The Geological Influence on Water Quality in Alberta's Paskapoo Aquifer System
The Paskapoo Formation represents Alberta's most significant groundwater resource, supporting approximately one-third of all water wells in the province and serving as a critical water supply for thousands of communities and industries. This extensive aquifer system exhibits distinctive water quality characteristics that directly reflect its complex geological structure and composition. Understanding how the formation's geology influences water chemistry is essential for sustainable management of this vital resource that underlies much of Alberta's most populated corridor.
The Paskapoo Formation developed during the Paleocene epoch as part of the Western Canadian Sedimentary Basin, forming a wedge-shaped body of sedimentary rock that extends across approximately 65,000 square kilometers of southwestern Alberta. This heterogeneous fluvial unit reaches thicknesses between 0-800 meters, with its greatest depths occurring near the foothills of the Rocky Mountains before thinning eastward across the Alberta plains4.
The formation consists of a complex succession of interbedded sandstones, siltstones, and mudstones with minor coal beds that were deposited in varied fluvial environments. Modern stratigraphic analysis has divided the Paskapoo Formation into three distinct members, each with unique lithological characteristics that significantly influence their water-bearing properties. The lowermost Haynes Member comprises thick, massive, coarse-grained sandstones that form the principal aquifer zone within the formation. The middle Lacombe Member consists predominantly of channel sand complexes encased in finer-grained floodplain deposits characterized by siltstone, mudstone, shale, and minor coal. The uppermost Dalehurst Member, present primarily near the Hinton area adjacent to the foothills, represents an erosional remnant containing sandstones and notable coal seams45.
This internal stratification creates a complex three-dimensional groundwater flow system with predominantly horizontal movement through the more permeable sandstone channels and limited vertical exchange between units. The Paskapoo's heterogeneous structure means that water yield and quality can vary dramatically over relatively short distances, presenting challenges for water resource development and management5.
The groundwater chemistry within the Paskapoo Formation exhibits distinct patterns that correlate directly with its geological structure and regional depositional history. Water quality is generally characterized as fresh and suitable for various domestic, agricultural, and industrial applications across much of the formation6. However, geochemical analyses reveal systematic variations in water chemistry both laterally and vertically throughout the aquifer system.
The bulk water geochemistry varies considerably with Total Dissolved Solids (TDS) content across the aquifer, ranging from calcium-magnesium-bicarbonate (Ca-Mg-HCO₃) type waters, to sodium-bicarbonate (Na-HCO₃) waters, to sodium-sulfate (Na-SO₄) dominated waters2. TDS values typically range from 375 to 2500 mg/l, with an average of approximately 1000 mg/l. A pronounced east-west transition in water chemistry has been documented, with groundwater in the western portion of the Paskapoo generally exhibiting lower TDS compared to the eastern regions2.
This lateral transition is marked by increasing concentrations of dissolved solids, sodium, sulfate, and alkalinity from west to east, along with decreasing concentrations of calcium and carbon-14 (as dissolved inorganic carbon)1. The most dramatic chemical shift occurs across an approximately north-south trending boundary that corresponds with glacial till deposits of different origins that overlie the formation2.
The Paskapoo Formation also displays vertical variations in groundwater chemistry that reflect the internal stratigraphic zonation of the formation. Field pH values generally increase with depth throughout the formation, while concentrations of dissolved calcium and magnesium typically decrease with greater depth1. This vertical stratification results from the different lithological characteristics of the three main members and the progressive evolution of groundwater chemistry along flow paths from recharge areas.
Dissolved oxygen concentrations throughout the formation are generally low, with most samples measuring below 0.01 mg/l and nearly all below 3.0 mg/l1. These reduced conditions influence the mobility and speciation of various dissolved constituents, particularly affecting iron, manganese, and sulfur compounds. The predominantly anoxic conditions likely result from the consumption of dissolved oxygen by organic matter and minerals containing reduced species as water percolates through the formation.
