Use of environmental isotopes to infer flow in the highly exploited aquifer system of the Diass region (Senegal)

audience: researcher, professionalThe Diass horst multilayered aquifer constitutes a complex hydrogeological system. But stable isotopes are illustrated to be powerful tools for clarifying the origin of recharge water, and the groundwater dynamics due to high exploitation of the system. Used with 3H and 14C, data confirms that most of the investigated groundwater are palaeowaters. Pumping has an impact on groundwater flow evidenced by the different water isotope compositions that illustrates the transient conditions of the system. Mixing of old waters and recently recharged (tritiated) waters occurs in some exploited boreholes, indicates lateral flow to the pumping field through the main groundwater flow directions

Identifying the flow systems in a karstic-fissured-porous aquifer, the Schneealpe, Austria, by modelling of environmental 18O and 3H isotopes

The Schneealpe karst massif of Triassic limestones and dolomites with the altitude up to 1800 m a.s.l., situated 100 km SW of Vienna in Kalkalpen, is the main drinking water resource for the city. The catchment area of about 23 km2 is drained by two springs: the Wasseralmquelle (196 l/s) and the Siebenquellen (310 l/s). This karstic aquifer is approximated by two interconnected parallel flow systems of: (a) a fissured-porous aquifer, and (b) karstic channels. The fissured-porous aquifer is of a high storage capacity and contains mobile water in the fissures and stagnant water in the porous matrix. The water enters this system at the surface and flows through it to drainage channels, which are regarded as a separate flow system, finally drained by both springs. The channels are also connected with sinkholes, which introduce additional water directly from the surface. Measurements of 18O and tritium in precipitation and springs were modelled by a combined application of lumped-parameter models. Modelling yielded information on the mean values of the following hydraulic parameters: (1) The volume of water in the whole catchment area is 255×106 m3, of which about 1.8×106 m3 are in channels and 253×106 m3 in the fissured-porous aquifer. (2) The total volumetric flow rate is 506 l/s, of which 77 l/s comprises direct flow from sinkholes to springs and 429 l/s are contributed to fissured-porous aquifer. (3) As the volume of the massif is 16.6×109 m3, the total water saturated porosity (fissures and micropores of the matrix) is 1.5% and the channel porosity is about 0.01%

A simple bioclogging model that accounts for spatial spreading of bacteria

An extension of biobarrier formation and bioclogging models is presented that accounts for spatial expansion of the bacterial population in the soil. The bacteria move into neighboring sites if locally almost all of the available pore space is occupied and the environmental conditions are such that further growth of the bacterial population is sustained. This is described by a density-dependent, double degenerate diffusion-equation that is coupled with the Darcy equations and a transport-reaction equation for growth limiting substrates. We conduct computational simulations of the governing differential equation system

Regional transport modelling for nitrate trend assessment and forecasting in a chalk aquifer

Regional degradation of groundwater resources by nitrate has become one of the main challenges for water managers worldwide. Regulations have been defined to reverse observed nitrate trends in groundwater bodies, such as the Water Framework Directive and the Groundwater Daughter Directive in the European Union. In such a context, one of the main challenges remains to develop efficient approaches for groundwater quality assessment at regional scale, including quantitative numerical modelling, as a decision support for groundwater management. A new approach combining the use of environmental tracers and the innovative ‘Hybrid Finite Element Mixing Cell’ (HFEMC) modelling technique is developed to study and forecast the groundwater quality at the regional scale, with an application to a regional chalk aquifer in the Geer basin in Belgium. Tritium data and nitrate time series are used to produce a conceptual model for regional groundwater flow and contaminant transport in the combined unsaturated and saturated zones of the chalk aquifer. This shows that the spatial distribution of the contamination in the Geer basin is essentially linked to the hydrodynamic conditions prevailing in the basin, more precisely to groundwater age and mixing and not to the spatial patterns of land use or local hydrodispersive processes. A three-dimensional regional scale groundwater flow and solute transport model is developed. It is able to reproduce the spatial patterns of tritium and nitrate and the observed nitrate trends in the chalk aquifer and it is used to predict the evolution of nitrate concentrations in the basin. The modelling application shows that the global inertia of groundwater quality is strong in the basin and trend reversal is not expected to occur before the 2015 deadline fixed by the European Water Framework Directive. The expected time required for trend reversal ranges between 5 and more than 50 years, depending on the location in the basin and the expected reduction in nitrate application. To reach a good chemical status, nitrate concentrations in the infiltrating water should be reduced as soon as possible below 50mg/l; however, even in that case, more than 50 years is needed to fully reverse upward trends.FP6 Project AquaTerra (Project nb. 55428

