Groundwater Monitoring Systems

October 10th, 2019

Groundwater contamination and low groundwater levels are serious issues. These problems can affect drinking water, irrigation, and municipal water supplies. NexSens groundwater monitoring systems offer an in-situ solution for both challenges, tracking groundwater quality and level in real-time.

Learn all about groundwater, monitoring groundwater, and instruments for protecting groundwater quality here.

What is groundwater?

Groundwater simply refers to water originating beneath the Earth’s surface. An important part of the planetary water cycle, groundwater begins as precipitation. As rainwater and snowmelt percolate through the surface of the ground, whether by natural or artificial means, that moisture collects as groundwater.

Precipitation that doesn’t become groundwater or get used by animals and plants, such as stormwater runoff, might be wasted. It can even cause damage as it reenters the ecosystem, as when agricultural runoff that is too high in nutrients causes dead zones in waterways.

How much precipitation can become groundwater depends on how much water the ground can absorb. This is determined by soil type. Very porous soils, which may be sandier, soak up water far more quickly than less porous soils such as clay. (Loam or “loamy” soil is good for growth because it is porous enough to allow water to move through it, but not so porous that it becomes waterlogged easily.)

Where groundwater is located, the saturated soil acts like a sponge. In these places, that saturated soil is called an aquifer. The boundary of an aquifer is its basin. Basins that hold groundwater form naturally over time—depending on the local geology, it could take several years to centuries.

Aquifer groundwater is among the most abundant sources of drinking water. It is also naturally filtered by soil, sand, and rock.

This ongoing process helps remove potentially dangerous organisms and organic material from groundwater, something that doesn’t happen automatically in surface water. Even so, it is still critical to treat groundwater for quality.

Who Uses Aquifer Water

The aquifer is an underground layer of water that rests in the nooks and crannies of sand, soil, and rock in certain porous areas. Communities that rest on or close to an aquifer rely on this groundwater for irrigation, and drinking water supplies, both municipal and private.

In fact, groundwater represents at least 30 percent of the global freshwater supply. The United States is even more reliant on groundwater.

More than 13 million households across the US depend on private wells that draw on groundwater. Approximately one-third of public water supply systems in the US draw on groundwater sources. Altogether, around 44 percent of Americans source drinking water from groundwater between private wells and public supplies.

Without groundwater, American agriculture would look much different. Currently, agricultural irrigation systems across the United States use about 53.3 billion gallons of groundwater daily. This doesn’t include the aquaculture and livestock industries, which use another 3.2 billion gallons of groundwater daily.

Beyond drinking water and agricultural uses, groundwater is used in industries such as mining and manufacturing, and to generate thermoelectric power. These uses consume almost 80 billion gallons more groundwater every day.

Why Monitor Groundwater Levels?

It is critical to monitor groundwater levels for several reasons. Water managers and other decision-makers must monitor groundwater levels to determine how aquifer levels change when groundwater is pumped compared to when conditions are static. They need to explore how surface development affects groundwater levels and the aquifer. Finally, they need to better understand how local sources of surface water and groundwater interact.

Pumping Groundwater: Consequences of Excessive Extraction

Of any human activities that can affect the levels of groundwater an aquifer stores—and how fast it recharges—pumping for use on the surface is the most impactful. In fact, excessive groundwater pumping can lower the water table.

One reason it is essential to monitor groundwater levels is to better predict and prepare for the impact of new wells. For wells to function, they must draw groundwater from beneath the water table. It’s important to know where to place a new well, and how much it is likely to draw down the aquifer locally.

Water managers must also monitor groundwater on an ongoing basis to ensure that wells don’t go dry. Knowing the groundwater levels helps decision-makers know how much groundwater to pump safely, without harmful impact on the aquifer.

When groundwater levels get too low, a host of new problems ensue. Existing wells must be deepened, or in some cases, their pumps must be lowered. In either case, these are expensive solutions.

In many situations, new wells must be drilled. This is even more costly, and as the water table drops, the energy it takes to pump it to the surface increases. The task of reaching the groundwater may eventually prove impossible, or at least cost-prohibitive.

