recharge conceptual model and mineralization of groundwater in a semi arid region essaouira basin morocco

Peer-review under responsibility of the organizing committee of WRI-15 doi: 10.1016/j.proeps.2016.12.036 ScienceDirect 15th Water-Rock Interaction International Symposium, WRI-15 Rechar

Procedia Earth and Planetary Science 17 ( 2017 ) 69 – 72

1878-5220 © 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of WRI-15

doi: 10.1016/j.proeps.2016.12.036

ScienceDirect

15th Water-Rock Interaction International Symposium, WRI-15 Recharge conceptual model and mineralization of groundwater in a

semi-arid region; Essaouira basin (Morocco)

M Bahira, P M Carreirab,1, S Ouhamdouchc, H Chamchatic

a

Departement of Geology, E.N.S of Marrakech, Cadi Ayyad University, Morocco

b

Centro de Ciências e Tecnologias Nucleares,CTN/IST, Universidade de Lisboa, Portugal

c

3GEOLAB, Department of Geology, Faculty of Sciences Semlalia, Cadi Ayyad University, Morocco

Abstract

The geological setting of PQ and T aquifers system of Wadi Ouazi basin plays an important role in the geochemical and hydrodynamic characteristics of the groundwater systems, the most important source of water supply in Essaouira basin The combination of hydrogeochemical and isotopic methodology applied in this investigation allowed a better understanding of the conceptual model of the aquifer response under present climatic regime The chemical data indicate that dissolution of evaporate minerals (mainly halite and anhydrite), cation exchange process and sea spray, as the main processes controlling groundwater mineralization The isotopic data show that the recharge is assured by precipitation of oceanic origin without notable evaporation; stable isotopes content allowed the identification of different recharge areas at different altitudes between the two aquifer systems.

© 2017 The Authors Published by Elsevier B.V

Peer-review under responsibility of the organizing committee of WRI-15

Keywords: Essaouira basin, groundwater, hydrogeochemistry, stable isotopes, semi-arid area

Nomenclature

PQ Plio-Quaternary aquifer

SI Saturation Index

T Turonian aquifer

TDS Total dissolved solids

* Corresponding author Tel.:+ 351-219946179; fax: + 351-219946185

E-mail address: carreira@ctn.tecnico.ulisboa.pt

© 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license

( http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

Peer-review under responsibility of the organizing committee of WRI-15

1 Introduction

The continuous abstraction increase of groundwater resources to meet industrial, agricultural and domestic needs, are leading to growing deficit of groundwater resources, sometimes worsened by severe drought periods The continuous decreasing in the piezometric levels simultaneously to a progressive degradation of water quality are the main consequences of such intensive exploitation1,2 The Wadi Ouazi basin (WO), with an area of 996 km², is located at the Atlantic coast of Morocco, NE of Essaouira city (Fig 1) The prevailing climate is semi-arid with very variable rainfall averaging 300 mm/year Temperatures hover around 20 °C From the hydrogeological point of view, the study area is composed by two main aquifers (Fig.1): i) the Plio-Quaternary situated downstream and ii) the Cenomanian-Turonian formations are located upstream3 The PQ is composed by sandstones and limestones of marine origin, with a hydraulic conductivity around 3.2x10-2 m/s and a primary porosity This is an unconfined aquifer, the wall is formed of senonian formations and can be in direct contact with the Triassic and Cretaceous other levels The C-T aquifer is composed by limestones and dolomitic limestones formations The base of the system corresponds to lower Cenomanian grey clays overlay by the Senonian white marls4 The aim of this investigation is the characterization of groundwater resources in Wadi Ouazi basin in order to a better understanding

of the mineralization processes (water-rock interaction) occurring in the two systems down-gradient

Fig.1 Location and geological map of study area

2 Methodology

Water samples for chemical and isotope analyses were collected from 20 water point (borehole, well and spring) Physico - chemical parameters (temperature, pH, electrical conductivity) were measured in the field, alkalinity in the laboratory shortly after sampling Major ions (Ca2+, Mg2+, Na+, K+, Cl-, SO42-,) were determined in the laboratory at the Faculty of Sciences Semlalia, Marrakech (Morocco) Stable isotope composition of water samples (18O, 2H) was determined by isotope mass spectrometry at the Isotope Hydrology Laboratory of the International Atomic Energy Agency (IAEA) in Vienna The isotopic results are expressed to V-SMOW, in delta notation in per mil (‰)

