Monitoring and Modeling the Effects of Groundwater Flow on Arsenic Transport in Datong Basin

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

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 carried 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 irrigation is the primary factor in determining the groundwater flow paths. Irrigation can not only increase 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()

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). 3, commonly rich in organic matter, are usually 1, 3 and 2 m thick, respectively. The upper alluvial silt layer is generally 10 m 2.4 Numerical Modeling thick and contains several thin clay lenses (Figs. 1c, 1d). The Numerical modeling was employed to further evaluate the bottom of sand 3 cannot be strictly fixed due to the limited bore- groundwater flow conceptualization. Using a data set including hole depth of only 25 m below the ground surface. geological structure, hydraulic constants, tube well information (numbers, locations, screen depth), transient water head records 2.2 Groundwater Levels for three observation wells with three different depths at the All the monitoring wells were used for groundwater level same site, precipitation and evapotranspiration, a transient measurements. To avoid the effect of drilling activities on three-dimensional groundwater flow model in undisturbed con- groundwater flow regime, monitoring work began in February ditions was created with MODFLOW (McDonald and Har- 2011, four months after the wells were completed. The water baugh, 1988) to simulate the saturated flow paths of level was measured monthly in each well using an electronic As-contaminated groundwater within the shallow aquifer of the water level tape from February to November 2011. Absolute riparian zone at the site over 270 days within the model domain elevations of the top of each well from the site were determined (Fig. 3a). The model domain was discretized into 75 columns using an Enhanced Global Positioning System (EGPS) static (x-direction), 30 rows (y-direction), and 10 layers (z-direction), survey technique. The observed temporal and spatial variations covering a horizontal distance of 75 m in the x-direction and 30 of groundwater level were shown in Fig. 2. The groundwater m in the y-direction and a vertical thickness of 24 m. The bot- level increases from January to May and then decreases from tom of the grid was set as a no-flow boundary condition. The May to October with a 0.5–0.6 m fluctuation (Fig. 2b), which uppermost layer was treated as a net recharge boundarybecause does not correspond with the pattern of local precipitation that is both irrigation and evapotranspiration are so intensive in this mostly concentrated in July and August. The most likely reason area that the vertical groundwater recharge should not be ne- is the effect of long-term irrigation activity. From April to June, glected. During the rainy season and irrigation season, the pre- large-scale irrigation together with flushing of saltwater and cipitation and irrigation return water vertically crossed the va- sluicing from the Dongyunlin reservoir that is located upstream doze zone and infiltrated into the aquifer with an annual net of the Sanggan River (Fig. 1a), can cause ascent in groundwater recharge rate of 80 mm (Dong et al., 2008). The time-varying level. The major irrigation period occurs in May in Datong Basin, constant-head (CHD) package was used to define the boundary about two weeks of duration time. Saltwater widely distributed conditions of the four sides of the model domain (plan view, during the irrigation period in the centre of the basin, especially Fig. 3a). Due to the sparse monitoring intervals, however, the around this field site; and its flushing will lift the groundwater lateral boundary conditions could not be represented accurately level. Sluicing from Dongyulin Reservoir along the Sanggan (a) N River, another import source of irrigation water, flow through this 30 field site, would directly raise the groundwater level although a 1 006.48 1 006 36 very short irrigation period. Unfortunately, the information of 1 006.40 1 006.44 20 1 006 31 1 006 32 1 006 30 . . saltwater flushing amount and sluicing amount of Dongyulin . . (m) Reservoir had not been correctly determined up till now. It 10 Y should be noted that large numbers of irrigation pumping wells Sanggan River were distributed in the basin, but no one located around this field 0 0 10 20 30 40 50 60 70 75 site. A clear trend of increase in groundwater level from the X (m) (b) 1 006.5 Sanggan River can be also seen from Fig. 2b, except the Well 1-2, Well 5-2D suggesting the groundwater flows toward the river as a whole at Well 4-2D 1 006.4 the site. Well 1-2, close to the river, has a higher groundwater Well 3-2D Well 2-2D levels than those of wells 2-2 and 3-2, it may be related to the 1 006.3 Well 1-2D river, which potentially affects the groundwater level even though dried up almost all the year, except during the short irri- 1 006.2 River elevation gation period. 1 006.1 2.3 Groundwater Chemistry Data Collection

