Effects of Seasonal Soil Temperature into Contaminant NSZD Rates
Microbial activity is highly sensitive to temperatures. Refrigeration helps us preserve foods intended for human consumption and allows us to outcompete food microbes. In this regard, soil microbes behave similarly to food microbes: they operate at a certain temperature range, and within that range, they operate faster at higher temperatures (and vice versa). If a maximum temperature is exceeded, enzymes (proteins involved in microbial metabolism) become denatured and microbes become inactive.
Laboratory studies measuring contaminant biodegradation in soils have reported biodegradation at a range of 10-45 °C, with maximum rates around 30 °C (Zeman, et al, 2014; Siddique, et al, 2008) (see Figure 1). Due to this range, most soil microbes are classified as mesophiles (i.e., operating near ambient temperatures). Microbes from arctic locations might be the exception, as they have been evolutionarily selected for low temperatures (called psychophiles or phsycotrophs).
Figure 1. Data from Zeman, et al, 2014 showing biodegradation rates of petroleum contaminant microcosms incubated at different temperatures.
How does this apply to contaminated field sites?
There is general agreement that contaminated sites show the highest microbial activities during late summer or fall. Ultimately, local soil temperatures at locations where biodegradation reactions occur will determine depth-integrated contaminant biodegradation rates in the field. Site-specific considerations include:
Ambient and groundwater temperatures: The soil temperatures are the result of a dynamic interaction between the soil properties (its capacities to transmit and store heat) and surrounding temperatures (ambient and groundwater). Soil temperatures typically lag ambient temperatures due to the time required for heat transfer, with deeper elevations showing longer lags.
Depth of Contamination: Shallower locations tend to be controlled by soil temperatures, while the temperature of deep locations might be primarily controlled by groundwater temperature.
Microbial degradation rates: Like compost piles, locations where microbes operate at high rates, produce heat and might raise local temperatures by up to 4 °C (Mc Coy, et al, 2015).
How should you consider these factors when assessing NSZD at your contaminated site?
Each site is different, but it is a good idea to consider the following when developing your conceptual site model (CSM):
What is the range of ambient/groundwater temperatures and their seasonal variability?
Where is the contaminant located (elevation-wise, in addition to an aerial perspective)?
Screening for indicators of microbial activity, such as electron acceptor use or the production of biodegradation products like CO₂ and CH₄. Do these indicators show seasonal trends?
If quantifying NSZD rates, does the required vertical gas transport exist when other indicators of microbial activity are highest?
References:
Keenleyside, Wendy. (2019). "Microbiology: Canadian Edition.
Available at: https://ecampusontario.pressbooks.pub/microbio/chapter/temperature-and-microbial-growth/
McCoy, K., Zimbron, J., Sale, T., & Lyverse, M. (2015). Measurement of natural losses of LNAPL using CO₂ traps. Groundwater, 53(4), 658-667.
Siddique, T., Gupta, R., Fedorak, P. M., MacKinnon, M. D., & Foght, J. M. (2008). A first approximation kinetic model to predict methane generation from an oil sands tailings settling basin. Chemosphere, 72(10), 1573-1580.
Zeman, N.R., Renno, M.I., Olson, M.R., Wilson, L.P., Sale, T.C. and Susan, K.. (2014). Temperature impacts on anaerobic biotransformation of LNAPL and concurrent shifts in microbial community structure. Biodegradation, 25(4), pp.569-585.
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