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, or biological
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
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. (Texas
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
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
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 into aquifers.
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 TCEQ (RRC).
From Texas Commission on Environmental Quality web page on Injection
Wells: Am I Regulated?
Diagram of an injection well. From Pollution Issues
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
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 in
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
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
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
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
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 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.
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
From U.S. Geological Survey Fact Sheet
155-99 on Fertilizers
-- Sustaining Global Food Supplies.
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.
Reston, Virginia, U.S. Geological Survey. Scientific Investigations Report
Pollution Primer, Virginia Tech.
23 December, 2008