Residents in cities and many rural homes use groundwater for drinking
and other household purposes. At the same time, groundwater is contaminated
by many sources. How dangerous are pollutants in drinking water, where
do they come from, and will they increase in the future?
Groundwater Contamination - The detrimental
alteration of the naturally occurring physical, thermal, chemical,
quality of groundwater. Further, groundwater contamination, for purposes
of inclusion of cases in the public files and the joint
groundwater monitoring and contamination report, shall be limited to
contamination reasonably suspected of having been
caused by activities of entities under the jurisdiction of the agencies
identified in the Texas Water Code §26.406, TGPC rules, and subsequent
legislative amendments. Groundwater contamination may result from many
sources, including current and past oil and gas production and related
practices, agricultural activities, industrial and manufacturing processes,
commercial and business endeavors, domestic activities, and natural
sources that may be influenced by, or may result from, human activities.
From Texas Groundwater Protection Committee (2005)
Sources of Contamination
Groundwater is contaminated by many activities such as
those shown here.
Sources of groundwater contamination. Click on the image for a zoom.
From US Environmental Protection Agency, Safe
Drinking Water - Protecting America's Public Health Poster.
Landfills and Hazardous Waste Facilities
Texans dispose of approximately 30 million tons of municipal solid waste
(durable and non durable goods, containers, food scraps, yard waste,
inorganic waste, sludge from water and wastewater treatment facilities,
septic tanks, construction and demolition debris) per year. 63% is
residential. 37% is commercial and institutional. In 2005, there were
186 active landfills in Texas. They received 29.67 million tons of
waste. Excluding construction waste, the per capita disposal rate in
Texas 5.5 pounds per person per day. In 2005, U.S. residents, businesses,
and institutions produced more than 245 million tons, which is approximately
4.5 pounds of waste per person per day. Almost all goes into landfills.
Environmental Almanac and EPA
Municipal Solid Waste).
Landfills contaminate groundwater when rain water leaks into aquifers
below the landfill. Many early landfills did not have liners to trap
rainwater that percolates through the landfill, and some newer landfills
have liners that leak. The percolating water leaches toxic chemicals
from batteries, broken fluorescent bulbs, electronic equipment, discarded
household chemicals, and paints and solvents. Although landfills now
prohibit toxic waste, and they are carefully regulated to prevent leakage
to groundwater, many older sites are unlined and leak.
Left: A municipal landfill. Right:
Design of a modern landfill.
Left from Aircraft
Owners and Pilots Association. Right from
Energy Information Administration A
Extensive herbicide use in agricultural areas
(accounting for about 70 percent of total national use of pesticides)
has resulted in widespread occurrence of herbicides in agricultural
streams and shallow ground-water. The highest rates of detection
for the most heavily used herbicides—atrazine, metolachlor,
alachlor, and cyanazine—were found in streams and shallow ground
water in agricultural areas. Insecticides were frequently detected
in some streams draining watersheds with high insecticide use but
were less frequently detected in shallow ground water because most
insecticides are applied at lower levels than herbicides and tend
to sorb onto soil or degrade quickly after application.
From USGS Water-Quality
Patterns In Agricultural Areas
Application of agricultural pesticides. Left:
By a crop dusted. Right: From the ground.
From Colorado State University Environmental
Health Advanced Systems Laboratory
Mining wastes include waste generated during the extraction, beneficiation,
and processing of minerals. Extraction is the first phase of hard rock
mining which consists of the initial removal of ore from the earth.
Beneficiation is the initial attempt at liberating and concentrating
the valuable mineral from the extracted ore. This is typically performed
by employing various crushing, grinding and froth flotation techniques.
Mineral processing operations generally follow beneficiation and include
techniques that often change the chemical composition of the ore or
mineral, such as smelting (iron and steel), electrolytic refining (aluminum)
and acid attack or digestion. Copper and gold mines comprise 80% of
the non-fuel facilities in the United States. They discard 90% to 99.99%
of the mined rock, generating 1.3 gigatons of waste.
