A Brief Review of the Physical Characteristics of PEI’s “Redbed Aquifer”
In anticipation of the proclamation of the Water Act and a suite of associated regulations there is renewed interest in the state of the Province’s water resources, with much attention focused on groundwater and the adequacy of government’s scientific knowledge base for its sustainable management.
Fortunately the systematic study of the Province’s “Redbed Aquifer” dates back more than half a century, and through the efforts of various levels of government, academia and private sector enterprises a solid understanding of the nature and processes of Prince Edward Island’s “hydrogeological setting” has been developed. This is complemented by the fact that our geology, physiography and climate are relatively uniform across the Province. While this knowledge base will continue to expand, sufficient hard scientific facts are known to allow the Department of Environment, Water and Climate Change to predict with reasonable confidence the impact of most groundwater management decisions.
A number of authoritative bodies have recognized the excellent capacity of PEI’s aquifer, deeming it one of the major water supply formations in the Maritimes Basin in eastern Canada (Stapinsky et al. 2002, Rivard et al., 2008, 2014,) and describing it as “one of the best water supply aquifers in Canada” (Jacques Whitford and Associates 1990a). In simple terms, the generous capacity of this or any other “highly productive aquifer” can be attributed to a few basic factors: the size of the aquifer, its storage capacity, its permeability, the amount of recharge and the relationship of recharge to groundwater discharge for the stream system.
Unlike other jurisdictions in Canada, the entire Province is underlain by a single bedrock aquifer, thus from a “size” perspective PEI does not suffer from the constraints of only having a select number of useful aquifers. While hydraulic properties do vary across the aquifer and there are some areas where well yields are limited, useable quantities of groundwater can be accessed almost anywhere.
One of the important characteristics of the geological formation hosting PEI’s groundwater resources is the relatively high proportion of porous sandstones in the geological section relative to many other sedimentary environments. This generous “storage capacity” is complemented by the high permeability of the formation, a result of the combination of intergranular porosity and the prevalence of water bearing fractures in the formation.
The Province also benefits from a relatively humid climate and thin, generally permeable soils overlying the aquifer, with the result that recharge to the aquifer is also very generous. These high recharge rates and good aquifer permeability support the large base-flow contributions groundwater makes to PEI’s fresh water bodies and the productive capacity of wells constructed in the aquifer. Overall groundwater use is low relative to the amount of recharge, although locally withdrawals have been sufficient to reduce baseflow contributions to streams below acceptable levels. On a broader, watershed scale groundwater extraction rates are in all cases below recharge rates, and there is no threat of groundwater depletion.
With the proposed Water Act Water Withdrawal Regulations out for public review there have been many questions relating to the quantity of water on PEI and the sustainability of its use. PEI is unique within Canada in its complete dependence on groundwater for potable water supply, needs that are met by a single bedrock aquifer underlying the Province. It is hoped that the following review of the basic features and processes related to this aquifer provides a sound scientific basis for discussion about the Province’s groundwater resources and whether concern is warranted. It is acknowledged at the outset that all water resources on the Island are intimately linked, however the focus here is directed at the groundwater component. Secondly, while the topic of interest here is water quantity, this work also draws on some of the many studies focused on groundwater quality where they shed light on the physical behavior of groundwater in the aquifer.
The history of “groundwater research” on PEI
As the province relies entirely on groundwater for its potable water needs, it is not surprising that groundwater has been subject to considerable study over the years. Combining this with the fact the geology and physiography of the province are relatively uniform permits a robust understanding of groundwater resources. Technical reports by the Geological Survey of Canada dating back to the mid 1950’s and 1960’s began systematically characterizing various elements of groundwater resources (see for example Pollitt, 1952; Brandon, 1966; Howie, 1966; Carr, 1968; Carr and van der Kamp, 1969).
