Dr. Thomas Harter provides a basic understanding of groundwater – what it is, how much groundwater is out there, how fast groundwater moves, and where it comes from and where it goes
Groundwater is an important part of California’s – and the nation’s water supply. Nationwide, groundwater makes up on average 26% of the water supply. In California, that number is significantly higher – groundwater provides nearly 40% of the water used by California’s farms and cities, and significantly more in dry years. But what is groundwater? In this post based on the first segment of the UC Davis shortcourse on groundwater, Dr. Thomas Harter, who is the Robert M. Hagan Endowed Chair for Water Resources Management and Policy at the University of California, Davis as well as Professor and Cooperative Extension Specialist in the Department of Land, Air, and Water Resources (among many other things), provides a basic understanding of groundwater – what it is, how much groundwater is out there, how fast groundwater moves, and where it comes from and where it goes.
Dr. Harter began by likening a glass that was filled with red beans and water to groundwater. The water that’s filling the spaces in between the beans is groundwater; the part of the beans that are above the water is called the unsaturated zone and the top of the water is the water table.
Groundwater is the water that is fills in the pore space between particles, gravels, and rock fractures that make up the earth. In the subsurface, where the pore space is completely filled with water, it is called groundwater; above where the pore spaces might be wet but not completely filled with water is called the unsaturated zone; and the boundary between the unsaturated zone and groundwater is called the water table.
The materials in the ground can have many different shapes and forms, such as loose materials with a lot of pore space, hard rocks that are fractured and filled with water, or even material that itself is porous and has a secondary porosity that also fills with water.
GEOLOGY 101 IN 60 SECONDS
There is liquid magma deep in the subsurface where it’s too hot for any water to be in liquid form. The magma cools downs and form rocks or comes up from below and forms volcanoes. The rocks and volcanoes become eroded, the material that settles down makes new sediments that get buried, and then become rocks themselves which are called sedimentary rocks. Under the influence of pressure form the overburden and the heat from the subsurface, these rocks may actually reform into other rocks and minerals which are called metamorphic rocks.
The table shows the relation of the commonly used names for rocks to where they came from, what they are, and how they got there. For example, glaciers deposit sand, silts, and clays which are called till and outwash. Wind deposits fine silts and the loss and smaller sands which makes sand dunes. All of these sediments can get lithified as they are sitting in the ocean or sitting on the land and sand becomes sandstone, clay becomes shale, and a mix of gravel, sand, and silt becomes a conglomerate.
The oceans have chemical sediments, such as calcium carbonate which comprises limestone and dolomite and calcium sulfate which comprises gypsum. Igneous rocks form from the cooling of lava; granite is magma that’s cooled underneath the land surface and basalt is magma that came to the surface as lava and cooled off very quickly. There are also the various metamorphic rocks that changes at depths, so for example, sandstone becomes quartzite, and shale becomes slate.
Geologists use geochronologic units to determine when rocks were deposited or were formed. These time eras are divided into large eras: the Cenozoic, the Mesozoic, the Paleozoic, and the Precambrian. The chart on the lower left shows the further divisions of those large eras.
The California map on the upper right shows the geology that’s observed on the surface. There are many types of rocks that are from very old to very young in the volcanic regions up in the northeast part of the state. There are hard rocks that make up the mountains of the Coast Range, the Sierra Nevada, and the Cascade range and the other ranges in the basin and range province.
The areas shown in light yellow are alluvial sediments deposited mostly by rivers in more recent ages; often old marine sediments can be found underneath them. Alluvial sediments are generally found in the flat areas of California, Nevada, and Arizona; Dr. Harter describes these as sediments sitting in the bathtub with the bathtub walls being the mountains of California. The sediments are divided into various sizes of particles: gravel, sand, silt, and clay, the smallest being clay particles at .0001 inch and ranging all the way up to medium pieces of gravel as big as a half an inch.
The unsaturated zone that is where there is soil moisture but the pores are not completely filled with water; the groundwater is where all of these pores are fully saturated.
In an unconfined aquifer, that water table that separates the unsaturated zone or vadose zone from groundwater can freely move up and down; there is no restriction in the sediments. It’s recharged from streams and from the rainfall that’s not used by plants. It might discharge elsewhere into a stream, in which case it would be a gaining stream; if the water table is too low, the stream will discharge into the aquifer, in which case it would be a losing stream.
