Hydrochemical characteristics and water quality of groundwater 1 in the thick loess deposits

: Water quality and quantity should be paid more attentions for regions with 20 arid climate and thick vadose zones since the limited groundwater cannot be 21 replenished rapidly once polluted. This study focused on the Loess Plateau of China to 22 investigate the geochemical mechanism affecting groundwater chemistry and to 23 calculate contribution rates of multiple sources to groundwater solutes. We employed 24 multiple methods (diagrams, bivariate analyses, hierarchical cluster analysis (HCA), 25 sodium adsorption ratio (SAR), water quality index (WQI), correlation analysis, 26 forward simulation) for the above purposes. We collected 64 groundwater samples in 27 the thick loess deposits in June 2018 (flood season) and April 2019 (dry season). The 28 average concentrations of cation were in the order of Ca 2+ > Na + > Mg 2+ > K + in the 29 flood season, and Na + > Ca 2+ > Mg 2+ > K + in the dry season. The order of anions 30 contents in the flood season and the dry season was HCO 3- > SO 42- > Cl - > NO 3- . The 31 major hydrochemical facies were Ca-HCO 3 and Ca·Mg-HCO 3 in the flood season, and 32 Na·Ca-HCO 3 ·SO 4 and Na-HCO 3 in the dry season, respectively. Most of the 33 groundwater (95% in the flood season and 96% in the dry season) was suitable for 34 drinking, and overall water quality (except samples F28 and D13) was acceptable for 35 irrigation. Mineral dissolution and cation exchange were important natural processes 36 affecting groundwater chemistry. A forward model showed that the contribution of 37 atmospheric input, anthropogenic input, evaporite dissolution, silicate weathering and 38 carbonate weathering to solutes in groundwater was 2.3±1.5%, 5.0±7.1%, 19.3±21.4%, 39 42.8±27.3% and 30.6±27.1% in the flood season, and 9.1±6.4%, 3.4±5.2%, 20.3±15.9, 40 56.6±23.2%, and 10.7±15.4% in the dry season, respectively. Although the overall 41 contribution of anthropogenic inputs was minor, it was the dominant source of solutes 42 for some groundwater samples. This study provides fundamental information for water 43 management in arid areas. 44


Sample collection and analysis
In this study, we focused on the unconfined groundwater that discharges into rivers 135 instead of getting recharge from rivers. The above-mentioned groundwater is generally 136 stored in high-altitudes areas where surface water is scarce and groundwater become 137 8 the only water source. The groundwater level in these areas is generally 10 m below the 138 ground surface. These groundwater samples exclude the impacts of surface water, thus 139 highlighting the impacts of anthropogenic activities. 140 We collected 64 groundwater samples in the thick loess deposits, among which 37 141 samples were collected during the flood season (August 2018) and 27 samples from the 142 dry season (April 2019). In order to eliminate the negative effects of stagnant water, the 143 wells were pumped for at least 10 minutes before sampling. All groundwater samples 144 were stored in plastic bottles and kept in a portable fridge. The samples were delivered 145 back to the laboratory by fast delivery at the end of each day to ensure that the 146 hydrochemical components can be determined in the next day. The temperature, pH, 147 total dissolved solids (TDS), electrical conductivity (EC) were measured by a portable 148 meter (HANNA, HI98130) in situ. Major cations (K + , Na + , Ca + , and Mg + ) were 149 determined by a coupled plasma-atomic emission spectrometry. Cl -, SO4 2-, and NO3 -150 were determined by ion chromatography (DIONEX ICS-1100, Thermal Fisher 151 Scientific, USA). The HCO3content was measured by titration with hydrochloric acid 152 (Table 1). 153 where Wi and wi are the weight and relative weight of the ith parameter, respectively 184 (Table 3)  contents of ions were in the order of Ca 2+ > Na + > Mg 2+ > K + and HCO3 -> SO4 2-> Cl -> 227 NO3 -. 228  is the same as the order of anion concentration in the flood season. As shown in Table  238 2, the standard deviation of major cations and anions was highly dispersive, indicating 239 that the chemical composition of groundwater exhibited a large spatial difference. Fig.  240 3 showed the Wilcoxon test results of major ions content in different seasons. As shown 241 in Fig. 3, the difference of most ions was not significant (p>0.05) except for Ca 2+ 242 (p=0.0078) and Na + (p=0.033), which implied the contents of Ca 2+ and Na + in 243 groundwater changed significantly in the two seasons. This is the reason why the order 244 of abundance of major cations is different in the flood season and the dry season.   the concentration of other water quality parameters exceeded their respective 289 acceptable limits. In the flood season, approximately 43%, 3%, 6%, 14%, 3%, and 3% 290 of groundwater samples had Ca 2+ , Na + , K + , NO3 -, Cl -, and TDS exceeding the 291 acceptable limits, respectively. In the dry season, 11%, 4%, 7%, 4%, and 4% of the 292 samples exhibited Ca 2+ , Na + , NO3 -, SO4 2-, and TDS exceeding the acceptable limits, 293 respectively. 294 The WQI values ranged from 12 to 132 (average 36) and from 11 to 149 (average 33) 297 in the flood season and the dry season, respectively (Table 1). Slight differences in 298 groundwater quality for domestic purpose were found in the two seasons. In the flood 299 season, about 81%, 14% and 5% of the samples were classified as excellent, good and 300 poor quality, respectively. Compared with the flood season, the percentage of excellent 301 quality water (89%) increased, while the percentage of good (7%) and poor quality (4%) 302 19 decreased in the dry season. 303

