Modelling Recharge from Irrigation Developments with a Perched Water Table and Deep Unsaturated Zone

Modelling of recharge under irrigation zones for input to groundwater modelling is important for assessment and management of environmental risks. Deep vadose zones, when coupled with perched water tables, affect the timing and magnitude of recharge. Despite the temporal and spatial complexities of irrigation areas; recharge in response to new developments can be modelled semi-analytically, with most outputs comparing well with numerical models. For parameter ranges relevant to the western Murray Basin in southern Australia, perching can reduce the magnitude of recharge relative to irrigation accessions and will cause significant time lags for changes to move through vadose zone. Recharge in the vicinity of existing developments was found to be similar to that far from existing developments. This allows superposition to be implemented spatially for new developments, thus simplifying estimation of recharge. Simplification is further aided by the use of exponential approximants for recharge responses from individual developments.

Coupled hydro-mechanical modelling of a 1995 Hong Kong landslide

The paper deals with the modelling of the instability mechanism induced by rainfall in an unsaturated cut-slope. A large-sized landslide occurred in 1995 in Hong Kong (the so-called “Fei Tsui Road landslide”). It was here analysed because it was characterized by unusual dimensions and very large runout distance for the study area. The slope failure was attributed to a decrease in soil shear strength due to the rise of a perched water table above a weak kaolin-rich layer, together with the loss of suction caused by water infiltration during a heavy rainfall event. The hydro-mechanical coupled analyses made through the commercial software Plaxis 2D aimed to investigate the relations between the hydrological variables (i.e., rainfall infiltration, suction, saturation) and the slope response in terms of changes in soil resistance and soil plastic deformations. The study demonstrates that the evaluation of the hydro-mechanical coupling effects on the hydraulic slope response as well as on the stability of the whole slope is a crucial issue to well capture the mechanical behaviour of the unsaturated cut-slope. Different failure scenarios have been also considered in order to match the field observations and to back-analyse the initial condition of the slope before landslide.

Modelling the genesis of equatorial podzols: age and implications for carbon fluxes

Amazonian podzols store huge amounts of carbon and play a key role in transferring organic matter to the Amazon River. In order to better understand their C dynamics, we modelled the formation of representative Amazonian podzol profiles by constraining both total carbon and radiocarbon. We determined the relationships between total carbon and radiocarbon in organic C pools numerically by setting constant C and 14C inputs over time. The model was an effective tool for determining the order of magnitude of the carbon fluxes and the time of genesis of the main carbon-containing horizons, i.e. the topsoil and deep Bh. We performed retrocalculations to take into account the bomb carbon in the young topsoil horizons (calculated apparent 14C age from 62 to 109 years). We modelled four profiles representative of Amazonian podzols, two profiles with an old Bh (calculated apparent 14C age 6.8 × 103 and 8.4 × 103 years) and two profiles with a very old Bh (calculated apparent 14C age 23.2 × 103 and 25.1 × 103 years). The calculated fluxes from the topsoil to the perched water table indicate that the most waterlogged zones of the podzolized areas are the main source of dissolved organic matter found in the river network. It was necessary to consider two Bh carbon pools to accurately represent the carbon fluxes leaving the Bh as observed in previous studies. We found that the genesis time of the studied soils was necessarily longer than 15 × 103 and 130 × 103 years for the two younger and two older Bhs, respectively, and that the genesis time calculated considering the more likely settings runs to around 15 × 103–25 × 103 and 150 × 103–250 × 103 years, respectively.

Modelling the genesis of equatorial podzols: age and implications on carbon fluxes

Amazonian podzols store huge amounts of carbon and play a key role in transferring organic matter to the Amazon river. In order to better understand their C dynamics, we modelled the formation of representative Amazonian podzol profiles by constraining both total carbon and radiocarbon. We determined the relationships between total carbon and radiocarbon in organic C pools numerically by setting constant C and 14C inputs over time. The model was an effective tool for determining the order of magnitude of the carbon fluxes and the time of genesis of the main carbon-containing horizons, i.e. the topsoil and deep Bh. We performed retro calculations to take in account the bomb carbon in the young topsoil horizons (14C age from 62 to 109 y). We modelled four profiles representative of Amazonian podzols, two profiles with an old Bh (14C age 6.8 × 103 and 8.4 × 103 y) and two profiles with a very old Bh (14C age 23.2 × 103 and 25.1 × 103 y). The calculated fluxes from the topsoil to the perched water-table indicates that the most waterlogged zones of the podzolized areas are the main source of dissolved organic matter found in the river network. It was necessary to consider two Bh carbon pools to accurately represent the carbon fluxes leaving the Bh as observed in previous studies. We found that the genesis time of the studied soils was necessarily longer than 15 × 103 and 130 × 103 y for the two younger and the two older Bhs, respectively, and that the genesis time calculated considering the more likely settings runs to around 15 × 103–25 × 103 and 150 × 103–250 × 103 y, respectively.

Vulsino volcanic aquifer in Umbria Region : Hydrogeological survey for the characterization of the presence of arsenic and aluminium and the correct use of groundwater

In December 2009 and the first months of 2010, a large water crisis took place in the Orvieto area, because of sudden high concentration of aluminum (Al) in the groundwater of the vulsino aquifer. This represents a supply for Orvieto’s population and other near municipalities (about 20,000 people). The contamination had reached values of about 3000 μg/l. Water crisis was made worse because of the expiring, in the same period, as expected, of the derogation of European Commission that allowed Arsenic concentrations above 10 μg/l (up to 50 μg/l). The contamination by Al occurred after intense and persistent rains, that mobilized a large amount of aluminum hydroxides in perched water table, in the form of colloidal particles. The field analysis showed that the potable water catchments are not interested in the same way by the contamination, i.e. the vulsino aquifer was not wholly conditioned by the presence of Al; in addition, in the same period in which the Al contamination occurred, there were no changes in the levels of As in groundwater. This paper shows the study of the complex hydrogeological Vulsino system; the aim is to identify technical solutions for realizing new catchments in order to manage the resource, in qualitative and quantitative terms, replacing/integrating the current equipments, which represent a risk because of the presence of Al and, secondly, As. A numerical flow and transport model was implemented to support the hydrogeological study, that has allowed us to formulate reliable predictions regarding the risk of Al contamination of future new wells.

Biological control of pedological and hydro-geomorphological processes in a deciduous forest ecosystem

This study describes the effect of soil fauna and vegetation on the development of landscapes and how these actually control soil formation, geomorphological development and hydrological response. The study area is located in a semi-natural deciduous forest on marl in Luxembourg, with a strong texture contrast in the soil at 15–25 cm depth (luvic planosols).The methodology applied is both based on hydrological and geomorphological field measurements on runoff, sediment yield, perched water table dynamics, geomorphological survey, pedological survey and measurements related to in situ ectorganic horizon dynamics and litter decay, soil animal activity, as well as measurements of dynamic soil properties such as soil moisture and swelling and shrinkage.The results show that there is a positive feedback between tree type, soil fauna activity and the development of pipes, partial areas, soils and geomorphology. The landscape can be divided into two main types: Areas where Stellario-Carpinetum vegetation and partial areas are common and areas with Milio-Fagetum vegetation on dry slopes, which are differentiating more and more over time as a result of ongoing geo-ecosystem processes, and which also reflected in their sediment yield. The hydrological response is highly different for both landscape compartments as they are dominated by matrix (Beech) and pipe flow (Hornbeam) respectively. Soil fauna and tree type drive both soil and geomorphological evolution and they both can be considered as important ecosystem engineers.