Groundwater level (m) 1 006.0 All the monitoring wells were sampled once a month from February to November 2011 using a peristaltic pump at a flow 1 005.9 rate of 800 mL·min-1. The oxidation-reduction potential (ORP), pH, and temperature were measured on-site using HACH meters 1 005.8 Feb. Mar. Apr. May. Jun. Jul. Aug. Sep. Oct. Nov. in a flow-through cell. The redox-active water quality parameters, Date - such as HS content, were determined by a HACH DR2800 spec- Figure 2. (a) Contour map of water level observed in May trophotometer. Groundwater samples filtered through a 0.45 μm 2011; (b) temporal change in the deep water level at wells Millipore filter were stored in 50 ml polyethylene terephthalate 1-2, 2-2, 3-2, 4-2 and 5-2, which are 5, 10, 20, 45 and 80 m bottles and acidified with ultra-purified HNO3 to pH<2 for use in away from the Sanggan River, respectively (D stands for the As analysis by inductively coupled plasma mass spectrometry deep groundwater).

Monitoring and Modeling the Effects of Groundwater Flow on Arsenic Transport in Datong Basin 389

N estimate unknown hydrologic parameters. The Calibrated CHD boundary 30 model was then applied to estimate the various fluxes in and (a) out of different formations.

20 2-3 3-3 4-3 2.5 Simulation Conditions (m)

Y Based on the hydrogeological setting described above, a 10 2-2 3-2 4-2 simplified 10-layer system was used to describe the hydraulic

CHD boundary CHD CHD boundary CHD conductivity field (Fig. 3b) with the upper silt layers (L1, L2, CHD boundary 0 L3, L4) and the underlying three sand layers (L6, L8, L10) 0 5 15 25 35 45 55 65 75 being separated by three clay layers (L5, L7, L9). Using typical X (m) N sand aquifer media and clay aquitard media with a relatively Net recharge boundary 1 008.8 high macroscopic uniformity, all the material properties, in- (b) L1 cluding the three-dimensional distribution of hydraulic conduc- 1 006.4 tivity, porosity and specific yield, were initially sourced from Silt L2 1 004.4 the widely accepted empirical values (Konikow and Neuzil, L3 2007; Fetter, 2001; Thangarajan et al., 1999) (Table 1). The 1 002.4 L4 horizontal hydraulic conductivities were assumed to be ten 1 000.3 times greater than those in the vertical direction and were Clay 1 L5 998.8 Sand 1 L6 treated as fitting parameters in the subsequent calibration. The 997.3 groundwater level data for February 2011 were used as the Clay 2 L7 initial head within the numerical model. And the direction of 994.3 Altitude (m) Altitude horizontal groundwater flow is from south to north, with very Sand 2 L8 low hydraulic gradient. 991.8 boundary Constant-Head Clay 3 L9

Constant-Head boundary Constant-Head 989.3 3 RESULTS AND DISCUSSION 3.1 Numerical Model Calibration Sand 3 L10 To verify the applicability of the model, calibration and 984.8 sensitivity analyses were performed by checking the agreement No flow boundary between the calculated and observed results in Fig. 4. The hy- Figure 3. Model domain in plan view (a) and cross section draulic conductivity of the medium is the most sensitive factor (b) with 75 m (16 cells)×30 m (6 cells) ×24 m (10 layers). in this model. Figure 4 compares the observed and simulated groundwater head at four observation well groups. Almost all in fact. Accordingly, measured monthly groundwater level data the calculated groundwater head variations are consistent with from the existing wells within the site were used to linearly the observed values with 0.6 m fluctuation, suggesting the rea- interpolate the time-varying groundwater head at the four lat- sonability of the hydraulic parameters and the reliability of this eral model boundaries. flow model. The Sanggan River is not considered in this groundwater The initial hydraulic conductivity of the clay, silt and sand flow model due to a lack of surface water level records (as layers was 8.64×10-4, 0.5 and 15 m·d-1, respectively. But the mentioned above, the river has been dry for most the time in calibration results show that a better fit to the observed seasonal recent ten years). The impact of groundwater pumping for irri- trends in the well levels was obtained using values of 0.001, 1 gation on the flow model can also be ignored because there are and 10 m·d-1, respectively. no pumping wells nearby. All the data used in this study, in- Generally speaking, a remarkably good fit between the cluding the temporal groundwater level and the spatial and time simulated seasonal variations and the observed trends was - variations of ORP, HS , and As concentrations, were collected achieved using the stratigraphic structure and conceptual model. by our manual monitoring in the field from February to No- However, the modeling results remain under-constrained. The vember 2011. uncertainty of the hydraulic conductivity, in particular, has a The model is used to estimate the fluxes among the sand, strong impact on the transient flow model, which prevents the the silt and the clay formation. First, the model was used to precise determination of groundwater fluxes in the flow model.

