Colin J. Whitfield

The importance of atmospheric base cation deposition for preventing soil acidification in the Athabasca Oil Sands Region of Canada

Industrial activities in the oil sands region of Alberta, Canada have resulted in greatly elevated emissions of SO2 and N (NO(x) and NH3) and there are concerns over possible widespread ecosystem acidification. Acid sensitive soils in the region are common and have very low base cation weathering rates: the median base cation weathering rate estimated for 63 sites using PROFILE was just 17 mmol cm(-2) yr(-1). Deposition of S and N in throughfall was approximately twice as high as deposition measured with open collectors and could be as high as 360 mmol cm(-2) yr(-1) within 20 km of the main industrial center, although deposition declined logarithmically with distance from the industrial activities. Base cation deposition however, mostly exceeded the combined inputs of S and N in bulk deposition and throughfall, particularly during the summer months. The potential for soil acidification at a site close (<3 km) to the largest mine was assessed using the dynamic ecosystem acidification model, MAGIC (Model of Acidification of Groundwater in Catchments). Despite very low base cation weathering rates (~6 mmol cm(-2) yr(-1)) and high (~250 mmol cm(-2) yr(-1)) acid (S+N) deposition at the site, soil base saturation and soil solution pH and molar Ca:Al ratio were predicted to increase in the future assuming acid and base cation deposition constant at current rates. This work shows that despite extremely low soil base cation weathering rates in the region, the risk of soil acidification is mitigated to a large extent by high base cation deposition, which in contrast to S emissions is derived from fugitive dust sources in the mines, and is poorly quantified for regional modeling studies.

Controls on greenhouse gas concentrations in polymictic headwater lakes in Ireland

Freshwater lakes are known to release carbon dioxide (CO(2)) and methane (CH(4)) to the atmosphere; however, the importance of lakes in global nitrous oxide (N(2)O) budgets is not yet known. Further, despite the abundance of small lakes on the landscape, neither emissions of these gases nor their drivers are well described. Dissolved concentrations of CO(2), CH(4) and N(2)O greenhouse gases were related to water chemistry, hydrology and catchment characteristics in order to identify factors controlling gas concentrations for 121 small Irish headwater lakes (median area: 2.0ha) in relatively undisturbed catchments; lake-atmosphere gas fluxes were also calculated. The majority of lakes were supersaturated (relative to the atmosphere) with CO(2) and N(2)O while CH(4) was above saturation in all lakes. Dissolved gas concentrations were correlated with land cover (rock, forest and grassland), deuterium excess (an indicator of hydrologic character) and lake organic carbon concentrations, although dissolved CO(2) exhibited few significant relationships. Principal components analysis indicated that higher levels of CH(4) and N(2)O supersaturation were exhibited under different conditions. Methane supersaturation was highest in low elevation catchments with an evaporative hydrologic character and high organic carbon concentrations. In contrast, lakes characteristic of N(2)O supersaturation were low in carbon and located in more rapidly flushed higher elevation catchments. Estimated fluxes of CO(2), CH(4) and N(2)O to the atmosphere averaged 14, 0.36 and 1.3×10(-3)mmolm(-2)d(-1), respectively.

A regional approach for mineral soil weathering estimation and critical load assessment in boreal Saskatchewan, Canada

In boreal regions of the province of Saskatchewan, Canada, there is concern over emerging acid precursor emission sources associated with the oil sands industry. Base cation weathering rates (BC(w)) and steady-state critical loads of sulfur (CL(S)) were identified for upland forest soil plots (n=107) in 45 ecodistricts according to a new method for approximation of BC(w) in the region. This method was developed by regression of simple soil and site properties with BC(w) calculated through application of a soil chemical model (PROFILE). PROFILE was parameterized using detailed physicochemical data for a subset (n=35) of the sites. Sand content, soil moisture and latitude emerged as important predictive variables in this empirical regression approximation. Base cation weathering varied widely (0.1-8000 mmol(c) m(-3) yr(-1)) across the study sites, consistent with their contrasting soil properties. Several sites had lower rates than observed in other acid-sensitive regions of Canada owing to quartz dominated mineralogy and coarse-textured soils with very low surface area. Weathering was variable within ecodistricts, although rates were consistently low among ecodistricts located in the northwest of the province. Overall, half of the forest plots demonstrated CL(S) less than 45 mmol(c) m(-2) yr(-1). Historically, the acidification risk in this region has been considered low and monitoring has been limited. Given the very low CL(S) in many northern ecodistricts and the potential for increased acid deposition as oil sands activities expand, soil acidification in these regions warrants further study.

Predicting the partial pressure of carbon dioxide in boreal lakes.

