The document discusses groundwater contamination sources and processes. It describes how contaminants from various human activities like storage tanks, septic systems, hazardous waste sites, landfills, chemicals and road salts can seep into and travel through groundwater. The main transport processes are advection via groundwater flow, dispersion which dilutes contaminants, and retardation from interactions with soils and aquifers. Contaminant plumes can migrate significant distances depending on the aquifer properties and contaminant characteristics.

1. Ground Water
Contamination

2. Groundwater contamination occurs when
man-made products such as gasoline,
oil, road salts and chemicals get into
the groundwater and cause it to
become unsafe and unfit for human
use. Materials from the land's surface can
move through the soil and end up in the
groundwater.

3. Drinking contaminated groundwater can have
serious health effects. Diseases such as
hepatitis and dysentery may be caused by
contamination from septic tank waste.
Poisoning may be caused by toxins that have
leached into well water supplies. Wildlife can
also be harmed by contaminated groundwater.
Other long term effects such as certain types of
cancer may also result from exposure to
polluted water.

4. Most contaminants are introduced to the
subsurface by percolation through soils.
The interactions between soil and a
contaminant are important for assessing
the fate and transport of the contaminant
in the groundwater flow system.
Contaminants that are highly soluble, such
as salts (e.g. sodium chloride, NaCl) move
readily from surface soils to saturated
materials below the water table. This often
occurs during and after rainfall events.

5. Those contaminants that are not highly
soluble may have considerably longer
residence times in the soil zone. Some
contaminants adsorb readily onto soil
particles and slowly dissolve during
precipitation events, resulting in dissolve
fraction concentrations of contaminants
migrating to groundwater. This mode of
transport is common for trichloroethylene.

6. Liquids spilled onto surface soils can
migrate downward or can evaporate, which
limits their potential for reaching the water
table. Once below the water table,
contaminants are also subject to dispersion
(mechanical mixing with uncontaminated
water) and diffusion (dilution by
concentration gradients).

7.  Storage Tanks. May contain gasoline, oil, chemicals, or other
types of liquids and they can either be above or below ground.
There are estimated to be over 10 million storage tanks buried in
the United States and over time the tanks can corrode, crack and
develop leaks. If the contaminants leak out and get into the
groundwater, serious contamination can occur.
 Septic Systems. Onsite wastewater disposal systems used by
homes, offices or other buildings that are not connected to a city
sewer system. Septic systems are designed to slowly drain away
human waste underground at a slow, harmless rate. An
improperly designed, located, constructed, or maintained septic
system can leak bacteria, viruses, household chemicals, and
other contaminants into the groundwater causing serious
problems.

8.  Uncontrolled Hazardous Waste. In the U.S. today, there are
thought to be over 20,000 known abandoned and uncontrolled
hazardous waste sites and the numbers grow every year.
Hazardous waste sites can lead to groundwater contamination if
there are barrels or other containers laying around that are full of
hazardous materials. If there is a leak, these contaminants can
eventually make their way down through the soil and into the
groundwater.
 Landfills. Landfills are the places that our garbage is taken to be
buried. Landfills are supposed to have a protective bottom layer
to prevent contaminants from getting into the water. However, if
there is no layer or it is cracked, contaminants from the landfill
(car battery acid, paint, household cleaners, etc.) can make their
way down into the groundwater.

9.  Chemicals and Road Salts. The widespread use of chemicals
and road salts is another source of potential groundwater
contamination. Chemicals include products used on lawns and
farm fields to kill weeds and insects and to fertilize plants, and
other products used in homes and businesses. When it rains,
these chemicals can seep into the ground and eventually into the
water. Road salts are used in the wintertime to put melt ice on
roads to keep cars from sliding around. When the ice melts, the
salt gets washed off the roads and eventually ends up in the
water.
 Atmospheric Contaminants. Since groundwater is part of the
hydrologic cycle, contaminants in other parts of the cycle, such as
the atmosphere or bodies of surface water, can eventually be
transferred into our groundwater supplies.

10.  Arsenic & Fluoride
 Pathogens
 Nitrate
 Organic Compounds
 Metals
 Pharmaceuticals
 Others

11. Generally three processes occur:
i) Advection
ii) Dispersion and
iii) Retardation

12.  Contaminants that are dissolved in water are solutes and the
water is the solvent and the combination is the solution. As the
water flows, the contaminants are transported with the water a
process known as advection.
 As the water flows around the soil particles, it is mixed, a process
known as mechanical dispersion. The result is dilution or
reduction in the contaminant concentration.
 A one time introduction of pollutants is termed a slug and opposed
to a continuous source. If the pollutant is introduced at a discrete
location, it is known as a point source as opposed to a non-point
source

13.  The distribution and extent of contaminants migrating in the
subsurface is known as the plume.
 Contaminates, particularly the ionic and molecular constituents, will
move from areas of high concentration to areas of low concentration
and this process is termed diffusion.
 In general, advective transport and associated mechanical
dispersion dominate the contaminant transport in formations of
medium to high hydraulic conductivity. In formations of low
hydraulic conductivity, including clay liners, diffusive transport is
frequently the controlling mechanism.

14. 

15. 1. A pollutant has entered into groundwater and is traveling
towards a well. When will the pollutant reach the well? The
aquifer is a sandstone with a hydraulic conductivity of 5m/d
and an effective porosity of 15%.
2. A chlorine spill has infiltrated through a well that crosses a
confined level and reaches the piezometric level of a 20m
thick sand and gravel aquifer immediately below the first
one, with a hydraulic conductivity of 80m/d and effective
porosity of 205.
The piezometric level in this well is located at 400m, whereas
the piezometric level in a well located 1km downgradient from
the first one is 398.
What is the groundwater and pollutant velocity? How long will
it take for the pollutant to reach the second well?

16. Two ponds of water are 1000 ft. apart with the water surface in
one pond being 150 ft. higher than in the other. Both ponds have
a square shape, measuring 200 ft. on a side. A 2-ft thick sandy
layer connects the ponds and the sand has a hydraulic
conductivity (K) = 5 x 10-2 cm/sec with a porosity of 25%.
• How long does it take for water to travel from the upper pond
to the lower one?
• How much water seeps from the upper pond into the lower
one on a yearly basis?
Assuming that K for the soil = 1 x 10-3 cm/sec and the gradient
of the water table is 1%, how long does it take for water to travel
50 and 100 ft. through this soil? Assume a porosity of 15%