Research papersSurface–ground water interactions and hydrogeochemical evolution in a fluvio-deltaic setting: The case study of the Pinios River delta
Introduction
Over 500 million people live in river deltas, despite these areas account for only 5% of the global land mass. River deltas are areas of ecological and societal value, because they host fresh surface and groundwater resources, sustain biodiversity, and provide large flat areas for agricultural and urban uses. These multiple conflicting human activities apply significant pressures and risks to deltaic and coastal ecosystems (Kuenzer and Renaud, 2012).
The status of fresh surface water and groundwater quality in the deltaic areas is affected by natural factors, such as the high lateral and vertical granulometric variability of deltaic depositional sequences, the hydrogeochemical reactions between water and sediments and the surface water-groundwater interactions, which modify the hydrochemical characteristics of the surface water and groundwater resources on a spatial and temporal basis (Brunke and Gonser, 1997, Ezzy et al., 2006, Menció and Mas-Pla, 2008, Torres-Rondon et al., 2013).
Seasonal variations of precipitation, river discharge and water table level can cause a reduced replenishment of the water system leading to limited availability of water. Groundwater over-extraction can trigger sea water intrusion, causing a degradation of the deltaic freshwater quality and shortage of drinking water supplies. Nutrients, such as nitrogen and phosphorus, in agricultural practices can contaminate aquifers and surface waters via infiltration and direct runoff, respectively (Barrett et al., 2002, Kim et al., 2009, Mondal et al., 2011, Sarma et al., 1982).
The detailed knowledge of the factors controlling the surface water and groundwater quality is recognized as critical for water resources management, especially in the semi-arid southern coastal parts of Europe (i.e. Mediterranean coast). Conventional hydrochemical methods of data analysis combined with multivariate statistical techniques, such as Principal Component Analysis (PCA), are widely applied in surface and groundwater studies to identify the natural and anthropogenic factors that control water quality (Ahmed Baig et al., 2010, Cloutier et al., 2008, Fan et al., 2010, Matiatos et al., 2014, Okiongbo and Douglas, 2015, Tanasković et al., 2012, Qin et al., 2013, Ujević Bošnjak et al., 2012).
Water Quality Indices (WQIs) are used to describe the overall ‘quality’ of a water body by assigning to a set of critical parameters a single value (e.g. Abbasi and Abbasi, 2012, Debels et al., 2005, Dhanasekarapandian et al., 2016, Şener et al., 2017). The ‘quality’ of a water system can be evaluated in a more easily and rapidly comprehensible manner, assisting decision makers and regulatory authorities to determine priorities and evaluate the impact of various environmental management practices (Pusatli et al., 2009, Sadiq et al., 2010, Song and Kim, 2009). The application of WQIs facilitates the comparison of the water quality between different sampling sites and sampling events in a single or multiple geographical areas.
The deltaic plain of Pinios River lies in central Greece and is of significant ecological importance. It is characterized by rich fresh surface water availability supporting a great biodiversity of flora and fauna. It is classified as “Special Protection Area” of the NATURA 2000 network (GR1420015) and a CORINE biotope (A00020006) and includes riparian forests, an estuary with riparian woodland, marshes, small freshwater lakes, sand dunes and coastal zones. The area combines the natural-biological features of the regions placed under protection by the international treaties of Bonn (Convention on the Conservation of Migratory Species of Wild Animals), Bern (Convention on the Conservation of European Wildlife and Natural Habitats), Barcelona (Convention for the Protection of the Marine Environment and the Coastal Region of the Mediterranean) and Washington (Convention on International Trade in Endangered Species) (Economou, 2005).
This study uses hydrochemical and multivariate statistical techniques and aimed to a) define the possible interactions between groundwater and surface water bodies in the deltaic plain of Pinios River; b) identify the main hydrogeochemical processes and mechanisms that control the water quality in the deltaic environment and the suitability of the water for drinking purposes, and c) test the suitability of a modified Water Quality Index for describing the overall water quality. The major pressures exerted on deltaic aquifers worldwide are reviewed and compared to the results of this study. The outcomes of this work could be easily used by the local authorities, stakeholders and water managers as an operational tool for the effective management of the water resources.
Section snippets
Study area
The deltaic plain of Pinios River (Thessaly, Greece) covers approximately 70 km2, where the river flows in a WSW-ENE direction, discharging into Thermaikos Gulf (Fig. 1). Literature data show that river discharge levels have their maxima between November and April and minima between May and September (Lazogiannis et al., 2014). Before exiting Tempi valley, the river is recharged from karstic spring waters (Fytianos et al., 2002, Migiros et al., 2011). The deltaic plain is characterized by a
Sampling and analytical procedures
Groundwater levels and river water discharge were measured monthly from October 2012 to September 2013. River water discharge (in m3/s) was measured at two river sites, one just before the river enters the delta area (site GA) and one close to the river mouth (site GP, see Fig. 1), using a current flow meter (Valeport BFM 001/002). Groundwater levels were measured in 13 boreholes with a contact gauge (OTT KL 010). RTK-GPS was used to map the position of the boreholes to an accuracy of ∼10 mm.
Hydrological conditions
The temporal variation of groundwater levels in the middle part of the deltaic plain (e.g. P7, P19; Fig. 2a) follows roughly the seasonal patterns of precipitation with higher water table values observed during the wet season and the lower ones during the dry season. In the beginning of the dry season (October–November) water levels of the shallow aquifer were relatively low, increased until February and dropped gradually to reach the lowest values during the end of the dry season
Conclusions
This work presents a regional hydrogeochemical study of the deltaic plain of River Pinios (central Greece), an evaluation of the water quality status and the identification of the main geochemical processes controlling water hydrochemistry and its spatial evolution. The river is primarily recharged by groundwater influxes from the surrounding mountains and secondarily by precipitation and the shallow aquifer inside the deltaic plain.
The results of the Water Quality Index classification showed
Acknowledgements
A significant part of this work was financially supported by project DAPHNE (MIS: 375908), which is co-funded by Greece and the European Union through the O. P. “Education and lifelong learning, 2007-2013” of the Ministry of Education and Religious Affairs. Acknowledgement is given to Mr A. Panagopoulos (PhD), Mr E. Vassilakis (Asst. Professor), Ms D.E. Sifnioti (PhD), Ms D.I. Giannouli (MSc), Mr K. Tsanakas (PhD) and Ms A. Tsoutsia (MSc) for their contribution in the project. We would like to
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