Quantifying shallow subsurface flow and salt transport in the Canadian Prairies Andrew Ireson GIWS, University of Saskatchewan www.usask.ca/water Uri Nachshon Garth van der Kamp GIWS, University of Saskatchewan Environment Canada
Introduction Who I am Flow in unsaturated fractured porous media my research in the Chalk, and general implications for application to other settings An introduction to the hydrology/hydrogeology of the Canadian prairies A conceptual model of flow processes at the St Denis, SK, field site A conceptual model of salt distribution at the St Denis, SK, field site An early attempt at a physically based quantitative model of the system
Unsaturated fractured porous systems Macropores are ubiquitous in soils, yet frequently not considered explicitly in models, or interpretations of observations Some geological formations underlying soils also include Fractures Functionally, I will consider these as the same thing: fractured systems I will describe how different fractured systems operate I will describe how fractured systems may or may not be associated with preferential flow, dependent on a number of factors I will show how fractured systems may affect observations I will then move to the Canadian prairies, which are dominated by fractured-porous Glacial Till.
3 decades researching soil macropores Beven & Germann (1982, 2013): When does water flow through macropores in the soil? How does water flow through macropores in the soil? How does water in a macropore interact with water in the surrounding soil? How important are macropores in terms of volumes of flow at the hill slope or catchment scale? What are the implications of macropores for movement of solutes and chemical interactions in the soil?" After 30 yrs these questions still haven t received adequate attention An adequate physical theory is missing (they reject Richards equation) Beven & Germann, 2013, WRR doi:10.1002/wrcr.20156
5 decades of research in Chalk
5 decades of research in Chalk Researchers in UK debated fracture flow vs matrix flow for last 5 decades Contradictory observations included: Bacteria in deep wells (fracture flow) Slow transport of tritium pulses (piston flow through matrix) Diffusion of tritium from flowing fractures into matrix (fracture flow) Rapid deep water table responses (fractures) Delayed slow summer groundwater recharge (matrix) For the past decade it has been clear that both F&M transmit flow Fracture flow dominates if soil/rock is wet and/or in response to high intensity rainfall Matrix flow probably transmits the largest volume of recharge annually
Different types of fractured systems Key to how unsaturated fractured systems work are the properties of the matrix Impermeable matrix Flow and storage in fractures only Porous, low K matrix Dual porosity Porous, moderate K matrix Dual permeability For more discussion of this see Doughty, 1999, J. Cont. Hydrol. 38: 69-106 and Ireson & Butler, 2011, JoH doi:10.1016/j.jhydrol.2010.12.015
Flow in dual permeability systems: Three modes of flow are possible: Matrix flow Equilibrium matrixfracture flow (Non-preferential fracture flow) Equivalent continuum model Low intensity rainfall Continuou`s slow drainage from the matrix which persists throughout the summer Moderate intensity rainfall Recharge via matrix and partially saturated fractures, lags of 10s of days. Preferential fracture flow Dual continua model Discrete fracture model In the English Chalk, all three are operative at different times: High intensity rainfall Rapid preferential recharge, through fractures, lags of <1 day. Ireson & Butler, 2011, JoH doi: 10.1016/j.jhydrol.2010.12.015
Preferential flow: Preferential flow occurs when flow through the fractures is too fast to equilibrate with matrix water. Occurrence of preferential flow depends on rainfall event characteristics and antecedent soil moisture NOT ALL FRACTURE FLOW IS PREFERENTIAL! Ireson & Butler, 2011, JoH doi: 10.1016/j.jhydrol.2010.12.015
Observations from fractured systems
Observations from fractured systems Matrix: low K high n Fractures: high K low n
Water content Instrument: Profile probe (same principle applies to neutron probe and other dielectric methods) Measurement integrates water located in Matrix pores Fractures (Access tube gaps) Hence, bulk q
Matric potential Instrument: Pressure transducer tensiometer (same principle applies to other methods) Measurement dominated by water in Fractures (Access tube gaps) Hence, fracture y If fracture and matrix are in pressure equilibrium, this is also bulk y
Soil moisture characteristic: Hysteresis or fracture-matrix disequilibrium? Ireson et al., 2012, HR doi: 10.2166/nh.2012.030
The Canadian Prairies Nachshon et al, 2013, JoH doi:10.1016/j.jhydrol.2013.07.001
16 Classical unconfined aquifer systems Typical conceptualisation for a humid climate: Precipitation > Potential evaporation Groundwater feeds stream Fetter, C.W. 2000. Applied Hydrogeology Fetter, C.W. 2000. Applied Hydrogeology Typical conceptualisation for an arid climate: Precipitation < Potential evaporation Intermittent stream feeds groundwater
The Canadian Prairies 17 Climate = semi-arid: Precipitation < Potential evaporation But very strong seasonal pattern caused by snow accumulation/melt Perhaps more similar to an arid climate But flow can potentially be in both directions, as illustrated by Fetter: Fetter, C.W. 2000. Applied Hydrogeology
Seasonally frozen hydrogeological processes Ireson et al., 2012. HydroGeol. J. doi: 10.1007/s10040-012-0916-5
Prairie snowmelt dominated hydrology 29 th April 2013 11 th May 2013 29 th April 2013 7 th June 2013
Deep Groundwater Flow? A. Lissey (1971)
St Denis Conceptual Model: Hydrology Basic general principle: Liquid water that avoids being evaporated or transpired at the surface will always continue to move vertically downwards with gravity, until it reaches some impediment that causes it to migrate laterally or stagnate.