From Environmental Protection Agency web
pages on mining.
Coal mines are another major source of contaminants. When pyrite rocks
associated with coal mining are exposed to oxygen they are oxidized to
generate acid mine drainage. The waste then flows into streams and infiltrates
A complex series of chemical weathering reactions
are spontaneously initiated when surface mining activities expose
spoil materials to an oxidizing environment. The reactions are analogous
to "geologic weathering" which takes place over extended
periods of time (i.e., hundreds to thousands of years) but the rates
of reaction are orders of magnitude greater than in "natural" weathering
systems. The accelerated reaction rates can release damaging quantities
of acidity, metals, and other soluble components into the environment.
For example, the overall pyrite reaction series [which occurs in
high-sulfur coal mines] is among the most acid-producing of all weathering
processes in nature.
From US Department of the Interior Factors
controlling acid mine drainage formation.
Left: Pollution due to acid mine drainage
in the Blackwater River of West Virginia. Right:
Water collecting in the open-pit Adams Mine, an open-pit iron mine
in Ontario, Canada.
Left: From US Department of the Interior Office
of Surface Mining. Right: From Adams Mine Landfill Proposal.
Above Ground and Underground Storage Tanks
Gasoline stations, dry cleaners, and other industrial establishments
store large quantities of liquids in tanks. Some are above ground,
some are below ground. Homes is cold areas store heating oil in underground
tanks or in basement tanks. Underground tend to cause groundwater contamination
because small leaks often go undetected.
Nearly one out of every four underground
storage tanks in the United States may now be leaking, according
to the U.S. Environmental Protection Agency. If an underground petroleum
tank is more than 20 years old, especially if it's not protected
against corrosion, the potential for leaking increases dramatically.
Newer tanks and piping can leak, too, especially if they weren't
installed properly. Even a small gasoline leak of one drop per second
can result in the release of about 400 gallons of gasoline into the
groundwater in one year. Even a few quarts of gasoline in the groundwater
may be enough to severely pollute a farmstead's drinking water. At
low levels of contamination, fuel contaminants in water cannot be
detected by smell or taste, yet the seemingly pure water may be contaminated
to the point of affecting human health. Petroleum fuels contain a
number of potentially toxic compounds, including common solvents
such as benzene, toluene and xylene, and additives such as ethylene
dibromide (EDB) and carbon-based lead compounds. EDB is a carcinogen
(cancer-causing) in laboratory animals, and benzene is considered
a human carcinogen.
From University of Missouri web page on Assessing
the Risk of Groundwater Contamination From Petroleum Product Storage.
The EPA identified over 460,000 leaking underground storage tanks up
to September 30, 2006. Steady cleanup work has progressed for over a
decade and more than 350,000 contaminated sites have been cleaned up.
The main concern now is contamination by methyl tertiary-butyl ether
MTBE. The additive, or other additives with similar ability to oxygenate
fuels, is required by the EPA to help reduce carbon monoxide emissions
from cars in cold weather.
Gasoline storage tank being removed from site.
From Virginia Tech Groundwater
Homes not connected to municipal sewer systems usually use septic systems
to dispose of wastewater from toilets and drains. Waste water drains
first into a septic tank where solids are separated from the liquid.
Light solids such as fats rise to the surface, heavy solids sink to
the bottom. The light solids remain until the tank is cleaned. Some
of the heavy solids are decomposed by bacteria, some remain until the
tank is cleaned. Relatively clear water from the tank drains into a
field of pipes, the drain field or leach field, which slowly leak water
into the ground. Most percolates downward and enters the water table,
some is taken up by plants, some evaporates. The water is cleaned by
natural remediation processes.
Water discharged into the ground includes nitrates and phosphorus which
can contaminate aquifers or nearby streams.
From Thurston County (Washington State) Public Health & Social Services
Department web page on Inspecting
Your Septic Tank.
