Journal of Earth Science, Vol. 25, No. 2, p. 386–396, April 2014 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-014-0421-y Monitoring and Modeling the Effects of Groundwater Flow on Arsenic Transport in Datong Basin Qian Yu, Yanxin Wang*, Rui Ma, Chunli Su, Ya Wu, Junxia Li State Key Laboratory of Biogeology and Environmental Geology, School of Environmental Studies, China University of Geosciences, Wuhan 430074, China ABSTRACT: Although arsenic-contaminated groundwater in the Datong Basin has been studied for more than 10 years, little has been known about the complex patterns of solute transport in the aquifer systems. Field monitoring and transient 3D unsaturated groundwater flow modeling studies were car- ried out on the riparian zone of the Sanggan River at the Datong Basin, northern China, to better un- derstand the effects of groundwater flow on As mobilization and transport. The results indicate that ir- rigation is the primary factor in determining the groundwater flow paths. Irrigation can not only in- crease groundwater level and reduce horizontal groundwater velocity and thereby accelerate vertical and horizontal groundwater exchange among sand, silt and clay formations, but also change the HS- concentration, redox conditions of the shallow groundwater. 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. The maximum value of As enriched in the riparian zone is roughly estimated to be 1 706.2 mg·d-1 for a horizontal water exchange of 8.98 m3·d-1 close to the river and an As concentration of 190 μg·L-1. KEY WORDS: arsenic, groundwater flow model, Datong Basin. 1 INTRODUCTION semi-arid areas (Masuda et al., 2010; Farooqi et al., 2007) or Natural As groundwater contamination is a serious prob- strongly reducing conditions in geologically sedimentary basin lem in many areas around the world, especially in Asian coun- (Smedley and Kinniburgh, 2002). Therefore, the major proc- tries (Berg et al., 2007; Charlet and Polya, 2006; Islam et al., esses of arsenic mobilization are most likely linked to As de- 2004; McArthur et al., 2004; Smedley et al., 2003; ven Geen et sorption from Fe oxides/oxyhydroxides and the reductive dis- al., 2003; Smedley and Kinniburgh, 2002; Nickson et al., 2000, solution of the Fe-rich phase in the aquifer sediments under 1998; Smith et al., 2000). In China, approximately 0.6 million reducing and alkaline conditions (Wang et al., 2009). Argilla- people are affected by groundwater As contamination, primar- ceous deposits have a broader range and higher average As ily in Shanxi, Inner Mongolia, Xinjiang and Taiwan (Yu et al., concentration than sandstones, reflecting the large proportion of 2007). Long-term intake of high-As groundwater has caused sulphide minerals, organic matter and clays (BGS and DPHE, endemic As poisoning in the Datong Basin, Shanxi Province 2001). Hence, Fe oxide/hydroxide reduction would be con- (Li et al., 2005; Guo et al., 2003). Our work has shown that the trolled by the biodegradation of organic matter and the increase elevated dissolved As concentrations are limited to depths of in alkalinity can further promote the competitive sorption be- -1 - 10–60 m (Xie et al., 2008), with a maximum of 530 μg·L in tween HCO3 and As (Duan et al., 2009). In fact, the hydrology this field site. Extensive studies have been made in the past ten condition can play an important role in As release. In recent years to elucidate the mechanism of As mobilization in years, many studies have demonstrated the effect of hydrody- groundwater. It was proposed that the arsenic in the Quaternary namic conditions on dissolved As distribution in the aquifer aquifer systems mainly originates from the Archean metamor- (Postma et al., 2007; Stute et al., 2007; Harvey et al., 2006; phic rocks and Mesozoic coal-bearing strata around the basin Klump et al., 2006). Benner et al. (2008) created a simple 2D (Pei et al., 2005; Guo and Wang, 2004). High As concentration groundwater flow model to elucidate the importance of the groundwater is typically associated with elevated pH in arid or hydrologic flow system on As release in Mekong Delta and suggested that the shallow sediments (upper 2–10 m of *Corresponding author: [email protected] fine-grained material) were important As sources to the under- © China University of Geosciences and Springer-Verlag Berlin lying aquifers. Klump et al. (2006) coupled 3H/3He groundwa- Heidelberg 2014 ter dating with conceptual flow modeling to propose that re-infiltrating irrigation water was the direct