Assabet River Stream Watch

Organization for
the Assabet River
9 Damonmill Sq., Suite 1E
Concord, MA 01742
Tel. (978-369-3956)

Water
[Tour of Groundwater | Water Use | Water Balance | Groundwater Model | Back to Main]

A Tour of Groundwater and Surface Water

What is groundwater?
Aquifers
Where are our aquifers?
Glaciers created our aquifers
Where are the good public well sites?
Groundwater in the watershed
Water in the well

These pages are adapted, with permission, from a series of Groundwater Information Flyers published by the Massachusetts Audubon Society. This information may be copied. Please give credit to the Massachusetts Audubon Society. Read on or download a pdf version of this document (Groundwater.pdf).

Adapted from:
Massachusetts Audubon Society: Groundwater Information Flyer #1 "An Introduction to Groundwater and Aquifers". November/December 1983; Revised January 1993.

Massachusetts Audubon Society: Groundwater Information Flyer #2 "Groundwater and Contamination from the Watershed into the Well". January/February 1984; Reprinted May 1985.

WHAT IS GROUNDWATER?
Groundwater is the water beneath the surface of the ground in the zone of saturation where every pore space between rock and soil particles is saturated with water. Above the zone of saturation is an area where both air and moisture are found in the spaces between soil and rock particles. This is called the zone of aeration. Water percolates (moves downward) through this zone until it reaches the zone of saturation. The water table is the top of the saturated zone.

Where Does It Come From?

It's not as mysterious as it seems. The real sources of groundwater are rain and snow. Rain and melting snow percolate into the ground and saturate the pores between rock and soil particles. Geologists call this process groundwater recharge and, the places where it occurs, recharge areas.

Where Does It Go?

Once it reaches the zone of saturation under the ground, groundwater begins to move slowly by the force of gravity through the interconnecting pore spaces until it reaches a discharge area, where it seeps or flows out into a wetland, spring, river, or pond to become part of the surface water.

Water evaporates from surface water bodies and from land surfaces and returns to the atmosphere. Plants transpire water into the atmosphere. Water in the atmosphere condenses into rain. Some of the rain recharges the groundwater, and the cycle keeps repeating. Groundwater, in other words, is part of the hydrologic cycle. Groundwater and surface water are interconnected; groundwater becomes surface water when it discharges to surface water bodies. Most streams keep flowing during the dry summer months because groundwater discharges into them from the zone of saturation - this flow is called baseflow. Under certain conditions the flow may be reversed and the surface water may recharge the groundwater. Only a portion of the water that falls as rain or snow in Massachusetts actually recharges the groundwater. The rest runs off into surface water bodies, is taken up by plants and transpired, or evaporates.

Groundwater Movement

Groundwater is always moving from higher recharge areas to lower discharge areas; however, it moves slowly. Groundwater movement is measured in feet per day or, in some cases, in feet per year. In contrast, surface water movement is measured in feet per second. The speed at which groundwater moves is determined by the types of material it must flow through and the steepness of the gradient from the recharge area to discharge area. Water moves more easily through the large pores of sand and gravel, for example, than through material that contains fine silt and clay.

The water table is at the top of the zone of saturation, but it doesn't remain at one level all the time. The rise and fall of the water table is a natural part of the groundwater system. It occurs seasonally each year. In the late winter and early spring (February, March, April), melting snow and rain percolate into the ground to raise the water table to its annual high level. During the growing season, rainwater is used by plants for transpiration or it evaporates. As a result, little or no groundwater recharge occurs during the late spring and summer months. During that time, however, groundwater continues to discharge into streams, lakes, and wetlands, so the water table drops. By fall (October and November), the water table has dropped as much as fifteen feet to its lowest annual level. The groundwater is recharged again by rain that falls after the growing season. There is no recharge in the winter when the ground is frozen, but recharge can occur during midwinter thaws. During the winter, water is stored in the snow pack. In the spring, the melting snow recharges the groundwater, raising the water table to its annual high level again.

The water table also responds to cyclical periods of drought and heavy precipitation that last for several years. For example, starting in August and over the winter of 2001/ 2002 Massachusetts has suffered drought conditions so that by April groundwater levels at the monitoring well in Acton (USGS real-time groundwater conditions: http://waterdata.usgs.gov/nwis/gw) were more than 2 feet below normal for the month.

