Oxygen migration through a cover with capillary barrier effects colonized by roots

Abstract

2 Covers with capillary barrier effects (CCBE) are multilayered oxygen barrier covers used in humid 3 climates to reclaim reactive mine tailings and limit the generation of acid mine drainage. Once 4 constructed, CCBEs are colonized by surrounding plants. Roots modify water storage and respire 5 oxygen. The performance of CCBEs could evolve over time due to root colonization. Twenty-five 6 plots with varying vegetation were investigated at a seventeen-year-old CCBE in the mixed forest 7 of Québec, Canada. Geotechnical parameters and root colonization of the moisture-retaining layer 8 (MRL) of the CCBE were characterized. The performance of the MRL to control oxygen migration 9 was assessed using oxygen consumption tests and numerical modeling. Despite root colonization 10 at the surface of the MRL, oxygen fluxes generally complied with the CCBE's design criteria. 11Roots presence created oxygen consumption in the MRL, which could be expressed with a 12 reactivity coefficient (Kr). A positive correlation (R 2 = 0.65) was found between root length density 13 and Kr. Oxygen consumption by root respiration helped to lower oxygen fluxes by 0.5 to 76 g/m 2 /yr 14 with an average of 13 g/m 2 /yr. These results will help to better understand the influence of roots 15 on CCBEs' performance to control oxygen migration. 16

Natural analogue of a cover with capillary barrier effects to improve the long-term performance evaluation and the design of the cover

“…The MRL is a fine-grained material placed between two layers of coarse-grained material (used as the upper and bottom capillary barrier layers) to maintain a high (e.g. >85%) degree of saturation (Sr) in the MRL and create an oxygen diffusion barrier (Bussière et al 2003;Proteau et al 2020a). CCBEs are recommended to reclaim sites that are exposed to positive water budget that allows for periodic water recharge of the MRL, as well as sites with water table levels naturally several metres below the surface of the reactive mine wastes to be reclaimed.…”

“…However, after mine closure, plants, animals, and microorganisms from the surrounding ecosystem (Smirnova et al 2011;Proteau et al 2020b) may colonise the CCBE and alter its hydraulic properties. For example, plant roots can pump water towards the surface and decrease the Sr (degree of saturation) of the MRL (Guittonny et al 2019;Proteau et al 2020a). Root colonisation can also modify the materials' in situ hydrogeological properties, such as the saturated hydraulic conductivity (ksat) and water retention curve (WRC), that control cover performance (Bussière & Guittonny 2021;Proteau et al 2020a).…”

Aboveground and Belowground Colonization of Vegetation on a 17-Year-Old Cover with Capillary Barrier Effect Built on a Boreal Mine Tailings Storage Facility

“…It has a high saturated hydraulic conductivity of approximately 10 −3 cm/s, which is 10 2 to 10 3 times higher than the silt used in the middle layer. In comparison, the n of the MRL is on average 0.34, with a high degree of saturation (≈0.93) [25]. Finally, the top layer of the Lorraine site CCBE is the protection layer (PL), which is about 30 cm thick and made of sand and gravel (n > 0.39).…”

“…The intensity of root colonization is usually expressed through parameters such as root length density (RLD) [21,22], root volume density (RVD) [23], or root mass density (RMD) [24]. Proteau et al [25] have worked on establishing links between root parameters and a CCBE's performance and showed that a higher RLD could increase the reactivity coefficient (K r ) of the CCBE because of root's oxygen consumption while decreasing the degree of saturation (S r ) of the MRL. However, studies on root colonization of cover materials remain sparse [15,[26][27][28][29][30], especially quantitative studies measuring root traits.…”

“…Root studies are known to be time consuming [25,[42][43][44] and imply soil excavations that could negatively affect the integrity of an engineered cover. Since some below-and aboveground parameters of vegetation can be correlated, it could be possible to predict the degree of root colonization present in the MRL and the possible threat to the CCBE's performance from measurements on aboveground vegetation.…”