Identification of anthropogenic and natural inputs of sulfate into a karstic coastal groundwater system in northeast China: evidence from major ions, <i>δ</i><sup>13</sup>C<sub>DIC</sub> and <i>δ</i><sup>34</sup>S<sub>SO<sub>4</sub></sub>

Abstract

Abstract. The hydrogeochemical processes controlling groundwater evolution in the Daweijia area of Dalian, northeast China, were characterised using hydrochemistry and isotopes of carbon and sulfur (δ 13 C DIC and δ 34 S SO 4 ). The aim was to distinguish anthropogenic impacts as distinct from natural processes, with a particular focus on sulfate, which is found at elevated levels (range: 54.4 to 368.8 mg L −1 ; mean: 174.4 mg L −1 ) in fresh and brackish groundwater. The current investigation reveals minor seawater intrusion impact (not exceeding 5 % of the overall solute load), in contrast with extensive impacts observed in 1982 during the height of intensive abstraction. This indicates that measures to restrict groundwater abstraction have been effective. However, hydrochemical facies analysis shows that the groundwater remains in a state of ongoing hydrochemical evolution (towards Ca-Cl type water) and quality degradation (increasing nitrate and sulfate concentrations). The wide range of NO 3 concentrations (74.7-579 mg L −1 ) in the Quaternary aquifer indicates considerable input of fertilisers and/or leakage from septic systems. Both δ 13 C (−14.5 to −5.9 ‰) and δ 34 S SO 4 (+5.4 to +13.1 ‰) values in groundwater show increasing trends along groundwater flow paths. While carbonate minerals may contribute to increasing δ 13 C DIC and δ 34 S SO 4 values in deep karstic groundwater, high loads of agricultural fertilisers reaching the aquifer via irrigation return flow are likely the main source of the dissolved sulfate in Quaternary groundwater, as shown by distinctive isotopic ratios and a lack of evidence for other sources in the major ion chemistry. According to isotope mass balance calculations, the fertiliser contribution to overall sulfate has reached an average of 62.1 % in the Quaternary aquifer, which has a strong hydraulic connection to the underlying carbonate aquifer. The results point to an alarming level of impact from the local intensive agriculture on the groundwater system, a widespread problem throughout China.

Temporal variations of groundwater table and implications for submarine groundwater discharge: A three-decade case study in Central Japan

“…The SGD flux we estimated is 8 and 22 m off Uozu. However, the fresh SGD flux at 40-100 m is about 2 to 4 times 0-40 m (Hatta and Zhang, 2013). Compared to the results of the water budget (Table 2), the fresh SGD may be underestimated.…”

“…The source of the freshwater is the precipitation in the Toyama region. Furthermore, this area has shown a rapid response to climate change, with a recent reduction in the year-round snow extent on the top of nearby mountains, which is shown here to affect SGD flux into the coastal environment (Hatta and Zhang, 2013).…”

“…However, the increased head in the groundwater system at the coast can be easily produced, due to the highly permeable Holocene and Pleistocene layers (Kokusai Kogyo Co. Ltd., 2002;Oude Essink et al, 2010). The average concentration of NO − 3 in fresh SGD (0.69 mg L −1 ) is larger than riverine input (0.18 mg L −1 ) (Hatta and Zhang, 2013). The estimated SGD flux is described in Table 2 and Fig.…”

“…The groundwater flows down a gradient, and SGD occurs wherever a coastal aquifer is connected to the sea (Chen et al, 2005;Zhang and Mandal, 2012). SGD has been recognized as not only an important source of freshwater discharge into the ocean, but also a valuable component of the hydrological cycle between the terrestrial groundwater system and the marine environment (Church, 1996;Taniguchi et al, 2002;Hatta and Zhang, 2013;Liu et al, 2014). The estimation of global SGD varies from 0.2 to 10 % of the river flow (Burnnet et al, 2001).…”

“…Thus, an accurate estimate of SGD flux is essential to predict future coastal environments under climate change conditions. Toyama Bay is an ideal site to study SGD because it is easily accessible and has been studied in previous reports on several SGD sites off the coastal area (Fujii et al, 1986;Zhang and Satake, 2003;Nakaguchi et al, 2005;Hatta and Zhang, 2013). The SGD flow rates in the Katakai alluvial fan ( ∼ 680 m yr −1 ; Zhang et al, 2005) are greater than most of those reported worldwide (∼ 454 m yr −1 ; Taniguchi et al, 2002).…”