AQUIFERS

Although groundwater can be found beneath most land surfaces in Massachusetts, not all groundwater can be drawn into wells. To yield significant quantities of water, wells must be located in aquifers. An aquifer is a geologic formation that is capable of yielding a significant amount of water to a well or spring. All the spaces and cracks, or pores, between particles of rock and other material in an aquifer are saturated with water. Water can move through the pores toward a spring or other discharge area or toward a pumping well.

More Pores, More Water: Porosity

The porosity of a material determines how much water it will hold--the more pores, the more water. Porosity is expressed as a percentage of the total volume of a material. For example, the porosity of a certain sand might be 30 percent; that is, 30 percent of the total volume of the sand is pore space and 70 percent is solid material. That means that 30 percent can be filled with water, or more than 2 gallons of water per cubic foot!

Water Moves Through Pores: Permeability

The ability of a material to transmit water is called permeability. It is important to understand this concept because permeability determines whether groundwater can actually be drawn into a pumping well. In consolidated rock, such as granite, permeability depends on how well the fractures in the rock are interconnected. In an unconsolidated material, such as sand and gravel, permeability depends on the size of the pore spaces between the grains of material.

Porosity and Permeability Are Different

Porosity and permeability are related, but they are not the same thing. A material can be very porous and hold a large volume of water but not be very permeable. For example, clay may be twice as porous as sand, but a pumping well will not be able to pull the water from the pores between clay particles fast enough to supply the well. Very small pore spaces create a resistance to flow that reduces permeability. Porosity determines the capacity of the material to hold water. Permeability determines its ability to yield water.

WHERE ARE OUR AQUIFERS?

Bedrock as an Aquifer

Solid rock can't yield water. Groundwater in rock can only be found in cracks, fractures, or in channels created when water enlarges the fractures in certain carbonate rocks (such as limestone).

Bedrock is the rock that lies beneath all the unconsolidated materials (soil and loose rocks) on the surface of the earth. It is the earth's crust. (In New England, bedrock is commonly called ledge.) If a well is drilled into bedrock fractures that are saturated with water, bedrock can serve as an aquifer. In most of Massachusetts, however, bedrock is not highly fractured. Fractures generally occur within the first 100 to 150 feet of the surface, and they tend to be rather small, with few interconnections. Consequently, in Massachusetts, wells that intercept rock fractures can usually yield only enough water for private, domestic supplies. However, there are some highly fractured zones known as faults, where yields in the range of 200,000 to 400,000 gallons per day (gpd) have been developed, primarily for industrial use.

Surficial Deposits as Aquifers

What are Surficial Deposits?

Most people would call it soil, but geologists call the sand, gravel, soils, rocks, and other loose material that lie on top of bedrock "surficial deposits". Because bedrock can seldom yield enough water for a public supply well in Massachusetts, public water supplies must be found above bedrock in surficial deposits.

Porous, Permeable Surficial Deposits Make Good Aquifers

Some surficial deposits are porous and permeable. Most are not. What makes the difference? Most surficial deposits are heterogeneous. They consist of a wide variety of material types and sizes. In these deposits, almost all of the spaces between the large materials are filled with smaller particles. For example, the spaces between pebbles and large stones are filled with sand, and the spaces between the grains of sand are tilled with clay. This leaves few pore spaces for groundwater storage and makes it difficult for water to move through the pores. Thus, deposits that are a mixture of types and sizes of materials are not usually porous and permeable enough to serve as aquifers.

In other surficial deposits, particles are similar in size and do not fit closely together. This creates many interconnecting pore spaces that can hold water. Some of these deposits are very fine-grained silt and clay. They are porous but not permeable because the pores are too small to transmit water easily. In some surficial deposits of similar-sized particles such as coarse sand, the pores are large and water can flow through them easily. These deposits are both porous and permeable and are excellent aquifers.

Large Volume Wells Need Large Sources of Water

The capacity of an aquifer to produce water is determined by the amount of porous, permeable materials that are present and the quantity of water that is available in that material. These factors can be determined for specific aquifers by geologic studies and pumping tests.

