Drawing by Bernie Amell
A Short Course on
Groundwater and Aquifers
We’ve all heard of groundwater and aquifers. Here I fill out those concepts and explore how they relate to geology, biology, and human activities.
Simply, groundwater is any water under the surface of the ground. Groundwater lies in aquifers, which are underground reservoirs of water. For obvious reasons, groundwater and surface waters such as seasonal streams, wetlands, lakes, and ponds are often managed together, often as whole watersheds. Clearly, they are related.
The Big Picture
All Quadra Island water comes from Quadra Island precipitation – – rain, snow, and a little dew and fog. Depending on where, how fast, and at what time of year it falls, water either runs off or soaks in. If it soaks in, it is groundwater, first in shallow, near-surface aquifers that follow the shape of the land. From there, it may or may not reach deeper aquifers. If it runs off fast enough, water carves canyons, washes out roads, scours streams, and fills ponds and lakes with its sediment. It makes marshes and meadows of them over time. Running water is a powerful force, and some Quadra residents turn that power into electricity.
Like surface water, groundwater runs downhill. Underground siphons carry it over humps, and pumps can pump it up again, but groundwater usually doesn’t really “run” the way it does on the surface. There usually aren’t underground rivulets or rivers, and the flow doesn’t usually accumulate in underground lakes.
Groundwater isn’t usually free to run like that for an interesting reason. Not only is groundwater under the ground, but we can think of it as part of the ground. Aquifers are the “hydro” part of “hydrogeology“, and they vary with the geological structures that hold them. Because the underlying geology varies from place to place on Quadra Island, so do its aquifers. And because Quadra Island geology is remarkably complex, so are its aquifers.
As one of many kinds of examples, consider one 400-foot well drilled recently on Quadra Island. Its top 29 feet were fine-grained glacial deposits, some of it silty and sticky like clay. That top part of the well penetrates a shallow surface aquifer regenerated by each year’s rainfall. Directly under that is dry, impermeable bedrock all the way to a tiny bit of water at 160 feet, then through dry bedrock to 385 feet, where water enters the hole fast enough to accumulate and pump.
That is one of several kinds of wells on Quadra Island, and it penetrates 3 aquifers. Groundwater in the layer of soil near the surface feeds a seasonal stream. It and its branches feed several fire ponds and shallow wells. The two bedrock aquifers beneath it are different. That groundwater runs in fractures in the bedrock, which is otherwise solid, dry, and impermeable to water. Water runs through the fractures, enters the well, and is available to be pumped.
If water runs downhill in an aquifer but can’t get out, that aquifer is called confined and its water is under pressure. In that particular well, the static pressure in both fractures is high enough to push water up to the top of the bedrock, but not all the way to the surface. What confined the water in that case is impermeable bedrock above and below the fractures. If it had come all the way to the surface or higher, the well would be called an artesian well (Fig.1).
Figure 1. A slice of imaginary landscape showing two separate aquifers at different depths, one above the other. The shallow aquifer is unconfined. Its water is free to run downhill, run off in streams, and so on. It feeds a shallow well and a pond, and “processes” any wastes added to it through runoff and septic fields. This deep aquifer resides in fractures in impermeable bedrock; other deep aquifers reside in other material, such as Quadra Sand, and they may or may not be confined. Being confined, the water in this deep aquifer either can’t get out or leaks away slowly enough that it accumulates “in the bottom”, to a certain water level. Water level varies depending on how fast precipitation recharges the aquifer, how fast it leaks away, and how fast water is pumped out of it.
It is important to notice that all three deep wells have the same water level as the aquifer, including the artesian one. They share the same water, and except for local variations in permeability and other factors that affect how fast the aquifer recharges, they share the same water level. Very little is known yet about Quadra Island aquifers.
Seasonal streams can run for weeks or months after the last serious rainstorms each spring. That tells us that shallow aquifers can hold water. It seeps slowly downhill in watersheds, feeding streams that run along the bottoms of them. Shallow aquifers also feed ponds and wetlands. The fact that ponds’ water level varies seasonally measures the capacity of that shallow aquifer to feed that pond, under that particular pattern of weather conditions.
Shallow aquifers feed shallow wells, so-called “dug wells”, often at the limit of the machines that dig them (18 – 22 feet). As many Quadra Islanders know, water level in shallow wells varies seasonally and some have run dry in recent years. Too many septic tanks too close to too many shallow wells can be a health hazard. Fortunately, testing can detect both fecal and many other kinds of contaminants in well water. The more we know about our shallow aquifers, the better we can prevent contamination like that from happening, as well as contamination of the deep aquifers they feed.
An important function of shallow aquifers is that if they are underlain by impermeable strata, their water can feed wetlands (Fig. 2). In the figure, a pond or stream lies at the bottom of a shallow depression, and supports a wetland community in its shallows and verges. So-called “emergent vegetation” is rooted in shallow water and grows up into the air. Other vegetation grows in moist soil above the waterline. Wetlands teem with both plant and animal life, and contribute significantly to the diversity of broader areas. Although wetlands drain slowly, they do drain, and are a significant source of water to recharge deeper aquifers below.