Oil, Gas, and Industrial Injection Wells
Chapter 27 of the Texas Water Code (the Injection
Well Act) defines an “injection well” as “an artificial
excavation or opening in the ground made by digging, boring, drilling,
jetting, driving, or some other method, and used to inject, transmit,
or dispose of industrial and municipal waste or oil and gas waste
into a subsurface stratum; or a well initially drilled to produce
oil and gas which is used to transmit, inject, or dispose of industrial
and municipal waste or oil and gas waste into a subsurface stratum;
or a well used for the injection of any other fluid; but the term
does not include any surface pit, surface excavation, or natural
depression used to dispose of industrial and municipal waste or oil
and gas waste.” All injection wells are regulated by either
TCEQ (the commission in the Act) or the Railroad Commission of Texas
From Texas Commission on Environmental Quality web page on Injection
Wells: Am I Regulated?
Diagram of an injection well. From Pollution
The US Environmental Protection Agency defines five classes of injection
wells, all of which have important uses.
- Deep Wells Used to Inject Hazardous and Non-hazardous Waste Deep
Below the Surface, EPA Class I
Injection of hazardous waste into deep wells began in the United
States in the 1960s. At that time, the chemical industry was
looking for a safe, relatively inexpensive method for disposing
of high volumes of waste that could be considered toxic. Technology
was borrowed from the oil and gas industry to develop this new
form of disposal ... There are 163 Class I hazardous waste injection
wells located at 51 facilities. These are the only facilities
that can accept hazardous waste generated offsite for injection.
From EPA web page on Deep
Wells (Class I)
Class I wells are also used to dispose of non-hazardous industrial,
low-radiation and municipal wastes. Because 89 % of the hazardous waste
that is disposed of on land is disposed through Class I wells, they
are the most strictly regulated type of well.
- Oil and Gas Injection Wells, EPA Class II
This is the most common type of injection well.
The oil and gas production industry accounts
for a large proportion of the fluids injected into the subsurface.
Typically, when oil and gas are extracted, large amounts of salt water
(brine) are also brought to the surface. This salt water can be very
damaging if it is discharged into surface water. Instead, all states
require that this brine be injected into formations similar to those
from which it was extracted. Over 2 billion gallons of brine are injected
daily into injection wells in the US.
The largest proportion of these brines are injected into formations
that contain trace portions of extractable oil and gas. Injection
of the brine can have the effect of enhancing production of oil and
gas from the formations, thus secondary recovery of oil and gas depends
heavily on injection. Furthermore, when States started to implement
rules that prevented the disposal of brine to surface water bodies
and soils, injection of this waste fluid became the prevalent form
Class II wells exist wherever there is production of oil and gas.
There are approximately 167,000 oil and gas injection wells in the
US, most of which are used for the secondary recovery of oil. In
this process water is pumped into the formation that contains some
residual hydrocarbons. A portion of the hydrocarbons are recovered,
along with the injected water, by extraction or production wells.
In a common configuration, one injection well is surrounded by 4
or more extraction wells. The recovered fluid is treated to remove
most of the hydrocarbons in a device called a separator. The other
type of oil and gas injection well is a disposal well. In this type
of well, excess fluids from production and some other activities
directly related to the production process are injected solely for
the purpose of disposal.
From EPA web page on Oil
and Gas Injection Wells (Class II)
- Mining Wells, EPA Class III
Wells are used to mine salt, sulfur, and uranium.
A number of minerals are mined by using injection
wells. In general the technology involves the injection of a fluid
that contacts an ore which contains minerals that dissolve in the fluid.
When the fluid is nearly saturated with components of the ore it is
pumped to the surface where the mineral is removed from the fluid.
More than 50% of the salt used in the US is obtained this way.
From EPA web site on Mining
Wells (Class III)
- Shallow Hazardous and Radioactive Injection Wells, EPA Class IV
These wells are prohibited unless the injection wells are used to inject
contaminated ground water that has been treated and is being injected
into the same formation from which it was drawn.