To supply a public well, there must be a large volume of water in storage in the aquifer or a nearby source such as a river or a lake that is connected hydrologically with the aquifer. Often, several wells are needed to supply all the water a municipality requires, but even small public wells are considerably larger than private wells serving single households. A small public well might yield 100,000 gpd, while a private well serving a single home might yield only 500 to 1,000 gpd.

Though most areas contain aquifers that will yield enough water for private, domestic wells, public wells must be located in aquifers that are large enough to sustain a consistently high yield over a long period of time. Aquifers that are large enough to supply public wells are found only in locations with certain geologic and hydrologic conditions.

GLACIERS CREATED OUR AQUIFERS

In Massachusetts, porous, permeable surficial deposits were created by melting glaciers. Most of Massachusetts was covered by continental glaciers a number of times in the past 2 million years. The last glacier melted from Massachusetts between 10,000 and 13,000 years ago. Each glacier moved down over Massachusetts from the north carrying with it large quantities of rocks and soil that it had scraped and plucked from the bedrock as it moved across the land surface. When the glacier finally melted, it redeposited this material as glacial debris. Surficial deposits in Massachusetts are made up mostly of glacial debris, topped with a thin layer of soil that has formed since the last glacier melted.

Stratified Drift: Good Aquifer Material

Some glacial debris was carried away by torrents of water that flowed off the melting ice in meltwater streams. At the front of the glacier, these streams flowed so fast that they could transport glacial debris of all sizes except large boulders. As the meltwater moved further away from the glacier, it slowed down. The slower moving water could no longer carry pebbles and gravel, so that debris settled out. Further along, when the water slowed more, sand grains settled out. Still further downstream, the water reached a lake or the ocean, and slowed completely. By this time, only very small particles remained suspended in the moving meltwater stream. When the water stopped flowing after it entered the lake or ocean, the small particles settled out to form very fine deposits of silt and clay on the bottom of the lake. Thus, as they moved away from the glacier, the meltwater streams sorted the rock fragments they carried into separate layers of gravel, sand, and fine sand. These sorted deposits are called stratified drift.

Many stratified drift deposits were eroded and redeposited over and over again by flowing water. This repeated sorting action created porous, permeable, stratified drift deposits that often make excellent aquifers.

Glacial Till: Poor Aquifer Material

Most glacial debris was plastered onto the landscape as the last glacier advanced. Some slumped off the glacier into piles, was left up against the sides of valleys, or was formed into spoon-shaped hills called drumlins, when the glaciers moved over the debris. These types of glacial debris are called glacial till. They consist of an unsorted mixture of all sizes of soil and rock fragments and are usually not very porous or permeable. Therefore, public supply wells are not located in glacial till.

WHERE ARE THE GOOD PUBLIC WELL SITES?

Aquifers in Ancient River Valleys

In most of Massachusetts, meltwater moving away from the front of the glaciers flowed into existing river valleys. These valleys had been carved into the bedrock over millions of years by the rivers that drained the continent. In these ancient streambeds, the glacial debris settled out of the meltwater as stratified drift in the process described previously. Some of these ancient streambeds contain more than 200 feet of porous, permeable stratified drift. These buried valley aquifers are the sites of the majority of the public supply wells throughout Massachusetts including the Assabet River basin, with southeastern Massachusetts as the only exception.

Most public wells are located in buried valley aquifers that are connected hydrologically with a nearby river or stream. The pumping well may lower the water table below the level of the river, drawing water from the river into the well. This phenomenon is called induced recharge.

Most ancient streambeds correspond to present day river and stream valleys, but the courses of a few rivers have changed since the glaciers melted. In those places, the aquifer is not located under the present river. Other small tributary streams have completely disappeared, leaving valleys filled with stratified drift behind. Geologic investigations can locate these ancient buried valleys, and the aquifers can be tapped for water supply. Most new public wells in Massachusetts are being developed in this type of aquifer.

Aquifers in Outwash Plains

In southern Plymouth County, Cape Cod, Martha's Vineyard, and Nantucket, glacial meltwater also formed excellent aquifers, but not in ancient river valleys. The last ice sheets to cover Massachusetts stopped there. When they melted, the meltwater carried glacial debris from the front of the ice in a myriad of small parallel streams. Eventually, the stratified drift from the meltwater streams formed broad surfaces called outwash plains. These are excellent aquifers. They differ from valley aquifers in that they are generally spread out over a larger area, but usually they have no large sources of induced recharge.

