The evolution of alkaline groundwaters in the continental intercalaire aquifer of the Irhazer Plain, Niger

Abstract

This paper discusses the hydrochemical evolution of groundwaters in the multilayer artesian aquifer of the Continental Intercalaire (the mainly Jurassic Agadez-Dabla sandstones) aquifer of northern Niger. Isotopic and chemical changes in relation to the geological setting suggest that the increasing HCO• alkalinity of the groundwaters is caused by interaction of deep CO2 with the aquifer matrix. Alteration of feldspars by CO2 with a •3C value of -3.0700 accounts for the observed alkalinity and isotopic and trends. Carbon 14 ages have been estimated for recent and Holocene groundwaters which have a distinct stable isotopic (2H and •80) signature compared with 14C dead groundwaters from further west on the Irhazer Plain. Recharge temperatures, estimated by analysis of noble gas contents, show that the 14C dead waters were recharged under conditions cooler than present averages in the region, possibly at the beginning of deglaciation (•. 16 ka B.P.) or during the period 23-30 ka B.P. Trends in the hydrochemistry of U are related to the deep CO2 alteration process. Radiogenic He contents increase along the flow direction and extremely high values occur in the proximity of U ores. Denitrification in the palcowaters of the Irhazer was assessed by measurements of nitrogen/argon ratios.

Noble Gases in Lakes and Ground Waters

“…The result of this particular study was challenged on grounds of insufficient modelling of the excess air fractionation by Ballentine and Hall (1999), but later confirmed by using the CE-model for excess air . Other noble gas studies from tropical sites also yielded similar coolings (Andrews et al 1994;Edmunds et aI. 1998;Weyhenmeyer et al 2000), suggesting that the tropics and subtropics (Stute et al 1992a~Clark et al 1997; Stute and Talma 1998) cooled rather uniformly by around 5°C during the LGM.…”

A paleotemperature record derived from dissolved noble gases in groundwater of the Aquia Aquifer (Maryland, USA)

“…The lack of constraints on the amount of calcite that was exchanged renders 14 C useless as dating tool. The same situation was encountered by Andrews et al (1994) in an aquifer in Niger. These authors stressed that such an exchange with secondary calcite deposits can occur if the groundwater is saturated with respect to calcite.…”

Hydrochemical Conditions and Environments Problems in the Low Land Zone of Pinios River of Ilias Department

“…(1) Διαδικασία ιοντοανταλλαγής (Andrews et al, 1994): Na 2 X + Ca 2+ -* CaX + 2Na + CaC0 3 + H 2 0 -• Ca 2+ + HC0 3 " + OH" Όπου Χ" υποδηλώνει τον ιοντοανταλλάκτη που είναι ο σκελετός του υδροφόρου. Η υδρόλυση των πλαγιοκλάστων (Albite, Anorthite and Anaclite) απαιτεί την κατανάλωση CÜ2, σύμφωνα με τις αντιδράσεις (Andrews et al, 1994): NaAI 2 Si 3 0 8 + 5 Λ Α H 2 0 + C0 2 -Na + +HC0 3 " + 1 / 2 AI 2 (SÌ20 5 )(OH)4 + 2H 4 Si0 4 CaAI 2 Si 2 0 8 + 3H 2 0 + 2C0 2 -» Ca 2+ + 2HC0 3 " + AI 2 (Si 2 0 5 )(OH) 4 NaAISi 2 0 6 + 3 1/2H 2 0 + C0 2 -• Na + +HC0 3 " + 1/2AI 2 (Si 2 0 5 )(OH) 4 + H 4 Si0 4 Ο σχηματισμός των παραπάνω ορυκτών οδηγεί σε απομάκρυνση επίσης του Σεπιολίτη (Mg 2 Si 3 0750hx3H 2 0) από το νερό.…”