anticipated temperature effects on biogeochemical reaction rates in ...

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1 Nationaal Congres Bodemenergie Utrecht, Nederland, 13 - 14 Oktober 2011

ANTICIPATED TEMPERATURE EFFECTS ON BIOGEOCHEMICAL REACTION RATES IN SEASONAL AQUIFER THERMAL ENERGY STORAGE (ATES) SYSTEMS : AN EVALUATION USING THE ARRHENIUS EQUATION Niels Hartog Utrecht University—Department of Earth Sciences Deltares—Soil and Groundwater Systems Princetonlaan 6 Utrecht, 3584 CB, Utrecht, The Netherlands e-mail: [email protected]

Introduction Aquifer thermal energy storage (ATES, Figure 1) is increasingly used for indoor climate control and involves the seasonal alternation of producing, storing and using warm and cold groundwater for heating and cooling during winter and summer. During the last two decades more than a 1000 ATES systems were implemented in The Netherlands. More recently, national and municipal ambitions in reducing CO2 emissions provides a driving force for further rapid expansion. With 10,000 additional ATES systems expected in the coming decade, questions have been raised on their impact on overall groundwater quality. Within a Dutch national research project (MMB, 2011), the effects of ATES systems on groundwater quality are currently assessed through lab experiments and field monitoring. Earlier research on groundwater chemistry in ATES systems mainly focussed on operational aspects such as clogging by mineral precipitation, particularly of carbonates and silica, due to the temperature effect on mineral equilibria (e.g. Brons et al., 1991; Griffioen and Appelo, 1993; Perlinger et al., 1987). Hot Well

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Temperature Mixing Gaspressure gradients

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4. Aquifer-Groundwater interaction 5. Interaction at ATES interface

Figure 1 – Principle of ATES and the factors influencing groundwater quality. e

However, many of the reactions that influence the overall quality in groundwater systems are not determined by chemical equilibrium but are driven by thermodynamic imbalances (e.g. (Appelo and Postma, 1993)). It has frequently been found that the rates of such (bio)geochemical reactions in groundwater systems are temperature-sensitive (Brons et al., 1991; Holm, 1986; Perlinger et al., 1987; Prommer and Stuyfzand, 2005), with higher rates observed at higher temperatures. Temperature is therefore one of the key factors (Figure 1) that may contribute to the overall effect on groundwater quality in ATES systems. However a thorough assessment of the influence of temperature differences in ATES systems on the rates of kinetically controlled reactions is currently lacking. In addition to the fact that the effect of temperature is accompanied by the effects of the other key factors in ATES systems (Fig. 1), most current ATES systems in the Netherlands have relatively narrow temperature ranges (∆T 70 kJ/mol) and ∆Ts larger than 15 ºC do rate constants increase more than a factor 1,5. The relatively small increase of reaction rate constants for reactions with low activation energies in low ∆T ATES systems is easily offset when groundwater in the ATES system spends on average more time in the cold volume than in the hot volume. This is illustrated in Figure 4, where the 50:50 distribution used so far is plotted with the conditions of the Uithof ATES system (Zuurbier et al., 2011) in which the groundwater is on average 60% of the time in


fATES (-)

the cold volume and 40% of the time in the hot volume. In such case the fATES can be smaller than 1 over the whole practical range of ∆T. For the reaction with the higher activation energy (“recalcitrant”) the negative impact on fATES is compensated at higher ∆Ts.

rate increase would only be expected to become significant at millennial or longer timescales. To illustrate this point of relative reaction time scales for fast and slow reactions further, an approximate reaction time scale is included in Figure 3, where the variation of reaction rate constants is expressed relative to the rate constant of the labile model compound “glucose”. As can be seen, the reaction rate constant for the recalcitrant 8 model compound “tannin” is about 10 times slower than the rate constant for glucose degradation. Now if the degradation of glucose would be characterized by a half-life in the st order of 1 day (1 order) than the half-life for tannin degradation would be in the order of 100,000 years. Conclusions

Figure 4 – The nett relative rate constant increase (fATES) as a function of the temperature difference (∆T) in ATES systems for reactions with Ea=30 kJ/mol (labile) and Ea=75 kJ/mol (recalcitrant). Dashed lines indicate condition where groundwater is in the cold volume for 60% of the time. Reaction time scales quality changes

