Environ Monit Assess (2015) 187: 774 DOI 10.1007/s10661-015-4998-0 Assessing groundwater quality in Greece based on spatial and temporal analysis Zoi Dokou & Nektarios N. Kourgialas & George P. Karatzas Received: 5 May 2015 /Accepted: 20 November 2015 /Published online: 26 November 2015 # Springer International Publishing Switzerland 2015 Abstract The recent industrial growth together with the critical thresholds (set as the parametric values of the urban expansion and intensive agriculture in Greece has contaminant of interest in each case). The resulting increased groundwater contamination in many regions groundwater contamination maps could be used as a of the country. In order to design successful remediation useful tool for water policy makers and water managers strategies and protect public health, it is very important in order to assist the decision-making process. to identify those areas that are most vulnerable to groundwater contamination. In this work, an extensive Keywords Groundwater contamination . Major ions . contamination database from monitoring wells that cov- Heavy metals . Kriging . Geostatistics er the entire Greek territory during the last decade (2000–2008) was used in order to study the temporal and spatial distribution of groundwater contamination Introduction for the most common and serious anionic and cationic trace element pollutants (heavy metals). Spatial and Water resources worldwide are currently under an in- temporal patterns and trends in the occurrence of creasing pressure due mainly to the urban expansion groundwater contamination were also identified that, together with the high population density and in- highlighting the regions where the higher groundwater dustrial development, has resulted in an increase in contamination rates have been detected across the coun- water demand and at the same time has contributed try. As a next step, representative contaminated aquifers significantly to the contamination of the existing water in Greece, which were identified by the above analysis, bodies. Greece is no exception to water-related prob- were selected in order to analyze the specific contami- lems. Groundwater systems in Greece experience sig- nation problem in more detail. To this end, geostatistical nificant pressure on their quantity, due to increased techniques (various types of kriging, co-kriging, and water abstraction and lowering of the water levels, and indicator kriging) were employed in order to map the their quality, related to pollution from point and non- contaminant values and the probability of exceeding point sources. A wide variety of materials have been identified as Electronic supplementary material The online version of this contaminants found in groundwater such as organic and article (doi:10.1007/s10661-015-4998-0) contains supplementary inorganic compounds, pathogens, heavy metals, and material, which is available to authorized users. hydrocarbons. The most common contamination prob- lems encountered in Greece are seawater intrusion, ni- Z. Dokou (*) : N. N. Kourgialas : G. P. Karatzas School of Environmental Engineering, Technical University of trate pollution caused by intensive agriculture activities, Crete, Chania, Greece and heavy metal contamination, especially near mines e-mail: [email protected] or industrial areas. Saltwater intrusion is a problem that 774 Page 2 of 18 Environ Monit Assess (2015) 187: 774 occurs in many coastal aquifers and can jeopardize sewage sludge as a soil conditioner, Pb arsenate pesti- groundwater quality if not detected in time (Dokou and cides and leaded gasoline. Pb has a limited mobility in Karatzas 2012). The adverse effects of seawater intrusion groundwater and usually migrates sorbed onto colloidal- in productive aquifers involve the reduction in the avail- sized metal hydroxides. As is usually concentrated in able freshwater storage volume and the simultaneous sulfide-bearing mineral deposits, especially those asso- contamination of production wells, given that less than ciated with gold mineralization. It is relatively soluble in 1 % of seawater is capable of rendering freshwater groundwater compared to other heavy metals depending unsuitable for drinking and irrigation purposes (Werner on the pH, redox conditions, temperature, and solution et al. 2013). Groundwater contamination by nitrates is a composition. Man-made sources of As include mineral problem related mainly to the spreading of organic and extraction, industrial activities, waste processing, and chemical fertilizers by farmers and, to some extent, to pesticides (Smedley and Kinniburgh 2002). effluents from domestic sewage systems (Psarropoulou The need to avoid further deterioration of water qual- − and Karatzas 2014). Nitrate (ΝΟ3) contamination