Net Infiltration Research Articles

Experimental Investigation on Scouring v.s. Mass Failure of Unsaturated Soil Bed: Implications for Debris Flow Initiation and Erosion

AbstractScouring and mass failure are two common mechanisms used to describe soil bed erosion, but their combined effects are often not considered. To better understand how these mechanisms compete and under what conditions they prevail, it is essential to consider infiltration and a more realistic unsaturated soil bed. This study investigates soil bed erosion by considering unsaturated soil mechanics, a wetting front, and both erosion mechanisms of scouring and mass failure. Physical experiments were conducted on model water runoff over an unsaturated sand bed to investigate the effects of soil water content and flow velocity on erosion. Experimental results show that current understanding of soil bed erosion can be enhanced by adopting unsaturated soil mechanics and considering the combined effects of scouring and mass failure. The scouring rate is found to be independent of the bed water content because it only affects the uppermost soil particles, which immediately become saturated once water flows over them. Mass failure, on the other hand, is initiated at the wetting front when the rate of infiltration exceeds that of scouring. The depth of mass failure can be described by the net infiltration depth, which is defined as the difference between the infiltration and scouring depths. The net infiltration depth is jointly governed by the soil water content and flow velocity. The crucial role of the coupled effects of the hydro‐mechanical behavior of unsaturated soil in the realistic modeling of soil bed erosion is demonstrated. Outcomes present advancement toward improved hazard assessments of debris flows.

Multi-year in situ hydrogeochemical monitoring of hard rock lithium mine tailings in a large-scale experimental pile

Spodumene, a lithian mineral found in granitic pegmatites, is a major source of lithium. In situ, multi-year and large-scale (>10s of tons) predictive hydrogeochemical studies can be of great value for informing mine waste rock and tailings management, not least because the material is exposed to the actual climate where it is meant to be stored. However, such studies are rare given their cost and size, and previous ones have focused mainly on sulfide-bearing materials. An experimental field cell filled with spodumene beneficiation tailings was built at the Whabouchi lithium mine site in northern Québec (Canada) and monitored during four consecutive years. In parallel, column laboratory kinetic testing on the same material was conducted for water quality parameters to compare the effect of testing scales on the geochemical behavior. Geochemical results and release rates were overall similar between laboratory and field, and consistent with previously published laboratory results for materials from the same site. The leachates were neutral to slightly alkaline, with ions from feldspars (Ca > Na > K) and residual spodumene (Li) being notable solutes. Concentrations for most solutes surveyed remained higher in the field after four years than in columns at the end of the experiment. One major difference between laboratory and field was a brief (less than 2 years) increase in iron concentrations from the field cell, resulting in Fe-oxyhydroxides precipitation, that was not observed in laboratory. The source of this iron is arguably residual ferrosilicon (used for dense medium separation) and this difference is attributed to the different testing conditions and configurations in the laboratory versus in situ. Field hydrogeological results highlight strong seasonal patterns and the rapid response of this sand-like, sulfide-poor material to ambient temperature changes and wetting-drying events. The tailings both wet and drain rapidly and easily given their water retention curve and the range of matrix suctions recorded during frost-free months. Net infiltration through the tailings was estimated to represent 55% of total precipitation in summer (June–October) 2021. This study provides a comprehensive assessment of the environmental behavior of hard rock lithium mine tailings under real, sub-arctic climatic conditions and outlines similarities and differences between laboratory- and field-generated geochemical results.

Assessing water dynamics and net percolation rates within engineered cover systems for mine waste rock piles: A long-term field monitoring study

Mine waste rock piles (WRPs) typically contain trace sulfidic minerals that can react with the atmosphere and produce toxic acid mine drainage (AMD) leachate. Water flow through the waste rock affects both the chemistry and transport of the leachate, and a common remedial solution is the installation of engineered cover systems over the WRP to control net percolation (NP) into the waste rock. As NP is mainly controlled by meteorological conditions and the hydrological properties of cover materials, the water balance (WB) method can be employed to investigate NP and water dynamics within cover systems. However, studies on comprehensive WBs at full-scale WRPs have been limited, and most NP studies have relied on controlled, small-scale laboratory and field tests. This unique study contains the long-term field monitoring data at multiple WRPs needed to comprehensively assess NP rates within cover systems. Field monitoring data with high temporal resolution (hourly) was collected from 2012 to 2018 at four covered WRPs located within the Sydney Coalfield (Nova Scotia, Canada), consisting of one simple soil cover and three multi-layer covers lined with the same geomembrane but differing drainage layer materials. Each WB component – precipitation (PPT), evapotranspiration (ET), water/snow storage, surface runoff, and interflow – was determined, with the residual inferred to be NP into the waste rock. The simple soil cover was not very effective, only reducing NP from the pre-cover 34% PPT to 30% PPT. In contrast, the geomembrane-lined cover systems reduced NP to 4% PPT (no drainage layer), 1.6% PPT (thick gravel drainage), and 1.8% PPT (geocomposite drainage net). While ET and water/snow storage changes at all WRPs were similar, the biggest influence on NP was runoff and interflow. This study provides new field information on the performance of cover systems for controlling NP and water dynamics at full-scale WRPs.