More recent works, many conducted in collaboration with federal government departments, have continued to expand our knowledge of groundwater resources and their behavior on the Island. Under Natural Resources Canada’s (NRCan) groundwater program, a significant body of published work on various facets of PEI’s groundwater resources has been developed (see for some examples: Stapinski et al., 2002; Liao et al., 2005; Rivard et al., 2008; 2014; Savard et al., 2007; Paradis et al., 2016, 2018). Also, an investigation into the effects of climate change on coastal aquifers in the Atlantic region, including PEI was conducted with the support of NRCan under the auspices of the Atlantic Regional Adaptation Collaborative (RAC) Climate Change Program, (Somers and Nishimura, Eds. 2012).
Other programs supported by Agriculture and Agri-Food Canada, while more directly focused on ground quality issues, have contributed significantly to the understanding of local scale groundwater flow dynamics (see for example Jiang and Somers, 2008; Somers and Savard 2011; and Jiang et al., 2014.) Collaboration with Environment and Climate Change Canada on the development and on-going maintenance of a comprehensive groundwater and surface water monitoring network, with continuous periods of record for some monitoring wells dating back to 1968 and stream flow gauges to 1961, continues to be the back bone of the province’s water surveillance activities.
Other important studies include:
- A detailed description of the hydrogeology of the Winter River Basin (Francis 1989),
- A compilation of hydrogeological and water supply data of the province (Jacques Whitford and Associates 1990a)
- A study on the availability of salt water wells as a resource to the aquaculture sector (Jacques Whitford and Associates 1990b)
- A study by Dillon Consulting (Dillon Consulting, 2000) on recommendations for a well field protection strategy for PEI.
On a more local scale, a series of “groundwater assessments”, many including aquifer testing programs, were conducted in a number of Island communities during 1989-90 by the Province as part of Canada-PEI Water Management Agreement.
In addition to specific groundwater studies, routine activities related to management of groundwater in the province provide much of the raw data to support the Department’s management of the resource. Well logs submitted by well drillers provide valuable information on local geological and hydrogeological conditions including depth to the water table, rock types encountered in well bores, distribution of water bearing zones and overall well yields. Pumping test data required as part of the approval process for high capacity wells provide much of the data describing the basic geology of the aquifer, and include additional information on key aquifer characteristics, response to pumping and the local configuration of the water table. For larger water supply development projects, the consultant’s reports submitted in support for approvals for groundwater extraction are comprehensive scientific/technical documents prepared by highly qualified professional staff, providing long-term environmental impact assessment of proposed projects with respect to impacts on stream flow, domestic wells, saltwater intrusion, and groundwater resources in general. Monitoring of landfill and Construction and Demolition disposal sites or investigation of contaminated sites adds additional information on groundwater behavior such as local aquifer characteristics and groundwater flow characteristics and velocities.
Collectively, this body of information provides a solid foundation to inform the groundwater management functions of the Department, allowing science based decision making on important matters relating to the effect of groundwater withdrawals on overall groundwater conditions, other groundwater users, and ecological functions such as supporting environmental flows in streams. The approval process for larger water supply programs draws on this knowledge base, and is augmented by additional site specific data that typically includes pumping test programs for detailed characterization of local aquifer characteristics, characterization of local stream flow regimes, and numerical modelling of groundwater and stream flow response to pumping.
The Department’s current approach to assess groundwater allocation proposals was recently reviewed by an independent party. The goal of the review was to provide an independent, scientific opinion regarding the methodologies presented by the PEI Department of Environment, Labour & Justice (DELJ) to assess groundwater availability”. In particular, the focus of this review was on the science basis for groundwater extraction assessments and groundwater quantity estimates. (MacQuarrie, 2014).
Some groundwater basics
An assessment of the state of groundwater resources can be looked at in several ways but first it is important to understand what exactly is being referred to when discussing “groundwater” and to have a basic appreciation of how it behaves.
The ground below us is divided into two distinct zones. There’s an upper zone where the spaces between rock and soil particles are partially filled with air and partially with water (called the unsaturated zone and where we might refer to the water as “soil moisture”). Below that is a region where all the “pore spaces” in the rock or soil are saturated with water (called the “saturated zone” and where we refer to the water as “groundwater”). The term “water table” refers to the boundary between the “unsaturated zone” and the “saturated zone”. Thus groundwater is simply the water filling in the voids between soil or rock particles, and generally is not water held in vast underground rivers or caverns (there are some exceptions to this, but not in PEI). Well water is simply groundwater accessed via a hole drilled into the saturated zone.