Regional setting St Denis test hole St Denis NWA
270 m Stratigraphy Gravel, Sand, Silt, Clay Gravel, Sand GROUNDSURFACE Till Till Till Till Silt & Clay Sand & Silt Silt & Clay Sand & Silt St Dennis Silt & Clay Hydraulic conductivity mm/d (log scale)
Depth St Denis Conceptual Model: Hydrology Water at the surface can infiltrate relatively easily (in unfrozen conditions) It becomes progressively harder to percolate downwards through the till Hydraulic conductivity mm/d (log scale) Macroporosity At some point, it may become easier for the water to move laterally (interflow/perched GW flow) The higher the water table, the more rapid the lateral saturated flow.
St Denis Conceptual Model: Hydrology High K effective transmission zone ~5-10 m Low K zone with minimal flow Oxidation front, coincident with base of High K zone Thin confined sandy aquifer
St Denis Conceptual Model: Hydrology High K effective transmission zone ~5-10 m ESSENTIALLY IRRELEVANT FOR THE WATER BALANCE BUT IMPORTANT FOR SALTS
Completing the picture Riparian zones Wetland Pond Recharge pond Throughflow pond Discharge pond Possibly terminal
Flow patterns: post snowmelt profile Riparian zones Wetland Pond Mobile groundwater Potential connections Surface connections during snowmelt Subsurface connections may persist after surface connections stop flowing
Flow patterns: riparian evaporation There is also water above the water table shouldn t be ignored Wetter soil is more conductive Conductive soil can transmit water by capillary flow to an evaporation front very important in the riparian zone Below the top of the uplands, soil becomes drier, conductivity reduces and capillary flow stalls hence less water lost to transpiration and less abundant vegetation.
Flow patterns: typical profile Riparian zones Wetland Pond Mobile groundwater Minimal/no connections
St Denis Conceptual Model: Salts Basic general principle: Salts of geological origin oxidize and dissolve into the water and move with flow (advection, diffusion, dispersion), accumulating in evaporation front locations. Intimately linked to, and diagnostic of, the hydrology.
Salinization of the prairies Nachshon et al, 2013, JoH doi:10.1016/j.jhydrol.2013.07.001
Landscape Units: We have proposed a number of distinct zones in the landscape that describe how salts migrate and accumulate: Recharge Ponds Ponds that tend to lose water by infiltration, and have low salinities. Discharge Ponds Ponds that tend to gain water from exfiltration and surface flow, and lose water mainly by evaporation, and have elevated salinities. Saline Ring A ring of high salinity around the ponds in the riparian zone, where salts are drawn from ponds for evapotranspiration. Surface salt belt Accumulation of salts due to evaporation and transpiration in the very near surface, across the landscape. Deep salt belt Perhaps less a zone of accumulation than one of reduced flushing, this deep region is also close to where salts are still being produced by oxidation. Freeze out may also contribute to this elevated salinity region.
Observed salt distribution Nachshon et al, 2013, JoH doi:10.1016/j.jhydrol.2013.07.001
Salt distribution: conceptual model Nachshon et al, 2013, JoH doi:10.1016/j.jhydrol.2013.07.001
Flow patterns: intense summer rainfall Riparian zones Wetland Pond Mobile groundwater Somewhat speculative! Rapid infiltration into macropores Restricted downward movement due to reduction in K with depth Perched water tables, lateral flows from uplands to ponds Typical flow directions may be reversed in many places
Flow patterns: sustained wet conditions Riparian zones Wetland Pond Mobile groundwater Effective transmission zone almost completely saturated Flow directions may be reversed in many places
A wetter future = a saltier future? Nachshon et al, 2013, JoH doi:10.1016/j.jhydrol.2013.07.001
A physically based model Finite volume, unstructured mesh Solving Richards equation Equivalent Continuum Representation for dual permeability Fracture porosity, hence bulk hydraulic conductivity, increases near ground surface due to weathering For details of the model as applied to the English Chalk see Ireson & Butler, 2013, HESS, doi:10.5194/hessd-9-12061-2012
Depth [m] Fractured-till Hydraulic Conductivity
Fractured-till unsaturated K CHALK TILL I think this relationship is a fundamental control on flow/transport processes in surficial fractured porous media. Ireson et al., 2013. Proc. Env. Sci. doi:10.1016/j.proenv.2013.06.037
Semi-hypothetical hillslope simulations PRAIRIES CHALK Ireson et al., 2013. Proc. Env. Sci. doi:10.1016/j.proenv.2013.06.037
Semi-hypothetical hillslope simulations Ireson et al., 2013. Proc. Env. Sci. doi:10.1016/j.proenv.2013.06.037
Conclusions Prairie hydrology/hydrogeology is complex Most of the action is in the shallow subsurface, where fractures provide enhanced permeability We (particular Garth van der Kamp and Masaki Hayashi) have developed a very good field based, qualitative understanding of the processes We lack rigorous quantitative models which are needed to understand the potential impacts of change (climate/land use) My research combines newly instrumented transects with physically based models of the system. So far models behave well, but not fully tested Ongoing work is looking at transient hydrological and geochemical observations and attempting to establish a physically based model that is consistent with these data