Figure 2. Slice of a wetland. Shallow aquifers feed ponds and wetlands. Ponds and wetlands feed deep aquifers.
Any wells deeper than shallow aquifers are drilled deep wells. Because aquifers vary in their depth below the surface, drilled wells can be from a few dozen to 500 or more feet deep. Some extend well below sea level. Aquifers can be broad enough to span several provinces or states, and Quadra Island has several small ones. Aquifers also vary in how much water they can hold, how fast they can recharge in rainy seasons, and how fast they will give up their water to pumps. An important question is whether polluted shallow aquifers can pollute deep aquifers.
Quadra Island Geology
Quadra Island comprises four geologically distinct regions (Fig. 3). Two are igneous (mainly granitic or basaltic bedrock), one is sedimentary (limestone karst), and one is glacial till. These regions are most obvious in a 3D topographic map. I discuss each region and its associated aquifers next.
Figure 3. Topographic map of Quadra Island, showing geological regions: Igneous bedrock, sedimentary bedrock, and unconsolidated glacial till. The igneous bedrock is mainly granitic: diorite, granodiorite, and quartz diorite, but there are also areas of basaltic bedrock. The sedimentary bedrock is a typical limestone Karst landscape that is newer, geologically, than the igneous rock beneath it. See text for details. Boundaries are estimated from geological maps. Topography is from Google Maps.
South Quadra. From the southern tip of the island to Heriot Bay is what we call South Quadra. It is home to the highest human densities, its web of roads is extensive, and its communities of Quathiaski Cove and Heriot Bay are the commercial and communal hubs of the island. South Quadra comprises about 10% of Quadra’s area and supports most of its human population.
Geologically, South Quadra is distinct from most of the rest of the island. It is a relatively flat-topped mound, and its bedrock is buried under glacial deposits, or glacial till left behind by retreating ice age glaciers. Geologists recognize many types of glacial till and use specific terms to describe them. Here, “till” refers to all unconsolidated glacial deposits, except when it is important to be more specific.
Types of till are created by different processes and behave differently with respect to water. South Quadra has several of those types, but deep, relatively uniform deposits of “Quadra sand” (Fig. 4, one of those types) are especially important for our aquifers. In general, sand has very high permeability, and also high water-holding capacity, but only if it is underlain by impermeable layers.
Figure 4. Photo of Quadra Sand, taken in the late Roy Dahlnas’ gravel pit on Heriot Bay Road. The vegetation at the top shows that this layer functions as a shallow aquifer, and its visible ripples reflect the series of events that built up this layer of till. The material at the bottom is loose sand that fell from the steep cliff above it.
South Quadra is mainly built up of rubble dropped from glaciers that came from the Bute Inlet region of the mainland. Thick layers of ice bulldozed bedrock, then retreated, leaving the till behind. Whiskey Point and the some of the terrain around Gowlland Harbour are exceptions, being mainly bedrock of various types. Quadra Island bedrock includes a complex pattern of older sedimentary limestone with younger granitic and basaltic igneous layers above it. South Quadra, Marina and South Cortes Islands mark the extreme northern tip of the kind of till-based landforms that characterize many other Gulf Islands and large portions of southern Vancouver Island.
In terms of South Quadra aquifers, unconsolidated glacial deposits are still more or less rubble (they are “unconsolidated”). Particles vary in size from clay, fine silt, and sand to boulders and larger. The permeability of the material to water and the amount of water the aquifers can hold varies from place to place, depending largely on the sizes of particles in different places. Not much is known about the aquifers on South Quadra, other than that there are probably at least two of them. But their size, the extent to which they share water with each other, how effectively they recharge, and many other qualities are still unknown.
There are also two small pockets of glacial till in low-lying places north of Heriot Bay.
North of Heriot Bay, Quadra Island is predominantly mountainous, especially along its east and west coasts. Those mountainous regions are hard igneous bedrock, resistant to erosion by water and ice and covered by a thin skin of soil. Their aquifers are based on crack systems in the impermeable bedrock stone. Because fracturing varies widely from place to place, aquifers in the bedrock are complex, and their size, depth, and other qualities can vary accordingly, even within local neighbourhoods. Think of these aquifers as residing in “fracture systems” in the bedrock.
Between those mountain ridges, in a band running from Open Bay in the south to Granite Bay in the north, is a lower-lying landscape of limestone. It is a typical Karst topography, complete with sinkholes, underground caverns, and undoubtedly connections between them. Village Bay Lakes Provincial Park and its set of interconnected lakes is part of that landscape. Figure 5 illustrates general features of Karst landscapes and limestone hydrogeology.
Figure 5. A typical Karst topography. Fractures in the limestone matrix are partly breaks in the rock and partly channels etched away by water over time. Note that the depth of the aquifers varies from place to place depending on the channels water has created. You can also see that the deep aquifer is confined by the underlying layer of impermeable rock and that its water level is determined by the height of the spring that feeds the stream that drains water out of the aquifer.
An exception to a general rule about aquifers. In general, aquifers aren’t like underground rivers or lakes. But we’ve all seen stunningly beautiful pictures of those features and we may have visited some of them. Underground rivers and lakes in karst are an important exception to a general rule about aquifers.