Groundwater does not stand still, but just like surface water, moves from higher elevations to lower elevations. The path starts where water soaks into the ground and percolates down to the water table, a process called recharge, and flows through the aquifer to points where it either discharges to streams or the coast (groundwater discharge), or to a point where it is extracted by a well. The water table typically rises and falls throughout the year depending on the relative rate of recharging to and discharging water from the aquifer. When water is being extracted from a well, a cone shaped depression (called a cone of depression) forms in the top of the water table, and when pumping ceases, the cone of depression fills in again, restoring the original water table elevation.
The behavior of groundwater and the capacity and sustainability of an aquifer are determined by a number of physical characteristics including:
- the overall size of the aquifer,
- the ability of the aquifer to store water,
- the ease with which water can be transmitted through the aquifer, and
- the relative rate at which water is recharged to, and discharged or extracted from the aquifer.
When these factors are known, they can be used to predict the behavior of groundwater in response to a variety of natural or man-made activities or circumstances.
A general characterization the aquifer
A number of scientific authorities have commented on the enviable characteristics of the aquifer underlying PEI. A report on water supply capacity on PEI states “The Province has one of the best water supply aquifers in Canada considering water quality and quantity.”(Jacques, Whitford and Associates, 1990a).
In a report produced as part of the Natural Resources Canada’s Maritimes Groundwater Initiative, Stapinsky et al., (2002) write “A preliminary review of the data indicates that groundwater in the Carboniferous Maritimes Basin is available for water supply in high abundance from several rock formations in both sedimentary rock units and in unconsolidated sediments. Five geological formations of interest have been deemed major aquifers in the Carboniferous Maritimes Basin. These hydrostratigraphic units are located in southeastern New Brunswick, northeastern Nova Scotia, and in Prince Edward Island and are described hereafter.”
A quote from the Groundwater Information Network (GIN, 2014) referring to two geological formations in the Maritime region notes: “Boss Point and PEI are located under a till with a sandy matrix. They have the most important aquifer potentials and recharge rates of the area”.
In light of such assessments it is worth examining the specific features that have contributed to these conclusions. Presented below, is a description of the important major aquifer characteristics as they apply to groundwater resources in PEI. The first three of these factors can be considered as constants, as they are determined by the nature of the province’s geology. The fourth component – the relative rate of recharge and discharge is subject to change, in response to changes in climatic conditions as well as human activities such as groundwater extraction.
An aquifer is a permeable material that can transmit significant quantities of water to a well, springs or surface water bodies (Rivera, 2014). The aquifer supplying groundwater in PEI is hosted by a single geological formation (often referred to as the “PEI Redbeds”) which form part of Maritimes Sub-basin, located in the broader Appalachian system. While in many jurisdictions, productive aquifers may be limited in areal extent and thus not accessible to all locations, in PEI there are no such limitations. In a comment on a report on groundwater mapping in Canada, it notes “since there is a single sandstone aquifer covering the province, further aquifer mapping is unnecessary from a geological perspective” )(Canadian Council of Academies, 2009. It is important to note that while the province is underlain by a single aquifer, groundwater movement can be separated into many individual groundwater flow systems, defined by topography (i.e. watershed boundaries).
The PEI aquifer also has a significant vertical extent. Van de Poll (1983) describes the geological formation that comprises the framework for PEI aquifer as being comprised red bed units totaling more than 850 m however, not all this thickness represents accessible groundwater. First, the permeability of the aquifer declines with depth, and at such depths well yields would be expected to be very low. Secondly as water-rock interactions proceed over time, deeper, slow moving groundwater becomes more and more saline.