Forecast climate change impact on porewater pressure regimes for the design and assessment of clay earthworks

Understanding and mitigating the impact of climate change on the built environment is becoming increasingly important worldwide. Earthworks (embankments and cuttings) supporting road and rail transportation networks often have direct contact with the atmosphere and are therefore influenced by extreme weather events and seasonal weather patterns. Atmospheric wetting and drying alters porewater pressures (PWPs) within earthworks, potentially contributing to the deformation and failure of earthwork slopes. Consequently, it is essential to understand the influence of climate change on PWPs within earthwork slopes, to inform strategies for their design, assessment and maintenance. Extensive 1D seepage analyses were carried out for typical railway embankments in the London area. The analyses showed that forecast hotter, drier summers will increase the water storage capacity of earthworks. This will lead to increased net infiltration in the winter months owing to both a forecast increase in rainfall and a longer time being required to saturate the soil pores and bring the water table back to the slope surface. Hence, despite the forecast increase in winter rainfall, this will not lead to higher design PWP regimes. The analyses were conducted for the London area, but this approach and conceptual framework can be readily adapted for other locations. Thematic collection: This article is part of the Climate change and resilience in Engineering Geology and Hydrogeology collection available at: https://www.lyellcollection.org/topic/collections/Climate-change-and-resilience-in-engineering-geology-and-hydrogeology

Examination of Measured to Predicted Hydraulic Properties for Low Impact Development Substrates

To counter the impacts of climate change and urbanization, engineers have developed ingenious solutions to reduce flooding and capture stormwater contaminants through the use of Low Impact Developments (LIDs). The soil is generally considered to be completely saturated when designing for the LIDs. However, this may not always be an accurate or realistic approach, as the soil could be variably unsaturated leading to inaccurate designs. To analyse the flow under variably unsaturated conditions, Richards’ equation can be used. To solve the Richards’ equation, two nonlinear hydraulic properties, namely soil water characteristic curve (SWCC) and the unsaturated hydraulic conductivity function are required. Laboratory and field measurements of unsaturated hydraulic properties are cumbersome, expensive and time- consuming. Pedotransfer functions (PTFs) estimate soil hydraulic properties using routinely measured soil properties. This paper presents a comparison between the direct measurement obtained through experimental procedures and the use of PTFs to estimate soil hydraulic properties for two green roof and three bioretention soil medias. Comparison between the measured and estimated soil hydraulic properties was accomplished using two different approaches. Statistical analyses and visual comparisons were used to compare the measured and estimated soil hydraulic properties. Additionally, numerical modelling to predict the water balance at the ground surface was conducted using the measured and estimated soil hydraulic properties. In some instances, the use of predicted hydraulic properties resulted in overestimation of the cumulative net infiltration of as much as 60 % for the green roof substrate, but was considered negligible for the bioretention substrate. Design performance criteria for green roof and bioretention facilities were examined using the measured and estimated soil hydraulic properties under extreme precipitation analysis. Results indicate that there is a high level of uncertainty when using PTFs for LID materials. A percent difference between the measured and predicted properties for the green roof peak time delay under a 2-year storm can be as much as 300%. For the bioretention design criteria of a 25-year storm, the surface runoff was overestimated by 14.7 cm and by 100% for the ponding time percent difference.