In contrast to groundwater lying in fractures in solid bedrock or in unconsolidated material like glacial till, limestone-dominated geology is different. Limestone is made mainly of calcium carbonate, which dissolves weakly in water. In general, running water erodes rock surfaces mechanically, wearing and breaking it away and carrying it away in pieces, and it does that to limestone as well. But water also dissolves limestone away slowly in karst topographies and deposits it later in other places. Water can carve whole underground mazes of openings in limestone, molecule by molecule, so slowly we can hardly imagine. Small caves open up, systems of cavities develop, water accumulates in the cavities, and runs downhill in the rock as well as over its surface. Lakes and ponds accumulate in its hollows.
In a word, Quadra Island hydrogeology is complex.
Runoff. How much precipitation runs right off depends on where it lands, whether it falls as rain or snow, how fast it comes down, and several other factors. If it lands on bare bedrock, roofs, or paved surfaces, it all runs off. At the other end of that spectrum, if it lands on plants growing in healthy soil with high carbon content, almost all of it either soaks right in or evaporates from the surfaces of the plants. The deeper the topsoil, the higher its carbon content, the richer its microbial life, and the more organic material (mulch) that lies on its surface, the more water it can hold. Any soil can hold only so much water, of course. Clean sand or gravel can hold hardly any unless it is trapped by an impermeable layer beneath it. In general, any water the soil can’t absorb and hold runs off.
To photosynthesize, grow, and produce seeds for following generations, plants take water and nutrients from the soil as they grow. They also secrete nutrients into the soil, both while they are living and after they die. Those nutrients attract and nourish many microbes, including the mycorrhizal fungi that connect many kinds of plants together in local areas and nurture them. The more this kind of thing happens in any kind of soil, wherever it is in the world, the more water it can hold regardless of those other factors.
Soils over igneous bedrock tend to be thin on Quadra Island. Because igneous stone strongly resists erosion by water and glaciers, those soils tend to be higher in elevation here than other types, despite their greater age, and steeper. Even on level ground, granite is impermeable to water except through cracks. Basically, what that means is that if water falls on the green areas of the geology map (Fig. 3), much of it soon finds streams that run to lakes or the sea and the forest takes the rest. Most of Quadra Island’s fish-bearing streams are in those uplands (Fig. 6). Some of the water in those shallow aquifers enters deeper aquifers through fracture systems.
Figure 6. Fish-bearing streams on Quadra Island. Information is from a provincial water use study that monitored stream flow of several of those streams over time.
Limestone is also relatively impermeable to water, except via fractures and channels the water itself etches in the stone. The depth and water-holding capacity of soils derived from limestone bedrock are similar to those on granite bedrock, but on Quadra Island the limestone band is lower in elevation, its streams are less steep, and water accumulates as lakes in its depressions before running to the Salish Sea.
There is no bedrock at the surface on South Quadra. It is buried under glacial till. Whether rain falling on till soaks in or runs off depends on the composition of the till, in addition to all those other factors. Sandy till absorbs water faster and dries out faster than silty till. If the till is rich enough in silt, it acts like clay. When soaked, it is impermeable enough to make a good liner for fire ponds. If topsoil above a layer of till is thick enough and rich enough biologically, it can support a shallow aquifer. But once water reaches an impermeable layer of silt, it runs downhill or builds static pressure if it is confined.
In terms of its aquifers, we could think of the igneous and sedimentary areas of Quadra Island as one big rock with cracks, with a thin layer of soil draped over it. In contrast, glacial till has everything from boulders and bigger to gravel and sand and silt. Silt is sticky when saturated, like clay, and can be as hard as a rock when it’s dry. Most South Quadra soils are a mixture of sands, silts, and gravels, and a few cobbles and boulders, and organic material accumulated on it, in it, and growing up out of it.
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Notes: When I joined the Water Security Team of Quadra Island ICAN, a charity devoted to developing strategic plans for long term sustainability and resilience of the entire island, I knew next to nothing about water. My first task was to begin to educate myself, and this document is what I have learned so far.
I called it a Short Course because though there is an enormous amount that needs to be said about groundwater and aquifers, on Quadra Island as in other places, we can’t say much more than this so far. Our mission and our objective as a Water Security Team is to learn as much as we can about our groundwater. Where it is, how much there is, how it recharges after rainstorms, and how much water is available for use by humans without depleting it.
There is a lot to know about Quadra Island aquifers, and even more to understand about them. The more we talk with each other about what we do and don’t know about water and the more we learn about it, the more able we will be, as a community, to manage that and other resources sustainably and resiliently in an uncertain future.
Acknowledgments: Christian Gronou’s many comments helped me present Quadra Island geology in a simple but understandable way. Ken Lertzman helped integrate geological, hydrological, and biological aspects of the story, and members of the Quadra ICAN Water Security Team also offered many useful comments and criticism.
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On World Water Day, March 22, 2022, I was interviewed about the work of the Water Security Team by Cortes Cooperative Radio. You can listen to that interview here.