Table 1 Hydraulic properties of aquifers used in model simulations

Hydraulic conductivity Specific storage Specific yield Effective Total -1 a -1 b a a a K(m·d ) Ss (S ) Sy porosity porosity Silt 0.5 1×10-4 0.16 0.22 0.35 Clay 8.64×10-4 5×10-4 0.01 0.2 0.45 Sand 15 1×10-4 0.22 0.25 0.25

a. Fetter, 2001; b. Konikow and Neuzil, 2007; Thangarajan et al., 1999.

390 Qian Yu, Yanxin Wang, Rui Ma, Chunli Su, Ya Wu and Junxia Li

Furthermore, the uncertainty in the aquifer thickness of sand 3, cussed in the following part). From February to March and which is estimated to be at 25 m below the ground surface due from October to November, horizontal groundwater velocities to the borehole depth, may also introduce some uncertainty into were moderate, from 0.35 to 0.44 m·d-1. the model results. Neglecting the local hydraulic conductivity Groundwater flow was noticeably unsystematic within the heterogeneity, the idealized conceptual model has been proven 20 m range from the riverbed and then became orientated be- to be effective on the macroscopic scale. More detailed work is tween 20 and 60 m. This phenomenon is most likely related to still needed to take into account the heterogeneity of sediments the Sanggan River, which may affect the groundwater flow, that can cause complex variations in flow paths of groundwater although the river was almost dried up throughout the year and on smaller scale of modeling. the affected range was 20 m away from the riverbed. At locations further away, the impact of the river is insignificant. Gen- 3.2 Horizontal Groundwater Flow Velocities erally speaking, the model domain should be wider than the Figure 5 shows the simulated monthly horizontal ground- study area to avoid boundary artificially effects for the flow water flow velocity variations at the depth of 13 m from Febru- field (Nakaya et al., 2011), which does not apply to our ex- ary to November 2011. The horizontal groundwater flow ve- perimental site because there are not any observation wells locities exhibited marked differences between the around the site. Boundary effects are therefore inevitable, non-irrigation and irrigation periods. Between April and June in which may explain the abnormity occurring in the southeastern the irrigation reason, the horizontal groundwater velocity was corner of the model, especially in March and July. The domi- -1 relatively low, with a maximum value (Vmax) of 0.38 m·d . nant groundwater flow direction in different seasons was from Irrigation and salt flushing activities can cause abundant water south to north and toward the Sanggan River, indicating that to accumulate on the ground surface. Part of this accumulated irrigation did not change the macroscopic flow directions. water is transported upward to the atmosphere via evapotran- Irrigation has not only caused disturbances of groundwater spiration, and the rest vertically infiltrates the subsurface to the flow, such as decrease of groundwater flow rate and increase of aquifers by passing through the vadose zone. Moreover, the groundwater level, but also changed groundwater fluxes among Dongyulin reservoir, as another important source of irrigation the sand, clay and silt formations, which in turn promotes the water, sluiced out water into the Sanggan River during the irri- evolution of groundwater chemistry and solute transport within gation season, which may affect the regime of groundwater the groundwater system, as discussed below. close to the riverbed, although the sluicing lasts only for two weeks. Slightly higher horizontal groundwater velocity up to 3.3 Evaluation of Groundwater Flux and Its Impact on 0.49 m·d-1 was observed in the rainy season, due to precipita- Hydrochemistry tion recharge from July to September, which can well be used The model was used to examine the net groundwater to interpret the time-variations of vertical fluxes (Fig. 6, dis- fluxes among sand, clay and silt formations. Most of

1 006.5 Simulated (S) Observed (S) Simulated (S) Observed (S) Well 2-2 Simulated (M) Observed (M) 1 006.5 Well 3-2 Simulated (M) Observed (M) 1 006.4 Simulated (D) Observed (D) Simulated (D) Observed (D) 1 006.4 1 006.3 1 006.3 1 006.2 1 006.2 1 006.1 1 006.1 1 006.0 1 006.0 1 005.9 1 005.9 (m) level Groundwater

