Geostatistics as a tool to improve the natural background level definition: An application in groundwater

The Natural Background Level (NBL), suggested by UE BRIDGE project, is suited for spatially distributed datasets providing a regional value that could be higher than the Threshold Value (TV) set by every country. In hydrogeochemically dis-homogeneous areas, the use of a unique regional NBL, higher than TV, could arise problems to distinguish between natural occurrences and anthropogenic contaminant sources. Hence, the goal of this study is to improve the NBL definition employing a geostatistical approach, which reconstructs the contaminant spatial structure accounting geochemical and hydrogeological relationships. This integrated mapping is fundamental to evaluate the contaminant's distribution impact on the NBL, giving indications to improve it. We decided to test this method on the Drainage Basin of Venice Lagoon (DBVL, NE Italy), where the existing NBL is seven times higher than the TV. This area is notoriously affected by naturally occurring arsenic contamination. An available geochemical dataset collected by 50 piezometers was used to reconstruct the spatial distribution of arsenic in the densely populated area of the DBVL. A cokriging approachwas applied exploiting the geochemical relationships among As, Fe andNH4+. The obtained spatial predictions of arsenic concentrationswere divided into three different zones: i) areas with an As concentration lower than the TV, ii) areas with an As concentration between the TV and the median of the values higher than the TV, and iii) areas with an As concentration higher than the median. Following the BRIDGE suggestions, where enough samples were available, the 90th percentile for each zone was calculated to obtain a local NBL (LNBL). Differently from the original NBL, this local value gives more detailed water quality information accounting the hydrogeological and geochemical setting, and contaminant spatial variation. Hence, the LNBL could give more indications about the distinction between natural occurrence and anthropogenic contamination

Subsoil geostatistical modeling as tool for hydrogeological modeling: transitional probability approach applied upon a heterogeneous site

Hydrogeological modeling, 3D simulation, Venice lagoon, reactive transport numerical mode

defining the hydrogeological behavior of karst springs through an integrated analysis a case study in the berici mountains area vicenza ne italy

Knowledge of the hydraulic and geological properties of karst systems is particularly valuable to hydrogeologists because these systems represent an important source of potable water in many countries. However, the high heterogeneity that characterizes karst systems complicates the definition of karst hydrogeological properties, and their estimation involves complex and expensive techniques. In this study, a workflow for karst spring characterization was used to analyze two springs, Nanto spring and Mossano spring, located in the Berici Mountains (NE Italy). Based on the data derived from 4 years of continuous hourly monitoring of discharge, water temperature and specific electrical conductivity, a hydrogeological conceptual model for the monitored springs was proposed. Flow rate measurements, which combined recession curve, flow duration curve and autocorrelation function techniques, were used to evaluate the spring discharge variability. Changes in spring discharge can be related both to the degree of karstification/permeability and to the size of the karst aquifer. Moreover, combining monitored parameters and rainfall—analyzed by the cross-correlation function and VESPA (Vulnerability Estimator for Spring Protection Areas) index approach—permitted assessment of the spring response to recharge and the behavior of the drainage system. Although the responses to the recharge events were quite similar, the two springs showed some differences in terms of the degree of karstification. In fact, Mossano spring showed a more developed karst system than Nanto spring. Three systems (two karsts and one matrix/fractured) are outlined for Mossano spring, while two systems (one karst and one matrix/fractured) are outlined for Nanto spring

Unravelling the importance of fractured zone in regional fluid flow: insights from the hydrothermal modelling of the Euganean geothermal system (ne Italy)

Euganean Geothermal System, fault system, 3D coupled flow and heat transport numerical model

Concentrazioni Soglia di Contaminazione (CSC) e valori di fondo dell'arsenico nella prima falda confinata: uno studio geostatistico nell'area centrale del bacino scolante in laguna di Venezia

The study want to show an example of geostatistical approach to understand the arsenic distribution in the shallow groundwater of the drainage basin to the Venice lagoon. The datasets used are the result of some characterization studies made by ARPA Veneto from 2009 to 2014. They were got by four season fieldtrips. At the beginning these data were modified to consider only the parameters as As, Eh, pH and NH4+. Then these data were interpolated with two geostatistical methods to explore the arsenic distribution in the area and note the zones where arsenic concentrations exceed the contamination thresholds (CSC) or the arsenic background values. The CSC values are defined by D.Lgs 152/06, whereas, the background values are proposed by A.Li.Na study. To make the interpolation were used the Ordinary kriging and the Ordinary cokriging methods. Moreover, this study allowed to compare kriging and cokriging methods

The Natural Background Level problem: A Hydro-geochemical study on the natural occurrence of Arsenic located in the Veneto region (NE Italy)