In many regions, groundwater levels and water quality are closely connected. For example, ocean water can invade a freshwater aquifer in coastal regions at times. Particularly since groundwater withdrawal demand typically exceeds supply in these densely populated areas, this means that over-taxed coastal groundwater aquifers sometimes cannot achieve a sufficient recharge rate.

Land Development and Groundwater Levels

Surface development changes the face of the land and groundwater levels beneath the surface. Draining wetlands, cutting down forests, and developing urban areas can all cause much faster water runoff than normal. This, in turn, slows the recharging of the aquifer.

As the water table sinks lower, it takes more energy to pump groundwater to the surface. This means that as groundwater levels drop, the cost of water increases.

When the water table drops, it can cause land subsidence or a loss of underground support. Pumping groundwater can lead to this kind of subsidence as extraction leaves a void, allowing the soil to dry, contract, and settle.

More groundwater extraction means more settling, and potentially more serious damage to local communities, including cracks in walls, roads, or foundations, and even sinkholes. Real-time groundwater monitoring is the most effective way to protect the groundwater for use by the community.

Surface Water and Groundwater Interaction

Most people see groundwater and surface water as totally distinct. However, far more interaction takes place between surface water—such as rivers and lakes—and groundwater than meets the untrained eye.

Some of the water in most streams and rivers seeps into the waterway from groundwater stores—about 30 percent on average in most climatic and physiographic settings. In some locations, large amounts of river and stream water come from groundwater.

Historical information on groundwater levels is essential to ensuring accurate forecasts of surface water and groundwater interactions. During droughts, streamflow relies even more on groundwater contribution. In fact, drinking water and agricultural use tend to climb during dry spells as well, so climate change has a major effect on groundwater levels.

Today, the Environmental Protection Agency (EPA) points out that the American West in particular faces groundwater supply challenges. These problems will only worsen as the already limited groundwater supplies in these regions shrink, and rising populations increase demand. Furthermore, if weather patterns continue, the Western US will see less rain and more periods of drought in the coming years.

How to Monitor Groundwater Levels

There is more than one way to measure groundwater levels. Selecting the right instrument for taking groundwater level measurements depends on various factors. The kind of wells located nearby, the type of pumping they engage in, the range of local water quality issues that exist, and both ease and accuracy of measurement all affect your choice of groundwater instruments.

Most wells are small and difficult to access. This makes steel tape that is weighted at the end a good option in many locations because it avoids variability in measurements and does not stretch.

In other locations, tape sounders or electronic measuring tapes are a better choice. These are a pair of insulated wires, separated. When the electrode is lowered into a well and makes contact with water, the tape makes a sound or lights up so the user knows the water completed the circuit.

There are even specialty tape sounders for unique conditions, such as those which can measure a layer of oil on top of the water in wells contaminated with hydrocarbons. Newer sonic technologies have also arrived, allowing users to measure the water level’s depth.

Many wells are difficult from a monitoring standpoint because they are capped or constructed in ways that make access difficult or impossible. In these situations, an air line may be used to measure groundwater levels.

An air line is really just a small diameter tube or pipe linked to an air pump and a pressure gauge. The line is inserted into the well to a depth of about 10 feet lower than the lowest predicted water level. The air pump then pushes air into the line until it displaces the water. The gauge subtracts the submerged length from the total air line length, leaving you with the water level.

Of course, each of these tapes and lines is not a real-time, continuous monitoring solution. However, in most cases, that technology is now available.

Automatic data loggers and pressure transducers provide optimal continuous, long-term groundwater level monitoring. Pressure transducers are underwater sensors that use ceramic, silicon, or stainless steel membranes and gauges to generate current. They then convert that electrical current into a water level rating by calibrating it to a pressure rating in PSI.

Data loggers work in tandem with pressure transducers, recording the pressure readings and either saving them for future use or transmitting them, depending on telemetry options. Data loggers also work with software that can enable calibration of the pressure transducers in the field, improving accuracy.

Groundwater Quality

It’s critical to confirm that groundwater supplies are sufficient, but that is not the end of the inquiry. A full well that is contaminated is a danger—sometimes forever.

Water is a natural solvent and can contain any number of dissolved compounds. Groundwater has even more opportunity to dissolve substances as it passes through soil and rocks, so it is often even more prone to contamination despite the Earth’s natural filtering mechanisms.