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While the presented Arrhenius-based equation for fATES can be used to describe changes in relative reaction rates as a function of Tref, Ea and ∆T, it is the change in the absolute rate of reactions that impacts groundwater quality. Many of the reactions in groundwater systems are intrinsically slow, such as the degradation of sedimentary organic matter in aquifer sediments or the weathering of feldspar, with reaction time scales ranging from 1000s to millions of years or more. For example, the naturally available sedimentary organic matter typically consists of relatively recalcitrant lignin-derived compounds (e.g. Hartog et al., 2004), that have with structural similarity to tannin. Therefore, at a specific ∆T, a stronger increase in their decomposition rate would be expected than for much more labile organic compounds with lower activation energies for their degradation. However, the rates for the degradation of these recalcitrant compounds would still be intrinsically slow due to the higher activation energy and the quality changes induced by the temperature-induced

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Based on this evaluation of the Arrhenius equation for seasonal ATES systems, several primary conclusions can be drawn. First of all, the generalized assumption that reaction rates increase with every 10 ºC neglects the dampening effect by reduced rates in the cold volume of an ATES system and insufficiently considers their dependency on activation energies for reactions in groundwater systems. In addition, for the practical limit of ∆T for thermally balanced seasonal ATES systems, the effect of temperature is likely insignificant for relatively fast reactions (e.g. half-lives < year) and, while water quality changes due to temperature differences could become significant with time for slower reactions this might take longer than the typical lifespan for an ATES system (~25 years). Considering the limited relative increases in the calculated nett reaction rates, the distribution of heat and cold demand over time a particular seasonal ATES system may have an overriding effect on the nett change in of reaction rates. Overall changes in groundwater quality are expected to be controlled by other factors than temperature effects on reaction rates (Figure 1). With respect to temperature effects of ATES systems on groundwater remediation goals, where degradation is dependent on naturally recalcitrant compounds, these are likely to be insignificant within the remediation timeframes, compared to physical and (bio)geochemical changes induced by ATES systems (Zuurbier et al., 2011) With respect to allowing injection temperatures higher temperatures than 25 ºC this will increase the potential for significant groundwater quality changes, particularly depending on the extent to which a thermal imbalance is allowed. These changes will particularly be expressed


at longer timescales, at which also the heat storage efficiency as influenced by regional groundwater flow may diminish overall groundwater quality changes. References Appelo,

C.A.J. and Postma, D., 1993. Geochemistry, Groundwater and Pollution. Balkema, Rotterdam, 536 pp. Arrhenius, S., 1889. Über die Reaktionsgeschwindigkeit bei der Inversion von Rohrzucker durch Säuren. Z. Phy. Chem., 4: 226-248. Brons, H.J., Griffioen, J., Appelo, C.A.J. and Zehnder, A.J.B., 1991. (Bio)Geochemical Reactions in Aquifer Material from a ThermalEnergy Storage Site. Water Research, 25(6): 729-736. Davidson, E.A. and Janssens, I.A., 2006. Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature, 440(7081): 165-173. Girardina, C.P. and Ryan, M.G., 2000. Evidence that decomposition rates of organic carbon in mineral soil do not vary with temperature. Nature, 404: 858-861. Griffioen, J. and Appelo, C.A.J., 1993. Nature and extent of carbonate precipitation during aquifer thermal energy storage. Applied Geochemistry, 8(2): 161-176.

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Hartog, N., Griffioen, J. and Van Bergen, P.F., 2004. Reactivity of organic matter in aquifer sediments: geological and geochemical controls. Geochimica et Cosmochimica Acta, 68(6): 12811292. Holm, T.R., 1986. Groundwater Geochemistry of Aquifer Thermal-Energy Storage. Abstracts of Papers of the American Chemical Society, 192: 77-GEOC. MMB, 2011. Meer Met Bodemenergie. Perlinger, J.A., Almendinger, J.E., Urban, N.R. and Eisenreich, S.J., 1987. Groundwater Geochemistry of Aquifer Thermal Energy Storage: Long-Term Test Cycle. Water Resources Research, 23(12): 2215-2226. Prommer, H. and Stuyfzand, P.J., 2005. Identification of TemperatureDependent Water Quality Changes during a Deep Well Injection Experiment in a Pyritic Aquifer. Environmental Science & Technology, 39(7): 2200-2209. Zuurbier, K., Hartog, N., Valstar, J. and van Nieuwkerk, E., 2011. Sterk verbeterde analyse van interactie warmte/koudeopslag en verontreinigd grondwater H2O, 3: 33-36.
