Model-Based Analysis of the Link between Groundwater Table Rising and the Formation of Solute Plumes in a Shallow Stratified Aquifer

Simone Varisco,Luca Raffaelli,Giovanni Pietro Beretta,Paola Raimondi,Daniele Pedretti

doi:10.3390/pollutants1020007

Abstract

Groundwater table rising (GTR) represents a well-known issue that affects several urban and agricultural areas of the world. This work addresses the link between GTR and the formation of solute plumes from contaminant sources that are located in the vadose zone, and that water table rising may help mobilize with time. A case study is analyzed in the stratified pyroclastic-alluvial aquifer near Naples (Italy), which is notoriously affected by GTR. A dismissed chemical factory generated a solute plume, which was hydraulically confined by a pump-and-treat (P&T) system. Since 2011, aqueous concentrations of 1,1-dichloroethene (1,1-DCE) have been found to exceed regulatory maximum concentration levels in monitoring wells. It has been hypothesized that a 1,1-DCE source may occur as buried waste that has been flushed with time under GTR. To elucidate this hypothesis and reoptimize the P&T system, flow and transport numerical modeling analysis was developed using site-specific data. The results indicated that the formulated hypothesis is indeed plausible. The model shows that water table peaks were reached in 2011 and 2017, which agree with the 1,1-DCE concentration peaks observed in the site. The model was also able to capture the simultaneous decrease in the water table levels and concentrations between 2011 and 2014. Scenario-based analysis suggests that lowering the water table below the elevation of the hypothesized source is potentially a cost-effective strategy to reschedule the pumping rates of the P&T system.

Highlights

In recent decades, socio-economic problems connected to groundwater table rising (GTR), referred to as groundwater rebound [1] or groundwater inundation [2], have become increasingly frequent in several areas around the world [3,4,5,6,7,8]
MODFLOW-NWT provided a well-defined correlation between the trends of the calculated groundwaters heads and the measured concentrations of 1,1-DCE
This study showed that cancan generate new solute plumes and, create a potential risk groundwater pollution presence undetected contaminant a potential risk of of groundwater pollution in in thethe presence of of undetected contaminant sources located in the vadose zone

Summary

Introduction

Socio-economic problems connected to groundwater table rising (GTR), referred to as groundwater rebound [1] or groundwater inundation [2], have become increasingly frequent in several areas around the world [3,4,5,6,7,8]. Groundwater table rising can be due to the reduction or complete decommissioning of large-scale groundwaterbased water supply facilities, such as public well fields [9,10]. It may be driven by natural processes, as in the case of coastal aquifers threatened by climate-changedriven sea-level fluctuations [11,12,13]. Groundwater table rising causes well-documented problems to the safety of underground structures and infrastructures [14,15,16,17]. Relatively less attention has been paid to the environmental problems that can be directly triggered by GTR [19,20]

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