Hydrochemical data and groundwater dating to infer differential flowpaths through weathered profiles of a fractured aquifer

Keywords:Stable isotopes (d 18 O and dD) CFCs and SF 6 Shallow groundwater origin and age Shallow groundwater recharge Lake Nyos -Cameroon s u m m a r yThe shallow aquifer in the vicinity of Lake Nyos (Northwest, Cameroon) is one of the main water supply sources to meet the water needs of the inhabitants to be resettled after 1986s tragedy. Unfortunately, there is a lack of knowledge on the groundwater recharge and flow systems in the area. Multiple environmental tracers (d 18 O, dD, Cl À , CFCs and SF 6 ) and a yearly record of rainfall, surface waters and groundwater were employed to characterize the recharge mechanism of the shallow groundwater seeping in the fractured rock of the Lake Nyos catchment (LNC). The d 18 O-dD relationship of the rainfall events gave the Nyos Meteoric Water Line: dD = 8.28 d 18 O + 11.87. Inland moisture vapor may have impacted the isotopic composition of original vapor masses from Gulf of Guinea prior to precipitation. Shallow groundwater in the LNC shows a similar trend of enrichment in 18 O and D as surface waters indicating a well-mixed aquifer. The proportions of surface waters and rainfall in the groundwater reservoir were 87% and 13%, respectively. The high annual recharge rate (941 mm/yr) and the seasonal variability in the isotopic signatures of groundwater indicate a renewable aquifer system. CFCs apparent ages-based piston flow model revealed a young age (average of 24 a.) of the groundwater in the LNC. SF 6 -based ages were biased young as compared to relatively younger than CFCs-based ages, implying an additional terrigenic production of SF 6 . The conceptual model for groundwater flow suggests that three main flow regimes, mainly controlled by the physical properties of the rock heterogeneities govern the movement of water in the aquifer. The piston flow model appears, however, to be the better model to explain the flow regime in the highly faulted and fissured area where recharge occurs ($1200-1600 masl). The rapid circulation and the low solubility lead to low mineralization. In the middle-lower area where waters circulate more in the weathered layer, exponential mixing model dominates, yielding more mineralized water. Downward to the valley, with prevailing gentle slope, interaction between surface and recently infiltrated groundwater better explain the observed similarity in the isotopic compositions and the mineralization of water.

Section: Groundwater Flow Systemmentioning

confidence: 98%

A multi-tracer approach for assessing the origin, apparent age and recharge mechanism of shallow groundwater in the Lake Nyos catchment, Northwest, Cameroon

Kamtchueng

Fantong

Wirmvem

et al. 2015

“…The apparent ages of water samples were determined by measuring Chlorofluorocarbons (CFCs) concentrations. These anthropogenic gases provide information on groundwater resident times up to 50 years and can give information on mixing processes between different water sources during flow changes (Ayraud et al, 2008;Busenberg and Plummer, 1992;Cook and Solomon, 1997;Cook et al, 2005;Jaunat et al, 2012).…”

Section: Geochemistry and Groundwater Datingmentioning

confidence: 99%

Hydrological behavior of a deep sub-vertical fault in crystalline basement and relationships with surrounding reservoirs

Roques

Bour

Aquilina

et al. 2014

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International audienceCrystalline-rock aquifers generally yield limited groundwater resources. However, some highly productive aquifers may be encountered, typically near tectonic discontinuities. In this study, we used a multidisciplinary experimental field approach to investigate the hydrogeological behavior of a sub-vertical permeable fault zone identified by lineament mapping. We particularly focused our investigations on the hydrogeological interactions with neighboring reservoirs. The geometry of the permeable domains was identified from geological information and hydraulic test interpretations. The system was characterized under natural conditions and during a 9-week large-scale pumping test. We used a combination of piezometric analysis, flow logs, groundwater dating and tracer tests to describe the interactions between permeable domains and the general hydrodynamical behaviors. A clear vertical compartmentalization and a strong spatial heterogeneity of permeability are highlighted. Under ambient conditions, the vertical permeable fault zone allows discharge of deep groundwater flows within the superficial permeable domain. The estimated flow across the total length of the fault zone ranged from 170 to 200 m3/day. Under pumping conditions, hydrological data and groundwater dating clearly indicated a flow inversion. The fault zone appears to be highly dependent on the surrounding reservoirs which mainly ensure its recharge. Groundwater fluxes were estimated from tracer tests interpretation. This study demonstrates the hydrogeological capacities of a sub-vertical fault aquifer in a crystalline context. By describing the hydrological behavior of a fault zone, this study provides important constrain about groundwater management and protection of such resources

“…Indeed, the groundwater residence time documents on contaminant transport and sustainability of water resources (Baudron et al, 2014;Cook et al, 1995;Cook and Herczeg, 2000;Gourcy et al, 2009;Murgulet et al, 2016;Vautour et al, 2015). It is also a powerful integrative approach of groundwater origin and flow path organisation within aquifers (Anders et al, 2014;Bertrand et al, 2010;Gooddy et al, 2006;Jaunat et al, 2012;Kamtchueng et al, 2015). Furthermore, groundwater residence time provides information to constrain recharge area, discharge rate, flow directions and velocities, all necessary to either quantitatively validate a numerical or conceptual hydrogeological model Post et al, 2013;Turnadge and Smerdon, 2014;Zuber et al, 2005).…”

Section: Introductionmentioning

confidence: 99%

Residence time, mineralization processes and groundwater origin within a carbonate coastal aquifer with a thick unsaturated zone

Santoni

Huneau

Garel

et al. 2016

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