Excess of arsenic in groundwater is a worldwide problem threatening the health of the millions of people directly exposed to Arsenic-rich water intake. The problem is particularly acute in naturally occurring unconsolidated aquifers where Arsenic-rich groundwater is an easily accessible resource of drinking water, such arsenic in India, Bangladesh and Vietnam. In Italy, arsenic is found in aquifers from north to south and it is associated to different geological settings (i.e. volcanic areas and alluvial plains). The shallow aquifer of Venetian Alluvial Plain (VAP) is notoriously affected by arsenic contamination, characterized by a patchy distribution with variable extensions and concentrations. With concentrations exceeding the WHO limit of 10 μg/L, this metal poses a risk for locals, arsenic the aquifer is exploited for agricultural purposes. Although empirical evidences exist about the relationship between arsenic occurrence and other factors, several aspects regarding the physical and geochemical processes controlling arsenic in the VAP aquifers remain unclear. In this line, the present study aims to elucidate both the geochemical processes fostering arsenic mobility and their correlation with the subsurface heterogeneity, in order to improve the knowledge about arsenic-controlling processes and use them arsenic aid for the environmental management. To this end, we focused in detail on an agricultural zone nearby the Venice lagoon, affected by arsenic contamination (called “Western Agricultural Areas”, WAA). The available data, collected by several hydrogeological surveys, show a spatial and temporal variability of arsenic concentration, which can be associated to a variety of hydro-geochemical processes such arsenic redox variations, sorption or reductive dissolution of Arsenic-rich iron oxy-hydroxides. In order to point out the consistency and the importance of these processes, we structured the study in two main phases: 1) evaluation of geochemical processes by means of a PHREEQC batch-like reactive model and 2) creation of a 3D reactive transport model based on previous results. The former allows us for testing whether the supposed geochemical processes are able to fit the geochemical conditions of the study system, identifying the main actors for arsenic mobility. The latter couples the obtained reactive processes with a 3D flow model, in order to evaluate the spatial and temporal distribution of dissolved arsenic as function of the subsurface heterogeneity. The results highlighted a strong effect of oxy-reductive potential on arsenic mobility, and it seems to be strictly correlated to organic matter degradation. The uprising of reduced condition, then, affects other mechanism such arsenic reductive dissolution of iron hydroxides, ion exchange and sorption processes, causing arsenic mobilization. Moreover, this study shaded light on the existence of oxygen ingress arsenic function of local water recharge events, which seems to be responsible of space/time redox variation. The 3D reactive transport model showed a strong dependence between the aforementioned processes and subsurface heterogeneity. The material distribution, indeed, plays an import role affecting the arising of the main chemical reactions

Probability of Non-Exceedance of Arsenic Concentration in Groundwater Estimated Using Stochastic Multicomponent Reactive Transport Modeling

Stochastic multicomponent reactive transport modeling is a powerful approach to quantify the probability of non-exceedance (PNE) of arsenic (As) critical concentration thresholds in groundwater. The approach is applied to a well-characterized shallow alluvial aquifer near Venice, Italy. Here, As mobility depends primarily on rainfall-controlled redox-dependent precipitation-dissolution of iron hydroxides. A Monte-Carlo analysis based on a calibrated three-dimensional flow and transport model targeted the geochemical initial conditions as the main source of uncertainty of As concentrations in the studied aquifer. It was found that, during 115 simulated days, the fraction of the entire aquifer volume with As > 10 μgL−1 decreased on average from ~43% to ~39% and the average As concentration from ~32 μgL−1 to ~27 μgL−1. Meanwhile, PNE increased from 55% to 60% when 10 μgL−1 was set as target threshold, and from 71% to 78% for 50 μgL−1. The time dependence of As attenuation can be ascribed to the increase of oxidizing conditions during rainfall-dependent aquifer recharge, which causes As sorption on precipitating iron hydroxides. When computing the same statistics for the shallowest 6 m, As attenuation was even more evident. The volume fraction of aquifer with As > 10μgL−1 dropped from 40% to 28% and the average As concentration from 31 μgL−1 to 20 μgL−1, whereas PNE increased from 58% to 70% for As < 10 μgL−1 and from 71% to 86% for As < 50 μgL−1. Thus, the wells screen depth in the aquifer can be a critical aspect when estimating As risk, owing to the depth-dependent relative change in redox conditions during rainfall events

Subsoil Reconstruction in Geostatistics beyond Kriging: A Case Study in Veneto (NE Italy)

The reconstruction of hydro-stratigraphic units in subsoil (a general term indicating all the materials below ground level) plays an important role in the assessment of soil heterogeneity, which is a keystone in groundwater flow and transport modeling. A geostatistical approach appears to be a good way to reconstruct subsoil, and now other methods besides the classical indicator (co)kriging are available as alternative approximations of the conditional probabilities. Some of these techniques take specifically into account categorical variables as lithologies, but they are computationally prohibitive. Moreover, the stage before subsoil prediction/simulation can be very informative from a hydro-stratigraphic point of view, as the detailed transiogram analysis of this paper demonstrates. In this context, an application of the spMC package for the R software is presented by using a test site located within the Venetian alluvial plain (NE Italy). First, a detailed transiogram analysis was conducted, and then a maximum entropy approach, based on transition probabilities, named Markovian-type Categorical Prediction (MCP), was applied to approximate the posterior conditional probabilities. The study highlights some advantages of the presented approach in term of hydrogeological knowledge and computational efficiency. The spMC package couples transiogram analysis with a maximum entropy approach by taking advantage of High-Performance Computing (HPC) techniques. These characteristics make the spMC package useful for simulating hydro-stratigraphic units in subsoil, despite the use of a large number of lithologies (categories). The results obtained by spMC package suggest that this software should be considered a good candidate for simulating subsoil lithological distributions, especially of limited areas