From EPA web page on Shallow
Hazardous and Radioactive Injection Wells (Class IV)
- Shallow Injection Wells
These are are injection wells that are not
included in Classes I through IV. Class V wells inject non hazardous
fluids into or above an aquifer. They are typically shallow, on-site
disposal systems, such as floor and sink drains that discharge into
dry wells, septic systems, leach fields, and similar types of drainage
From EPA web page on Shallow
Injection Wells (Class V)
The two most numerous types of Class V wells are storm water drainage
and large capacity septic systems. Large cesspools and shallow waste
disposal systems that receive or have received fluids from vehicular
repair or maintenance activities, such as auto body or automotive repair,
car dealerships, or other vehicular repair work, are now prohibited.
Some Class V wells inject surface water to recharge aquifers, to control
land subsidence, and to limit salt-water intrusion provided the injected
water does not endanger underground sources of drinking water.
Despite the widespread use of injection wells, the Texas Commission
for Environmental Quality has found few wells contaminating ground water
supplies, and these have been cleaned up.
Types of Contaminants
Methyl Tertiary-Butyl Ether MTBE
Methyl tertiary-butyl ether (MTBE) is produced
in very large quantities (over 200,000 barrels per day in the U.S.
in 1999) and is almost exclusively used as a fuel additive in motor
gasoline. It is one of a group of chemicals commonly known as "oxygenates" because
they raise the oxygen content of gasoline. At room temperature, MTBE
is a volatile, flammable and colorless liquid that dissolves rather
easily in water. MTBE has been used in U.S. gasoline at low levels
since 1979 to replace lead as an octane improver (helps prevent the
engine from "knocking"). Since 1992, MTBE has been used
at higher concentrations in some gasoline to fulfill the oxygenate
requirements set by Congress in the 1990 Clean Air Act Amendments.
Oxygen helps gasoline burn more completely, reducing harmful tailpipe
emissions from motor vehicles. In one respect, the oxygen dilutes
or displaces gasoline components such as aromatics (e.g., benzene)
and sulfur. In another, oxygen optimizes the oxidation during combustion.
Most refiners have chosen to use MTBE over other oxygenates primarily
for its blending characteristics and for economic reasons. The Clean
Air Act Amendments of 1990 (CAA) require the use of oxygenated gasoline
in areas with unhealthy levels of air pollution. The CAA does not
specifically require MTBE. Refiners may choose to use other oxygenates,
such as ethanol.
From EPA web page on MTBE
The health effects of MTBE are not well understood. When inhaled in
high concentrations, it causes cancer in some research animals. There
is little data on its effects when humans ingest the chemical. EPA's
Office of Water has concluded that available data are not adequate to
estimate potential health risks of MTBE at low exposure levels in drinking
water but that the data support the conclusion that MTBE is a potential
human carcinogen at high doses. The EPA reviewed the available information
on health effects in a 1997 advisory and stated that there is little
likelihood that MTBE concentrations between 20 and 40 ppb in drinking
water would cause negative health effects. The EPA Drinking Water Advisory
recommends, but does not require, that concentrations be below 20 ppb
in drinking water.
Chlorinated solvents are volatile organic
(carbon-based) compounds (VOCs) that contain chlorine. In general,
chlorinated solvents have low water solubility and high volatilities
and densities relative to other VOCs. They are used in aerospace
and electronics industries, dry cleaning, manufacture of foam, paint
removal/stripping, manufacture of pharmaceuticals, metal cleaning
and degreasing, and wood manufacturing. Solvents also can be found
in a variety of household consumer products including drain, oven,
and pipe cleaners, shoe polish, household degreasers, typewriter
correction fluid, deodorizers, leather dyes, photographic supplies,
tar remover, waxes, and pesticides.
According to the [EPA Toxic Release Inventory] TRI, during 1998–2001,
total on- and off-site releases of methylene chloride, PCE [perchloroethene],
TCE [trichloroethene], and TCA [1,1,1-trichloroethane] averaged about
33 million pounds, 4 million pounds, 11 million pounds, and 0.5 million
pounds, respectively. PCE is still the solvent of choice for 85 to
90 percent of the approximately 30,000 dry cleaners and launderers
in the United States.