PERCHED WATER TABLES
A perched water table is groundwater that is separated from the main groundwater body by an unsaturated zone. It is typically recharged from the land surface from rainfall not used by the vegetation that percolates down until it hits a unit that’s less permeable than the surrounding materials; water literally piles up on that unit because it can’t move through fast enough as it’s being recharged.
These perched water tables in many cases are relatively shallow, anywhere from a foot to five feet thick, and whether or not they can be used for economic purposes really depends on the local conditions, said Dr. Harter. Perched water tables aren’t generally used for large production wells, but sometimes are used for domestic wells. He also noted that since perched water tables are generally shallow, they are the first groundwater encountered and therefore the most vulnerable to any contamination that’s coming down from the surface.
AQUIFERS AND AQUITARDS
Aquifers are geologic units that provide a significant amount of groundwater in the regional context; it is somewhat relative because some geologic units that are aquifers in one region might not be called an aquifer in another region as they might not actually be the most productive unit and instead act more as an aquitard, meaning a slower moving groundwater geologic body than what the actual aquifer would be in that region.
“If you have a fine sand unit sandwiched between a lot of clays, the clays don’t move a lot of water but the sand does move a lot of water, so that sand would be the aquifer,” explained Dr. Harter. “But in another region, you might have that fine sand sandwiched between a lot of gravel which really moves water very fast, and the sand in that context becomes what we call an aquitard, which is something that actually makes water move slower relative to what would be called the aquifer there. So what is an aquifer is not tied to the particular material; it’s really relative to the other material in your region. The more permeable material in your region is what we would call the aquifer.”
In California and other places, there are semi-confined aquifers which have portions which are unconfined with clays interspersed between the sands and gravels, which are the units that move most of the water, so it’s sort of unconfined as well as sort of confined.
“By confined, I mean there are aquifers that are sandwiched fully between an aquitard, like a clay unit here in gray at the top and another clay unit below the aquifer itself, which is shown in blue and is mostly sand and gravely material,” said Dr. Harter. “These are conditions that we find all around California and the Southwest where there are basins with alluvial sediments sitting in between mountain ranges, which are like bathtub walls and the bathtub itself is filled with these sediments. Then the sediments in turn are filled with water and so really that’s what it all comes down to.”
If a well is sunk into an artesian aquifer, water would rise until equilibrium is reached; such a well is called an artesian well at least as long as the pressure remains. If the well is flowing for a long time, the pressure will go down and eventually the artesian well would be lost, unless there is more supply coming in.
HOW MUCH WATER IS IN AN AQUIFER? POROSITY, SPECIFIC RETENTION, AND SPECIFIC YIELD
How much water an aquifer can hold depends not on the total volume of the aquifer, but rather on the porosity of a material – the more pores, the more water. This porosity varies greatly between the different rocks and sediments that comprise the subsurface. The size, shape, regularity and continuity of the pore space determines how much water can be extracted or stored in the aquifer, how fast water can move through the pore space, and how contaminants are distributed in the subsurface.
Groundwater fills the entire pore space, but not all of the groundwater will be available to be pumped as some pores may be entirely isolated from other pores or rock fractures may be disconnected from each other. Groundwater that is available for domestic wells or irrigation water or that is flowing to a spring or providing base flow is the water that is flowing out of the connected pore spaces or connected fracture spaces. So the total pore space can be divided into effective porosity, which is the interconnected pore space, and isolated pores.
Effective porosity can be further divided into specific retention and specific yield. Specific retention is the amount of water remains as moisture in the sediments when the aquifer is drained. Specific yield is the amount of water that is available for pumping when sediments or rocks are drained out due to the lowering of groundwater near a well.
Specific retention and specific yield vary greatly for different materials. In a flowerpot, the potting soil is mainly peat soil so it’s very porous; if you filled the flowerpot with water, a lot of water would come out the hole in the bottom of the pot; that’s the specific yield, which is about 40% for peat. The amount of water left behind is the specific retention and is the water the plants will use until it is watered again. The specific retention is very high in peat soils and clay soils; in fact with clay, the specific retention is so much that the clay will yield very little, even though it has a lot of water.