It has been suggested that a useable “fresh groundwater zone” can be expected to extend to depths in the ranging from 100 to 300 m (Council of Canadian Academies, 2014). Given typical well yields of deeper wells in PEI, a practical thickness in the range of 100 metres for the useable portion of the aquifer is probably reasonable. This depth is an important factor to keep in mind when looking at seasonal changes in water table elevation. Typical water table elevations, as recorded from observation wells in the Provincial monitoring network, show annual fluctuations from less than two meters to as much as seven metres in some upland locations (Canada – PEI Water Agreement 1991), this represents only a small portion of the overall aquifer volume or availability of groundwater. Unfortunately since streams often represent the very top of the water table, they are much more vulnerable to changes in water table elevation than wells, which can commonly endure water table changes of many metres without serious consequences.
Water Storage Capacity
(porosity and specific yield)
Porosity is a measure of the volume of the void spaces in an aquifer available to store water, expressed as a percentage of the total rock volume. Typically porosity is measured in a laboratory, using core samples of the aquifer material. This porosity can be classified as being “intergranular” or “matrix” porosity, meaning the space between sand grains. “Secondary” porosity includes void spaces created by fracturing or dissolution of rock material and is usually volumetrically smaller than intergranular porosity. It can be important in contributing to the permeability of an aquifer. When both intergranular and secondary porosity are significant, the aquifer is often described as having “dual porosity”, and when the secondary porosity is dominated by fractures, the description “fractured porous aquifer” is often applied. The PEI redbed aquifer can be described as a dual porosity aquifer, and has the characteristics typical of a “fractured porous aquifer” (Jacques Whitford and Associates, 1990a).
In a compilation of aquifer characteristics conducted by Jacques Whitford and Associates (1990a) the authors note that “The massive sandstone aquifer exhibits moderately high intergranular porosity”. Studies by Brandon (1966) and Carr (1971) report average matrix porosity to be 17 to 23 percent. Francis (1989) reported a matrix porosity of 16 percent. In essence this means that once you are below the water table, some 15 percent or more of the rock volume is in fact stored water. Considering the areal extent and potential thickness of the aquifer, this represents a very substantial water storage capacity for the aquifer.
While overall porosity is important in the ability of an aquifer to store water, not all of this water is readily available and some portion of this stored water is bound to the aquifer materials by capillary forces etc. There are parameters that characterize the capacity of an aquifer to release groundwater such as specific yield. Specific yield is defined as the volume of water released from storage by an unconfined aquifer per unit surface area of aquifer per unit decline of the water table. Rivard et al., (2008) suggest specific yield values in the range of 2% to 7% for PEI. Rivard et al., (2005) note that “These values, in agreement with total porosities obtained from thin sections (5-10% on average, see the MGWI Bulletin), confirm that water can easily circulate through this very well fractured and porous media.".
(hydraulic conductivity / transmissivity)
In addition to the ability to store water, to be useful, water must be able to be transmitted through an aquifer. The permeability of an aquifer is a key determinant in such features as the velocity of groundwater flow, the rate of water discharge to streams, well yields and the size and shape of the cones of depression formed when wells are pumping. In groundwater studies, permeability is usually described as “hydraulic conductivity”, being defined as the volume of flow across a specific cross sectional area of the aquifer. Hydraulic conductivity values are generally derived from pumping tests. In practice, data from pumping tests usually provides information on the hydraulic conductivity averaged across the thickness of the test interval in the aquifer (called transmissivity) and then hydraulic conductivity being determined by dividing the transmissivity by the thickness of the test interval. In other cases, individual layers of an aquifer can be tested to provide a vertical profile of hydraulic conductivity within a well bore.
Hydraulic conductivity values are a critical input to numerical groundwater flow models used to describe groundwater flow conditions in a particular region, and to make predictions about the behavior of a groundwater flow system in response to changes in factors such as groundwater withdrawals.
Rivard et al., (2014) reports that “several sedimentary formations of the Maritimes Basin, including those of Prince Edward Island, Îles-de-la-Madeleine, the eastern Annapolis Valley and the Acadian littoral of New Brunswick show large values (>10-3 m2/s), typical of high capacity formations.” Similar values for hydraulic conductivity (as well as the related parameter “transmissivity”) have been reported in numerous other studies (for example Jacques Whitford and Associates, 1990; Rivard et al., 2008; Francis, 1989; Dillon, 2000; Paradis et al., (2016) and Jiang et al., 2015)
These high values of hydraulic conductivity and aquifer thickness are largely responsible for the generally excellent well yields possible throughout much of the Province.