Solvents have been associated with both acute
and chronic human-health problems. Some are suspected human carcinogens,
and USEPA has set Maximum Contaminant Levels (MCLs) for solvents
in drinking water at very low concentrations. Many of the solvents
have water solubility that are high relative to their MCLs. This
means that even small spills of some solvents can result in substantial
ground-water contamination problems with respect to human health.
From USGS Occurrence
and Implications of Selected Chlorinated Solvents in Ground Water and
Source Water in the United States and in Drinking Water in 12 Northeast
and Mid-Atlantic States, 1993–2002.
They become a problem when they leak from tanks, pipelines, and land
fills, when they are spilled, and when they are disposed of improperly.
They are the most commonly found volatile carbon-based compounds found
in groundwater. They are strongly correlated with urban areas with high
population densities and with groundwater having high oxygen concentrations.
Perchloroethene was found most often. It was detected in 10% of the samples
at levels exceeding 0.02 microgram per liter, and in 4% of the samples
at levels exceeding 0.2 micrograms per liter. (Moran, 2006).
Percent of water samples with chlorinated solvent levels that exceeded EPA
Maximum Contaminant Levels. From Moran (2006).
Pesticides are any substance or mixture intended to prevent, kill, or
repel any pest, including insects, weeds, mice, fungi, or bacteria.
Thus, household chemicals to disinfect surfaces or to remove mildew
are legally classified as pesticides.
Total pesticide use in the United States
has remained relatively constant at about 1 billion pounds per year
[excluding chlorine/ hypochlorates (2.6 billion pounds per year),
wood preservatives (1 billion pounds per year), and special biocides
(0.3 billion pounds per year)] , after growing steadily through the
mid-1970s because of increased use of herbicides. Agriculture now
accounts for 70 to 80 percent of total pesticide use. Most agricultural
pesticides are herbicides, which account for about 60 percent of
the agricultural use. Insecticides generally are applied more selectively
and at lower rates than herbicides. Major changes in insecticide
use have occurred over the years in response to environmental concerns,
which have resulted in various restrictions on the use of organochlorine
insecticides, such as DDT. Specifically, as the use of these persistent
pesticides declined, the use of other, less persistent insecticides
From USGS Sources
of nutrients and pesticides
Glyphosate is by far the most commonly used herbicide, but it is of
little concern because glyphosate doesn’t readily leach into water
systems. Instead, it latches tightly to soil particles and degrades within
weeks into harmless byproducts. By contrast, herbicides such as atrazine
have been widely implicated in contaminating groundwater. (Service, 2007).
The US Geological Survey conducted a national survey of Pesticides in
the Nation's Streams and Ground Water, 1992–2001, and issued a
summary in 2006.
Among the major findings are that pesticides
are frequently present in streams and ground water, are seldom at
concentrations likely to affect humans, but occur in many streams
at concentrations that may have effects on aquatic life or fish-eating
wildlife. Human-health benchmarks were seldom exceeded in ground
water. One or more pesticides exceeded a benchmark in about 1 percent
of the 2,356 domestic and 364 public-supply wells that were sampled.
The greatest proportion of wells with a pesticide concentration greater
than a benchmark was for those tapping shallow ground water beneath
urban areas (4.8 percent).
(Pesticides in the Nation's
Streams and Ground Water, 1992–2001—A Summary).
From US Geological Survey Pesticides
in the Nation's Streams and Ground Water, 1992–2001—A Summary.
Atrazine is the pesticide most commonly found in rural water, being
found in 20% of shallow groundwater sites surveyed by the US Geological
Service as reported in Barbash (1999). Prometon is the most common pesticide
in urban water, being found in 5% of shallow groundwater sites surveyed.
Atrizine use in US in 1997. The map may be somewhat misleading because
farmers now use mostly glyphosate to control weeds in corn.
