Spatial and temporal analysis of shallow groundwater quality using GIS, Grombalia aquifer, Northern Tunisia

Highlights

• Spatio-temporal maps (2000–2015) for each ion indicate different patterns.

• Cations/anions exchanges and reverse exchanges were quite evident within the aquifer system.

• Over-saturation of Cl⁻ and SO₄²⁻ impact clearly the aquifer components.

• Slow hydrochemical process maintains the aquifer quality with a moderate variability.

• Degradation level is induced by high salinization hazards from anthropogenic sources.

Abstract

This study aimed to analyze the spatial and temporal variations of water quality from 2000 to 2015 in the Grombalia basin aquifer, northern Tunisia. Some physicochemical parameters of collected samples were measured and processed in a GIS environment. Quantitative assessments of their distribution patterns and spatio-temporal variability were performed.

Inverse distance weighting (IDW) interpolator and map algebra procedures were the used GIS tools. During 2000, 2015 campaigns 21 groundwater samples were collected from georeferenced boreholes and analyzed for Ca²⁺, Na⁺, Mg²⁺, K⁺, SO₄²⁻, HCO₃⁻, Cl⁻, EC, pH, T°, O₂, and Salinity. Hydrochemical indices were also estimated such as sodium adsorption ratio, total dissolved solids, and soluble-sodium percentage. Findings were examined in terms of water quality, salinization degree and deterioration rate. During the considered interval, a mixed facies of Cl–SO₄–Na–Ca, which remained roughly unchanged, characterizes the aquifer. While the spatial distribution of almost all ions is the same, the obtained spatio-temporal maps (2000–2015) for each ion indicate different patterns. Significant variations of concentration were observed in the pediplain area. This region is susceptible to contain most suitable sources for irrigation purposes; it represents 23.8% of groundwater samples that are among the good and safe category. Also, there is no significant variations of water quality indices are observed in the aquifer, except for some northern samples (coastal area). Therefore, a slow hydrochemical process maintains the aquifer quality with a moderate variability while an ongoing salinization hazard locally induces the deterioration level.

Introduction

Groundwater in arid and semi-arid areas is an essential domestic, agricultural and industrial source of fresh water. As a result of the rising demand for clean drinking water, management of groundwater quality is a vital task, especially in developing countries. From a strategic point of view, it frequently implies the implementation of control-monitoring studies. The assessment of water quality has an important implication for the groundwater's potential as a resource. It can indicate where negative impacts may be mitigated, also it is possible to conduct an evaluation scheme for the water conservation program.

Usually, groundwater studies involve various methods, techniques, and analytical procedures to allow the understanding of certain aspects of such a complex natural system. Water quality analysis, primarily based on the hydrochemical study, is one of the most decisive approaches. Coastal areas are subject to cumulative impacts from a variety of pollutants and from the heavy concentration of activities (e.g., Sebei et al., 2004; Petalas and Lambrakis, 2006; Boughariou et al., 2018; Ntanganedzeni et al., 2018). Thus, water quality assessment seems to be very important and helpful. Also, these limited resources are under pressure from population growth, urban water use, and overuse in irrigation. In general, several complex interactions between different heterogeneous factors control and influence the groundwater system.

From an analytical point of view, spatial and temporal variability approach could be efficient to depict and analyze the current hydrochemical picture of the groundwater (Huang et al., 2018). It provides quality data of the present baseline aquifer with an evaluation of both natural and anthropogenic pollution.

Geographical information system (GIS) with spatial analysis approaches allow new and useful insights in groundwater studies. Obtained results allow new insights and understanding concerning complexities of the hydrologic system. Valuable 2-D spatial mapping (e.g., Howari et al., 2007; Mosaferi et al., 2014; El Alfy et al., 2015; Nasir et al., 2016; Sebei et al., 2018) and 3-D modeling (Trabelsi et al., 2013) are also performed. The GIS framework is a synergistic processing of both multi-scale and multi-source spatial data. It allows, by simplification of obtained results, efficient data exploitation by policymakers and other stakeholders (Mosaferi et al., 2014). Spatial mapping of georeferenced hydrochemical data by gridding approach is a valuable source of useful interpretations. Using adequate cartographic semiotics and conventions are also helpful for efficient decisions concerning the studied groundwater (Nasir et al., 2016). Furthermore, a GIS-based raster operation using Map Algebra allows extraction of geospatial variability and temporal analysis (Dokou et al., 2015). Therefore, significant changes in time and space could be carried out with an evaluation of the groundwater chemical composition and deterioration rate.

The GIS-based spatial analysis approach was performed using hydrochemical data collected from sampling points within the Grombalia Basin (GB) aquifer, northeast of Tunisia. The hydrogeologic system of GB structure is regarded as an unconfined aquifer and represents the only way to overcome the water scarcity during dry years. This aquifer is considered to be the essential hydrogeological system in the Cap Bon region; it closely influences the ongoing development of the agro-industrial complex. In contrast, the groundwater of this unconfined aquifer is considered among the most vulnerable of the region.

Expanding anthropogenic, agricultural, and industrial areas have resulted in a rapid increase in water demand from this shallow aquifer. Also, the long-term withdrawals have engendered several deleterious problems such as water-level decline, salinization by seawater intrusion, and pollution of groundwater resources (Trabelsi et al., 2011).

The main objectives of this study are to: (i) investigate the geospatial model of major physicochemical parameters extracted from 21 water samples collected over a distinctive two-year period (May 2000 and April 2015), (ii) analyze the temporal variations of these parameters during the considered interval (15 years), and (iii) examine the spatial and temporal variations of principal hydrochemical indices such as Total dissolved solids (TDS), Sodium percent water class (SSP), and Sodium adsorption ratio (SAR). All collected variables, parameters, and indices were processed and examined using GIS-based spatial procedures to map patterns of spatial distribution and temporal variation. These results will provide a medium-term geospatial and temporal assessment of the hydrochemistry of the studied aquifer. Analyses of the geospatial variability combined with assessment tools were needed to evaluate water quality and to identify sensitive and high-risk areas.