The vertical exchange of groundwater between different members of the formation is limited by the presence of lower-permeability siltstones and mudstones in the Lacombe Member, which function as partial barriers to flow. This stratified arrangement creates a complex hydrochemical system where water chemistry can vary significantly with depth even at the same location, presenting challenges for water well development and quality prediction.
One of the most significant geological factors affecting Paskapoo water quality is the nature of the glacial deposits that overlie the formation. Research has documented a striking correlation between water chemistry in the Paskapoo aquifer and the type of glacial till that serves as its cover material2. The formation is overlain by two distinct till types: Cordilleran till derived from the Canadian Rocky Mountains to the west, and Laurentide till originating from the Canadian Shield to the east.
Groundwater analyses reveal that portions of the Paskapoo Formation beneath Cordilleran till typically contain lower sulfate concentrations, while areas beneath Laurentide till exhibit significantly higher sulfate levels2. This dramatic shift in sulfate concentrations occurs across a boundary that precisely matches the contact between the two till types. Stable isotope analysis of sulfur (δ³⁴S) confirms this relationship, showing that low-sulfate waters have isotope values consistent with Devonian-carbonate-associated sulfate present in the Cordilleran tills, whereas high-sulfate waters have isotope values indicative of pyrite oxidation in the Laurentide till2.
The Laurentide till contains relatively high concentrations of pyrite (iron sulfide minerals) derived from the Canadian Shield. Under oxidizing conditions, these sulfides weather to form sulfate, which subsequently infiltrates into the underlying Paskapoo Formation. In contrast, the Cordilleran till is dominated by carbonate materials from the Rocky Mountains, which contribute different dissolved constituents to infiltrating water. This relationship demonstrates how surficial geology can profoundly influence bedrock groundwater quality through recharge processes, even in deep aquifer systems.
The complex hydrogeological properties of the Paskapoo Formation further influence water quality patterns throughout the aquifer system. The formation exhibits highly variable transmissivity values, ranging from less than 1.5 m²/day to over 100 m²/day, depending primarily on the degree of fracturing5. These variations in hydraulic conductivity affect groundwater flow rates and residence times, which in turn influence water-rock interactions and resulting water chemistry.
Areas with higher transmissivity and more extensive fracturing generally exhibit more rapid groundwater flow and potentially younger, less mineralized water. In contrast, zones with lower permeability may contain water with longer residence times, allowing for more extensive water-rock interactions and typically higher dissolved solid concentrations. Regional permeability data for the Paskapoo indicate values ranging from 10 to 1000 millidarcies where the fracture network is present, creating significant spatial variability in flow characteristics5.
Storativity values throughout the formation range from 5 × 10⁻⁵ to 10⁻⁴, indicating that most aquifers within the Paskapoo are confined to varying degrees5. These confined conditions limit vertical recharge from the surface in many areas, potentially preserving distinct water quality zones and contributing to the chemical stratification observed throughout the formation.
The mineralogical composition of the Paskapoo sandstones directly influences groundwater chemistry through water-rock interactions. The sandstones are primarily litharenites, with major framework grains including quartz, feldspar, and various rock fragments (mainly chert, volcanic, metamorphic, and sedimentary)4. These minerals undergo gradual dissolution when in contact with groundwater, contributing different ions to solution depending on their composition.
Feldspar weathering contributes sodium, potassium, calcium, and aluminum to groundwater, potentially explaining some of the variations in cation composition throughout the formation. The dissolution of carbonate minerals, both as framework grains and as cementing material, releases calcium, magnesium, and bicarbonate ions. These dissolution processes are influenced by factors such as pH, temperature, residence time, and initial water chemistry, creating spatial variations in groundwater quality.
Diagenetic processes that occurred during and after the formation's deposition have also affected its hydrochemical properties. Post-depositional cementation by minerals such as calcite, silica, and iron compounds has reduced primary porosity in some areas while creating secondary porosity through partial dissolution in others. These diagenetic alterations influence not only the hydraulic properties of the aquifer but also the types and rates of water-rock interactions that determine water chemistry.
The Paskapoo Formation is characterized by a regional groundwater flow system that is predominantly topographically driven, with recharge occurring along regional topographic highs and discharge in valleys and lowland areas. This flow pattern creates predictable evolutionary trends in water chemistry as groundwater moves from recharge to discharge areas.