The role of physical geography on Puerto Rico’s water budget

Study regionPuerto Rico, a tropical island in the Caribbean Sea. Study focusThis study examines how water stress in Puerto Rico is influenced by its physical geography using a new USGS Soil-Water-Balance Model (Version 2.0) applied to three periods: a baseline climatology (1981–2010), a recent decade (2010–2019), and multiple drought years (1991, 1994, 1997, 2015). New hydrological insights for the regionOceanic islands often experience chronic water stress due to their size, population, land use, and geomorphology. Yet more severe, acute stress arises due to atmospheric forcing droughts, in which decreased precipitation is accompanied by increased temperatures, evapotranspiration, and water withdrawals. Based on our analysis of the physical geography of Puerto Rico, we find that water stress is more likely in southern Puerto Rico during both baseline and recent decade conditions due to reduced net infiltration within the rain shadow of the Cordillera Central. On the contrary, chronic stress is lower in mountainous and vegetated areas on the remainder of the island, as well as where coastal aquifers are available for augmenting surface water. During drought events, water stress is more common in central and eastern Puerto Rico mainly due to reductions in precipitation and net infiltration, increased evapotranspiration, and limited groundwater availability. Drought imposes additional chronic stresses by reducing recharge to coastal aquifers. These coastal aquifers are recharged by direct rainfall and streamflow from central Puerto Rico; the decreased precipitation and net infiltration in those areas constrain water availability during drought.

Datasets of Groundwater Level and Surface Water Budget in a Central Mediterranean Site (21 June 2017–1 October 2022)

This note makes available five years of data gathered in a measurement site equipped with a micrometeorological station and two monitoring wells. Series of data of hydrological and atmospheric variables make it possible to estimate the flux of water across the atmosphere-land interface and to calculate the water budget, which are crucial topics in climate and environmental sciences. The water-table measures began during 2017, one of the driest years of the whole instrumental period of climate history for the Central Mediterranean. Data from the micrometeorological station have been used to construct two more datasets of daily and monthly totals of different terms of the surface water budget, from which the net infiltration has been estimated. An apparent decreasing trend characterizes both the data time series of groundwater level and estimated infiltration in the considered period.

Spatiotemporal difference of leachate production and its impact on the development and dynamics of LCS clogging

Due to the complexity of industrial solid waste, heavy metals and organics can be enriched in leachate. While leachate is difficult to handle, it can also cause clog of the leachate collection system (LCS), increasing the risk of leakage. Therefore, it is necessary to study the clogging process of LCS in industrial solid waste landfills (ISWLs). In this study, the prediction of the clogging process and hydraulic conductivity evolution of the LCS in ISWL were carried out through laboratory experiments and model simulations. The results show that the LCS of ISWLs in China faces severe clogging challenges. First, the rate of clogging is inversely proportional to the rate of leachate production. Then, it was found that the main influencing factor was infiltration conditions (precipitation and capping systems). Under accelerated infiltration conditions, the time for complete clogging of the leachate drainage pipes was shortened from the initial 26–735 years to 11–315 years. The time to complete LCS clogging was shortened from the initial 78–2205 years to 32–945 years. In addition, the acceleration of the clogging process was fully consistent with the increase in leachate production. In particular, when the net infiltration volume increases from 0 to 50 mm, the clogging process is significantly accelerated. After greater than 50 mm, the effect on the clogging process gradually decreases. This provides a reliable theoretical basis for accurately predicting the clogging process of LCS.

Implementation of Climate Change Effects on Slope Stability Analysis

The objective of this study is to determine the impacts of expected climate change on slope stability. For this purpose, the case study of a slope instability, that was triggered in 2021 was selected. The stability analysis was performed considering the theory of rainfall infiltration and using Geo-Studio’s SEEP/W module for the surface infiltration model of the slope. A parametric stability analysis of the slope was conducted to determine the importance of climate change on slope stability. Conditions for changes in volumetric water content, water permeability, porewater pressure, and groundwater flow are important. When soil permeability is low, the factor of safety decreases during rainfall events and on the days following, while when permeability is higher, safety increases after rainfall events. The effect of lower cohesion is nearly linear, with the factor of safety decreasing by 0.1 for every 1 kPa less cohesion. The increase in net infiltration of water may be the most critical factor for slope instability. The results of the analysis indicate that timely reduction of water net infiltration through planting and proper surface water runoff from the upper road and slope would be a relatively simple and inexpensive measure compared to the cost of remediating the landslide, considering expected climate change. Therefore, it is advisable to analyze all slopes with respect to the expected climate change, taking into account the potential impacts of climate change.