In fact, those natural processes are far more effective at removing larger particulate matter from groundwater, such as insects, leaves, human-made debris, and soil. However, dissolved gases and chemicals can still easily occur in groundwater in large enough amounts to be problematic.

Agricultural, domestic, and industrial chemicals from various industries and even private landowners can contaminate groundwater through runoff. This includes compounds and chemicals such as herbicides, fungicides, and pesticides that many homeowners use in their gardens and lawns, and the use of road salt by municipalities.

Particularly in the northern regions of the US, groundwater contamination by road salt is a serious issue. Salt is highly soluble in water, so as it is spread to melt ice, chloride and sodium levels in groundwater rise.

In rural areas across the nation, septic tanks and the bacterial contamination they cause are the most frequently seen groundwater quality problems. In urban areas with sewage treatment systems, chlorine is often used to kill harmful bacteria. In these rural areas, chlorination may also be necessary if effluent—or leakage and overflow—from a septic tank seeps into wells or the water table.

Groundwater Contaminants

It is easy to find human-induced and natural chemicals and contaminants in groundwater.

Naturally-occurring contaminants and metals such as manganese and iron are present in sediment and rock. As groundwater flows through them, these contaminants dissolve and flow along with the water.

Naturally-occurring chemicals such as hydrogen sulfide may occur in groundwater. Human activity can also introduce oil, chemicals, and other compounds. For example, land near farms, gas tanks, highways, and other human concerns can all cause chemicals to leach into groundwater.

Furthermore, even land that was once used for some other purpose can pose a risk when it comes to drilling a well. An aquifer’s location and physical property certainly play into whether or not the groundwater there will be contaminated. The type of sediment or rock and the overall thickness of the aquifer all influence the percolation process and how readily land surface contaminants can reach water underground.

There is a higher contamination risk for water table aquifers which are unconfined. These water tables have no confining layer stopping contaminants from moving into the groundwater, and they are typically closer to the land surface as well.

It can take years, or even centuries for groundwater to recover from pollution. This is because water moves slowly underground, and it takes time for the sediments to absorb the contaminants.

Why Monitor Groundwater Quality?

Since millions of Americans rely on groundwater, we all need to ensure it is clean and safe. As we monitor groundwater quality, we look for contaminants and other threats from both point sources (specific locations) and nonpoint sources (from broader areas).

Point sources of contaminants that can hurt groundwater include chemical storage areas, landfills, leaking effluent treatment ponds and septic tanks, leaking underground fuel pipelines and tanks, mines and waste tailings, timber treatment sites, and waste disposal sites. Non-point sources include agricultural areas, fertilizer and pesticide applications, and saltwater intrusion.

One frequently overlooked source of groundwater contamination arises from naturally-occurring toxic chemicals such as selenium and arsenic. Although these chemicals can be found in surrounding soil and rock in some regions, such as the American West, it is critical to test for them before using the local ground water.

Today, there are safety procedures in place that guide how we dispose of potentially contaminated industrial waste in all industries. However, there is still ample reason to test groundwater for contamination from industrial use. Many chemicals used in processing, cooling, cleaning, and other industries can persist in soil and water for years and years—and those safeguards were not always in place.

In addition, we’re all only human, so to speak. Even trying to follow safety regulations, chemicals may be handled improperly, leaked, or spilled, especially in transport.

Furthermore, some industries are inherently risky, in terms of contaminants. Mining and ore extraction generates large amounts of hazardous waste, for example, as do very large factory farms and slaughterhouses.

Landfills present a particular problem with relation to the aquifer and groundwater. Modern landfills have liners designed to guard the soil underneath them against hazardous chemicals leaching through—although these are not completely leak-proof forever. Older landfills lack even these liners, and rainwater carries chemicals from discarded batteries, household cleaning items, electronics, and other sources of hazardous compounds down into the groundwater.

The issue of groundwater quality testing is even more complex because not all contaminates directly mix into the aquifer. Instead, some become longer-term sources of contamination as they pool under the soil, tainting any groundwater that eventually reaches it.

For example, the US EPA estimates that many underground storage tanks leak petroleum or other hazardous substances into the soil around them. In fact, tanks over 20 years old are significantly more likely to leak. Areas around and beneath these tanks can present problems that demand monitoring for years to come.

