What is groundwater?
Simply put groundwater is water beneath the surface of the Earth. Groundwater starts as precipitation
and the portion of the rain water that infiltrates beneath the ground's surface, either naturally or artificially,
becomes groundwater. The remaining portion of the precipitation is used by plants, evaporates, or becomes
surface water runoff which can either add to groundwater levels in other areas or be increased by groundwater
outflows depending on the geology the surface water travels through. The amount of precipitation that
gets absorbed and becomes groundwater depends on the soil type. Highly porous soils, such as sandy
soils, absorb water much faster than soil such as clay which has very small pores. The saturated soil
acts like a sponge and the area where groundwater is present, or saturated, in the soil is called an aquifer,
which generally has a boundary defined as its basin. Groundwater basins are formed naturally over a
period ranging from several years to more than a millennium in some geologies.
Who uses groundwater
Groundwater is critical to the communities that are built on or near the aquifer, or the underground layer of
water that fills cracks in the rock or sand that makes up the soil. This groundwater layer which makes
up about 30 percent of the world's freshwater supply is used for many purposes including irrigation, private
drinking water, and municipal water supplies.
Groundwater is a key part of the United States ability to irrigate its farmland. Irrigation systems
account for the use of about 53.3 billion gallons of groundwater per day for agriculture watering. This
usage can be compared to the 1900s when the US only used 2.2 billion gallons for irrigation. Additionally
the livestock and aquaculture industries consume approximately 3.2 billion gallons of groundwater per day.
Information regarding groundwater use for drinking water worldwide is limited, however it is estimated that
one-third of the world's population depend on groundwater as the main source of their drinking water. Over
13 million US households regularly depend on private ground water wells.
Groundwater sources are used by about 33 percent of the public water systems in the United States. Between
public and private water wells about 44 percent of the US population depends on groundwater for their drinking
water. Including drinking water and farming along with other uses of groundwater such as manufacturing,
mining, and thermoelectric power to name a few, 79.6 billion gallons a day of fresh groundwater are used.
Why should we monitor groundwater levels?
Knowing the groundwater level is important for several reasons, including understanding aquifer levels under
static conditions and pumping conditions, determining how the levels interact with local surface water sources,
and understanding how surface development has impacted the aquifer.
Groundwater extraction from pumping
In some areas reducing the water table level can have a great impact on the groundwater's quality. In
coastal freshwater aquifers salt water intrusion can occur when the different densities of both the saltwater
and fresh water allow the ocean water to intrude into the fresh water aquifer. Often coastal groundwater
aquifers support large populations where the demand for groundwater withdrawals exceeds the fresh water
recharge rate allowing salt water to make its way into the aquifer contaminating the water.
Surface water interaction
Pumping groundwater for surface use has the greatest effect on the amount of groundwater stored in an aquifer
and the rate at which it refills or recharges. The most severe consequence of excessive groundwater pumping
is that the water table can be lowered. It is important to monitor and understand the groundwater levels
prior to drilling any wells which will have significant drawdown of the aquifer because the water table levels
will give a good idea of the impact of the new well. For water to be withdrawn from the ground, water
must be pumped from a well that reaches below the water table. The data collected by monitoring groundwater
can be used to determine the amount of groundwater that can safely be withdrawn before no more water can be
pumped. In this way, local water managers can prevent wells from going dry and prevent the movement of
poorer quality groundwater into the aquifer. If groundwater levels decline too far, then the well owner
might have to deepen the well, drill a new well, or, at least, attempt to lower the pump which could become
very expensive for the owner. In addition to the cost of increasing the depth of the well, drilling a new
well, or moving the pump down as the depth to water increases, the water must now be lifted higher to reach the surface. If
pumps are used to lift the water (as opposed to artesian wells), more energy is required to drive the pump which
results in more expensive water. Eventually a deep well could become prohibitively expensive to pump water
There is more of an interaction between surface water, such as lakes or rivers, and groundwater than most people
think. Some, and often a great deal, of the water flowing in rivers come from seepage of groundwater into
the streambed. Groundwater contributes an average of 30 percent of the water in streams and rivers in most
physiographic and climatic settings. The proportion of stream water that comes from groundwater inflow
varies according to a region's geography, geology, and climate. Having historical groundwater level
information is important to forecast the impact of the level on local streams or rivers. During dry periods
the groundwater contribution to the stream flow becomes especially important. In fact, climate change has
a significant effect on groundwater levels. During droughts, not only is the groundwater aquifer contributing
a greater percentage to any stream flows, but it's depletion is generally increased significantly by greater need for farming
and drinking water use. According to the EPA (Environmental Protection Agency) many areas of the United
States, especially the West, currently face groundwater supply issues. The amount of groundwater available
in these areas is already limited, and demand will continue to rise as population grows. The Western United
States has experienced a growing reduction of rain throughout the past 50 years, as well as increases in the
severity and length of droughts; this has been especially concerning to Southwestern communities.