Groundwater level (m) level Groundwater 1 005.8 1 005.8 Feb.Mar.Apr.May.Jun.Jul.Aug.Sep.Oct.Nov. Feb.Mar.Apr.May.Jun.Jul.Aug.Sep.Oct.Nov. Date Date 1 006.6 1 006.5 Well 3-3 Simulated (S) Observed (S) Well 4-2 Simulated (S) Observed (S) Simulated (M) Observed (M) Simulated (M) Observed (M) 1 006.4 Simulated (D) Observed (D) 1 006.5 Simulated (D) Observed (D) 1 006.4 1 006.3 1 006.3 1 006.2 1 006.2 1 006.1 1 006.1 1 006.0 1 006.0

Groundwater level (m) level Groundwater Groundwater level (m) level Groundwater 1 005.9 1 005.9 1 005.8 Feb.Mar.Apr.May.Jun.Jul.Aug.Sep.Oct.Nov. Feb.Mar.Apr.May.Jun.Jul.Aug.Sep.Oct.Nov. Date Date Figure 4. Model calibration results for the tube wells (S is shallow, M is intermediate, D is deep).

Monitoring and Modeling the Effects of Groundwater Flow on Arsenic Transport in Datong Basin 391

VZ-1 VZ-1 max=0.4 m·d , = -13 m (a) max=0.44m·d , = -13 m (b) 30 30

20 20

(m)

Y 10 Y 10

0 0 0 10 20 30 40 50 60 70 75 0 10 20 30 40 50 60 70 75 X (m) X (m) VZ-1 VZ-1 ma =0.33m·d , = -13 m (c) max=0.38m·d , = -13m (d) 30 30

20 20

(m)

Y Y 10 10

0 0 0 10 20 30 40 50 60 70 75 0 10 20 30 40 50 60 70 75 X (m) VZ-1 VZ-1 ma =0.32m·d , = -13 m (e) max=0.49m·d , = -13 m (f) 30 30

20 20

(m)

Y 101 Y 10

0 0 0 10 20 30 40 50 60 70 75 0 10 20 30 40 50 60 70 75 X (m) X (m) VZ-1 VZ-1 max=0.45m·d , = -13 m (g) max=0.46m·d , = -13 m (h) 30 30

20 20

(m)

Y 10 Y 10

0 0 0 10 20 30 40 50 60 70 75 0 10 20 30 40 50 60 70 75 X (m) X (m) VZ-1 VZ-1 max=0.35m·d , = -13 m (i) max=0.4m·d , = -13 m (j) 30 30

20 20

(m)

Y 10 Y 10

0 0 0 10 20 30 40 50 60 70 75 0 10 20 30 40 50 60 70 75 X (m) X (m) Figure 5. Horizontal groundwater flow velocities simulated at z= -13 m in February (a), March (b), April (c), May (d), June (e), July (f), August (g), September (h), October (i) and November (j). precipitation occurs in July and August at Datong. During the infiltration of precipitation in the rainy season and irrigation irrigation season (mostly in May), the net groundwater ex- return water and salt flushing water in the irrigation season. change varies extensively with time (Fig. 6). The simulated Consequently, a significant difference in the groundwater hy- values of the vertical net groundwater fluxes for L1, L2, L3, L4, drochemistry such as dissolved sulfide (HS-) concentration and L5 and L6 (L1 to L4 are silt layers, L5 is a clay layer and L6 is oxidation reduction potential (ORP) exists between irrigation a sand layer) increased steeply with time from March to July and non-irrigation period (Fig. 7, Table 2), During the irrigation and then decreased gradually from July to November, fluctuat- period, the sulfide concentration and ORP in groundwater in- ing widely from 0 to 0.2 m3·d-1 (Fig. 6a). Vertical downward creases sharply from 0.4 μg·L-1 to 5.2 μg·L-1 and decreases groundwater flux (positive) implies that frequently vertical sharply from -10 to -140 mV, respectively. Oppositely, during groundwater exchange is always present during the modeling the non-irrigation period, these two indexes increase from 2 to period (a positive vertical net groundwater flux value indicates 12 mg·L-1 and decrease from -60 to -135 mV, respectively. downward flux, whereas a negative value indicates upward The relationship between vertical net groundwater flux and flux). Vertical downward water exchange would promote aero- aquifer depth is readily obtained by comparing the vertical net bic conditions and be heavier in organic matter (Harvey et al., groundwater flux of every pair of neighboring layers from L1 to 2006) and this process could be accelerated by the downward L6. With increasing depth, the vertical recharge was diminished