“Most of our aquifers in California are sand and gravel units which have a specific yield around 15-25%,” said Dr. Harter. “The lower percentage is because they’re all mixed in with clay and have relatively little specific retention; they are coarse and there’s not much water held against gravity in these materials. So if I have an aquifer that has a total volume of a million acre-feet and if all of it was gravel, that means there would be about 300,000 acre-feet of water in that gravel. Maybe I can get 200-250,000 acre-feet out if I was to completely drain that aquifer. That’s the meaning of the specific yield.”
WHERE DOES GROUNDWATER FLOW?
Groundwater moves from higher elevations to lower elevations and from areas of higher pressure to areas of lower pressure. Dr. Harter said that doesn’t always mean downward. Water flows from a place with high pressure to a place with low pressure, and that can be uphill, such as in an artesian well.
He presented a map showing water levels in an unconfined aquifer shown at the upper right, noting that it is quite similar to a topographic map; the contour lines indicate all the places where water level is at that level.
“Basically, these water level maps allow us to actually draw a direction in which water is moving and allows us to give an understanding of the direction of groundwater flow,” he said. “Water level maps are really critical to understand regional groundwater flow. To make water level maps, we need a lot of water level measurements over time to understand what the dynamics are of this water table surface that then determines in which direction groundwater is flowing. So water level maps are critical to our understanding of the direction of groundwater flow.”
HOW FAST DOES GROUNDWATER FLOW?
To answer the question of how fast groundwater flows, Dr. Harter gave a though experiment. Think of a lab experiment there is a column of sand, and at the top, there is a water beaker at the top with a constant water level, and at the bottom is a water beaker at the bottom where the water level is also kept constant by some sort of overflow mechanism (lower, left).
“As a result of keeping the water level constant at the top and constant at the bottom, what we will observe is that water is going to flow through the sand at a certain velocity and you can measure that by looking at how much water is flowing out of this beaker on a continuous basis,” he said.
If instead of sand, it is filled with clay or loam (upper, right) which is a much finer material, there will still be water coming out but it will be much, much slower.
“The material in this column is tighter and it’s denser,” he said. “There might be the same amount of pore space in there but the pores are much, much smaller so the water actually experiences a lot more friction and therefore comes out a lot slower. This capacity of a geologic material, in this case sand and loam, to transmit water is what we call the hydraulic conductivity. It is an intrinsic property of the geologic material through which groundwater is flowing.”
There is another factor in determining how fast groundwater flows. If, going back to the sand column, if the water level is raised on the downhill side, will the outflow on that side decrease or will it be the same as before?
“Because the water level now is higher on this side than it was before, therefore the difference in water level between the inside part of the sand column and the outflow part of the sand column is less than it was before, and since the distance is less than it was in the previous experiments, I have less water flowing through, so my flow is going to be a function of both the hydraulic connectivity and the gradient of this water level,” explained Dr. Harter. “The shallower the gradient between the pressure on one point and the pressure at another point, the slower the water, and in fact, if there is not gradient at all, even in sand or gravel, there is no water movement which is exactly why my water isn’t moving from the left to the right in this cup, because at the top of the cup, the water level is flat and so there’s no force that would move this water from the left side to the right side of this cup.”
Darcy’s law says that the groundwater flow is equal to the gradient in hydraulic head or pressure gradient times the hydraulic conductivity. Darcy’s Law describes the fundamental relationship for understanding how fast and how much groundwater moves.
In order to understand that, you need to have a lot of data on water levels in different places in the groundwater basin in order to understand the water level gradients and the pressure gradients, and you need to know the hydraulic connectivity of the materials making up the groundwater system. To measure hydraulic connectivity, the bore hole logs are used to determine the type of material; there are many tables that provide estimated hydraulic connectivities.
“For example, the range for well sorted gravel is over two orders of magnitude; it’s the same for sand,” said Dr. Harter. “There are two orders of magnitudes between the low end and the high end of the range which means water could flow at a velocity of 10 feet per day or water could flow at a velocity of 1000 feet per day, based on the range of estimates given here.”
“Sometimes we take our own material into the lab and we do exactly the kind of experiment that I just described,” he continued. “Or we may go out in the field and do what we call an aquifer test. In an aquifer test, we pump water in a well and we observe the change in water table in another well, and that change in water table can then be translated into a hydraulic connectivity and transmissivity, and also tell us something about how much water is stored in the groundwater.”
