Cassidy Aquifers: How they work

To aid public discussion regarding concerns about risk of contamination of groundwater in the Cassidy area posed by ongoing industrial activity, TAKE 5 is presenting a two part article on the Cassidy aquifers. The fi rst part may be found in our previous issue.

BY QUENTIN GOODBODY

Cassidy Aquifers 101

All our fresh water ultimately comes from rainfall. If we do not collect rainwater directly from our roofs, we access fresh water in one of two ways:

The simplest way of getting water is by drawing or pumping it directly from a nearby lake, river, stream or spring. The Town of Ladysmith, Diamond Improvement District, Stz’uminus First Nation and Saltair all draw water from two stream-fed artificial lakes (reservoirs) in the hills west of Ladysmith – Holland Lake and Stocking Lake. A lot of money has recently been spent on filtration systems to ensure that the water from these reservoirs is clean enough for domestic consumption.

However, if you live in the surrounding district, chances are that you do not have a body of surface water from which you can draw, nor do you have access to a municipal water system. Instead, you have to go looking for water in the ground – in other words, you must fi nd an aquifer. When you fi nd one, you want the water you take from it to be uncontaminated so you can use it in your home: there are all sorts of protocols associated with water well completion to ensure this.

Aquifers are not underground rivers or water-filled caverns. They consist of rock or unconsolidated sediment such as gravels and sands which hold water in spaces between the mineral grains making up the rock or sediment. These often tiny spaces are collectively called Porosity, which typically makes up 10-20% of the bulk volume of a rock aquifer, but can be more than 30% in unconsolidated sediments.

Just holding the water in the pore spaces is not enough for an aquifer to be useful. The water must be able to move through the rock or sediment so we can get it out. This property is called Permeability. The degree of permeability depends on the level of connectedness between the pores in the rock or sediment (Figure 1).

The more porosity and permeability there is, the faster water can move through the medium, and the higher the rate water can be sustainably produced from a well dug or drilled into it.

For an aquifer to provide a stable supply for water, the rate of natural fresh water recharge to the aquifer must equal or exceed the rate of water withdrawal from pumping wells. If this is not the case, the water level in the aquifer will go down and wells may become dry because they are not deep enough to encounter the modified (deeper) groundwater level.

Geology of the Cassidy Area Aquifers

Geology refers to the rocks and unconsolidated sediments that make up the ground we walk on.

The geology of the Cassidy area has been determined by mapping the rocks and sediments at ground surface and getting an idea of the subsurface set-up through examination of the logs of over 2000 water wells drilled in the area. Several aquifers have been recognized in the Cassidy area which are currently being used for domestic, agricultural and industrial purposes. The following description of the geology and the aquifers comes from a number of scientific reports that can be found online (references can be provided on request to the author).

There are two types of aquifers in the Cassidy area: 1/. Bedrock – composed of 80 million year old Cretaceous sandstones which have low to moderate porosity and permeability from which wells generally produce water at low rates. 2/. 1200014000 year old unconsolidated sands and gravels lying on top of the Cretaceous bedrock which are much more porous and permeable and from which water can be produced at higher rates.

Whereas the ground surface in the vicinity of Cassidy and The Nanaimo Airport is essentially flat today (Figure 2), geological mapping of the top of the Cretaceous bedrock reveals a valley system which was eroded into it during and just after the last glacial period about 14,000 years ago. This ‘fossil’ valley system extends from Ladysmith Harbour north toward where the Nanaimo River now flows to its estuary in Nanaimo Harbour (Figure 3).

Glacially fed streams filled this ‘fossil’ valley system with unconsolidated sediment. There is a distinct layering (stratigraphy, in geologic terms) to this valley fill (Figure 4). The aerial distribution of each layer (stratigraphic unit) is shown in Figure 5.

Unit 1, the oldest and deepest part of the unconsolidated valley fill, consists of densely packed sands and gravels with moderate porosity and permeability. These are distributed only locally in the deepest portion of the fossil valley and lie directly on Cretaceous bedrock.