Land development projects
Surface development projects have a huge impact on the groundwater level. For example deforestation, drained
wetlands, and urban development cause water to runoff much faster than it would normally. This leads to
reduced charging of the underlying aquifer. In addition to the surface water interaction mentioned earlier
additional concerns include the increased cost of water and land subsidence. The cost of water increases as
the depth of the water table increases due to costs associated with the energy required to lift the water
further. Land subsidence, or sinking in, is caused by the loss of support underground. Groundwater
extraction can cause subsidence by leaving a void where the water was and drying the soil allowing it to shrink
and settle. As groundwater is continuously taken out of the soil the likelihood increases that the ground
will settle to fill the empty spaces left behind. This settling can be the source of major damage to the
local communities including cracks in foundations, walls, roads or potentially even sinkholes. Effective
groundwater monitoring is the best way to protect the local community, ensure a dependable and affordable groundwater
supply, and protect the quantity available for future use.
How to monitor groundwater levels?
Groundwater level measurements can be made with many types of instruments. Choosing the right type of
equipment depends on factors such as accuracy or ease of the measurement, water quality issues, the type and
pumping activity of the well or nearby wells.
Steel tape is the most accurate method of taking a water level reading. Steel tape does not stretch
like some of the other instruments, so it doesn't introduce significant variability in the measurements. It
is graduated and has a weight on the end to ensure it hangs vertically in the well.
Electronic measuring tapes or tape sounders are made up of a pair of insulated wires which are separated and
when the electrode contacts the water the powered circuit is completed causing a light and/or sound to indicate
the contact. Some tape sounders, like Global Water's WL500,
include a steel core to achieve greater accuracy like the standard steel tape. For wells contaminated
with hydrocarbons such as oil special tape sounders, like Global Water's WL550,
can be used to measure the oil layer that sits on the top of the water. In recent years an innovation on
the standard tape sounder has emerged. These next generation sounders use sound waves to measure the depth
of the water level. Sonic sounders, such as Global Water's WL650,
can measure up to 1200ft (365m) saving operators significant time during the measurement process.
There are wells where there is no access port or the well cap is not easily removed. Wells of this nature
will often use an air line for groundwater level measurement. Using this method a small diameter pipe or
tube is inserted from the top of the well to a point about 10 ft (3 m) below the lowest expected water level. The
tube is connected to a pressure gauge and an air pump is used to pump air into the line until the water inside
the line is displaced. The gauge reading indicates the length of submerged air line which can be converted
into the water level by subtracting the submerged length from the total length of the air line.
Pressure transducers and automatic dataloggers are ideal for long term or continuous monitoring of groundwater
levels. Pressure transducers are submersible sensors that use some sort of membrane, silicon, stainless
steel, or ceramic, as a strain gauge to generate an electrical current. The electric current is calibrated
to a pressure rating in pounds per square inch which can then be converted into a water level rating. Pressure
transducers, such as Global Water's WL400, generally
use a vented cable to eliminate atmospheric pressure changes
from affecting the reading; however some types of pressure transducers do not. These pressure transducers
use an absolute pressure value to determine their pressure readings and must be compensated by using external
barometric pressure readings. Dataloggers are generally attached to pressure transducers so that the pressure
readings can be recorded for future use. Datalogger software, such as for Global Water's
WL16 Water Level Logger, allow a great amount of flexibility
in groundwater level measurements ranging from logarithmic recording modes for pump tests to fixed recording
intervals of up to 10 samples per second. The software can also allow you to field calibrate the pressure
transducers providing better accuracy. Standard dataloggers require field personnel to visit the site to
download the recorded data. As an alternative pressure transducers can also be connected to remote telemetry
systems such as the iRIS350 Cellular datalogger, the
RM100 Radio System, or SIT65
Satellite System. With a telemetry solution the groundwater level readings can be transmitted to a remote
location and gathered centrally.
Why should we monitor groundwater quality?
A groundwater well may have high water levels; however it may still be unsuitable for drinking water. Water
can easily become contaminated because it is a very good solvent and can contain many dissolved chemicals. Rain
water or surface water can come into contact with contaminated soil while seeping into the ground, from that point
it can become polluted and carry the pollution from the soil to the groundwater aquifers. Groundwater can
also become contaminated when liquid hazardous substances soak down through the soil or rock into the groundwater.
One source of groundwater contamination often overlooked by the general media is contamination from naturally occurring
chemicals found in the surrounding rock and soil. Groundwater is not static and as it moves through the ground
it dissolves the rock and soil along with any chemicals they may contain. For example, toxic chemicals such
as arsenic and selenium are common in some types of rock formations and groundwater near those sites should not be used
as drinking water sources until the it is treated.