392 Qian Yu, Yanxin Wang, Rui Ma, Chunli Su, Ya Wu and Junxia Li and the groundwater exchange also became increasingly sluggish has been consensus that the reductive dissolution of Fe oxides until the saturated zone (L4 to L5 and L5 to L6) where it was is an important mechanism in As release into the aquifers (Is- nearly zero. This behavior indicates that much higher water ex- lam et al., 2004; McArthur et al, 2004; Nickson et al., 2000). change often occurs approximately 5 m below the ground surface, Our work has provided evidences that arsenic-rich sedimentary whereas a relatively low exchange rate is present in deeper layers minerals and organic-rich lacustrine clay may be the main As due to the heterogeneous hydraulic conductivity and the fine sources in the shallow aquifers in Datong Basin (Xie et al., material lens, as reflected by the obvious spatial variations of HS- 2012a). A series of geochemical and biogeochemical processes, and ORP in different depths (Fig. 7). such as the adsorption or desorption of Fe and Mn oxyhydrox- Like vertical groundwater flux, the simulated horizontal ides under high-pH/low-ORP conditions, the oxidation of or- net groundwater flux for an AA’ profile of 17.5 and 57.5 m in X ganic matter by microbes in an anaerobic environment, the direction (Fig. 6b) first increased, peaking in July, and then reductive dissolution of Fe oxides (Xie et al., 2009) and decreased, In this model, positive values indicate groundwater long-term water rock interactions with exchangeable arsenic in exchange from A to A’, whereas negative values indicate ex- extremely slow groundwater flow aquifers (Guo et al., 2003) change from A’ to A. In the fluctuation range from 0 to 12 would lead to a relatively high-arsenic groundwater system in m3·d-1, almost all the horizontal net groundwater flux values this region. Additionally, the formation of sulfide (Lowers et al., were positive, clearly indicating that the groundwater flow 2007; Kirk et al., 2004) and oxidation of sulfide (Peters and direction was from A to A’. The consistency between the hori- Blum, 2003; Schreiber et al., 2000; Mandal et al., 1996) can zontal water exchange and groundwater flow direction explains remove and transport As into aquifers. The oxidation of organic why the groundwater chemistry varies along the flow path (Fig. matter in the oxidizing environment can produce a reducing 7). environment. Under this condition, Fe oxides/oxydroxidate and An estimation of vertical and horizontal water exchange sulfate will be reduced at low ORP, producing interrelated can provide some new insights into groundwater chemistry and high-solubility and low-valence ions (Xie et al., 2009). All the impact of hydrogeology on solute transport. Downward these processes can result in As release into groundwater. groundwater exchange occurs continually among layers from L1 to L6 throughout the simulation periods. Precipitation, irri- 3.4 Linking Arsenic and Groundwater Flow gation return water and salt flushing water travel downward New insights are provided into the relationships between through the vadose zone (L1), silt layers (L2, L3, L4), clay groundwater flow and water chemistry by mapping the simu- layer (L5) and nearly horizontally across the sand aquifer (L6) lated groundwater flow field on As concentrations across the to the river as is the case for most hydrogeological settings. aquifer (Fig. 7) in the Datong Basin, although the results should Therefore, the dissolved sulfide concentration increases with be judged with caution, given the idealized nature of our simu- depth from the surface to 15 m below and then increases along lation work. Along the flow path, the As concentration in- the flow paths. It is noteworthy that although vertical water creased with distance. However, a steeper gradient in As con- exchange exists in the aquifers, it is much lower than that in the centration was observed in the vertical direction, with the As horizontal direction which dominates the groundwater flux and concentration varying from 0 to 190 μg·L-1 over a depth of 20 has a primary control on hydrochemistry and solute transport in m (in October). The highest As concentration is located be- the groundwater system. tween 20 and 25 m below the ground surface (zone 3 and zone The variations in HS- concentrations and ORP values 4, Fig. 7), which agrees with the research of Pei et al. (2005), likely lead to As transport in the groundwater system. Advec- who reported that the high As section was located from 20 m to tion, dispersion and reactive transport of As occurs along the 40 m below the surface in Shanyin City. An increase in As con- flow paths. The spatial distributions of dissolved HS- concen- centration along the vertical profile can be interpreted as a re- tration, ORP values and As concentrations match well with flection of increases in As concentration along a flow path each other (Fig. 7, Table 2): the high As concentration zone (Benner et al., 2008). usually has a low ORP value and high HS- concentration. There

