause different chemical reactions
with sand grains and liquids moving through it. The
inherent differences in the structure and nature of
contaminating chemicals influenced hydraulic
conductivity such that the observed decrease was
greater for aliphatic than aromatic and chlorinated
hydrocarbons.
21
Conclusion (3)
Three possible mechanisms for hydraulic conductivity
reduction seem to be: (a) decreased porosity due to fine
particles filling the voids, (b) overall increase in surface
tension and other intragranular forces caused by adsorption
of hydrocarbons around sand grains, aided by large surface
area of the fine fragments, and (c) control of chemical
structure of contaminating fluids on hydraulic conductivity
reduction in the following way:
Chlorinated hydrocarbon < aromatic << aliphatic
22
THANK YOU
Questions?
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