It is important to monitor groundwater quality around roads and highways, especially those with high volumes of traffic. Auto fluids, corroding metals, exhaust emissions, lignin or oil on dirt surfaces, road salt, and wear from pavement and tires can all cause contaminants to seep into groundwater around roadways.

To ensure that drinking water stays safe in rural areas where septic tanks are in use, it is essential to monitor aquifer and well water quality. When tank wastewater can leak into soil and contaminate the underlying aquifer, the drinking water supply of local homeowners is at risk—principally from bacteria such as E. coli.

In agricultural areas, pesticides and natural or manufactured fertilizers might contaminate groundwater. Fertilizer’s nitrogen turns into nitrate which water then readily moves through the soil and into the aquifer.

For infants, the US EPA has established a safe drinking water standard for nitrate of 10 mg/L, because babies are vulnerable to nitrate poisoning, which can cause fatally low oxygen levels in the blood. Nitrate contamination can accumulate and persist for years, so monitoring for nitrates in agricultural areas is important.

Similarly, pesticides can persist in groundwater for many, many years. In addition, without monitoring and testing, it isn’t always obvious to water consumers when their groundwater has been tainted by pesticides—until it is too late.

The Ground Water Rule (GWR) and Monitoring

There are legal reasons to monitor groundwater, including the US EPA’s 2006 Ground Water Rule (GWR). The Ground Water Rule guards against microbial pathogens by requiring disinfection of small groundwater systems. Although formation of byproducts from disinfection processes is less of a worry with groundwater since it is typically low in organic matter, these groundwater systems may be vulnerable to contamination by fecal matter or other disease-causing pathogens.

The GWR most frequently impacts public groundwater treatment systems, but that’s not the extent of its reach. It also affects any system that directly blends ground- and surface water and then distributes the water without treatment.

If you’re not sure whether the Ground Water Rule affects your system, check the EPA’s website for more information.

Designing a Comprehensive Groundwater Monitoring System

With so many potential groundwater contaminates out there, more and more municipalities, universities, states, and other entities are designing comprehensive groundwater monitoring programs. Whether an area has agricultural regions, trace elements of concern in naturally higher concentrations, or just industrial and urban areas to watch, there are countless possible sources of groundwater pollution.

A comprehensive groundwater monitoring system can help ensure groundwater retains its beneficial uses and stays usable for generations to come. Such a system works best when it sets forth programs and rules for protecting groundwater quality and outlines which individuals or agencies will implement those programs and enforce the rules.

There is no reason to start from scratch when you’re trying to ascertain how to monitor groundwater, or how to develop a ground water monitoring system. The United States Geological Survey (USGS) through its National Water-Quality Assessment (NAWQA) program has defined groundwater assessment in three stages: status, trends, and understanding. In many cases, there is also a corrective action stage directly after assessment.

Status refers to assessing groundwater quality as a resource right now. Trends focus on watching for overall changes to groundwater quality over time. Understanding refers to studying how groundwater quality changes based on natural and human factors.

To ensure any comprehensive groundwater quality assessment or monitoring system is effective, its standards and protocols must be applied consistently and uniformly. On the other hand, local problems must be addressed flexibly. One way to achieve this is to assess local conditions from basin to basin carefully, establishing conditions for each hydrogeologic province.

Collecting Groundwater Quality Data

Groundwater quality assessment is limited without the right data. Ancillary and existing data are important to use in the assessment as well as data from ongoing monitoring.

The most important ancillary data for groundwater quality assessment include data that describe the characteristics and location of sampling points, such as open interval length and well depth. Hydrogeologic context such as data on rock type, sediment, or water levels is also critical for analysis of groundwater quality samples. Ancillary data on potential contamination sources is also important.

To find these kinds of ancillary data, look for state-level records on where pesticides have been applied, and where leaking underground fuel tanks are located. Records at the state level can also reveal things like potential mining contamination from drillers’ logs and where point sources of contamination are located.

Of course, it’s never safe to use groundwater at all without reviewing existing data, and that kind of review is an ongoing part of ground water quality monitoring and assessment. We, therefore, recommend that teams assess any available local, state, and federal groundwater quality data that relate to the assessment of aquifers in question.

