Long-term water balance of a combined capillary barrier-methane oxidising landfill cover system

Abstract

Capillary barrier systems (CBSs) have been demonstrated to be effective in deviating infiltration in a landfill cover. However, their performance when combined with a cover soil optimised for methane oxidation had not been tested in the field yet. This study aimed to describe the water balance of a test field where such a configuration was built, located on the landfill in the Wieringermeer area, the Netherlands, over the period from 2009 to 2023.
During that time period breakthrough and deviated infiltration were measured. A one-dimensional finite difference model was built to model evapotranspiration and storage in the cover soil as well as the moisture retaining layer (capillary layer; CL). This model performed well in describing overall seasonal trends but generally overestimated evapotranspiration in spring and outflow in autumn, consequently underestimating the storage in these seasons.
The results show that the annual precipitation ranges between 770 and 990 mm. On average, 59% (494 mm) of this precipitation is evaporated, 33% (281 mm) is diverted by the CBS and 7% (63 mm) breaks through. Compared to other test fields and design standards, this breakthrough is high. A weakened functioning of the CBS by construction errors or the ingress of sand could be a reason for that. Another factor might be the coarse grain size distribution of cover soil, necessary for the efficient oxidation of methane. This type of soil has a relatively high hydraulic conductivity which does not control infiltration rates into the CBS as well as cover soils in other studies.
Furthermore, the results suggest that storage in the cover soil and CL is the main determinant for breakthrough to occur. Major breakthrough only occurred at a storage larger than the storage in the soil associated with field capacity, both on a seasonal and daily time scale. The available storage is mainly affected by the evapotranspiration which shows a seasonal cycle. Consequently, outflows for the CL and breakthrough occur mostly when evapotranspiration is low (autumn, winter). The relation of breakthrough with precipitation is less straightforward as a high rainfall does not necessarily lead to breakthrough on a seasonal or daily scale, depending on the available storage in the system. The distribution of precipitation can matter however. On a seasonal scale, precipitation in late summer can result in high storage in the beginning of autumn, potentially leading to breakthrough. Furthermore, a precipitation amount uniformly distributed over a day leads to less breakthrough than more concentrated precipitation.
Overall, with some adjustments regarding the design of the CBS and the cover soil, this landfill cover design is a promising alternative for the current standard.