WHERE DOES GROUNDWATER COME FROM AND WHERE DOES IT GO?
“The land surface is the grand separator and what we call the critical zone: the soil and the land surface, as that’s where a lot of water decisions in terms of what happens with that water that’s coming out of the precipitation,” said Dr. Harter. “Some of it will runoff, some of it will stay in the snow for the season, or maybe for many, many years in the form of glaciers, some will sit in the soil and then be taken up by plants that evapotranspire it back into the atmosphere, and some will percolate down to the water table. And that groundwater will flow then to the lowest point and eventually either discharge into a lake or a stream, or into an ocean.”
Dr. Harter pointed out that freshwater is actually only a very small portion – only about 2 ½% – of all the water on the earth’s surface, and of that, the majority is held in ice caps and glaciers. However, groundwater is the other big part. Soil water, water in lakes, water in rivers – all the other water that’s not in the ocean and not in groundwater is a tiny, tiny fraction, so groundwater in fact is a really big part of the hydrologic cycle, he said.
Dr. Harter then returned to his ‘bathtub’ analogy for California aquifers, noting that it doesn’t apply to the volcanic areas in Northeastern California or some of the Northern Coastal Ranges. However, in most of California, the mountains are made up of hard rocks that are significantly less permeable, meaning they have a much lower hydraulic connectivity then the sediments that are sitting in the bathtub.
As the rivers flow out of the mountains, the water pours over the sandy and gravely sediments typically found on the mountain front, and if there wasn’t any water in the sediments, it would infiltrate into the subsurface and fill up the space in between the sediments. Also, some of the water is coming from precipitation that is not used by vegetation the infiltrates down to the water table. As the water table rises higher and higher, eventually the water table will rise to the surface at the lowest point in the basin, which is usually a river which has come out of the mountains and lost some of its water to groundwater and continued its journey to the valley floor and maybe to the next valley downstream or out to the ocean.
In the scenario on the slide, the aquifer has a flat gradient which means the water doesn’t move, and so more water will continue to flow into the aquifer, which will empty into the river, and so naturally the water table follows the gradient of the landscape. Eventually, there is an equilibrium that establishes itself so that there is a balance between water coming in from mountain front recharge, the recharge from precipitation, and water coming out as discharge to the river or what is called the base flow of the river.
“This is why we have water flowing in rivers in California a month after the last rain, two months after the last rain even if there’s no upstream reservoir,” said Dr. Harter. “It’s this contribution of groundwater as even though is no more water coming in at the back perhaps, this water table is slowly draining itself, meaning the slope is very, very slowly getting flatter and flatter as water is moving back out to the stream under completely natural conditions.”
“The reason why it’s a cone, it’s very easy to understand when you think about Darcy’s Law which says that the velocity with which water flows is equal to the hydraulic connectivity and the pressure gradient, so steep pressure gradient fast, shallow pressure gradient slow,” said Dr. Harter. “At the well, the water actually has to flow faster and faster because the well has a very small circumference and so at that point, it has to flow really fast into the well. In order to make water as it gets closer to the well flow faster, the hydraulic connectivity is not going to change; the only thing that can change is the slope of the gradient, so the closer to the well, the steeper the gradient in order to make that water flow faster and faster as that circle becomes smaller and smaller which represents the cross sectional area through which water flows.”
A lot of water is pumped during the summer for irrigation and to provide cities and domestic wells with water which develops cones of depression; over the winter, pumping is stopped or slowed and the aquifer is recharged. Often, it goes back and forth, and under ideal conditions, that doesn’t change, he said. However, in a lot of places in California, more water is taken out in the summer than is recharged in the winter and as a result, the cone of depression gets deeper and deeper. Rivers get disconnected from the water table, and instead of being gaining rivers, they become losing rivers and that creates the situation known as overdraft.
Dr. Harter pointed out that the groundwater pumping and resulting cones of depression doesn’t mean there isn’t a change to the river because the water that is pumped out is water that’s not going to a river.
“There’s always a relationship and an influence of this pumping on the river that would have otherwise received that water, or it could be riparian vegetation or it could be a wetland.”