0.20 12 (a)L1 to L2 (b) X=17.5 m L2 to L3 X=57.5 m L3 to L4 8 0.15 L4 to L5 L5 to L6

3-1 4 3-1 0.10 0

Rate (m ·d )

Rate (m ·d ) 0.05 -4 0.00 -8 Mar.Mar Apr.Apr May.Jun.MayJun Jul.Jul Aug.AugSep.Sep Oct.Oct Nov.Nov MarMar.Apr. May.Jun.MayJun Jul.Jul Aug.Sep.Oct.Oct Nov.Nov Date Date Figure 6. (a) Net vertical groundwater flux from L1 to L6 (positive values indicate movement downwards through the model layers); (b) net horizontal groundwater flux from L6 to L10 at X=17.5 and 57.7 m, respectively (the positive values indicate movement from south to north whereas the negative values indicate movement from north to south in the vertical view of the model).

Monitoring and Modeling the Effects of Groundwater Flow on Arsenic Transport in Datong Basin 393

(b) Non-irrigation season (October) (a) Irrigation season (May) N N Zone 3-2 4-2 2-2 3-2 A 2-2 A’ Zone A 4-2 A’ As (0-10) 1 008.8 As (0-10) 1 008.8 Eh (-10 - -90) 1 Eh (-60 - -90) 1 HS (0.4-2.5) HS (2-7) 1 003.8 1 003.8 Zone 1 As (10-18) As (10-100) 998.8 Zone 1 Eh ( -90- -100) 2 998.8 Eh (-90 - -100) 2 HS (2.5-2.8) HS (7-8.5) 993.8 993.8 As (18-38) Zone 2 As (100-170)

Altitude (m) Altitude Eh ( -100- -130) 3 Eh (-100 - -115) 3 988.8 (m) Altitude 988.8 Zone 2 HS (2.8-3.4) HS (8.5-10) Zone Zone Zone Zone 3 4 3 As (38-46) 4 As (170-190) 983.8 Eh ( -130- -140) 4 983.8 4 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 Eh (-115 - -135) HS (3.4-5.2) X HS (10-12) X (m) (m) Figure 7. Simulated flow field overlaid with As concentration in irrigation season (May) and non-irrigation season (October) in 2011.