in ity and quantity in Greek aquifers has attracted a great groundwater is attributed to various non-point sources deal of attention from governmental and regulatory including fertilizer application, use of animal manure, bodies. Currently, they are seeking actions and examin- − septic tanks, and geological origins. The amount of ΝΟ3 ing possible remediation methods to ensure sustainable that leach from agricultural areas into the groundwater management and protection of water resources. In order depends on various natural factors such as soil type and to design successful remediation strategies and protect climatic conditions (Mikkelsen 1992). In addition, shal- public health, it is very important to identify those areas − low aquifers are more vulnerable to ΝΟ3 pollution as that are most vulnerable to groundwater contamination compared to deep or confined aquifers; however, surface and analyze the temporal and spatial distribution of or near-surface outcrops of confined aquifers can allow contamination. Geostatistics offer an effective way to − deeper downward migration of ΝΟ3 contamination estimate (i) contaminant concentrations in un-sampled −2 (Daskalaki and Voudouris 2008). Sulfate (SO4 )con- areas using variants of kriging (D’Acqui et al. 2007; tamination is also common in Greece and is attributed to Varouchakis et al. 2012;Venterisetal.2014;Kourgialas dissolution of sulfate minerals (gypsum and anhydrite) and Karatzas 2015) and (ii) the probability that their commonly found in the Quaternary aquifer matrices concentration will exceed a pre-specified threshold while anthropogenic sources include mining activities, using indicator kriging (Lu et al. 2007). fertilizer application, and industrial wastes. In a previous work, Daskalaki and Voudouris (2008) Heavy metal contamination constitutes another major performed a synoptic review of the hydrochemical con- problem in various areas in Greece, receiving particular ditions of the porous aquifers in Greece, using collected concern because of their strong toxicity even at low groundwater quality data for Greece from 1998 to 2001. concentrations. Heavy metals are usually produced in As an extension of this work, a more recent, extensive mining areas and industrial zones but they can also be a contamination database from monitoring wells that cov- product of geological weathering. The most important er the entire Greek territory during the last decade heavy metals examined here for the region of Greece are (2000–2008) was used here in order to evaluate ground- chromium (Cr), lead (Pb), and arsenic (As). The exten- water quality in Greece. Based on these records, the sive use of Cr in industrial activities in combination to temporal and spatial distribution of groundwater con- − − −2 the geogenic origin of Cr from the leaching of tamination was studied for C1 , ΝΟ3,andSO4 as well ophiolithic rocks has resulted in multiple contaminated as the most commonly encountered heavy metals (Cr, sites in Greece. The high toxicity and mobility of the Pb, and As). Spatial and temporal patterns and trends in hexavalent oxidation state of Cr (Cr(VI)) has increased the occurrence of groundwater contamination were also the concern of scientists, grassroot organizations, and identified highlighting the regions where the higher the public, pressuring for the establishment of a para- groundwater contamination rates have been detected metric value specifically for Cr(VI). Currently, the para- across the country. Moreover, the seasonal distribution metric value of 50 μg/L for total Cr is being used in of the pollutants was investigated for the entire country. practice for Cr(VI). Pb contamination in groundwater Then, based on the above maps, representative contam- originates from the dissolution of Pb from soil or an- inated aquifers (Malia, Korinthiakos, and Asopos) were thropogenic sources such as mining and smelting, use of selected in order to analyze their water quality in more Environ Monit Assess (2015) 187: 774 Page 3 of 18 774 detail. To this end, geostatistical techniques (various the town of Malia, in Northern Crete, 40 km east of types of kriging and co-kriging) were used for the gen- the city of Heraklion. The study area of 13 km2 is eration of groundwater quality maps in each aquifer and characterized by a gentle slope to the north of the their performances were compared. In addition, indica- town of Malia and by mountains to the south. tor kriging was employed in order to map the probability Sclerofyllous vegetation and complex cultivation of exceeding critical thresholds (set as the parametric patterns cover most of the study area. Other land uses values of the contaminant of interest in each case). The include olive groves, natural grassland, non-irrigated resulting