Effects of a point source of phosphorus on the arsenic mobility and transport in a small fluvial system☆
Graphical abstract
Introduction
Arsenic is considered one of the most potentially dangerous elements for human health (ATSDR, 2019). Severe health problems can be caused by even low concentrations of dissolved As when mobilized from soils and sediments into freshwater systems (National Research Council, 1999). According to a recent estimation (Podgorski and Berg, 2020), 90–200 million people worldwide are potentially exposed to naturally occurring As-enriched freshwater as their only source of drinking water. The limit for As in drinking water was set to 10 μg/L (WHO, 1993); however, even concentrations <10 μg/L might cause inhibition to the DNA repair processes (Andrew et al., 2006).
The worldwide average As concentration in stream sediments is 5 mg/kg (Martin and Whitfield, 1983), rising with the clay or iron content and drastically increasing in the proximity of mineralized areas (Smedley and Kinniburgh, 2002). Mobilization of As from soils and sediments into freshwater systems happens under oxic as well as anoxic conditions. In anoxic conditions, the mobilization is mostly influenced by the bacterial reduction of FeIII or AsV (Islam et al., 2004). In oxic conditions, the two main pathways of As release are through (i) the oxidation of sulfide minerals (Chowdhury et al., 1999) and (ii) competition with other ions (e.g., carbonate, phosphate) for sorption sites, especially on iron, aluminum or manganese (oxyhydr)oxides (Appelo et al., 2002; Goldberg, 1986; Manning and Goldberg, 1996). The most significant competitor is phosphate due to the shared chemical and physical behavior e.g., similar ion-size and pKa constants (Anderson and Malotky, 1979). The extent of the As release by phosphate is mainly dependent on the As speciation and phosphate concentration. For example, a concentration of 10 mM of phosphate can release around 6% (∼15 mg/kg) of the total As from contaminated streambed sediments in gold mining areas (Rubinos et al., 2010). Nevertheless, even significantly lower concentrations of phosphate (∼0.03 mM) can mobilize above the As safety concentrations from contaminated soil (Mihaljevič et al., 2009). Although As and P competition is quite well described, studies focusing on the impact of this competition in fluvial systems are scarce and lack any long-term exposure effect (Rubinos et al., 2010, 2003; 2011). The published studies included laboratory experiments, showing that the addition of 0.1 mM phosphate substantially increases the amount of As released from polluted river sediments within one week. To our best knowledge, no study observed an As enriched fluvial system affected by permanent input of low phosphate levels on a scale of years.
The amount of P in fluvial systems has doubled under the effect of anthropogenic activities like the usage of fertilizers, sewage sources, and deforestation (Filippelli, 2008). It has been estimated that 25–45% of all the P in fluvial systems comes from treatment plants (Ekka et al., 2006; Parsons and Smith, 2008). The concentration of P in conventionally treated effluent is typically 4–10 mg/L (Carey and Migliaccio, 2009; Pitter, 2015). As a result, the maximal documented concentrations of P in fluvial systems can reach 7–10 mg/L downstream (Ekka et al., 2006; Haggard et al., 2005), which is a concentration ten times higher from the one documented to affect the mobility of As in soil (Mihaljevič et al., 2009). The fact that the discharged P increases the amount of sediment-bound P (Dorioz et al., 1998; Haggard et al., 2001) brings concerns about the fate of P as well as its interaction with As in As-enriched fluvial systems.
To our best knowledge, no field study has been conducted to investigate the long-term impact of discharged P on As-enriched streambed sediments. We studied a small stream with naturally elevated As concentrations, affected by a long-term discharge from a small municipal wastewater treatment plant (WWTP) to address this issue. This study aimed to assess the impact of the discharged P on (i) the mobility of As in the stream system and (ii) the As and P speciation in streambed sediments. We hypothesize that the long-term interaction of the discharged P with As in the streambed sediment has changed the chemical fractionation of As and P, and significantly lowered the fraction of adsorbed As in the sediment. This paper could provide a better understanding of the environmental impact of the discharged P on systems with elevated As concentrations.
Section snippets
Site characterization
The studied stream, Viničný potok, is situated in the eastern part of the historical Smolotely-Horní Líšnice gold district (central Czech Republic) around 60 km south of Prague. The stream is approximately 1 m wide with an average depth of 5 cm and an estimated average streamflow of 0.7 L/s during baseflow. The streambed consists of sand and organic matter with no visual evidence of Fe or Mn precipitates and an algal biofilm. Upstream of the sampling sites, the Viničný stream drains an
Streambed sediment
The streambed sediments were slightly alkaline (pHCaCl2 between 7.29 and 7.46) and oxic (as indicated by high Eh values; Table S5) with a loamy sand or a sandy loam texture (Table 1 and S2). The main minerals were consistent with the mineralogical composition of the bedrock (quartz, amphibole, and plagioclase). The fraction below <50 μm was enriched in chlorite and kaolinite, which dominated the clay fraction (Fig. S3). The heavy mineral fraction consisted mostly of amphibole (Fig. S4). The XRD
Impact of phosphate on the concentrations of dissolved As
Even though phosphate plays a critical role in the mobility of As (Smedley and Kinniburgh, 2002), the in situ impact of the competition in fluvial systems over long periods of time has not been studied. In this study, a small municipal wastewater treatment plant serves as a phosphate source to a stream with elevated As concentrations in streambed sediments (∼240 mg/kg). It brings between 3.8 and 8.4 mg/L of P to the stream, accounting for 91–98% of the dissolved P downstream (Table S7). The P
Conclusions
The long-term effect of phosphate on the As mobility and transport was studied in a small stream with naturally elevated As in the sediment (∼240 mg/kg). This fluvial system is significantly affected by the discharge from a small WWTP, bringing elevated dissolved P levels for 7 years. The high P concentration in the treated effluent (3.8–8.4 mg/L), which predominated as phosphate, increased the downstream concentration of aqueous P (1.0–3.2 mg/L). The long-term interaction of relatively low
Credit author statement
Conceptualization: Petr Drahota. Methodology: Petra Venhauerova, Petr Drahota. Validation: Petra Venhauerova. Formal analysis: Petra Venhauerova, Petr Drahota, Ladislav Strnad, Šárka Matoušková, Investigation: Petra Venhauerova, Petr Drahota, Recourses: Petra Venhauerova, Petr Drahota, Ladislav Strnad, Šárka Matoušková, Writing – Original Draft: Petra Venhauerova, Petr Drahota, Writing – Review & Editing: Petra Venhauerova, Petr Drahota, Ladislav Strnad, Šárka Matoušková. Visualization: Petra
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the Grant Agency of the Charles University (GAUK no. 790120), the Center for Geosphere Dynamics (UNCE/SCI/006), the Czech Science Foundation (GAČR no. 22-27939S), and the institutional support of the Institute of Geology ASCR, Czechia (RVO 67985831). The authors wish to thank our colleagues for the laboratory and instrumental support: Marie Fayadová (sample digestion), František Veselovský (heavy mineral fraction separation), Petra Vokurková (particle size
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This paper has been recommended for acceptance by Klaus Kümmerer.