The aquifer in PEI can be considered as semi-confined aquifer based on its structure and pumping test response. “Bulk” hydraulic conductivity can be broken into separate vertical and horizontal components, and in PEI, horizontal hydraulic conductivity is orders of magnitude greater than vertical hydraulic conductivity (Francis, 1989). This is in large part a function of the prevalence of sub-horizontal bedding plane fractures, especially well developed in the shallower portions of the aquifer. The result of this strong vertical/horizontal heterogeneity in hydraulic conductivity is that groundwater flow is dominantly horizontal in nature at least in shallower portions of the aquifer where flow is dominated by fractures. Jiang et al., (2007) conducted numerical modeling simulations of the groundwater flow in the Wilmot watershed. In a description of their work they conclude “Results from this exercise show groundwater between water table and 22 m below the land surface, which is the most contaminated and contributes >80% of base flow (>53% of total stream flow), has a residence time <4 years. Groundwater at depths tapped by typical domestic wells (22-32 m) has a residence time of 11 years.
In a broad sense, taking porosity and permeability characteristics together, it is the aquifer’s intergranular porosity that accounts for water storage, and the fracture network which dominates the aquifer’s permeability characteristics and groundwater flow dynamics.
Groundwater Recharge and Discharge and Water Budgets
Perhaps the most fundamental and important feature of an aquifer, a discrete groundwater flow system or a watershed are the relative rates of groundwater recharge and discharge; a balance frequently evaluated using a “water budget”. A simple form of a water budget for a watershed under natural conditions can be represented by equation 1 below:
Equation 1 P = ET + Qrunoff + Qgwd +/- ∆ Storage
where P represents precipitation, ET represents the combined quantities of evaporation and transpiration by plants, Q runoff represents direct runoff of precipitation, Qgwd represents groundwater discharge to streams or the coast and +/- ∆ Storage represents a change in the amount of water stored in the system. All units are reported in the equivalent depth across the system (i.e. watershed), and generally +/- ∆ Storage is considered to be negligible. When groundwater is extracted from the system, an additional term Q gw ext can be added to the right hand side of the equation as shown in equation 2 below:
Equation 2 P = ET + Qrunoff + Qgwd + Q gw ext +/- ∆ S
The three previous factors considered (aquifer size, storage capacity and permeability) are all fixed in time and represent the geological framework in which groundwater flow systems exist. On the other hand, groundwater recharge and discharge are both dependent on variable factors and the relative balance between the two factors determines the actual amount of water stored in an aquifer at any one time, as well as being an important consideration in managing water withdrawals. Over time, equilibrium is established between the rate of recharge and discharge. When conditions change (such as a change in the rate of groundwater extraction) the system adjusts to new equilibrium conditions. Under more extreme conditions, where the total of water discharges exceed the total of recharging water, continuous declines in the amount of water stored in the system will occur.
Recharge represents that portion of precipitation that soaks through the soil, through the unsaturated zone and reaches the water table. Recharge is typically quantified as an equivalent depth of water (in mm just as precipitation is recorded) spread evenly across the system, generally with boundaries defined by topography (i.e. watershed boundaries).
Recharge rates a) vary throughout the year and b) vary from year-to-year in response to changing climatic conditions. On a more local scale, recharge is also affected by the nature of ground cover and nature of the underlying geological materials as well. From a simplistic perspective, recharge occurs when the rate of precipitation (and snow melt if applicable) exceed the rate of evaporation from soils and free surfaces and transpiration by plants. Over shorter time frames, preceding soil moisture conditions affect whether water will move vertically through the soil profile to the water table or not. With these factors in mind we can see that recharge is greatest during the spring when the combination of precipitation and snow melt greatly exceed evaporation and transpiration, and is lowest to non-existent during late summer and early fall when evaporation and transpiration are at their peak. As a consequence, while direct overland run off (and thus total stream flow) may be vulnerable to prolonged periods without precipitation in the summer, groundwater recharge is negligible in this part of the year anyway, and short term weather patterns are far less influential on groundwater availability. Because of this seasonal variability, recharge is generally calculated and reported on an annual basis, and usually a large number of years are assessed to account for the variability in climatic conditions.
