T134 Streams and Aquifers: Integrating the Physical and Chemical (Posters)

Monday, 28 October 2013
9:00 AM - 6:30 PM
Hall D

RISK ASSESSMENT OF AQUIFER MIXING AND SALINITY INTRUSION IN THE NORTH WEST SAHARAN AQUIFER SYSTEM: A HYDROGEOCHEMICAL ANALYSIS- ALGERIA, TUNISIA, LIBYA

Paper number: T134-10
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

The North West Saharan Aquifer System includes the Plio-Quaternary (PQ), Complex Terminal (CT), and Continental Intercalaire (CI) aquifers located in northern Africa. These aquifers supply large quantities of water to rural and urban populations in Algeria, Tunisia, and Libya. The aquifers are considered non-renewable with little to no modern recharge. This region is water-stressed and thus is at risk for conflicts over water use. In addition, the water supply could be at risk of increased salinity due to aquifer mixing. Understanding the mixing processes in this multi-layer, trans-boundary aquifer will be useful knowledge for water managers. The principle areas for groundwater withdrawal of the three aquifers are surrounded by minor aquifers containing saline water. Changing the hydrologic equilibrium through pumping can cause previously isolated saline water to mix with the fresh water in the aquifers, resulting in increased salinity over time. In this regional study, it is hypothesized that certain areas are susceptible to increases in salinity due to aquifer mixing and upconing through wells.

Geochemical data compiled from fourteen studies on the three aquifers suggest mixing of more saline and less saline waters (greater or less than 3000 TDS) is occurring in some areas. Isotopes values range between -23 ‰ to -81 ‰ for δ2H, and -2 ‰ to -57 ‰ for δ18O (VSMOW). The major ions in meq/L were 1.4X10-6 to 1.0 for Br-, 0.34 to 5300 for Cl-, 1.1 to 165 for Ca2+, and 0.3 to 190 for SO42-. The isotope data indicate an evaporated signature, possibly due to the water rock interaction or evaporation during paleo-recharge. Dissolution of halite and gypsum is evidenced by the Br-/Cl- vs Cl- and Ca2+ vs SO42- graphs. End members were designated based on groundwater geochemistry, and were used to identify areas and extent of mixing in the multi-aquifer system. This study demonstrates how information about aquifer interactions can be obtained from historical datasets, and how regional hydraulic connections between fresh and saline aquifers can affect water availability as waters supplies are depleted.