cause of As mobi- Manuscript received October 21, 2013. lization. All these efforts have shown the important impact of Manuscript accepted February 27, 2014. groundwater flow on the subsurface geochemistry and biogeo- Yu, Q., Wang, Y. X., Ma, R., et al., 2014. Monitoring and Modeling the Effects of Groundwater Flow on Arsenic Transport in Datong Basin. Journal of Earth Science, 25(2): 386–396, doi:10.1007/s12583-014-0421-y Monitoring and Modeling the Effects of Groundwater Flow on Arsenic Transport in Datong Basin 387 chemistry. Furthermore, Aziz et al. (2008) illustrated that the northeastern China, covering 82 500 km2 (Wang et al., 2009; hydrological processes must be considered to unravel widespread Wang and Shpeyzer, 2000). The mean annual rainfall is but spatially variable As enrichments in South Asian deltaic aq- 300–400 mm (mostly in July and August), the mean evapora- uifers through geophysical surveying. Vertical recharge from tion rate is above 2 000 mm, and the yearly average air tem- surface water bodies and precipitation would tend to dilute any perature is 6.5 ℃. The Sanggan River, which is almost dried in As that is released from the sediment at depth in reducing aqui- recent ten years, is the major surface water system in this area, fers (Horneman et al., 2004; BGS and DPHE, 2001). deriving from Guancen Mountain and flowing throughout the To date, the studies on high-arsenic groundwater in the basin from southwest to northeast. Datong Basin have been mostly focused on the geochemical For this study, we selected a site for detailed monitoring and biogeochemical processes controlling As transport in the (Fig. 1a). The field experimental site is 75 m×30 m in size and groundwater system. No systematical investigations were con- located 5 m south of the Sanggan River, in Shanyin City (Fig. ducted to discuss the linkage between As concentration and 1b). A network of 20 nested monitoring wells was deployed at groundwater flow paths in this area. Since studies of ground- the site. Each well has three screened parts, corresponding re- water flow are helpful to understanding the enrichment of As in spectively to the upper (996.3–997.8 m), intermediate the groundwater affected by natural or anthropogenic changes (989.8–992.8 m) and lower (983.8–987.8 m) portions of the in the hydrological cycle (Stute et al., 2007), a aquifer sands that are abbreviated as sand 1, sand 2 and sand 3 three-dimensional transient groundwater flow model with real- respectively (Fig. 1c). The wells were installed using percus- istic assumptions of hydraulic constants and boundary condi- sion drilling (500 mm diameter) and well tubes composed of tions of the geological structure was conducted in this work to PVC screen of 90 mm outside diameter, 4.5 mm thickness and reveal the relationships between groundwater dynamics and As 1 m length. After installation, wells were backfilled with sand concentrations in shallow contaminated groundwater systems. over the screened interval and capped with clay to the surface. The field site is composed of typical Quaternary alluvial 2 METHODS and lacustrine sediments. Three Quaternary sand aquifers sepa- 2.1 Field Site Description rated by three Quaternary clay layers occur in this site (Figs. 1c, The Datong Basin is located in a semi-arid region of 1d). The lacustrine deposits of sand 1, sand 2 and sand 3 are (b) B’ N (a) 30 1-4 2-4 3-4 4-4 5-4 20 1-3 2-3 3-3 4-3 5-3 Shanyin Country () Ym 4378000 A A’ P! 10 1-2 2-2 3-2 4-2 5-2 Sanggan River Sanggan 1-1 2-1 3-1 4-1 5-1 0 0 10 20 30 40 50 60 70 80 B Xm() Field Site !!!! (d) Xiaogeda N 4374500 B 3-1 3-2 3-3 3-4 B’ 1 008.8 River 1 003.8 Silt ! Monitoring wells Clay 998.8 Clay 1 Sanggan P Country Sand 1 Clay 2 Village 993.8 Sand 2 Sanggan river Clay 3 ! (m) Altitude 988.8 Sand 3 0 1 km 983.8 0 10 20 30 40 50 19656500 19660000 19663500 Ym() (c) N A 1-22-2 3-2 4-2 5-2 A’ 1 008.8 1 003.8 Sanggan River Clay 998.8 Clay Clay Clay 1 Sand 1 993.8 Clay 2 Altitude (m) Altitude Sand 2 Clay3 988.8 Sand 3 983.8 -20 -10 0 10 20 30 40 50 60 70 80 90 100 Xm() Figure 1. (a) The location of the Shanyin field experimental site (SY site); (b) plan view of the Shanyin field experimental site and the experimental wells; (c) hydrogeological cross section perpendicular to the Sanggan River; (d) hydrogeological cross section parallel to the Sanggan River. 388 Qian Yu, Yanxin Wang, Rui Ma, Chunli Su, Ya Wu and Junxia Li usually 1.5, 3 and 4 m thick, respectively; clay 1, clay 2 and Clay (ICP-MS) (Perkin Elmer ELAN DRC-e).