Recharge cannot be measured directly but is inferred using a wide variety of methods from other data (Alan et al. 2014), but often utilizing stream flow hydrographs (hydrograph separation), monitoring well data, a combination of the two, or computer simulations employing numerical groundwater models. On PEI the most common methods for determination of annual recharge rates have been using hydrograph separation augmented by groundwater monitoring well data and computer simulations using numerical groundwater models.
Recharge rates in PEI are typically high compared with many jurisdictions as is the high proportion of groundwater discharge providing for a high volume of baseflow to streams. Paradis et al., (2016) quote an historical value of 369 mm/ year representing about 35% of annual precipitation. In an earlier study specifically examining groundwater conditions in the Wilmot, Paradis et al. (2007) estimate the recharge rate at 38% of annual precipitation. Similar values have been reported by other researchers: 345 mm (Rivard et al. 2014); 35-40% of annual precipitation (Jiang et al., 2015); 21-43% of annual precipitation depending on watershed (Francis, 1989); and 30% of annual precipitations (Jacques Whitford and Associated, 1990). For comparison a compilation by Alan et al. (2014) suggests potential recharge amounts of 50 to 100mm throughout much of the Prairie Provinces.
To put these quantities of water in perspective, the value of 369 mm cited above is equivalent to 369 million litres (roughly 81.2 million gallons) of recharging water for every square kilometer of aquifer area. Put another way, it is sufficient water to fill all the pore spaces in the top 2.5 metres of an aquifer with a porosity of 15%. In actual practice of course, groundwater is not static, and changes in water table elevation in response to groundwater recharge and discharge vary significantly depending on the position in the watershed, with larger changes occurring in upper portions of the flow system and very small changes in discharge portions of the watershed.
Natural Groundwater Discharge
Just as water is recharged to the aquifer, it also naturally discharges from the aquifer to streams or the coast, unless it is intercepted by withdrawal from a well. Taking the first case, the groundwater component to total stream flow can be estimated using the same hydrograph separation techniques described above for the determination of recharge. Indeed the assumption is made that over the long term (i.e. an annual cycle) the amount of recharge equals the amount of discharge and the same citations referenced for recharge would apply to groundwater discharge to watercourses.
Estimates of the annual groundwater contribution to stream flow generally fall in the range of 60 to 70%, (Francis, 1989; Paradis et al. 2007, Jiang and Somers, 2008; Rivard et al., 2014) however on a shorter time frame, especially during summer months, in the absence of recent rainfall, almost all water in streams will originate from groundwater discharge (Paradis et al. 2007).
In coastal areas, a significant amount of groundwater is also discharged directly to the ocean. In reporting on numerical model simulations of submarine groundwater discharges (SGD) in relation to salt water intrusion in the Summerside area, Hansen and Ferguson (2012) state “Upon comparison with measured surface run-off rates for the watershed, simulated SGD constitutes approximately 13% of the total freshwater discharge to the sea. “
The rate at which groundwater discharges to streams or the coast does not vary as much throughout the year as recharge does. The rate of groundwater flow is determined by Darcy’s Law in which relates the quantity of groundwater discharge to the area through which the water is flowing, the permeability of the formation and the hydraulic gradient (i.e. slope) of the water table. Since only the hydraulic gradient is subject to change in response to seasonal changes in water table elevation, it follows that changes to the rate of groundwater discharge are modest compared to seasonal changes in recharge.
Groundwater Discharge via Extraction
On a very local scale the factors relating recharge rates may be influenced by the type of land cover (influence of paved surfaces, forested vs. agricultural land), but on a watershed scale recharge rates and groundwater discharge to streams are largely beyond the influence of humans. However groundwater extraction is within our control. When water is removed from an aquifer by pumping from wells, several things occur: locally a cone shaped depression is created around the pumping well and on a watershed scale the “water budget” for the watershed is altered and water that would otherwise discharge to a stream or the coast is intercepted.