Outwash plains and many valley aquifers are large enough to supply public wells in Massachusetts. Smaller coarse-grained stratified drift deposits can be aquifers for private domestic wells. Coarse-grained stratified drift deposits also readily absorb precipitation and thus commonly serve as our most important recharge areas.

Confined Aquifers

The aquifers described so far are unconfined, or water table, aquifers. The top of this type of aquifer is identified by the water table. Above the water table, known as the zone of aeration, interconnected pore spaces are open to the atmosphere. Precipitation recharges the groundwater by soaking into the ground and percolating down to the water table. The majority of our public wells in Massachusetts, and many private wells, tap unconfined aquifers.

Some wells in Massachusetts, however, are located in confined aquifers. These aquifers are found between layers of clay, solid rock, or other materials of very low permeability. Little or no water seeps through these confining layers. Recharge occurs where the aquifer intersects the land surface. This may be a considerable distance from the well.

In confined aquifers, often called artesian aquifers, water is under pressure because the aquifer is confined between impermeable layers and is usually recharged at a higher elevation than the top confining layer. When a well is drilled through the top impermeable layer, the artesian pressure will cause the water in the well to rise above the level of the aquifer. If the top of the well is lower than the highest elevation of the aquifer, water will flow freely from the well until the pressure is equalized.

Aquifers Cannot Be Found Everywhere

Aquifers that are suitable for public supply wells were created by glaciers only under certain conditions that occurred only in specific kinds of places. In other words, these aquifers are not everywhere. They must be identified and protected. It is much easier and less expensive to protect aquifers from pollution and harmful development than to find new water supplies or restore groundwater quality after it has been contaminated.

GROUNDWATER IN THE WATERSHED

What Is a Watershed?

All land is part of one watershed or another. When rain falls, much of the water runs across the surface of the land toward a stream, river, pond, or lake as surface runoff. The land area that drains runoff to the stream, or other surface water body, is called a watershed, or drainage basin. Watershed boundaries, also called drainage divides, are the high lands that divide one watershed from another. Theoretically, a drop of rain that falls squarely on a drainage divide is split into halves; one half flows into one watershed and the other into the adjacent watershed.

Groundwater is found beneath the surface of the ground within drainage basins. It does not move in underground rivers from distant watersheds. The source of all groundwater in each watershed is the precipitation that falls there. Groundwater divides usually occur approximately beneath surface water divides. Occasionally, however, the groundwater divide may not coincide with the watershed boundary. In that case, a recharge area for an aquifer in one watershed may extend partially into the adjacent watershed, but these conditions are relatively rare, and, in any case, quite local.

Since groundwater occurs within watersheds, and groundwater divides are usually approximately beneath surface water divides, watersheds are often used as the basic hydrologic unit for both surface water and groundwater planning purposes. Massachusetts has been divided into 27 major drainage basins for water resources planning. Each one includes a number of tributary watersheds, or sub-basins, that are drained by smaller streams or rivers. The Assabet River watershed is a subbasin of the SuAsCo (Sudbury, Assabet & Concord), which is, in turn, a subbasin of the Merrimack River watershed.

How Groundwater Moves Through the Watershed

Groundwater moves slowly from recharge areas, where precipitation is absorbed, down to discharge areas, where it flows or seeps out of the ground and becomes part of the surface water. When groundwater discharges into surface water, they flow together. Streams and rivers flow down the valley of the watershed until they join larger rivers and, eventually, reach the ocean. Thus, groundwater typically flows toward a stream, while the stream flows toward the ocean.

The fact that groundwater becomes surface water when it reaches discharge areas can't be overemphasized. Groundwater and surface water are interconnected and can only be fully understood and intelligently managed when that fact is acknowledged. For example, if pumping wells remove too much groundwater, there will not be enough groundwater discharge to maintain stream flow and aquatic habitats such as wetlands and ponds. (Contrary to popular belief, wetlands in Massachusetts are not usually important groundwater recharge areas. Recharge through wetland soils occurs very slowly and introduces only minor amounts of surface water into the groundwater system.)