Unit 2, distributed over the entire valley system, consists of impermeable claystones deposited some 12000 years ago when sea level was about 150 meters higher than present.

Figure 5: i-iv — Aerial distribution of units within the Cassidy fossil valley fill.

Unit 1, the oldest and deepest part of the unconsolidated valley fill, consists of densely packed sands and gravels with moderate porosity and permeability. These are distributed only locally in the deepest portion of the fossil valley and lie directly on Cretaceous bedrock.

Unit 2, distributed over the entire valley system, consists of impermeable claystones deposited some 12,000 years ago when sea level was about 150 metres higher than present.

Unit 3, distributed over much of the buried valley system, consists of loosely packed highly porous and permeable sands and gravels. These are exposed at ground surface except for a small area north and west of the airport.

The impermeable clay-rich Unit 4 is ground surface to a small area underneath and west of the airport. It locally forms a barrier to surface water movement into the underlying sands and gravels of Unit 3.

Hydrogeology of the Cassidy aquifers

Hydrogeology refers to the movement of water within rocks and sediments.

The Cassidy area lies within the watershed of the Nanaimo River, which includes the Nanaimo River and its major tributaries such as Haslam Creek (Figure 6). The majority of the watershed lies within the Nanaimo Regional District (NRD); the southernmost part of the watershed, including part of the Cassidy area, lies within the Cowichan Valley Regional District (CVRD).

Analysis of groundwater levels in observation wells provides some understanding of how water flows into and out of the various geologic units.

The Cretaceous bedrock of the area is an important aquifer to those living outside of the distribution of the fossil valley-fill sediments. The BC Provincial Aquifer database recognizes two bedrock aquifers, numbered 162 and 964; the boundary between the two is roughly along a line continuing northwest parallel to the north shore of Ladysmith Harbour with 964 being west of this line and 162 east of it. Groundwater recharge for both aquifers is direct from rain, but due to the bedrock’s limited permeability and porosity it appears to be slow and care has to be taken not to over-produce from wells and running them dry. Fractures associated with faults in the bedrock appear to be an important factor in locally higher well production rates.

The water-bearing densely packed sands and gravels with moderate porosity and permeability of Valley-fill Unit 1 are numbered Aquifer 160 in the B.C. Provincial Aquifer database. There are relatively few well penetrations because it underlies a shallower good aquifer (Unit 3). Recharge occurs after a week’s lag following rainfall which indicates groundwater flows into it either from the underlying bedrock or via a slight connection to the overlying aquifer Unit 3. Aquifer 160 is termed a ‘confined’ aquifer due to its not being exposed at ground surface and because it is everywhere overlain by the impermeable claystone Unit 2. Only a few wells produce from aquifer 160; their low volume of production appears to have minimal effect as observation wells appear stable over time, the most recent data reviewed being from 2016.

The clay-rich ‘tight’ (ie non porous and impermeable) Unit 2 acts as a barrier to groundwater movement between Units 1 and 3 and is scientifically termed an ‘aquitard’.

Valley-fill Unit 3, numbered Aquifer 161 in the B.C. Provincial Aquifer database, is the most important aquifer in the area. Its high porosity and permeability sands and gravels provide good well flow rates. This aquifer is termed ‘unconfined’ due to its being exposed at the ground surface. While groundwater recharge is mainly direct from rainfall in the watershed, local recharge from streams has prompted its division into two connected aquifers (Figure 5iii) –Cassidy Aquifer 1 to the south which is locally recharged by Haslam Creek, and Cassidy Aquifer 2 in the north which is strongly in fluenced by the Nanaimo River. The strong recharge in the north from the Nanaimo River facilitates bulk water extraction for industrial purposes by Harmac. Data suggests that groundwater pumping signi ficantly a ffects water levels in Aquifer 161. Proximity to recharge sources, such as the Nanaimo River, locally allows for a stable high rate extraction, but more work with up to date information for the southern Cassidy Aquifer 1 (most recent data seen by the author dates from 2016) is required for determination of how much water can be taken from this aquifer without long term decline in groundwater water levels occurring.

