17 th International Conference on Environmental Science and Technology Athens, Greece, 1 to 4 September 2021 CEST2021_00573 On the geochemistry of Gialova lagoon, SW Peloponnesus, Greece Papakonstantinou M., Sergiou S., Dimas X., Fakiris E., Christodoulou D., Geraga M., and Papatheodorou G*. Laboratory of Marine Geology and Physical Oceanography, Department of Geology, University of Patras, Greece, 26 504. *corresponding author e-mail: gpapathe@upatras.gr Abstract. The spatial distribution of lithological characteristics, organic carbon and major / trace elements was studied in the surface sediments of the Gialova lagoon, a shallow water lagoon (<1.0 m) which is located at southwestern Peloponnesus (Greece). The sediment samples were collected on the basis of a detailed bathymetric map and the backscatter properties of the lagoon floor acquired using high resolution side scan sonar on board of an Unmanned Surface Vehicle (U.S.V.). Four main geochemical phases were identif ied based on the elements – grain size – organic carbon associations. Geoaccumulation index (Igeo) was estimated for heavy metal concentrations showed low to moderate contamination for Mo, Pb, Ni, and Cr. Keywords: Lagoon, sediments, heavy metals, Igeo 1. Introduction Lagoons are highly vulnerable environments, counted amongst the most threatened aquatic systems lying under various types of natural (such as global clim a te and sea level rise) and anthropogenic forces (intensive agriculture, aquaculture and input of heavy metals). The Gialova Lagoon is considered as an important wildlife refuge (Maneas et al. 2019). Even though it is protected by international conventions and belongs to the Natura 2000 European community Network as Special Protection Area (SPA) and Site of Community Interest (SCI), the lagoon has suffered from many human interventions during the last 70 years which resulted to the reduction of its extent from 7.5km 2 to 2.5km 2 (Avramidis et al. 2015, Maneas et al. 2019). The lagoon is separated from the Navarino bay by a 3.3km long and 0.15km wide natural sand barrier. In this barrier, a narrow canal connects the lagoon with the bay. The surrounding geological setting of the Gialova Lagoon consists of Holocene alluvial deposits a nd sa nd dunes, Plio–Pleistocene deposits of conglomerates, marls a nd fine grained sandstones, and Eocene to Oligocene flysch deposits (Avramidis et al., 2015). The main objectives of this study are to visualize the spatial distribution of sedimentological characteristics o f the lagoon and to examine potential heavy metal enrichment, since bottom sediments grain size and geochemical characteristics are important geological a nd environmental factors. 2. Material and methods 2.1. Field work The sampling survey was conducted during December 2020, on the basis of backscatter properties of the lagoon floor acquired using high resolution side scan sonar data on board of an Unmanned Surface Vehicle (U.S.V.). Subsequently, twenty-eight (28) surficial sediment samples were collected covering the entire different backscatter levels of the lagoon floor. Sediment samples were collected with the use of a Van-Veen grab while subsamples for laboratory analyses were taken f rom the central part of the grab to avoid contamination. 2.2. Analytical procedures Sediment samples were subjected to macroscopic description, granulometry and chemical analysis. All samples were visually described in terms of colour and textural features. Grain size analysis was performed using a Malvern Mastersizer 2000 particle analyzer. Hydrogen peroxide (H2O2) treatment was applied prior to analysis to eliminate organic matter. The definition of the main four grain size statistical parameters, mean (Mz), sorting (σi), skewness (Sk) a nd kurtosis (KG) were calculated with the GRADISTAT program (Blott & Pye, 2001) while sediment classification was based on Folk (1974) nomenclature. Bulk geochemistry was determined by four acid digestion followed by ICP-MS finish. Total Organic Carbon (TOC) was measured using a LECO Carbon analyzer after H CL (25%) pre-treatment to remove carbonates. The spatial distribution of sediment characteristics was visualized using the Surfer 9 and ArcGIS software. 2.3. Contamination indices Geoaccumulation index (Igeo) (Muller, 1979) was estimated for selected heavy metal concentrations in which,  =  2 [ /(1.5)]