Identification of processes mobilizing organic molecules and arsenic in geothermal confined groundwater from Pliocene aquifers

Highlights

• Polycyclic aromatic and polyphenols and As were co-mobilized in confined aquifer.

• Groundwater pH and temperature increased along the flow path in confined aquifer.

• High pH and temperature mobilized the polycyclic aromatics and polyphenols.

• The mobilized recalcitrant organic compounds were likely conducive to As release.

Abstract

Organic matter (OM) has been accepted as an important trigger fueling Fe(III) oxide reduction and arsenic release in the late Pleistocene-Holocene anoxic aquifers, whereas its fates and roles on arsenic mobility in the Pliocene aquifer are unclear. To fill this gap, groundwaters from a confined Pliocene aquifer (CG) and an unconfined Holocene aquifer (UG) were sampled in the Guide Basin, China, to monitor evolutions of groundwater geochemistry and OM molecular signatures along the groundwater flow path. The outcomes showed that groundwater pH, temperature, and arsenic concentrations in the CG samples generally increased along the groundwater flow path, which were much higher than those in the UG samples. The numbers and intensities of recalcitrant molecules (polycyclic aromatics and polyphenols) in the CG samples remarkably increased along the path, but relatively labile molecules (highly unsaturated and phenolic compounds and aliphatic compounds) showed the opposite trends. The arsenic-poor (<10 μg/L) UG samples contained more labile molecules than the arsenic-rich CG samples. High groundwater pH, temperature, and sediment age in the confined aquifers may be responsible for the selective mobilization of the unique polycyclic aromatics and polyphenols. The mobilized recalcitrant organic molecules may enhance arsenic release via electron shuttling, complexation, and competition. Furthermore, high temperature and pH may also facilitate arsenic desorption. The study provides molecular-scale evidences that the mobilization of recalcitrant organic molecules and arsenic were concurrent in the geothermal confined groundwater.

Introduction

Elevated arsenic (As) groundwaters have been world-widely observed in river deltas and low-lying inland basins (Guo et al., 2014), typically in the late Pleistocene-Holocene anoxic aquifers (Wang et al., 2019). Long-term drinking high-As groundwater (>10 μg/L) may pose severe health consequences, with an estimation of 94 to 220 million people being at the risk around the world (Podgorski and Berg, 2020). Understanding As enrichment mechanisms meets the emergent demands for effective solution of this global threat (Zheng, 2020).

In the late Pleistocene-Holocene anoxic aquifers, microbially-mediated reductive dissolution of Fe(III) oxides has been accepted as the primary mechanism of As mobility (Fendorf et al., 2010; Wang et al., 2019). Labile organic matter plays important roles in this process by acting as electron donors (Guo et al., 2019; Glodowska et al., 2020; Qiao et al., 2020). The recalcitrant OM may also be involved in As enrichment via electron shuttling, competition with As for surface sites, and complexation with As (Mladenov et al., 2015).

Groundwater from the Pliocene and older aquifers is typically considered as safe water sources, where low-As groundwater generally prevailed (Hasan et al., 2009; Hoque et al., 2017). However, high-As groundwater is found at depths of >200 m in the Pliocene aquifer in the Mekong Delta Basin (Erban et al., 2013, 2014), the Bengal Basin (Micheal et al., 2008; Mukherjee et al., 2011), the Pannonian Basin of Hungary and Romania (Rowland et al., 2011; Varsányi et al., 2017), and the Guide Basin of China (Wang et al., 2018b). In the Mekong Delta Basin, they attribute the pervasive occurrence of As hazard in the Pliocene aquifer to compaction of overlaying clay caused by extensive pumping, which expels high-As groundwater or As-mobilizing solutes (e.g., organic matter) into deep aquifers (Erban et al., 2013, 2014). It is still unclear how the mobilized OM facilitated As release in the Pliocene aquifers, although it is believed that organic matter should be rather refractory and the native labile OM should be near completely depleted due to the decrease in the bioreactivity of OM with sediment age and burial depth (Middelburg, 1989; Postma et al., 2016). In this case, the OM degradation rates and Fe(III) oxide reduction rates should be extremely low (Gao et al., 2020a). Therefore, it still remains open how OM is involved in As release processes in the Pliocene aquifer.

Identification of OM molecules by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) provides important information on their elemental compositions and relevant reactivity towards (bio)geochemical processes in aquifers (Valle et al., 2018; McDonough et al., 2020a, 2020b). By using FT-ICR MS, we have firstly delineated OM molecular signatures and their linkages to As mobility in the Hetao Basin (Qiao et al., 2020, 2021). In the typical late Pleistocene aquifer, the labile pools of water soluble sedimentary organic matter (SOM) (Qiao et al., 2020) and the recalcitrant pools of dissolved organic matter (DOM) (Qiao et al., 2021) are found to be responsible for As mobility by acting as electron donors and electron shuttles, respectively.

Geothermal groundwater typically contained toxic-level As concentrations (Grassi et al., 2014), especially in the western Latin America, the USA, Japan, Turkey, and New Zealand (Guo et al., 2012; Wang et al., 2018a; Knappett et al., 2020). The temperature-induced As mobility includes dissolutions of silicate minerals, desorption, SOM degradation, and the reductive dissolution of Fe(III) oxides (Schwenzer et al., 2001; Bonte et al., 2013). The hydrolysis of silicate minerals may generate alkaline pH groundwater, and thus facilitate As release in terms of desorption (Knappett et al., 2020). Furthermore, OM is inclined to be mobilized under high groundwater temperature and pH (Kleber et al., 2015; Chen et al., 2018), although molecular compositions of the mobilized OM are unclear. Since high temperature and pH could facilitate the enrichment of both DOM and As in geothermal groundwater, we may speculate that the enrichment of the two kinds of species is concurrent. Because of the thermal quenching effect on fluorescence intensity of DOM (Baker, 2005; Carstea et al., 2014), the FT-ICR MS technique may provide more reliable information on the intrinsic DOM properties of the geothermal groundwater. Nevertheless, few studies have investigated molecular characteristics of DOM in geothermal high-As groundwater (Kovács et al., 2010, 2012).

The Guide Basin is a special inland basin hosting high-As groundwater, where As enrichment in the deep confined Pliocene aquifer has been reported (Liao et al., 2013; Wang et al., 2018b). The deep confined groundwater has been used for drinking water in the Guide Basin since 1978, where more than 59,000 people were being at risk (Shi et al., 2010). In the Guide Basin, As is not believed to be directly sourced from geothermal water in the studied groundwater from the confined aquifer, since hot springs contain low As concentrations there (Zhang et al., 2016; Wang et al., 2018b). Geochemical processes may be conducive to As mobility in this aquifer. Therefore, investigation of the evolution of DOM and its roles in As mobility in geothermal groundwater from deep Pliocene aquifer at the molecular scale can be done in the Guide Basin.

The study aims to (1) delineate the differences in molecular-level compositions of DOM between the unconfined and confined groundwater samples, (2) identify geochemical processes controlling DOM molecular evolutions along a groundwater flow path, and (3) evaluate roles of DOM molecules in As mobility in the confined aquifer.