304
As shown in Table 1 flood samples and 10 dry samples were plotted in the C3-S1 zone, indicating moderate 322 quality for irrigation. Moreover, samples D13 and F28 were plotted in C3-S4 and S1-323 C3 zones, respectively, signifying unsuitable for irrigation owing to high salinity or 324 high sodium hazard. The Wilcox diagram (Fig. 5b)  ΔNO3accounted for 79%, 83%, and 88% of the study area, respectively, indicating that 348 the contents of TDS, Ca 2+ and NO3were generally higher in the flood season. 349 The Na + concentrations increased in most areas from the flood season to the dry season. 350 Especially, the largest increase of Na + concentrations (ΔNa + = 1185.0 mg/L) occurred 351 in Pucheng County, Shaanxi Province (sample D13) (Fig. 6c). ΔNO3was used to 352 determine the effect of anthropogenic activities on the spatiotemporal evolution (Fig.  353 6d). As shown in Fig. 6d 368 Gibbs plots (Gibbs, 1970) was used to illustrate three significant natural factors 369 controlling groundwater chemistry by plotting Na + / (Na + + Ca 2+ ) or Cl -/ (Cl -+ HCO3 -) 370 versus TDS (Fig. 7).

433
The contribution of halite and mirabilite dissolution can be confirmed by the scatterplot 434 of (2SO4 2-+Cl -) versus Na + (Sarin et al., 1989) (Fig. 9d). As shown in Fig. 9d,  435 approximately 75.7% of the samples in the flood season and 81.5% in the dry season 436 fell below the y = x line, signifying that Na + in groundwater may have other sources 437 such as cation exchange or silicate dissolution in addition to evaporite dissolution. 438

442
Cation exchange was considered a significant process to control groundwater chemistry 443 (Schoeller 1967). The chloro-alkaline indices (i.e., CAI1 and CAI2) were used to 444 interpret the occurrence of cation exchange in groundwater (Eq.11 and Eq.12) (Yong- which suggested that cation exchange between Ca 2+ and Mg 2+ in groundwater and Na + 451 and K + in aquifers was prevalent in the study area. Again, this indicated that cation 452 exchange played an important role for source of Na + in groundwater. 453

454
The calculation results of the contribution of different sources to the dissolved solutes 455 in groundwater in the two seasons were shown in Table 5. 456

508
Water quality is an important indicator for ensuring biological growth and socio-509 economic development, especially in arid regions such as the Chinese Loess Plateau. 510 In this study, the hydrochemical characteristics of dissolved major elements in 511 groundwater collected from the Loess Plateau were studied. The contents of Na + , Mg 2+ , 512 HCO3 -, and SO4 2in groundwater during the dry season were greater than those during 513 the flood season, while the contents of K + , Ca 2+ , Cl -, and NO3were lower than those 514 during the flood season. HCA and correlation analysis coupled with conventional 515 hydrochemical plots suggests that minerals dissolution and cation exchange are the key 516 factors in controlling groundwater chemistry. Moreover, anthropogenic activities (e.g., 517 agricultural activities) also have a certain impact on the formation of dissolved solutes 518 in groundwater, especially during the flood season. Most of the groundwater (95% in 519 the flood season and 96% in the dry season) was suitable for drinking, and overall water 520 quality was acceptable for irrigation. The calculation results based on the forward 521 model showed that the order of the contribution of different sources to dissolved solutes 522 in the flood season was silicate > carbonate > evaporite > anthropogenic input > 523 atmospheric input, while in the dry season was silicate > evaporite > carbonate > 524 atmospheric input > anthropogenic input. 525 Data availability 526 The datasets used and/or analyzed during this study are included in this published article. 527