From US Geological Service Pesticide
Metals (Natural and Anthropogenic)
Metals leach from landfills, old mines, and industrial sites. Batteries,
electronic equipment, metal plating operations, metal smelters, all
contribute. The US Geological Survey web page on Groundwater
Quality lists the major natural and human-produced sources.
Small amount of metals are essential to life. Higher concentrations
can be toxic. In addition, the toxicity of the metal also depends on
its chemical compound. For example, metallic mercury is not toxic. But
methyl mercury (CH3)2Hg is a highly toxic neurotoxin.
It is produced by sulfate-reducing bacteria living in environments with
low oxygen concentration. Chromium III is essential for humans, chromium
VI is very toxic. Chromium VI compounds are readily soluble in water.
Nutrients are chemical elements that are essential to plant and animal
nutrition. Nutrients include ammonia, urea, ammonium nitrate, and ammonium
sulfate, potassium chloride, and diammonium phosphate. They enter aquifers
from rain and irrigation water leaching the compounds from the soil
after fertilizer was applied by households and farmers.
Average annual commercial fertilizer application rates for nitrogen
From Natural Resources Conservation
Service, Model Simulation of Soil Loss, Nutrient Loss and Soil Organic
Carbon Associated with Crop Production
About 11.5 million metric tons per year (Mt/yr)
of nitrogen in all forms is used in fertilizers in the United States.
Ammonia represents about 32 percent of the total fertilizer nitrogen
used; urea and urea-ammonium nitrate solutions together represent 37
percent; ammonium nitrate, 5 percent; and ammonium sulfate, 2 percent.
Phosphate rock, when used in an untreated form,
is not very soluble and provides little available phosphorus to plants,
except in some moist acidic soils. Treating phosphate rock with sulfuric
acid makes phosphoric acid, the basic material for producing most phosphatic
fertilizers. Phosphatic fertilizers include diammonium phosphate (DAP)
and monoammonium phosphate (MAP), which are produced by reacting phosphoric
acid with ammonia, and triple superphosphate, produced by treating
phosphate rock with phosphoric acid. More than 90 percent of the phosphate
rock mined in the United States is used to produce about 12 Mt/yr of
phosphoric acid. Domestic consumption of phosphate in fertilizers has
averaged 4.5 Mt/yr since 1994.
Potassium is found in potash, a term that
includes various mined and manufactured salts; all contain potassium
in a water-soluble form. Potash is produced at underground mines,
from solution-mining operations, and through the evaporation of lake
and subsurface brines. Minerals mined for potash include potassium
chloride [KCl or muriate of potash (MOP)], potassium-magnesium sulfate
[K2SO4·MgSO4 or sulfate of potash magnesia (SOPM)], or mixed
sodium-potassium nitrate (NaNO3+KNO3 or Chilean saltpeter). Manufactured
compounds are potassium sulfate [K2SO4 or sulfate of potash (SOP)]
and potassium nitrate (KNO3 or saltpeter). The United States consumes
about 11 Mt/yr tons of potash of all types and grades.
From U.S. Geological Survey Fact Sheet 155-99 on Fertilizers
-- Sustaining Global Food Supplies.
Nitrate concentrations in groundwater is
highest in areas of well-drained soils and intensive cultivation
of row crops, such as corn, cotton, or vegetables. Low concentrations
are found in areas of poorly drained soils and where pasture or woodland
is intermixed with cropland in agricultural areas.
and Helsel (1996).
Alley, R. B., J. Marotzke, et al. (2003). Abrupt climate change. Science 299
Moran, M. J. (2006). Occurrence and Implications of Selected Chlorinated
Solvents in Ground Water and Source Water in the United States and in
Drinking Water in 12 Northeast and Mid-Atlantic States, 1993–2002.
U.S. Geological Survey. Scientific Investigations Report 2005–5268.
Mueller, D.K. and D.R. Helsel. (1996). Nutrients
in the Nations's Waters—Too Much of a Good Thing? U.S. Geological
Survey Circular 1136. 24pp.
Pollution Primer, Virginia Tech.
4 August, 2009