Application of isotopic tracers as a tool for understanding hydrodynamic behavior of the highly exploited Diass aquifer system (Senegal)

The Diass horst aquifer system located 50 km east of Dakar (Senegal) is exploited in two main aquifers covered by a sandy superficial aquifer: the confined/unconfined Palaeocene karstic limestone and the confined Maastrichtian sandstone aquifer underneath. This system has experienced intensive groundwater abstraction during the last 50 years to supply increasing water demand, agricultural and industrial needs. The high abstraction rate from 1989 to 2009 (about 109,000 m3/d) has caused a continuous groundwater level decline (up to 30 m), a modification of the groundwater flow and salinization in parts of the aquifers. The objective of the study is to improve our understanding of the system functioning with regards to high pumping, identify the geochemical reactions that take place in the system, infer origin and timing of recharge by using mainly stable (δ18O, δ2H, 13C) and radioactive (3H and 14C) isotopes. Water types defined in the Piper diagram vary in order of abundance from Ca–HCO3 (65%), Ca/Na–Cl (20%), Na–HCO3 (3%) and Na–Cl (12%). Values of δ18O and δ2H for the superficial aquifer range between −5.8 and −4.2‰ and between −42 and −31‰, respectively. For the Palaeocene aquifer they range from −5.8 to −5.0‰ and from −38 to −31‰, respectively; values in the Maastrichtian aquifer are between −5.9 and −4.3‰ for δ18O and −38 to −26‰ for δ2H. Plotted against the conventional δ18O vs δ2H diagram, data from the upper aquifer exhibit a dispersed distribution with respect to isotopic fractionation while those of the Palaeocene and Maastrichtian aquifers are aligned parallel and slightly below/or on the Global Meteoric Water Line (GMWL) evidencing ancient waters which had evaporated during infiltration. The low tritium (generally <0.7 TU) and 14C (0.7–57.2 pmc) contents indicate predominance of older water being recharged during the Pleistocene and Holocene periods. However, few boreholes which exhibit high tritium (1.2–4.3 TU) and 14C (65.7–70.8 pmc) values indicate some mixture with recent water likely through faulting and vertical drainage from the upper to deeper aquifers as well as lateral flow along flow paths to the piezometric depressions created by pumping

Direct Experimental Evidence of Non-first Order Degradation Kinetics and Sorption-Induced Isotopic Fractionation in a Mesoscale Aquifer: ¹³C/¹²C Analysis of a Transient Toluene Pulse

The injection of a mixed toluene and D₂O (conservative tracer) pulse into a pristine mesoscale aquifer enabled a first direct experimental comparison of contaminant-specific isotopic fractionation from sorption versus biodegradation and transverse dispersion on a relevant scale. Water samples were taken from two vertically resolved sampling ports at 4.2 m distance. Analysis of deuterium and toluene concentrations allowed quantifying the extent of sorption (<i>R</i> = 1.25) and biodegradation (37% and 44% of initial toluene at the two sampling ports). Sorption and biodegradation were found to directly affect toluene ¹³C/¹²C breakthrough curves. In particular, isotope trends demonstrated that biodegradation underwent Michaelis–Menten kinetics rather than first-order kinetics. Carbon isotope enrichment factors obtained from an optimized reactive transport model (Eckert et al., this issue) including a possible isotope fractionation of transverse dispersion were ε^equ_sorption = −0.31 ‰, ε^kin_{transverse‑dispersion} = −0.82 ‰, and ε^kin_{biodegradation} = −2.15 ‰. Extrapolation of our results to the scenario of a continuous injection predicted that (i) the bias in isotope fractionation from sorption, but not transverse dispersion, may be avoided when the plume reaches steady-state; and (ii) the relevance from both processes is expected to decrease at longer flow distances when isotope fractionation of degradation increasingly dominates