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

Partitioning a regional groundwater flow system into shallow local and deep regional flow compartments

International audienceThe distribution of groundwater fluxes in aquifers is strongly influenced by topography, and organized between hillslope and regional scales. The objective of this study is to provide new insights regarding the compartmentalization of aquifers at the regional scale and the partitioning of recharge between shallow/local and deep/regional groundwater transfers. A finite-difference flow model was implemented, and the flow structure was analyzed as a function of recharge (from 20 to 500 mm/yr), at the regional-scale (1400 km2), in three dimensions, and accounting for variable groundwater discharge zones; aspects which are usually not considered simultaneously in previous studies. The model allows visualizing 3-D circulations, as those provided by Tothian models in 2-D, and shows local and regional transfers, with 3-D effects. The probability density function of transit times clearly shows two different parts, interpreted using a two-compartment model, and related to regional groundwater transfers and local groundwater transfers. The role of recharge on the size and nature of the flow regimes, including groundwater pathways, transit time distributions, and volumes associated to the two compartments, have been investigated. Results show that topography control on the water table and groundwater compartmentalization varies with the recharge rate applied. When recharge decreases, the absolute value of flow associated to the regional compartment decreases, whereas its relative value increases. The volume associated to the regional compartment is calculated from the exponential part of the two-compartment model, and is nearly insensitive to the total recharge fluctuations

Conceptual modelling to assess how the interplay of hydrological connectivity, catchment storage and tracer dynamics controls nonstationary water age estimates

Acknowledgements We would like to gratefully acknowledge the data provided by SEPA, Iain Malcolm. Mark Speed, Susan Waldron and many MSS staff helped with sample collection and lab analysis. We thank the European Research Council (project GA 335910 VEWA) for funding and are grateful for the constructive comments provided by three anonymous reviewers.Peer reviewedPostprin

Groundwater recharge and age-depth profiles of intensively exploited groundwater resources in northwest India

Intensive irrigation in northwest India has led to growing concerns over the sustainability of current and future groundwater abstraction. Environmental tracers and measurements of groundwater residence times can help quantify the renewal processes. Results from 16 paired locations show the interquartile ranges for residence times in shallow alluvial groundwater (8–50 m deep) to be 1–50 years and significantly less than those from deeper groundwater (76–160 m deep) at 40–170 years. The widespread occurrence of modern tracers in deep groundwater (>60% of sites had >10% modern recharge) suggests that there is low regional aquifer anisotropy and that deep aquifers are recharged by a significant component of recent recharge via vertical leakage. Stable isotope and noble gas results at all depths conform to modern meteoric sources and annual average temperatures, with no evidence of significant regional recharge from canal leakage in this study area close to the Himalayas

Calibration of regional groundwater flow models: Working toward a better understanding of site‐specific systems

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95491/1/wrcr9512.pd

Illuminating hydrological processes at the soil-vegetation-atmosphere interface with water stable isotopes

Funded by DFG research project “From Catchments as Organised Systems to Models based on Functional Units” (FOR 1Peer reviewedPublisher PDFPublisher PD

Complementary methods to investigate the development of clogging within a horizontal sub-surface flow tertiary treatment wetland

A combination of experimental methods was applied at a clogged, horizontal subsurface flow (HSSF) municipal wastewater tertiary treatment wetland (TW) in the UK, to quantify the extent of surface and subsurface clogging which had resulted in undesirable surface flow. The three dimensional hydraulic conductivity profile was determined, using a purpose made device which recreates the constant head permeameter test in-situ. The hydrodynamic pathways were investigated by performing dye tracing tests with Rhodamine WT and a novel multi-channel, data-logging, flow through Fluorimeter which allows synchronous measurements to be taken from a matrix of sampling points. Hydraulic conductivity varied in all planes, with the lowest measurement of 0.1 md1 corresponding to the surface layer at the inlet, and the maximum measurement of 1550 md1 located at a 0.4m depth at the outlet. According to dye tracing results, the region where the overland flow ceased received five times the average flow, which then vertically short-circuited below the rhizosphere. The tracer break-through curve obtained from the outlet showed that this preferential flow-path accounted for approximately 80% of the flow overall and arrived 8 h before a distinctly separate secondary flow-path. The overall volumetric efficiencyof the clogged system was 71% and the hydrology was simulated using a dual-path, dead-zone storage model. It is concluded that uneven inlet distribution, continuous surface loading and high rhizosphere resistance is responsible for the clog formation observed in this system. The average inlet hydraulic conductivity was 2 md1, suggesting that current European design guidelines, which predict that the system will reach an equilibrium hydraulic conductivity of 86 md1, do not adequately describe the hydrology of mature systems

Heat as a tracer for understanding transport processes in fractured media: Theory and field assessment from multiscale thermal push-pull tracer tests

International audienceThe characterization and modeling of heat transfer in fractured media is particularly challenging as the existence of fractures at multiple scales induces highly localized flow patterns. From a theoretical and numerical analysis of heat transfer in simple conceptual models of fractured media, we show that flow channeling has a significant effect on the scaling of heat recovery in both space and time. The late time tailing of heat recovery under channeled flow is shown to diverge from the TðtÞ / t 21:5 behavior expected for the classical parallel plate model and follow the scaling TðtÞ / 1=tðlog tÞ 2 for a simple channel modeled as a tube. This scaling, which differs significantly from known scalings in mobile-immobile systems, is of purely geometrical origin: late time heat transfer from the matrix to a channel corresponds dimensionally to a radial diffusion process, while heat transfer from the matrix to a plate may be considered as a one-dimensional process. This phenomenon is also manifested on the spatial scaling of heat recovery as flow channeling affects the decay of the thermal breakthrough peak amplitude and the increase of the peak time with scale. These findings are supported by the results of a field experimental campaign performed on the fractured rock site of Ploemeur. The scaling of heat recovery in time and space, measured from thermal breakthrough curves measured through a series of push-pull tests at different scales, shows a clear signature of flow channeling. The whole data set can thus be successfully represented by a multichannel model parametrized by the mean channel density and aperture. These findings, which bring new insights on the effect of flow channeling on heat transfer in fractured rocks, show how heat recovery in geothermal tests may be controlled by fracture geometry. In addition, this highlights the interest of thermal push-pull tests as a complement to solute tracers tests to infer fracture aperture and geometry

Diffusion as the main process for mass transport in very low water content argillites: 2. Fluid flow and mass transport modeling

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94838/1/wrcr9843.pd

Large‐scale hydraulic conductivities inferred from three‐dimensional groundwater flow and 4 He transport modeling in the Carrizo aquifer, Texas

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94983/1/jgrb14187.pd