Many industrial businesses use water for cleaning, cooling, or processing. When this used water returns to the
natural system it can contaminate the groundwater if it has not been properly treated. Chemicals are often
transported and can cause groundwater contamination due to spillage, leakage, or improper handling. The mining
industry generates a large amount of potentially contaminated waste during the processing of extracting ore. This
waste is discarded to designated waste sites where rainfall can carry the contaminates into the groundwater aquifer
below. Older landfills were built without liners to protect the soil beneath. Rainwater leaches chemicals
from batteries, electronic equipment, discarded household chemical containers, and many other sources in these landfills
carrying them down to contaminate the groundwater. While newer landfills have liners under them there is the
possibility that the liners could leak transferring contaminates below. It is important to note that some
contaminates do not mix directly with the aquifer. The contaminates pool under the soil and can act as a long
term groundwater contamination source as groundwater comes in contact with the pool over time.
Many industrial businesses including gasoline stations and dry cleaners store fluids in underground or above ground
tanks. In cold areas heating oil is often stored in underground or basement tanks. Above ground tanks can
leak due to hose connections failing or spillage during fluid transfer. Tanks that are buried can cause even
greater groundwater contamination because small leaks often go undetected. The US EPA has estimated that one
out of every four underground tanks is leaking into the surrounding soil. If the tank is over 20 years old the
likelihood that the tank is leaking increases significantly.
Roadways, especially those with high traffic, are another source of groundwater contamination as contaminates deposited
on the road's surface are dissolved away by surface water runoff and then percolate into the local aquifer. Roadway
contamination can come from many sources including exhaust emissions, pavement and tire wear, auto fluids, and corrosion
of metals. In areas where oil or lignin is applied to roads to minimize dust those chemicals can be introduced
to the groundwater system. In northern areas road salt is a major source of groundwater contamination. In
these areas salt is spread on roads to melt ice which then makes its way to the groundwater below.
A common water quality issue in rural areas comes from septic tanks. In areas where septic tanks are used in
place of a sewage treatment system there is the possibility that wastewater from the tank could overflow or leak into
the surrounding soil. Once in the soil the wastewater can percolate through the soil eventually contaminating
the underlying aquifer and potentially the homeowner's drinking water supply.
Agriculture has two common sources of groundwater contamination, fertilizer (natural or manufactured) and
pesticide. Fertilizer contains nitrogen which turns into nitrate. Water can easily carry nitrate through
the soil and into the groundwater aquifer. This contamination can last for decades and can accumulate to high levels over
time. While nitrate is not generally an adult health threat the EPA established a drinking water standard of 10
mg/l to protect infants from a potentially fatal condition where nitrate can cause low oxygen levels in the blood.
Pesticide use in the United States grew as the desire and need for more crops and food for consumption or export
grew. The US is one of the largest producers of food in the world, partly due to the pesticides being used to
protect its crops. Pesticides have many paths into the groundwater supply including seepage from contaminated
surface water, spills and leaks during transportation or storage, improper disposal, and application to the
crop. Contamination from pesticides may take decades to become apparent to the people living on the contaminated
groundwater aquifer. It is estimated that 95 percent of the people living in agricultural areas rely on
groundwater for their drinking water.
How to Monitor Groundwater Quality?
Groundwater quality measurements can be made with many types of instruments. Choosing the right type of equipment
depends on factors such as what chemical or property is being monitored, how often you want to measure the water, and
what the conditions of the monitoring site are.
In many cases there is no way to measure a particular chemical directly in the groundwater aquifer. In these cases a
sample needs to be pulled up from the groundwater aquifer. Global Water offers several ways to obtain a sample
easily and conveniently. Groundwater pumps like the GP submersible pump series
or the WP purging pump series can pull a groundwater sample
up from a water table up to 90 ft (27.4 m) deep. Global Water's handheld
or automated samplers can pull up samples from about 20 ft
(6 m). For groundwater levels deeper than 90 ft (27.4 m) a larger pump will need to be used or a sample can
be taken from a tap in the well line.
Once a groundwater sample has been collected it could be taken to an analytical laboratory for analysis or, depending on
the chemicals in question, a portable photometer or spectrophotometer could be used to analyze the sample at the
monitoring site. The pHotoFlex photometer can measure
more than 35 different types of chemicals using reagents to test the groundwater sample. A portable spectrophotometer
such as the photoLab has the ability to measure over 50
chemicals in your groundwater.
To measure physical properties such as conductivity or pH a handheld meter is an easy way to take spot measurements. Global
Water offers a wide variety of water quality meters to measure these parameters such as the
pH3110 pH meter or the Cond3110
conductivity meter. Multiparameter meters are also available for those cases where you need to measure more than
one parameter. Instruments such as Multi3410 and the
Pro Plus can measure multiple parameters including pH, DO,
and conductivity using a single instrument. Some handheld meters also give you the ability to monitor certain chemical
ions such as nitrate, chloride, fluoride, or calcium. The Pro Plus
or the pH/ION 3400i meters offer this monitoring feature.
For longer term monitoring or groundwater aquifers that require more constant monitoring water quality sensors attached
to a datalogger are a good solution. Global Water offers several water quality sensors with 4-20mA output that can
be connected to an onsite datalogger, a
radio system, cellular system,
or satellite system. The water quality sensors available can
monitor pH, DO,
turbidity, and ORP.