Table 2 Seasonal variation in groundwater chemistry from the SY site

Sample ID 1-2S 2-2S 3-2S 4-2S 5-2S 1-2M 2-2M 3-2M 4-2M 5-2M 1-2D 2-2D 3-2D 4-2D 5-2D ORP (mV) Mar. 2011 - -18.7 -94.1 -17.1 -7.0 - -81.4 -177.8 -138.5 -34.3 -121.2 -139.4 -153.1 -117.7 -86.7 Apr. 2011 - - -82.2 - -112.1 - -112.3 -141.3 -160.2 -57.9 -108.7 -83.5 -127.4 -91 -91.2 May. 2011 -19.4 -6.6 -106.8 -29.5 -101.4 -7.7 -125.4 -141.1 -158.2 -104.5 -107.1 -129 -130.5 -102.6 -100.3 Jun. 2011 -56.2 -96.6 -92.2 -91.5 -164.1 -87.3 -148.0 -154.3 -199.4 -153.0 -108.2 -127.8 -138.9 -108.1 -86.2 Jul. 2011 -192.1 -125.1 -122.2 -143.3 -129.6 -171.5 -153.7 -201.7 -191.5 -156.2 -183.2 -112.9 -144.1 -95.1 -120.6 Aug. 2011 -195.4 -194.3 -124.2 -209.2 -212.8 -161.6 -174.8 -36.4 -211.3 -218.9 -194.5 -182.1 -64.5 -179.2 -207.5 Sep. 2011 -186.9 -174.7 -149 -170.5 -209.9 -193.9 -147.8 -187.3 -152 -195 -180.9 -177.8 -190.7 -149.7 -187.3 Oct. 2011 -85.7 -91.9 -61.1 -74.1 -74.9 -82.9 -104.2 -100.3 -97.1 -114 -105.1 -134.9 -114.4 -84.6 -106.1 Nov. 2011 - - -79.6 - - -93.6 - -113.2 - - -109.3 -134.1 -120.1 - -111.2 HS- (μg·L-1) Mar. 2011 - - 3 - 1 - - - 3 7 6 - 3 3 6 Apr. 2011 - 8 1 1 2 - 1 2 3 2 4 5 4 3 9 May. 2011 1 3 1 1 3 2 2 5 2 2 2 4 4 2 5 Jun. 2011 2 2 4 2 7 3 4 4 4 3 4 3 2 3 4 Jul. 2011 14 6 17 6 4 2 6 16 20 7 12 7 6 5 6 Aug. 2011 5 6 3 22 5 4 4 3 1 4 3 2 4 - 4 Sep. 2011 11 2 3 6 16 7 - 5 - - 11 1 6 - - Oct. 2011 5 2 3 6 5 4 8 9 5 7 7 12 10 8 9 Nov. 2011 - 4 2 2 9 3 6 10 1 4 1 2 2 1 5 As (μg·L-1) Mar. 2011 5.7 6.2 9.0 5.8 9.2 10.2 2.6 6.6 6.5 12.1 23.5 11.1 35.2 9.3 23.0 Apr. 2011 ------May. 2011 4.2 3.5 6.0 2.8 8.6 7.7 4.7 4.7 3.0 6.8 38.9 46.3 21.7 10.9 25.1 Jun. 2011 16.3 15.0 14.8 16.7 39.0 32.5 5.5 4.1 12.2 69.9 48.4 74.0 50.9 13.6 14.9 Jul. 2011 8.2 8.1 13.0 14.7 19.9 17.1 6.1 6.8 6.0 19.2 94.5 142.0 69.8 19.5 125.0 Aug. 2011 8.3 11.9 12.4 9.8 12.9 15.3 6.5 6.2 4.8 38.1 111.0 159.0 41.6 38.3 117.9 Sep. 2011 2.7 7.9 15.8 8.8 15.6 14.2 - 4.0 0.3 85.8 120.2 160.5 93.8 54.6 162.0 Oct. 2011 4.2 4.0 25.5 8.5 14.6 8.7 6.0 10.5 3.5 108.6 133.1 181.5 110.0 62.4 165.0 Nov. 2011 12.5 11.7 28.8 15.9 26.3 18.9 13.2 13.9 8.6 111.9 107.3 183.0 105.3 68.0 147.3 “-” no data.