One of the goals of a real-time groundwater monitoring system should ultimately be the ability to assess ground water resources consistently across regions at all scales. This means choosing a design and selecting wells that are distributed properly, and sometimes it can mean including some randomization in the process. Some networks might also add specific local sampling stations to address local concerns.

Most groundwater monitoring systems will focus on existing wells used for public supply, given the importance of maintaining ground water levels and quality. Using existing public wells allows decision makers to sample all major aquifers in a region systematically. It also ensures testers are sampling from sources with high pumping capacity and long well screens, and getting more of the aquifer’s volume in the sampling process. Furthermore, data from these public wells are easier to cross-check against other data since the wells are usually located near urban centers.

In some places, there may not be adequate coverage sampling solely from public supply wells. Where this is true, domestic supply wells, irrigation wells, or even monitoring wells can be sampled for target constituents.

Select target constituents based on which groundwater quality issues are most relevant in your region. Determine that based on several goals: protecting beneficial groundwater use, understanding how natural and human factors impact groundwater quality, and identifying and detecting unregulated “emerging contaminants” that are potentially concerning.

A tiered approach is often the most efficient way to achieve these goals. For example, the largest tier may make use of existing data to determine what beneficial groundwater use looks like in the local area. The second tier typically narrows a bit, sampling for a reduced list of contaminants across the board in a network of wells. The third tier may then target fewer wells, but sample for a “relatively expanded” list of constituents and emerging contaminants.

In this way, a program maximizes resources and tests groundwater optimally, ensuring goals are met.

Monitoring Data for Groundwater Quality Trend Assessment and Understanding

Showing changes or trends in the quality of groundwater over time is the end goal of any smart ground water monitoring system. To ensure this goal can be achieved, it is essential to sample and resample all of the same wells in the same networks at least twice over a period of time. Supplement these efforts with additional, ongoing sampling at some percentage of network wells.

The pattern and spacing of this kind of monitoring accomplish several things. First, it provides two complete sets of data conforming to set protocols within a period of time. This allows researchers to perceive any patterns over time and more confidently draw conclusions about those patterns. It also provides context and can tip off policymakers sooner as new contaminants are detected.

In places where scientists and water agencies already monitor groundwater basins frequently, that data is valuable. Where researchers are observing rapid changes and new trends, more frequent groundwater monitoring is useful.

A thorough assessment of both the natural and human factors that impact groundwater quality is also part of a groundwater monitoring system. Specifically, systematic sampling for a particular constituent helps answer why it is found in a groundwater system. This kind of focused sampling of selected wells enables an assessment of water quality parameters for indicators of contaminant and water sources and environmental tracers.

Groundwater Testing

Although groundwater is generally low on natural organic matter (NOM), colloidal particles may still remain in the water despite years of natural filtration through layers of soil. This means ground water may demand coagulants to treat NOM.

Under some fact patterns, especially groundwater under the direct influence of surface waters (GWUDI), the water requires no additional filtration. However, the possibility of bacterial contamination means that ground water testing is always important.

Groundwater may also contain compounds that present a challenge for certain treatment systems. These compounds, such as iron and manganese, don’t typically harm humans. However, as water treatment systems transition into using chlorine in particular, these compounds may be troubling.

Analytical testing with a groundwater monitoring system can help water managers and decision-makers achieve many goals more efficiently. You can treat with coagulants more effectively, and more easily comply with the GWR. This kind of ground water monitoring system can also enable a chlorine disinfection strategy based on measurements you know are accurate.

Any facility must monitor groundwater harmful contaminants and ensure it is safe to use. A better groundwater monitoring system means a more efficient treatment process and an optimal response time when problems arise. This, in turn, improves water quality even as it saves money.

Groundwater monitoring systems should monitor a variety of parameters to provide a full view of water health. External conditions such as rainfall levels, temperature, and surface water levels in holding tanks and rivers are all important to monitor. Standard water quality parameters, of course, provide a basic sense of water safety, and operational parameters for the system such as current, pump speed, and voltage all confirm the system is working well.

In monitoring wells, pumped wells, and piezometers, groundwater levels are critical to watch. Pumped flow rates from pumps and wells are also important.

Collect all this data with a logger electronically. This enables real-time data sharing via whatever telemetry option is available. It also alerts stakeholders to problems immediately, and implement planned remediation measures. Real-time data also empowers managers to detect longer-term trends and performance issues.