The size and shape of the cone of depression of a pumping well depends on the rate of water withdrawal, the duration of water withdrawal and the aquifer characteristics (hydraulic conductivity/transmissivity, and storativity). All of these factors can be readily quantified (aquifer characteristics have already been discussed.) With this information, the effect of pumping on local water table conditions, interference with other wells, discharge to streams etc. can be estimated with reasonable confidence. Pumping tests, required as part of the approval process for high capacity wells provide the necessary local data to make site specific predictions.
Beyond local considerations, groundwater extraction alters the water balance of a watershed (see equation 2 above), reducing the discharge of groundwater to streams Qgwd by an amount equivalent to the rate of groundwater extraction (+Q gw ext). Provided that the amount of groundwater extraction is less than the amount of the groundwater recharge to the watershed, the effect of pumping is the establishment of new “equilibrium” conditions with lower discharge rates to streams and lower water table elevations. In the special case where water withdrawals from a groundwater flow system exceeds rate of recharge of the system, disequilibrium conditions will persist and water table elevations will decline as long as the situation persists. This is often referred to as “groundwater mining” representing unsustainable water withdrawal rates. These are the same conditions often referred to in the media affecting parts of a number of states in the mid-west and south western U.S.A., parts of Mexico, India etc. This does not happen in PEI as withdrawals are held to only 50% of the recharge or less.
Statistics Canada (2017) report potable water use at 11.3 million cubic metres per year for PEI, somewhat lower than provincial estimates of 15.7 million cubic meters per year. When commercial and industrial water use is added in, total water use is estimated to be 35.7 million cubic metres per year. Total annual groundwater recharge is estimated to be 2.4 billion cubic metres per year. The estimate for recharge is based on an assumed average of 400 mm per year of recharging water, spread across all the land mass of the Province except for a 100 m zone along the coast.
In describing the parameters for a numerical groundwater flow and nitrate transport model for Prince Edward Island, Paradis et al. (2016) note: “The impact of pumping wells on the water table is thus expected to be low (<2 mm yr−1 based on a daily individual consumption of 200L), except in few localized areas where potable water is supplied by production wells (e.g., Charlottetown)”.
Thus taking PEI as a whole, total groundwater extraction represents less than two percent of total recharge. Find more details on water use
At the same time, groundwater extraction is not evenly distributed across the Province and the effects of pumping are largely restricted to the groundwater flow system (i.e. watershed) in which they occur. As a result there are some watersheds, such as the upper part of the Winter River Basin, where groundwater extraction equals approximately 50% of recharge to the system. While this is a completely sustainable situation from a groundwater perspective, the impact on stream flow is much more pronounced. This is in part because wells with depths of 10’s of metres are far less vulnerable to changes in water table elevations and groundwater discharge conditions than streams that may have a depth of less than a meter. It is considerations of this nature that have resulted in a shift in policy for approvals for groundwater extraction, from an assessment of pumping rates in relation to groundwater recharge to a regime where the focus of the assessment is on the effect of withdrawals on stream flow conditions.
Addressing some common concerns
Having discussed the major features affecting the capacity of an aquifer, and describing the PEI aquifer in this context, it is worth looking at some of the more important implications.
One of the common concerns voiced with respect to water use is the importance of leaving water for future generations. As the foregoing hopefully demonstrates, groundwater is constantly moving from its point of recharge to discharge, thus while in one sense, at any one time there may be a large volume of water “stored” in the aquifer this water is actually just slowly transiting through the groundwater flow system and cannot actually be stored.
Over time, all of precipitation falling on ground will be back either to the sea by stream flow and groundwater discharge or to the sky by evapotranspiration. Although groundwater transport is slow, groundwater discharge (via wells or direct discharge to streams) is continuously replenished by water stored in aquifer which in turn is replenished at the times of precipitation events with a very short lag time. Savard et al. (2007) documented surface water and shallow ground waters sharing the same seasonal isotopic signatures in nitrate in the Wilmot watershed, indicating very rapid transit through shallow portions of the aquifer and discharging to surface water. Flow and mass transport modeling as well as tritium age dating from the same study were used to estimate groundwater residence times in the watershed (Jiang et. al. 2007). Results from this exercise show groundwater between the water table and 22 m below the land surface, contributes more than 80% of stream base flow (>53% of total flow), and has a residence time in the ground of less than 4 years.