3 Results and discussion

3.1 Geochemistry

The TDS values of groundwater samples range from 584 to 3925 mg/L the correlation diagrams of different

major elements vs TDS (Fig 2) show that groundwater mineralization in the PQ aquifer is mainly dominated by Na+

and Cl- content while Ca2+, Mg2+, SO42- concentrations are characterizing the groundwater samples from the T

aquifer In order to understand the origin of groundwater salinity, scatter diagrams of some of the major chemical constituents present in the groundwater samples from both aquifer systems were built (Fig 3) The Ca2+ vs SO4

2-diagram (Fig 3b) shows that some groundwater samples from the T aquifer, perfectly cluster along the gypsum dissolution line The strong correlation between these two parameters is indicating that the contribution of gypsum/anhydrite in the total groundwater mineralization of T samples is important Besides, this dissolution process is corroborated through the parabolic proportional relationship in the plots of Ca+SO4 vs SI anhydrite (Fig

3f) The Ca2+ content is strongly correlated with Mg2+ (r²=0.98), indicating that they most likely drive from dolomite dissolution However, some water samples are placed below line 1:1 indicating a deficit of Ca2+ (Fig 3c) This depletion on Ca2+ indicate the presence of another water-rock interaction process able to modify the cation content The positive correlation between Na and Cl (r²=0.97 and 0.83 respectively for PQ and T aquifer) is indicating the contribution of halite dissolution to groundwater mineralization (Fig 3a) Halite dissolution hypothesis is supported

by the parabolic proportional relationship in the plots of Na+Cl vs SI with respect to halite (Fig 3e) Water samples

are placed under line 1:1 indicating deficit of Na+ vs Cl- This Na deficit is balanced by an excess of Mg contents indicating cation exchange process The Cl- excess is the result of sea spray effect The contribution of carbonate minerals to groundwater mineralization in the study area (calcite and dolomite) is not evident despite the existence

of carbonate deposits on the upstream part of study area This is suggested by the absence of correlation between HCO3

and TDS (Fig 2)

Fig.2 Correlation of Ca, Mg, Na, HCO 3 , Cl and SO 4 versus TDS content

Fig 3 Hydrochemical relationships (a) Na/Cl; (b) Ca/SO4; (c) Ca/Mg; (d) Mg/SO4; (e) IS Halite/Na+Cl; (f) IS Anhydrite/Ca+SO4

3.2 Stable Isotopes

Stable isotope composition of the groundwater samples ranges from -6.2 to -3.8 ‰ in 18O and from 38.4 to -20.7‰ in 2H The 18O and 2H content of the water samples were used to calculate the Local Meteoric Water Line

(LMWL = δ2H =7.95 δ18O+11.3 (n= 11, r=0.97) almost parallel to the Global Meteoric Water Line (GMWL) established by Craig in 19615 The LMWL is characterizing precipitation of oceanic origin (Fig.4) All points located above the GMWL and are dispersed around LMWL This reflects a recharge without evaporation The contents of 18O and 2H decrease going from downstream (group A representing PQ aquifer) to upstream (group B representing the T aquifer) This depletion is due to mostly to the altitude effect although the continental effect could also be partly responsible for the isotopic depletion recorded in the T groundwater samples Fig 5 is showing the relation between 18O and Cl content The groundwater samples collected from the T aquifer (group B) have 18O content around 1‰ reflecting as above mention an altimetry difference of the infiltration area of about 900 m, and with the lowest Cl concentration As for the group A, the Cl contents are high compared to Group B supporting the hypothesis of the effect of sea spray in addition to the effect of halite dissolution to the groundwater mineralization

in the area of study

Fig.4 2

H vs 18

O of the analyzed groundwater samples Fig 5 18

O vs Cl of the analyzed groundwater samples

4 Conclusion

The geological setting of PQ and T aquifers system of Wadi Ouazi basin plays an important role in the geochemical and hydrodynamic characteristics of the studied aquifers The combination of hydrogeochemical and isotopic tools applied in this study allowed a better understanding of the functioning of the aquifer under present pressure regime Analysis of chemical characteristics of the groundwater revealed that the observed mineralization is primarily linked

to dissolution of evaporates, with significant contribution of secondary processes such as cation exchange and sea spray Environmental isotope data provide a strong evidence that current recharge of the studied aquifer system occurs principally by precipitation without strong evaporation evidences The results obtain during this investigation can guide future groundwater management and protection of the water resources in the region

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