How to Monitor Groundwater Quality

There are many instruments that can be used to monitor groundwater quality. Which is the right one for your project depends on the conditions at your site, how often you plan to monitor, and which parameters are of interest.

Often, it’s not possible to directly measure a specific compound in the aquifer. When that’s true, sampling often takes place using boreholes as narrow as two inches across—much as it does when monitoring groundwater levels.

However, the right tools are still essential to this sampling process. To collect readings on water quality and simulate ambient flow, the low flow groundwater sampling technique is employed in many regions.

Low flow groundwater sampling is a methodology used around the world to simulate ambient flow conditions and take water quality readings. This kind of technique demands a pump that can draw the sample to the surface and work together with a flow controller. Various styles of pumps can be used in this role.

After collection, it’s time for analysis. The sample might be in transit to the lab, or in some cases, onsite analysis is possible. In these cases, depending on which compounds are the targets, a portable spectrophotometer or photometer be used on-site to analyze the groundwater.

Selecting the ideal multi-parameter probe for the job is crucial once you’ve identified the right sampling methodology. A portable spectrophotometer can measure many target chemicals in groundwater, or a handheld meter can take spot measurements of physical properties such as pH or conductivity.

Some instruments can even measure multiple parameters, such as conductivity, DO, and pH, or particular high-interest ions, such as calcium, chloride, fluoride, and nitrate. To monitor ground water quality and even larger underground aquifers over time, a data logger connected to water quality sensors is an ideal solution.

Monitoring Groundwater for Ammonia and Chlorine

Particularly near agricultural regions, ammonia levels in groundwater systems can reach several mg/L. These levels can also fluctuate with the season, enhancing the need to monitor ground water for ammonia.

Uncontrolled chloramination of water, which happens when ammonia and chlorine react to form chloramines, can cause water to smell and taste bad. It can also lead to nitrification issues in the distribution system.

Sometimes breakpoint chlorination, the addition of free chlorine to convert ammonia into chloramines, is used to destroy ammonia in groundwater. More free chlorine then converts chloramines until the remaining disinfectant is simply free chlorine.

However, free chlorine can prompt the formation of insoluble precipitates when it contacts manganese and iron. Filtration is one way to remove these precipitates.

In most places, water managers must also test groundwater for total chlorine to measure levels of total residual disinfectants.

Iron and Manganese in Groundwater

Most groundwater reservoirs contain iron, which is considered a secondary contaminant. This means its mere presence isn’t an issue, but there is a maximum contaminant level (MCL) for iron in groundwater of 0.3 mg/L.

The soluble form of the mineral, ferrous iron, is the form seen occurring naturally in groundwater most often. Levels of ferrous iron tend to remain fairly constant. The rusty stains we sometimes see on plumbing fixtures are caused by ferrous iron, oxidizing with air as it reaches the surface.

However, iron levels in groundwater are critical to monitor, because they can change and become dangerous. Iron might occur in water as plumbing corrodes or with iron-reducing bacteria. Furthermore, major changes to the water table caused by anything from well drilling to seismic events might impact groundwater iron levels.

Where iron is present in groundwater, manganese is often present as well. Manganese is also a secondary contaminant, and its maximum contaminant level (MCL) is 0.05 mg/L. When manganese levels in water become too high, you might see black stains on anything the water touches.

Monitoring Groundwater for pH, Hydrogen Sulfide

The pH of groundwater can fluctuate significantly by region. It’s important to monitor groundwater pH to determine any necessary adjustments for optimal chlorine disinfection. Monitoring the pH of groundwater is also a great way to detect events at the groundwater source rapidly.

As anaerobic decomposition of organic matter takes place in water, sulfate-reducing bacteria produce toxic hydrogen sulfide—the author of the rotten-egg odor you might sometimes smell in water. This nuisance is found mostly in groundwater supplies and is easily cured with chlorine or aeration.

The Bottom Line

The NexSens team has deep bench expertise in the design, development, and deployment of monitoring systems. We can help you create the bespoke groundwater monitoring system that achieves each of your goals.

 Top image: Gathering ground water samples to test for iron levels. (Credit: CSIRO [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)])

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