Interaction between groundwater and saline surface water in alluvial aquifers of ephemeral coastal streams, north-east of Tunisia
Anis CHKIRBANE (D3)
Abstract. Traditionally, management of water resources has focused on surface water and groundwater as separate entities because their interactions are difficult to observe and measure. However, the effective sustainable land and water management requires a clear understanding of the linkages between these resources. Anthropogenically induced groundwater salinization as consequence of its interaction with surface water is a serious problem threatening the safe use of water especially in arid regions. In Wadi Al Ayn and Daroufa plain, a sudden increase of groundwater salinity was registered since 2002. Nevertheless, the groundwater salinization origin and processes are still poorly understood.
The aims of this study are to assess and quantify the flux exchanges between the surface water and groundwater in the plain of Wadi Al Ayn and Daroufa, to delineate the extent of the saltwater plume, to clarify the mineralization / salinization mechanisms of groundwater and finally to simulate the saline plume dynamics and predict its behavior under the optimal remediation scenarios.
The ephemeral system of Wadi Al Ayn seems to be actively interacting with its relative alluvial aquifer since it constitutes an important recharge source mainly composed of non treated wastewater and oilfield brine. An evident interaction is occurring in the middle part where the wadi is gaining 1.3 l/s from aquifer seepage. However, in its downstream area, the wadi is losing 9 l/s as contribution in aquifer recharge. The total contribution of the ephemeral system of Wadi Al Ayn in the alluvial aquifer recharge was estimated as 30.24 % of its total water inflow.
Time Domain Electromagnetic data calibrated with geological and geochemical data are used for mapping the subsurface geology and the spatial extend of salt / brackish plume in Wadi Al Ayn and Daroufa plains. The saltwater plume derived from the percolated oilfield brine is longitudinally found in depth usually between 45 – 75 m with a transversal extend varying between 50 – 800 m under wadi al Ayn bed. While in the coastal part, near Daroufa area, the salt plume is located between 45 – 60 m depth and has an inland extent of 1 km from the shore line.
Using the Principal Component Analysis (PCA), the major factors controlling the water quality variation were identified and served to apply the End Member Mixing Analysis (EMMA) in order to quantify the contribution of every recharge origin in the contaminated part of the aquifer. The salinization of groundwater is due to salt intrusion: the previously infiltrated oilfield brine from the wadi bed for the middle area of wadi Al Ayn (f = 1.3 to 12%) and sea water intrusion for wadi Daroufa area (f = 0.06 to 21%).
Basing on the geochemical and isotopic data, the groundwater quality variation was investigated. In fact, groundwater mineralization in the study area is not a homogenous process, but it is related to different sources and dynamics with space variation. The fresh shallow and deep groundwater chemistry is mainly controlled by natural conditions of rock – water interaction. However, the groundwater salinization is due to the discharged oilfield brine in the sandy bed of wadi Al Ayn until late 2009. While the detected salinization in Daroufa area is mainly controlled by seawater intrusion. The main process controlling the salinization is the cation exchange phenomena.
The hydrodynamic, hydrochemical and geophysical data served to build a conceptual model and they constituted the input of a numerical model constructed with VISUAL MODFLOW and SEAWAT code in order to retrace the salt dynamics and predict its behavior under remediation scenario. The effect of oilfield brine and seawater intrusion in groundwater salinization were successfully reproduced and confirmed by the 3D numerical model. the oilfield brine plume needs at least 5 years to be naturally reduced to less than the half of its actual size, while the seawater – fresh groundwater interface can reach an inland extent of 1.3 km with a TDS more than 10 g/l if no countermeasures will be taken until the next 3 decades.
The proposed remediation plan consists of artificial recharge with treated wastewater which seems to be the best solution to stop seawater intrusion just after 2 years of its field installation. The natural remediation of the oilfield brine plume can be enhanced by imposing optimized pumping rates and the installation of a restricted pumping perimeter in the transition zone between Wadi Al Ayn and Wadi Daroufa.
Keywords: Interaction, oilfield brine, seawater intrusion, salinization, geochemistry, TDEM, factor analysis, EMMA, variable density flow, numerical simulation, SEAWAT, remediation