Net precipitation, infiltration and overland flow production in three types of community-managed forest in the Mid-hills of East Central Nepal

It is widely believed that community forest management concurrently improves the social, ecological and economic conditions of local populations. Accordingly, large areas of Nepal's Mid-hills’ forests are used and managed by local Community Forest User Groups (CFUGs). The CFUGs plant seedlings as well as thin and prune forest trees at varying intensities as determined by local forest conditions and the need for forest products (mainly timber, firewood, livestock fodder and compostable litter). However, the hydrological effects of these activities are uncertain. We present results of throughfall (Tf), stemflow (Sf) and overland flow (OF) measurements made between June 2015 and December 2016 (including two consecutive rainy seasons) in three types of community-managed forests subject to different levels of community usage in the Kavre district (East Central Nepal): (i) regenerating natural broad-leaved (BF), (ii) predominantly-planted pine (PF), and (iii) mixed pine-broad-leaved (MF). The BF and MF were used more intensively than the PF. Overall Tf fractions were 72.0%, 73.7% and 77.5% of incident precipitation (P) for the BF, MF and PF, respectively, with corresponding Sf fractions of 1.6%, 1.3% and 0.6%. Overall rainfall interception fractions were 26.4%, 25.1% and 21.9% for the BF, MF and PF, respectively, and reflected the trend in leaf area index. Total amounts of OF constituted 11.4%. 9.8% and 4.7% of net precipitation (Tf+Sf) inputs to the forest floor in the BF, MF and PF, respectively and broadly followed the relative intensity of forest usage. Observed amounts of OF in the study forests represent ca. 50–115 mm of foregone infiltration opportunity per year under average rainfall, implying reductions in the recharge of soil and groundwater reserves of ∼6% in the PF to 12–14% in the MF and BF. Our results thus show that regular harvesting of litter, fodder and firewood combined with surface compaction by human traffic constitute important determinants of hillslope hydrological functioning, particularly regarding amounts of foregone infiltration and runoff as OF in the more intensively used community-managed forests of Nepal's Mid-hills.

Hydrogeochemical Response of a Variably Saturated Sulfide‐Bearing Mine Waste‐Rock Pile to Precipitation: A Field‐Scale Study in the Discontinuous Permafrost Region of Northern Canada

AbstractGlobally, drainage from sulfide‐bearing waste‐rock piles, containing high concentrations of toxic metal(loid)s, can severely degrade surrounding ecosystems. Waste‐rock piles are typically deposited as unsaturated porous media. The complex physical and chemical heterogeneity of waste rock poses significant challenges to the evaluation of internal hydrological, (bio)geochemical, and mineralogical controls on the magnitude and duration of environmental impacts. An operational‐scale waste‐rock pile located in the discontinuous permafrost region of Northern Canada was well‐instrumented and monitored to examine hydrological processes (including precipitation, evaporation, and infiltration), hydrological and thermal responses to freeze‐thaw and dry‐wet cycles, and concentration‐discharge (cQ) relationships in drainage‐water quantity and quality. The net infiltration (subtraction of evaporation from precipitation) into the waste‐rock pile occurred from May to November, resulting in rainfall‐dominated recharge to pore water, surface seepage, and groundwater. Pronounced variations in water content and temperature in response to freeze‐thaw and dry‐wet cycles and variations in surface topography were observed in regions of the waste‐rock pile impacted by preferential flow pathways. In contrast, distinct variations in water content and temperature were not observed in regions of matrix‐dominated flow pathways. The cQ relationships show chemodynamically controlled dilution behavior for most dissolved constituents (e.g., SO4, Fe, Pb, Zn, Cu, Ca, Mn) in the drainage. Therefore, hydrological processes and (bio)geochemical and mineralogical reactions both play prominent roles in determining drainage‐water chemistry. This work presents an integrated approach to site‐specific monitoring and characterization to evaluate remediation and reclamation options for operational‐scale waste‐rock piles for long‐term ecosystem preservation.

