Groundwater Hydrochemical Characteristics in a Plain River Network Region: Effects of Dissolved Organic Carbon and Possible Traceability of Pollution Sources

The chemical composition of groundwater indicates the water quality and provides useful 23 information for identifying pollution sources. The aim of this study was to explore the effects 24 of dissolved organic matter inputs on the ionic composition of groundwater and identify ions 25 that can be used as indicators of pollution sources. Descriptive statistics, a Piper diagram, a 26 Gibbs plot, major ion ratios, and Pea rson’s correlation coefficients were used to analyze the 27 chemical data of 40 groundwater samples collected from the shallow aquifer surrounding 28 Lake Taihu. The results showed that the water quality index values of most sampling points 29 were less than 50 (excellent water quality), except for one point in the southeast direction of 30 the lake basin (good water quality). The dominant hydrochemical type of groundwater was 31 Ca – Mg – HCO 3 type, and rock dominance was the major mechanism controlling the 32 groundwater chemistry. With an increasing concentration of dissolved organic carbon, the 33 Na + , Mg 2+ , and HCO 3  concentrations all showed a sharp increase followed by a slow 34 increase, while the NO 3 – concentration initially decreased sharply and then decreased slowly. 35 The K + concentration was positively correlated with total dissolved nitrogen and phosphorus, 36 nitrate, As, and Cd concentrations (p < 0.05). The Ca 2+ , Na + , Mg 2+ , Cl  , HCO 3  , and CO 32  37 concentrations were all positively correlated with Pb concentration (p < 0.001). The results 38 indicated that high organic matter inputs can directly or indirectly change the hydrochemical 39 type of groundwater, and K + can be used as a tracer ion for the sources of As and Cd in 40 groundwater in the study area.


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Groundwater is an important water resource and has become a vital reserve source of 79 domestic water, agricultural irrigation, and industrial water. Particularly in arid and semi-arid the Eastern Coal Geology Corporation found that the groundwater in Hunchun Basin was 125 polluted by Fe, Mn, and NO3  -N. Subsequently, Woo et al. (2000) showed that the Cd and F 126 concentrations in groundwater in the same area also exceeded the standard levels, with 127 groundwater pollution often associated with anthropogenic activities. 128 Lake Taihu is often referred to as 'the land flowing with milk and honey' in China. This 129 lake basin is a typical plain river network region that includes the third largest freshwater lake 130 in China and nourishes more than 20 million people (Qin et al. 2007). Owing to the high 131 intensity of industrial and agricultural production, rivers and lakes in this region are facing 132 serious problems from eutrophication. Therefore, analyzing the groundwater quality and 133 identifying potential pollution sources in this region is imperative for protecting water 134 resources. Therefore, the aim of this study was to: investigate whether organic matter inputs 135 from recharge sources around Lake Taihu affect the ionic composition of shallow 136 groundwater, and explore whether specific ions can be used as tracers to indicate the sources 137 of pollutants in groundwater. The Lake Taihu Basin is located in the middle and lower reaches of the Yangtze River, with 142 a mean water depth of 1.9 m (Qin et al. 2007 In August 2018, groundwater samples were collected from 40 wells located in the northeast 147 (NE), northwest (NW), southeast (SE), and southwest (SW) directions of Lake Taihu (Fig. 1).

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The depth of these wells was approximately 5 m, and the sampling depth was 0.5-1 m below 149 the water surface. Ten sampling points were selected in each direction, and the distance from 150 the sampling point to the shore of Lake Taihu was in the range of 0-43.09 km.  Major cations (K + , Ca 2+ , Na + , Mg 2+ ) and heavy metal(loid)s (Cr, Cu, Zn, As, Cd, and Pb) 160 were determined by inductively coupled plasma-mass spectrometry (ICAP Qc; ThermoFisher, was measured using a total organic carbon analyzer (TOC-L CPN; Shimadzu, Kyoto, Japan). 166 All reagents used in this study were analytically pure, and standard solutions were prepared 167 using Milli-Q water. Furthermore, all experimental devices were precleaned using 5% 168 concentrated nitric acid solution and rinsed with Milli-Q water.

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Water quality index calculation 170 Groundwater quality was evaluated using the WQI, which was calculated as follows (Saleh

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(1) Calculation of proportional weight (Wi): where Wi is the proportional weight of the ith chemical parameter, wi is the weight of the ith 175 parameter, and n is the number of parameters tested in this study (n = 17; Table S1).

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(2) Calculation of the quality rating scale: where qi is the quality rating based on the ith chemical parameter, Ci is the value of the ith 179 parameter, and Si is the drinking water standard for the ith parameter.

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(3) Determining the sub-index value: (4) Calculation of the WQI value: Na + /(Na + +Ca 2+ ) was plotted on the x-axis and the TDS was plotted on the y-axis.  The WQI results showed that the groundwater around Lake Taihu did not have a serious  (Fig. S3). In the Na + -Cl  ratio diagram (Fig. S3a), some sampling points were The bivariate plot of Ca 2+ versus HCO3  (Fig. S3d) showed that most sampling points were 295 near the 1:1 line. This distribution pattern indicated that HCO3  in the groundwater mainly 296 originated from the dissolution of carbonates (such as calcite and dolomite), while Ca 2+ was 297 partly derived from carbonates in the study area. This conclusion was consistent with the 298 Ca 2+ -SO4 2 ratio results (Fig. S3c). To determine the occurrence of ion-exchange in 299 groundwater, the linear relationship between ((Ca 2+ +Mg 2+ )  (SO4 2 +HCO3  )) and 300 (Na + +K + Cl  ) was analyzed (Yang et al. 2016). Most of the groundwater sampling points 301 around Lake Taihu were near the 1:1 line (Fig. S3f), indicating strong ion-exchange occurring 302 in shallow groundwater in the study area.

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The results of this study showed that an increasing DOC concentration in groundwater leading to a slow increase in Na + , Mg 2+ , and HCO3  concentrations.

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In contrast to the trends of Na + , Mg 2+ , and HCO3  concentrations, NO3  concentration 315 showed an initial rapid decrease followed by a slow decrease in groundwater as the DOC 316 concentration increased. The reduction of nitrate in groundwater is mainly achieved through 317 denitrification by microorganisms (Spalding and Exner 1993   In this study, a significant positive correlation was observed between K + and several water Lake, 2018). In the present study, the mean TDN concentration in groundwater around Lake 345 Taihu (2.61 mg L 1 ) was higher than that in Lake Taihu and its surrounding river network.

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Therefore, NO3  leaching from soil in the Lake Taihu Basin might be the main source of 347 NO3  in groundwater.

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The results of this study also showed a significant positive correlation between the 349 concentrations of K + and As or Cd, indicating that the sources of As and Cd in groundwater 350 were similar to that of K + . Therefore, both As and Cd might come from the use of fertilizers