WATER IN THE WELL

Cone of Depression/Area of Influence

When a public supply well is pumping, groundwater flow changes direction in a portion of the watershed. Instead of moving toward the natural discharge area, the groundwater within the influence of the pump flows toward the well from every direction. The pumping well creates an artificial discharge area by drawing down (lowering) the water table around the well. This area of drawdown is called the cone of depression. Except for wells in bedrock, the cone of depression of a private residential well is usually very small; the cone of depression of a public supply well, however, can extend thousands of feet from the well.

The cone of depression is most easily illustrated by a diagram that shows a cross-section of the well and the cone. However, to make this concept clear in relation to the land surface, it is useful to visualize the area from above. From this perspective, the cone of depression is termed the area of influence. Although, technically, the terms refer to different views of the same phenomenon, they are often used interchangeably.

The Cone of Depression Changes Size

When the amount of groundwater that is withdrawn by the pumping well is equal to the amount of groundwater recharge within the area of influence, the cone stops expanding. However, the cone of depression does not always remain the same size. If there is no precipitation to recharge the aquifer, and the well keeps pumping, the pump will pull water from a greater distance, and the cone of depression will get deeper and wider. After heavy precipitation, with good recharge, it will get smaller.

Land use can change the size and shape of the cone of depression and the ability of the aquifer to supply water. If impermeable surfaces (such as parking lots) cover a portion of the area of influence or its upland recharge area, and the runoff from those surfaces flows overland to streams instead of recharging the groundwater, the cone of depression for the pumping well will have to expand to compensate for the lost groundwater recharge. If there is no porous, permeable land within reach of the pumping well that can provide the recharge needed, the yield of the well may decrease. If enough of the potential recharge area is covered with impermeable surfaces, or if nearby surface waters are diverted for other purposes, the yield can be reduced so drastically that the well must be abandoned.

Limit of the Well's Influence

A well draws water from only a portion of the watershed, specifically, the cone of depression and upland recharge areas. Outside these areas, collectively termed the areas of contribution, groundwater does not move toward the well. Instead, it moves in its normal pattern from the recharge area down to the discharge area.

Induced Recharge

Most public supply wells in Massachusetts are located in buried valley aquifers that are associated with a nearby stream or river. Most of those wells draw surface water from the stream in a process called induced recharge. Induced recharge occurs when the cone of depression reaches as far as the stream, thereby lowering the water table beneath it. If there are no impermeable barriers such as clay or thick deposits of organic muck in the streambed, the pump will pull water from the stream down through the aquifer and into the well. Under these conditions, polluted surface water can enter the well and degrade the quality of the water supply. In Massachusetts, induced recharge probably occurs in all but a few public supply wells located in valley aquifers.

Four Areas That Should Be Protected

There are four areas significant to groundwater supplies that must be identified and protected to prevent contamination. They are all a part of a watershed.

Aquifers -- Aquifers are geologic formations that are capable of yielding a significant amount of water to a well or spring. In Massachusetts, buried valley aquifers are the sites of most public supply wells. Coastal outwash plains in southern Plymouth County, Cape Cod, Martha's Vineyard, and Nantucket are also excellent aquifers.

Cone of Depression -- This is the area around the well where the water table is lowered when the well is pumped. Since water is withdrawn from this area to supply the well, it should receive utmost protection. Contamination that enters groundwater within the cone of depression will eventually reach the pumping well.

Recharge Areas -- Recharge areas are porous, permeable geologic deposits (usually sand and gravel) that can absorb precipitation and allow it to percolate down to the water table and flow into the aquifer. These areas usually include the land surface directly above the aquifer and the porous, permeable areas adjacent to the aquifer. Although the most important recharge areas are those that replenish the portions of an aquifer that supply a well, all aquifer recharge areas should be protected, especially if there is a potential for developing new wells in the aquifer in the future.

Surface Water - When the cone of depression intersects a lake, river, or stream, surface water may be drawn into the well via induced recharge. This occurs commonly in Massachusetts because most public supply wells are located in valley aquifers near rivers and streams. In these cases, both the quality and quantity of the surface water can affect the well. Therefore, it is important to protect the surface water that contributes to recharge. To do that, it is necessary to control land use in the watershed so that contaminants will not reach the river or stream, and to ensure that upstream use of the water does not decrease the quantity required to supply the well.