Porewater exchange drives nutrient cycling and export in a mangrove-salt marsh ecotone

Coastal wetlands regulate nutrient fluxes from the continents to the oceans. Salt marshes are rapidly encroaching into mudflat area in mangrove wetlands, shaping a mangrove-salt marsh ecotone, with unknown implications to coastal biogeochemical cycles. Here, we hypothesized that nitrogen and phosphorus cycling varied in mangrove and salt marsh, having significant implication on coastal waters. We investigated a tidal creek with a marked mangrove-salt marsh gradient in China using high-frequency time-series sampling of dissolved nutrients and observations of porewater exchange rate across the sediment–water interface over a spring-neap tidal cycle. The nitrogen transformation rates and microbiological activities were also investigated to explain the variability in nitrogen concentrations. The mangrove had net groundwater outflow rates of 3.6–4.3 mm d-1 while the salt marsh had net infiltration of surface water with rates of 0.5–2.9 mm d-1. Salt marsh had less capacity for ammonium (NH4-N) production (mineralization and dissimilatory nitrate reduction to ammonium DNRA) than mangrove. Denitrification dominated nitrogen removal reaching 97% and 83% in mangrove and salt marsh, respectively. Microbe distributions were consistent with nitrogen transformations with larger nirS and nrfA abundances for denitrification and DNRA in the mangrove than salt marsh. The mangrove had a net export of NH4-N but a net import of NOx-N (sum of nitrate and nitrite) and dissolved inorganic phosphorus (DIP) during the monitoring period. In contrast, the salt marsh had lower efflux of nutrient than influx leading to a net nutrient import during the monitoring period. Porewater released from the mangrove had a large DIN:DIP mole ratio (706 ± 236) due to high NH4-N concentrations, while NH4-N in the salt marsh were lower than in the mangrove. Overall, this study revealed that mangrove-salt marsh ecotone will push the native mangrove wetlands from being a source towards a sink of NH4-N to coastal waters by decreasing porewater exchange, modifying the nutrients stoichiometry, and ultimately alleviating the potential of N-associated eutrophication in nearby coastal waters.

Stemflow Infiltration Hotspots Create Soil Microsites Near Tree Stems in an Unmanaged Mixed Beech Forest

In stemflow, rainfall is collected and channeled to a concentrated soil water input. It can constitute up to 30% of incident precipitation in some ecosystems. However, the size of the zone influenced by stemflow is unclear, and statistically representative measurement of stemflow (on and in between sites) is scarce. Therefore, whether stemflow creates hotspots of infiltration and potential impacts on forest soils remain subject to controversy. In this study, we investigated the areal dimension of infiltrating stemflow fluxes as well as effects on near-stem soils. We measured throughfall, stemflow and soil properties in high-resolution statistical designs on a mixed forest plot in Germany receiving moderate stemflow. From this data, we modeled the spatial distribution of net precipitation infiltration depth on the plot. Furthermore, we examined soil chemical and physical properties around tree stems to test for and assess a stemflow impact. Results show that stemflow infiltration areas are much smaller than typically assumed and constitute strong infiltration hotspots compared to throughfall. This is also mirrored in soil properties, which are significantly altered near stems. Here, accelerated soil formation and enhanced translocation processes indicate increased soil water fluxes due to high inputs. Additionally, altered soil hydraulic properties enable quicker soil water fluxes near stems. Our findings attest that even comparatively low stemflow fractions (of gross precipitation) can generate strong hotspots of water and matter inputs, which are impactful to subsequent hydrological and biogeochemical processes and properties. Trees shape their direct soil environment, thereby establishing pathways of preferential water flow connecting the canopy and the deeper subsurface.

In situ monitoring of an inclined cover made with mine waste materials to control water infiltration on a reactive waste rock dyke

The development of a cover for inclined acid-generating areas, such as the external face of dykes and the slope of waste rock piles, is undoubtedly one of the biggest technical reclamation challenges at several mine sites. The LaRonde mine site, owned and operated by Agnico Eagle Mines (Quebec, Canada) is currently engaged to identify an optimal reclamation scenario for the Dyke 1 of its acid-generating tailings storage facilities. One of the promising reclamation options for controlling water infiltration in the acid-generating waste rock on the Dyke 1 is the use of an inclined cover built with available mine waste materials. An instrumented inclined cell with an inclination angle of 18.3 degrees was built on a slope of this dyke to validate if low sulfide tailings and non potentially acid-generating waste rock can be used as cover material to reclaim the Dyke 1. The instrumented inclined cell was monitored for 3 years (2017 to 2019) using volumetric lysimeters, suction sensors, and volumetric water content sensors. The monitoring was done under natural climatic conditions and artificial wetting events. Under natural conditions, less than 1% (5 mm) of incident rainfall percolated in the volumetric lysimeters installed along the slope of the inclined cell. Under controlled conditions associated with artificial wetting events of 6.4 mm/h over a period of 12 h, net percolation values between 1 and 9% (4 to 60 mm) of the sum of incident precipitation were measured. The distance between the top of the cell and the Down Dip Limit (DDL) point was greater than the slope length of the cover under natural conditions and the DDL point moved from the bottom toward the top to reach values between 12 and 20 m from the top of the slope when the wetting events were applied on the cover. These results confirmed the suitability of mining materials as an inclined cover material to control water infiltration in reactive mine waste rocks.