As a consequence government cannot save or reserve water for future generations, but can only set in place rules that safeguard the security of current water supplies and important ecological receptors, and work to ensure that future water management decisions provide similar considerations for balancing water demand and availability. In other words, the amount of water available for future generations will be determined by the water management practices at that time.
“Deep Water Wells”
Concerns are sometimes voiced about the effect of “deep water wells” with fears they are tapping deep, older water that can’t be replaced. The term “deep” in these circumstances is rather misleading as “high capacity” wells are not necessarily that much deeper than other wells, but rather differ mainly by the amount of water they extract. Furthermore, the largest portion of water withdrawn from these wells comes from the shallowest portions of the aquifer where aquifer permeabilities are highest. For example, Jiang et al.(2007) with respect to the Wilmot watershed note that the most productive part of the aquifer lies within the top 22 metres from the water table, accounting for 80% of baseflow to streams, and with an average residence time of only 4 years. In related work based on vertical profiles of hydraulic conductivity, Somers and Savard, ( 2008) estimated that the top 12 metres of the aquifer contributes 73% of flow to a typical well in the area. Thus for the most part these wells are actually tapping the youngest water in the aquifer.
In contrast, while in deeper parts of an aquifer water will be older, the permeability, and thus groundwater flow rates and well yields will also be lower. As a consequence these deeper parts of the aquifer, while storing an abundant amount of water, do not typically play an important part in the overall flux of groundwater in the system. For the same reason (low permeability), they are not promising targets for water supply development.
Aside from the fact that high capacity wells are not depleting reserves of deep, old groundwater, they are also more efficient in terms of energy and infrastructure costs to supply water for municipal and industrial needs. Compared to other alternative methods of water extraction, a high capacity well has a lower environmental risk than multiple low capacity wells and a lower environmental impact than ponds or reservoirs.
All of high capacity wells are subjected to a permitting process designed to safeguard existing wells, stream flow, and groundwater resources and to avoid water quality issues by a comprehensive environmental assessment. In addition the pumping rates of high capacity wells are required to be recorded and reported to the Department, and monitoring of stream flow and groundwater levels in environmentally sensitive areas may be required.
Climate Change / Sea Level Rise/ Salt Water Intrusion
From recent hydrological and groundwater modeling, over the next several decades, annual average stream flow and groundwater levels are projected to increase slightly. Models predict that seasonally, earlier snow melt will result in increases in stream flow in winter; decreases in spring; and little changed in the summer dry period. Groundwater levels are projected to increase moderately in late fall to early spring; decrease slightly in May and June; and be relatively unchanged in the summer dry period. The last 40+ years historical stream flow and groundwater level monitoring data in PEI, show very similar trends and patterns (Qing Li, personal communication, 2019).
Living on a relatively small island it is natural to consider the effects of sea level rise on the fresh groundwater supplies, and indeed this is a major issue in many parts of the world. This issue was examined through a collaborative project involving Atlantic Canada provincial government agencies and academic institutions under a program supported by NRCan under the auspices of the Atlantic Regional Adaptation Collaborative (RAC) Climate Change Program, (Somers and Nishimura, eds. 2012). The results of this work indicated that only slight changes in the extent of salt water intrusion can be expected as a result of projected sea level rise. They showed that management of groundwater withdrawals in coastal areas, regardless of climate change effects, are likely to be the most important and effective considerations in preserving the integrity of coastal aquifers.
The Province of Prince Edward Island is underlain by an extensive and productive aquifer, as documented by numerous researchers and groundwater professionals over the past half century. The relatively simple hydrogeological framework of this aquifer, and the body of knowledge compiled over the years, provide for the effective management of this important resource using standard scientific tools and approaches. In no case are groundwater resources themselves being over exploited or unsustainable, however groundwater withdrawals in some catchments are having a significant and un-acceptable impact on local stream flow regimes, and the environmental conditions in these cases are closely monitored and assessed.
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