In recharge zones, particularly along the western margin of the formation near the foothills, groundwater typically exhibits lower TDS values and is dominated by calcium, magnesium, and bicarbonate ions derived from relatively rapid dissolution of carbonate minerals. As water moves along regional flow paths toward the east, cation exchange processes progressively replace calcium and magnesium with sodium, while continued water-rock interactions increase overall dissolved solid concentrations1.
This evolutionary sequence explains the observed transition from Ca-Mg-HCO₃ type waters in western portions of the aquifer to Na-HCO₃ and ultimately Na-SO₄ type waters in eastern regions. The regional flow pattern also influences redox conditions throughout the formation, with dissolved oxygen being consumed along flow paths and creating increasingly reducing conditions that affect the mobility and speciation of various redox-sensitive elements.
Conclusion
The Paskapoo Formation's complex geology exerts a profound influence on groundwater quality throughout this extensive aquifer system. The formation's internal stratigraphic architecture, comprising three distinct members with different lithological characteristics, creates vertical zonation in water chemistry. Laterally, water quality transitions from calcium-magnesium-bicarbonate waters in the west to sodium-sulfate waters in the east, reflecting both the regional flow system and the influence of different overlying glacial deposits.
The interaction between bedrock geology and surficial deposits demonstrates how groundwater quality in the Paskapoo Formation is determined not simply by the formation itself but by the entire hydrogeological system, including recharge processes and water-rock interactions along flow paths. This relationship highlights the importance of considering both bedrock and surficial geology in predicting water quality patterns and managing groundwater resources sustainably.
As Alberta continues to rely heavily on the Paskapoo aquifer for water supply, understanding these geological influences on water quality will be essential for effective resource management. The heterogeneous nature of the formation means that water quality can vary significantly over short distances, requiring site-specific investigations for water supply development. By recognizing how geological factors determine groundwater chemistry, water resource managers can better predict potential quality issues and develop appropriate treatment strategies for this vital resource.
Citations:
- https://ags.aer.ca/publications/all-publications/ofr-2012-05
- https://members.cgs.ca/documents/conference2008/GEO2008/pdfs/217.pdf
- https://publications.gc.ca/collections/collection_2023/rncan-nrcan/m183-2/M183-2-8982-eng.pdf
- https://prism.ucalgary.ca/server/api/core/bitstreams/9096f553-1b48-4022-bd8c-5fa6b6db8aea/content
- https://prism.ucalgary.ca/bitstreams/3f998b61-7cc6-4d1d-b7be-17c60b8f008f/download
- https://gin.gw-info.net/service/api_ngwds:gin2/en/data/standard.hydrogeologicunit.html?id=75
- https://www.lacombecounty.com/en/business-and-development/resources/Documents/Highland-Park_GroundwaterAvailabilityStudyMay252009.pdf
- https://waterportal.ca/where-s-the-good-water/
- https://osdp-psdo.canada.ca/dp/en/search/metadata/NRCAN-GEOSCAN-1-226085
- https://members.cgs.ca/documents/conference2008/GEO2008/pdfs/206.pdf
- https://cdnsciencepub.com/doi/abs/10.1139/e08-069
- https://www.researchgate.net/publication/233528137_Regional_characterization_of_the_Paskapoo_bedrock_aquifer_system_southern_AlbertaGeological_Survey_of_Canada_Contribution_2008-0479
- https://cdnsciencepub.com/doi/10.1139/E08-069
- https://ags.aer.ca/document/OFR/OFR_2012_05.pdf
- https://cdnsciencepub.com/doi/full/10.1139/cjes-2016-0164?src=recsys
- https://publications.gc.ca/collections/collection_2022/rncan-nrcan/m183-2/M183-2-8903-eng.pdf
- https://hess.copernicus.org/articles/20/2759/2016/hess-20-2759-2016.pdf
- https://www.tandfonline.com/doi/full/10.1080/07011784.2018.1467796
No comments:
Post a Comment