Effect of climate change on earthen embankments in Southern Ontario, Canada

Soil embankments are an integral part of transportation networks and are vulnerable to climate change due to their continuous exposure to the environment. The direct and indirect costs associated with an embankment failure can be significant and it is critical to ensure against the adverse effects of a changing climate. This research aims to quantify the probable stability effects of regular and extreme climate change on a highway embankment in Southern Ontario, Canada. The impact of climate change was quantified for two different embankment fills (sand and silt) by comparing the stability results of historical climate with a 90-year future climate data set. This analysis was accomplished by coupling changes in pore-water pressures (using a two-dimensional (2D) transient unsaturated seepage finite-element model) with a 2D limit equilibrium unsaturated slope stability model. The results indicate that future climate could increase the cumulative annual net infiltration by as much as 41%, which would decrease the embankments’ factor of safety by as much as 30%. It was also found that hydraulic properties play a critical role in embankment stability for different climate scenarios and suitable fill material based on the geographic region should be selected to ensure the safety of the embankments against climate change.

A Multi-Method Approach for Quantification of In- and Exfiltration Rates from the Subterranean Estuary of a High Energy Beach

Accurate SGD (submarine groundwater discharge) mass export calculations require detailed knowledge of the spatial and temporal variability in SGD rates. In coastal aquifers, SGD includes a terrestrial freshwater component as well as a saline component originating from circulating seawater. Representative field measurements of SGD rates are difficult to conduct, because SGD is often patchy, diffuse, and temporally variable, especially under tidal influence and high wave activity. In this study, a combination of lysimeters, seepage meters, temperature sensors, pore water radon, and numerical modeling was used to estimate the volumes of infiltrating seawater and exfiltrating groundwater in the intertidal zone of a mesotidal, high energy beach on Spiekeroog Island, northern Germany. Additionally, a 3D-laser scanner was used over short (days) and medium time scales (months) to determine changes in beach topography. The results showed net water infiltration above mean sea level (MSL) and net exfiltration below MSL. Water exchange rates fluctuated between 0.001 and 0.61 m day−1, showing similar ranges within the multiple method approaches. The beach topography was subject to strong fluctuation caused by waves, currents, wind driven erosion and sedimentation, even over short time scales. A comparison of extrapolated in- and exfiltrating water volumes along a beach transect from the mean high water to mean low water line at different times highlights the variability of total in or outflow. The results show that exchange rates depend on beach topography, which in turn changes significantly over time.

Numerical simulation of a geotextile soil wall considering soil-atmosphere interaction

This paper presents a numerical method for considering soil-atmosphere interaction applied to infiltration into an unsaturated geosynthetic reinforced soil wall and its effect on wall stability. A hypothetical nonwoven geotextile reinforced soil wall was subjected to simulated conditions of evaporation and precipitation over 2 years considering local climate variation in São Paulo city, Brazil. Net infiltration and actual evaporation were quantified inside and outside the reinforced zone, allowing for the assessment of changes in soil suction and factors of safety. The study discusses the implications of using in-plane draining reinforcements (e.g. nonwoven geotextiles) and soil-atmosphere effects. Results show that soil suction and factors of safety variation are more dependent on consecutive days of precipitation than on isolated heavy rainfalls. For the climate conditions considered in this study, results show that approximately 50% of the precipitation and potential evaporation transformed into net infiltration and actual evaporation, respectively. Additionally, the numerical results indicate that after the first wetting of the soil inside the reinforced soil zone, the evaporation to the atmosphere did not remove water from the inside of the geosynthetic reinforced wall because of the capillary break.