Except where locally overlain by the impermeable clay-rich Unit 4, the sands and gravels of Unit 3/Aquifer 161 are exposed at ground surface beneath a variable soil cover.

The regional map of groundwater elevations in wells in Aquifer 161 (Figure 7) indicates that, on average, groundwater flows from the uplands west of Cassidy and diverges from the central Cassidy area, northeastward towards the Nanaimo River and estuary, and southeastward toward Ladysmith Harbour. However, this regional picture may be locally modified by production from water wells - as extraction of water from a well reduces the pressure around that wellbore and prompts groundwater movement toward it. This can cause local ground water movement against the regional flow.

More detailed analysis of many wells in the area is required to determine whether this aquifer is being over-taxed by the current level of withdrawal from wells or not, and how surface contamination might travel within the aquifer.

Aquifer Vulnerability and Industrial Activity

Increased land development pressures in the early 2000s, coupled with industrial and agricultural land use activities that were considered to potentially threaten the quality of ground and surface water, prompted the CVRD to participate in a multi-year study which focused on assessing the relative vulnerability of groundwater resources to surface contamination on Vancouver Island. This study was conducted jointly by the BC Ministry of Environment, Vancouver Island University, Natural Resources Canada, The Vancouver Island Health Authority and the Regional Districts on Vancouver Island. A rating system, called by the acronym ‘DRASTIC’ (relating to seven significant factors taken into account) was devised; each recognized aquifer was assigned a vulnerability rating of either Low, Moderate or High. A vulnerability map for surface aquifers on Vancouver Island was produced that shows high vulnerability in the Cassidy area (Figure 8).

Of the Cassidy area aquifers:

Cretaceous bedrock aquifer 162 which outcrops over a wide area was considered of High Vulnerability to surface contamination entering via exposed high permeability fractures.

Although also recognized as possessing highly permeable fractured zones, Bedrock Aquifer 964 was rated of Moderate Vulnerability as it only locally outcrops at surface.

The unconsolidated confined Aquifer 160 (valley-fill Unit 1) is considered to have Low Vulnerability to contamination from surface sources as it does not outcrop at surface and is everywhere overlain by the aquitard Unit 2.

Aquifer 161 (valley-fill Unit 3), the porous and permeable sands and gravels of which are at ground surface over much of the Cassidy area, is considered of High Vulnerability to surface contamination.

The quoted vulnerability study also provided an example of appropriate hydrogeological assessment for development permit applications situated on various aquifer types. If Schnitzer’s Cassidy activities, which are located on top of High Vulnerability Aquifer 161, are considered as a commercial ‘junk yard’ and of moderate hazard for surface contamination, the following were deemed required for assessment of an application to conduct these activities at that location:

• a detailed groundwater site investigation including an ongoing monitoring program

• specifics of the potential contaminants (toxicity, quantity, transport behaviour),

• details on the protection design factors (natural attenuation, physical barriers, etc.)

• A detailed emergency response plan

• An assessment of the financial capacity of the responsible party to enact the plan.

However, if Schnitzer’s activities are considered as an industrial activity posing a high risk of surface contamination to the aquifer, the possibility of complete prohibition of those activities is cited.

What the Neighbours Think

The southern portion of Aquifer 161 straddles the boundary between the regional districts of Nanaimo and the Cowichan Valley, with Schnitzer’s activities being located immediately adjacent to the boundary on the CVRD side. Given the high permeability of Aquifer 161 and its deemed High Vulnerability to surface contaminants, it seems reasonable to assume that the Regional District of Nanaimo (RDN) is interested in what the CVRD decides regarding Schnitzer’s rezoning application. ‘Actions and Best Practices’ cited from a 2010 RDN “Groundwater Assessment and Vulnerability Study” by GW Solutions Inc. indicates that industrial activities in a high vulnerability aquifer zone are not recommended and that auto-wreck yards are not allowed in such zones.

Thank you to Dr. Quentin Goodbody for this explanation of how aquifers work. We hope this has given our readers insight into the Cassidy aquifers.

Please visit www.ladysmithhistoricalsociety.ca for an online version of this article with full references.