394 Qian Yu, Yanxin Wang, Rui Ma, Chunli Su, Ya Wu and Junxia Li

Results of temporal monitoring of the groundwater As from the Sanggan River, the As horizontal exchange is 111.2, concentration revealed seasonal variation at the field site. In 130.8, 327.0, 784.9 and 883.0 mg·d-1, respectively. Likewise, October, which is during the non-irrigation season, the As con- when X=57.5 m, the As horizontal exchange is 257.5, 309.0, centration had a wide distribution (10 μg·L-1 to 100 μg·L-1; zone 721.0, 1 081.6 and 1 390.6 mg·d-1, respectively, based on a 2, Fig. 7b) and higher than that in May of irrigation season (10 maximum net horizontal water exchange of 10.30 m3·d-1 to- to 18 μg·L-1; zone 2, Fig. 7a). This lag difference is likely to be wards the Sanggan River and As concentrations of 25.0, 30.0, related to irrigation and salt flushing activities. Many studies 70.0, 105.0 and 120.0 μg·L-1 in L6, L7, L8, L9 and L10, re- have reported that groundwater irrigation could trigger As mo- spectively. Furthermore, the maximum value of As enriched in bilization in aquifers (Nakaya et al., 2011; Klump et al., 2006; the riparian zone is roughly estimated to be 1 706.2 mg·d-1 for a Stigter et al., 2006). Both Guo et al. (2013) and Han et al. (2013) horizontal water exchange of 8.98 m3·d-1 close to the river and have documented that higher As concentration occurred during an As concentration of 190 μg·L-1. Although these estimation the higher groundwater level seasons in the Hetao Basin and results of vertical and horizontal As exchange are rough, the Yinchuan Basin. Large-scale groundwater irrigation has been amount of As transported from the groundwater system to the implemented in Datong Basin since the 1980s (Zhang and Zhao, riverbed is remarkable. And the results highlight the risks of 1987). Although no detailed irrigation data or systematical test transport of toxic substances from groundwater system to the of As content in surface water have been reported, long-term river, and vice versa. More comprehensive and detailed moni- high-As groundwater irrigation was suspected to be responsible toring work is therefore urgently needed to better characterize for As mobilization at Datong by Xie et al. (2012b) who used and control the risks for the sake of ecosystem health and environmental isotopes (δ18O and δ2H) and Cl/Br ratios to safety. demonstrate that leaching and mixing are the dominant processes affecting the distribution of high-arsenic groundwater in 4 CONCLUSIONS Datong Basin. A simple numerical model has been developed considering Therefore, the possible mechanisms of As transport in the the hydrology of Shanyin field experimental site on a local aquifer can be discussed within the framework of groundwater scale to identify the principal controls on As transport within dynamics. The CM-B (conceptual model B) (Nakaya et al., the groundwater-flow system. Large variations of both hori- 2011) can be used to account for this relationship, in which As zontal groundwater flow velocities and net groundwater fluxes is released over the entire flow path where As is concentrated in indicate the effect of irrigation in As release, which not only the aquifer sediments. First, in the process of downward increases the groundwater lever and reduces the horizontal movement of irrigation return water and salt flushing water, groundwater flow velocity but also promotes the vertical water oxygen and organic matter are carried into the aquifers, which exchanges among different formations. The vertical net can not only oxidize the dissolved As in the vadose zone but groundwater fluxes of L1 to L2, L2 to L3, L3 to L4, L4 to L5 also desorbed As into the groundwater. Second, the horizontal and L5 to L6 and horizontal net groundwater fluxes of L6, L7, groundwater flux plays a dominant role since the vertical L8, L9 and L10 suggest that vertical infiltration is likely the groundwater flux is too small to be neglected within the satu- primarily control of As transport in the vadose zone; however, rated zone. Horizontal water exchange may also cause As dis- the horizontal exchange is dominant in controlling As migra- solution and release into the groundwater or promote the trans- tion within the sand aquifers. Irrigation return water and port of dissolved As toward a more reductive environment. flushed saltwater travel vertically downward through the va- Consequently, the As concentration increases along the flow dose zone (silt layers) and clay 1 layer and then almost hori- paths over the depth from 17 to 24 m below the ground surface. zontally through the sand aquifers to the river. Approximately 1 706.2 mg of As could be exchanged in the ripairan zone per 3.5 Quantitative Estimation of As Exchange in Aquifer day. The impact of such an exchange of arsenic on ecosystem System health and safety needs more detailed investigation and as- Based on the mechanism of As transport in groundwater sessment. Importantly, this model provides a physical frame- discussed above, the amount of As exchanged among different work within which the temporal and spatial changes in formations can be estimated using the results of net groundwa- groundwater geochemistry can be interpreted. Increases in the ter fluxes calculated by the conceptual flow model. Given the dissolved As and HS- concentrations and ORP values with maximum net vertical water exchange of 0.180 0, 0.072 7, depth and flow paths were observed, indicating that the combi- 0.022 7, 0.000 3 and 0.000 6 m3·d-1 downward from L1 to L2, nation of vertical infiltration in the vadose zone and horizontal L2 to L3, L3 to L4, L4 to L5 and L5 to L6, respectively, and water flow in sand aquifers may be the major processes con- dissolved As concentrations of 2.0, 4.0, 5.0, 6.0 and 10.0 μg·L-1, trolling As migration in the groundwater-flow system. respectively. The net vertical As exchange downward from L1 The groundwater flow model provides some new clues to L2, L2 to L3, L3 to L4, L4 to L5, L5 to L6 is 0.360 0, about the hydrogeochemistry of arsenic. The groundwater flow 0.290 8, 0.113 5, 0.002 0 and 0.005 6 mg·d-1, respectively. The regime proves to be an important control on the spatial vari- horizontal As exchange can also be calculated. Using the ability of the dissolved As concentration in the shallow aquifers, maximum net horizontal water exchange of 6.54 m3·d-1 towards as well as on relevant geochemical and biogeochemical proc- the Sanggan River and As concentrations of 17.0, 20.0, 50.0, esses. Although the underlying mechanism needs better under- 120.0 and 135.0 μg·L-1 in L6, L7, L8, L9 and L10, respectively, standing, the major reason for such a control is the effects of at the profile of X=17.5 m where X is the horizontal distance groundwater dynamics on the kinetics of As mobilization and

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