Using Statistical and Dynamical Downscaling to Assess Climate Change Impacts on Mine Reclamation Cover Water Balances

The oil sands industry in Canada uses soil–vegetation–atmosphere-transfer (SVAT) water balance models, calibrated against short-term (<≈ 10 years) field monitoring data, to evaluate long-term (≈60 years) reclamation cover design performance. These evaluations use long-term historical climate data; however, the effects of climate change should also be incorporated in these analyses. Although statistical downscaling of global climate change projections is commonly used to obtain local, site-specific climate, high resolution dynamical downscaling can also be used. The value of this latter approach to obtain local site-specific projections for mine reclamation covers has not been evaluated previously. This study explored the differences in key water balance components of three reclamation covers and three natural sites in northern Alberta, Canada, under future, site-specific, statistical, and dynamical climate change projections. Historical meteorological records were used to establish baseline periods. Temperature datasets were used to calculate potential evapotranspiration (PET) using the Hargreaves–Samani method. Statistical downscaling uses the Long Ashton Research Station Weather Generator (LARS-WG) and global circulation model (GCM) projections of temperature and precipitation. Dynamical climate change projections were generated on a 4 km grid using the weather research and forecasting (WRF) model. These climate projections were applied to a physically-based water balance model (i.e. Hydrus-1D) to simulate actual evapotranspiration (AET) and net percolation (NP) for the baseline and future periods. The key findings were: (a) LARS-WG outperformed WRF in simulating baseline temperatures and precipitation; (b) both downscaling methods showed similar directional shifts in the future temperatures and precipitation; (c) this, in turn, created similar directional shifts in future growing season median AET and NP, although the increase in future NP for LARS-WG was higher than that for WRF. The relative increases in future NP were much higher than the relative increases in future AET, particularly for the reclamation covers.

A Full-Scale Case Study on the Leaching Process of Acid Rock Drainage in Waste Rock Piles and the Net Infiltration Through Cover Systems

The leaching process and drainage chemistry of acid rock drainage is mainly controlled by geochemical reactions and how water carries soluble reaction products in waste rock piles. This paper studied how water flows through a full-scale waste rock pile at the Equity Silver mine site in British Columbia, Canada and elaborated on their effects on the leaching process and drainage chemistry. A revised dual-permeability model coupled with mass transport was adopted to investigate preferential flow and matrix flow in the pile simultaneously while considering water and geochemical products leaching/exchange between them. Furthermore, a particle-level water film model and also the effects of air flow, oxygen transport, and heat transfer in pile scale were integrated to account for geochemical reactions. Simulation results of full-scale iron discharge showed that aqueous concentrations in drainage water did not vary significantly relative to variations in drainage flow rate, which was confirmed by 12 years of field monitoring data. In addition, the comparison of pre- and post-cover simulations with measured lime consumption during ARD treatment, and mass-balance/dilution calculations for the entire pile, indicates that roughly 15–20% of total precipitation passes through the cover.

Climate-induced speleothem radiocarbon variability on Socotra Island from the Last Glacial Maximum to the Younger Dryas

Abstract. In this study, the dead carbon fraction (DCF) variations in stalagmite M1-5 from Socotra Island in the western Arabian Sea were investigated through a new set of high-precision U-series and radiocarbon (14C) dates. The data reveal an extreme case of very high and also climate-dependent DCF. For M1-5, an average DCF of 56.2±3.4 % is observed between 27 and 18 kyr BP. Such high DCF values indicate a high influence of aged soil organic matter (SOM) and nearly completely closed-system carbonate dissolution conditions. Towards the end of the last glacial period, decreasing Mg∕Ca ratios suggest an increase in precipitation which caused a marked change in the soil carbon cycling as indicated by sharply decreasing DCF. This is in contrast to the relation of soil infiltration and DCF as seen in stalagmites from temperate zones. For Socotra Island, which is influenced by the East African–Indian monsoon, we propose that more humid conditions and enhanced net infiltration after the Last Glacial Maximum (LGM) led to dense vegetation and thus lowered the DCF by increasing 14CO2 input into the soil zone. At the onset of the Younger Dryas (YD) a sudden change in DCF towards much higher, and extremely variable, values is observed. Our study highlights the dramatic variability of soil carbon cycling processes and vegetation feedback on Socotra Island manifested in stalagmite DCF on both long-term trends and sub-centennial timescales, thus providing evidence for climate influence on stalagmite radiocarbon. This is of particular relevance for speleothem studies that aim to reconstruct past atmospheric 14C (e.g., for the purposes of 14C calibration), as these would rely on largely climate-independent soil carbon cycling above the cave.