Sixteen boreal lakes in northern Alberta were sampled for a suite of water chemistry parameters, including dissolved carbon dioxide (CO2), using a headspace gas analysis technique. The lakes encompassed a wide range of pH and alkalinity but had very high dissolved organic carbon (DOC) levels (11–36 mg L−1) and were supersaturated in CO2 with respect to the atmosphere. While the partial pressure of carbon dioxide (pCO2) is regularly estimated from pH and dissolved inorganic carbon (DIC), pH was related to pCO2 at only 13 of 16 lakes and overall pH in combination with DIC was a poor predictor of pCO2. Similarly, despite very high DOC levels, pCO2 was unrelated to the DOC concentration of the lakes. Stepwise multiple linear regressions improved the prediction capability for the entire data set, when compared to simple regressions. Both physicochemical (alkalinity, temperature) and landscape descriptors (lake area, peatland relative area) were important predictors of pCO2. The best regression model included lake area, peatland relative area, and water temperature, and was better able to predict pCO2 than relationships based on DOC, and pH and alkalinity, but lakes with high pCO2 (> 1000 µatm) remain under-predicted and are likely subject to additional factors controlling pCO2 that were not considered in this analysis.

Predicting surface area of coarse-textured soils: Implications for weathering rates

Whitfield, C. J. and Reid, C. 2013. Predicting surface area of coarse-textured soils: Implications for weathering rates. Can. J. Soil Sci. 93: 621-630. The surface area of soil is an important determinant of mineral weathering rates, but is infrequently measured. Simple texture-based pedotransfer functions (PTFs) have been used to predict the specific surface area (SSA) of coarse-textured soils. Detailed physicochemical properties of 40 upland forest mineral soils from northeastern Alberta were used to evaluate three texture-based PTFs and to calculate weathering rates using a process-oriented soil-chemical model. Evaluation of the PTFs demonstrated that these equations predict only across a limited range of (low) surface areas. Moreover, the fit between predicted and measured SSA was generally poor for soils in this region of Alberta. Improved prediction of SSA was possible using a texture-based PTF calibrated for the region, although differences between measured and predicted values were often large. Mineralogy terms were used in a more comprehensive PTF to account for mineral-specific differences in surface area. This approach proved superior to texture-only approaches; however, it could not be used reliably for site-specific predictions (NRMSE=0.41). Soil-chemical model-generated weathering rates were strongly influenced by the SSA method used in parameterization; weathering estimates and corresponding critical load assessments based on measured SSA (and to a lesser extent SSA derived from the regional PTF) were the most robust. Methods for SSA prediction should be used with caution, particularly in cases where they are applied to soils with different character than those for which they were developed.

Estimating Base Cation Weathering Rates in the USA: Challenges of Uncertain Soil Mineralogy and Specific Surface Area with Applications of the PROFILE Model

The weathering release rate of base cations (BCw) from soil minerals is fundamentally important for terrestrial ecosystem growth, function, and sensitivity to acid deposition. Understanding BCw is necessary to reduce or prevent damage to acid-sensitive natural systems, in that this information is needed to both evaluate the effectiveness of existing policies, and guide establishment of further policies in the event they are required. Yet BCw is challenging to estimate. In this study, major sources of uncertainty associated with a process-based model (PROFILE) commonly used to estimate weathering rates were quantified in the context of efforts to quantify BCw for upland forest sites across the continental USA. These include uncertainty associated with parameterization of mineral content where horizon data are not available, stoichiometry of individual minerals, and specific surface area of soil and individual soil minerals. Mineral stoichiometry was not an important influence on BCw estimates (uncertainty < 1%). Characterizing B horizon mineralogy by averaging A and C horizons was found to be a minor (< 5%) contributor to uncertainty in some areas, but where mineralogy is known to vary with depth the uncertainty can be large. Estimating mineral-specific surface areas had a strong influence on estimated BCw, with rates increasing by as much as 250%. The greatest uncertainty in BCw estimates, however, was attributed to the particle size class-based method used to estimate the total specific surface area upon which weathering reactions can take place. The resulting uncertainty in BCw spanned multiple orders of magnitude at individual sites, highlighting this as the greatest challenge to ongoing efforts to produce robust BCw estimates across large spatial scales in the USA. Recommendations for improving estimates of BCw to support robust decision making for protection against terrestrial acidification are provided.

Acid deposition in the Athabasca Oil Sands Region: a policy perspective

Industrial emissions of sulphur (S) and nitrogen (N) to the atmosphere associated with the oil sands industry in north-eastern Alberta are of interest as they represent the largest localized source in Canada (with potential for future growth) and the region features acid-sensitive upland terrain. Existing emission management policy for the Regional Municipality of Wood Buffalo, where the industry is located, is based on a time-to-effect approach that relies on dynamic model simulations of temporal changes in chemistry and features highly protective chemical criteria. In practice, the policy is difficult to implement and it is unlikely that a scientifically defensible estimate of acidification risk can be put forward due to the limitations primarily associated with issues of scale, chemical endpoint designation (selection of chemical limit for ecosystem protection from acidification) and data availability. A more implementable approach would use a steady-state critical load (CL) assessment approach to identify at-risk areas. The CL assessment would consider areas of elevated acid deposition associated with oil sands emissions rather than targeted political jurisdictions. Dynamic models should only be (strategically) used where acidification risk is identified via CL analysis, in order to characterize the potential for acidification-induced changes that can be detrimental to sensitive biota within the lifespan of the industry.