Abstract
Superlative levels of arsenic (As) in groundwater and sediment often result from industrial pollution, as is the case for a coastal aquifer in Southern Italy, with a fertilizer plant atop. Understanding conditions under which As is mobilized from the sediments, the source of that As, is necessary for developing effective remediation plans. Here, we examine hydrogeological and geochemical factors that affect groundwater As concentrations in a contaminated coastal aquifer. Groundwater has been subject to pump-and-treat at a massive scale for more than 15 years and is still ongoing. Nevertheless, As concentrations (0.01 to 100 mg/L) that are four orders of magnitude more than Italian drinking water standard of 10 μg/L are still present in groundwater collected from about 50 monitoring wells over three years (2011, 2016, and 2018). As was quantified in three different locations by sequential extractions of 29 sediment cores in 2018 (depth 2.5 m to −16.5 m b.g.l.), combined with groundwater As composition, the aqueous and solid partitioning of As were evaluated by partition coefficient (Kd) in order to infer the evolution of the contaminant plumes. Most sediment As is found in easily extractable and/or adsorbed on amorphous iron oxides/hydroxides fractions based on sequential extractions. The study shows that As contamination persists, even after many years of active remediation due to the partitioning to sediment solids. This implies that the choice of remediation techniques requires an improved understanding of the biogeochemical As-cycling and high spatial resolution characterization of both aqueous and solid phases for sites of interest.
Highlights
Worldwide, more than 220 million people are at risk of exposure to groundwater containing elevated levels (>10 μg/L) of geogenic As [1,2,3,4]
Potential (ORP) filed measurements with a reference values for common Ag/AgCl electrodes [55]; this is because a variety of factors affect ORP measurements, including the effects of solution temperature and pH, irreversible reaction, multiple redox couples, and electrode poisoning
The results for MW07 (Figure 2 Upper panel), which is located within the immediate historic source area, shows opposite trends with depth for electrical conductivity (EC)
Summary
More than 220 million people are at risk of exposure to groundwater containing elevated levels (>10 μg/L) of geogenic As [1,2,3,4]. The average abundance of As in the upper continental crust is around 5.7 mg/kg [5], with the highest As values of about 13 mg/kg and the lowest ranging from 0.5 to 3 mg [6]. This issue has been intensively investigated all over the world. Besides geogenic sources [7,8,9,10], industrial, landfill [11], agricultural, and mining [12,13] activities have substantially contributed to the contamination of soils and groundwater with As over several decades, making it the third most common regulated inorganic contaminant to be detected at U.S Superfund sites [14]. The presence of multiple industrial inputs along the coastline, allow for contamination to persist long after the end of industrial activities [19]
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