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Université des géosciences de Chine (2014)

Monitoring and Coupled Modeling of the Hydraulic and Hydrogeochemical Processes of Arsenic Transport in Hyporheic Zone

余倩

Titre : Monitoring and Coupled Modeling of the Hydraulic and Hydrogeochemical Processes of Arsenic Transport in Hyporheic Zone

Auteur : 余倩

Grade : Doctoral Dissertation 2014

Université : Université des géosciences de Chine

Résumé partiel
Natural As groundwater contamination is a serious problem in many areas around the world, especially in Asian countries. The fate and transport of arsenic in groundwater system is influenced by hydrogeologic conditions and geochemical and biogeochemical processes. Due to the extremely complex geochemical reactions in aquifers system and many uncertain factors, numerical modeling has been regarded as the most cost-effective method.Datong Basin is located in an arid and semi-arid region of Shanxi Province, northern China. Groundwater has been the major source of potable water for drinking and irrigation purpose. High arsenic concentration has been detected in Datong groundwater, with the maximum value being up to1820μg/L. Long-term intake of high-As groundwater has caused endemic arsenic poisoning in Datong. Science1990s, a lot of work have been done to understand the genesis of Datong high arsenic groundwater. The results indicated that the arsenic in the Quaternary aquifer systems mainly originated from the Archean metamorphic rocks and Mesozoic coal-bearing strata around the Basin. The major processes of arsenic mobilization are most likely linked to As desorption from Fe oxides/oxyhydroxides and the reductive dissolution of the Fe-rich phase in the aquifer sediments under reducing and alkaline conditions.In fact, the hydrogeolocial conditions play an important role in As release. In recent years, many studies have demonstrated the effect of hydrodynamic conditions on dissolved As distribution in the aquifer. However, the previous studies on high-arsenic groundwater in the Datong Basin have been mostly focused on the geochemical and biogeochemical processes controlling As transport in the groundwater system. No systematical investigations were conducted to discuss the linkage between As concentration and groundwater flow paths in this area. Since studies of groundwater flow are helpful to understanding the enrichment of As in the groundwater affected by natural or anthropogenic changes in the hydrological cycle, clarifying the relationship between hydrodynamic conditions and arsenic behavior in groundwater is becoming essential.For this study, we selected a typical high arsenic groundwater site for detailed monitoring. At first, based on the one-year continuous monitoring work with one month interval for water level and water chemistry, a three-dimensional transient groundwater flow model with realistic assumptions of hydraulic parameters and boundary conditions of the geological structure was conducted with MODFLOW to reveal the relationships between groundwater dynamics and As concentrations in shallow contaminated groundwater systems. Then a short-term artificial flooding experiment was conducted to further understand the effects of groundwater and surface water interactions on arsenic transport in the adjacent aquifer. Finally, a one-dimensional reactive transport model occupied with biogeochemical processes of arsenic was conducted with PHREEQC to recognize the major processes controlling arsenic mobilization in adjacent groundwater system during the period of groundwater and surface water interaction. The main contents of this paper and findings are summarized as below.1. The relationships between groundwater dynamics and As concentrations in shallow contaminated groundwater systems were understood.A one-year continuous monitoring work was conducted under natural condition with one month interval for groundwater level and groundwater chemistry. Based on the three-dimensional transient groundwater flow model, the following main findings were obtained :(1) Groundwater arsenic concentration significantly increases from irrigation season to non-irrigation season, with the fluctuation range of2.8-46.3μg L-1and3.5-181.5μg L-1, respectively. A slight decrease of oxidation reduction potential (ORP) value presents from irrigation season to non-irrigation season, fluctuating between-6.6mV and-141.1mV and between-61.1mV and-134.9mV, respectively. During irrigation season, groundwater HS-concentration has a narrow fluctuation range of1-5μg L-1, but with a much wider fluctuation range of2-12μg L-1during non-irrigation season.(2) Groundwater numerical simulation indicates that irrigation can increase groundwater level and reduce horizontal groundwater velocity and thereby accelerate vertical and horizontal groundwater exchange among sand, silt and clay formations.(3) Results of net groundwater flux estimation suggest that vertical infiltration is likely the primary control of As transport in the vadose zone, while horizontal water exchange is dominant in controlling As migration within the sand aquifers. Recharge water, including irrigation return water and flushed saltwater, travels downward from the ground surface to the aquifer and then nearly horizontally across the sand aquifer.(4) The maximum value of As enriched in the hyporheic zone is roughly estimated to be1706.2mg day-1for a horizontal water exchange of8.98m3day-1close to the river and an As concentration of190μg L-1.(5) A possible mechanisms of As transport in the aquifer can be discussed within the framework of groundwater dynamics. First, in the process of downward movement of irrigation return water and salt flushing water, oxygen and organic matter are carried into the aquifers, which can not only oxidize the dissolved As in the vadose zone but also desorbed As into the groundwater. Second, the horizontal groundwater flux plays a dominant role in the saturated zone since the vertical groundwater flux is too small to be neglected. Horizontal water exchange may also cause As dissolution and release into the groundwater or promote the transport of dissolved As toward a more reductive environment.

Mots clés : Hyporheic zone; High arsenic aquifer; Groundwater-surface water interaction; Groundwater flow model; Reactive transport model;

Présentation (CNKI)

Page publiée le 13 octobre 2017