1. Introduction
The region of the Loess Plateau in China comprises an inland area with
limited freshwater resources and a considerable amount of brackish water
resources. The brackish water resource is valuable to the irrigated
agricultural in this region. However, the formation mechanism of this
water resource remains unclear, which is constraining the rational
utilization of brackish water. (Luo, Chen, & Han, 2010). Numerous
studies have been conducted over the past several decades to assess the
origin of brackish groundwater on a regional scale (Barth, 1998; Brenot,
Négrel, Petelet-Giraud, Millot, & Malcuit, 2015; Cartwright, 2004; Cary
et al., 2015; Jørgensen, Andersen, & Engesgaard, 2008; Werner et al.,
2013; Sahib, Marandi, & Schüth, 2016). Most of these previous studies
were focused on the coastal areas, where the formation of saline
groundwater was usually related to seawater intrusion. With the
increasing water demand, there is increasing study and focus on the
formation of brackish water in inland areas, where the geological
contexts and hydrological conditions are more complex than coastal areas
(Farid, Zouari, Rigane, & Beji, 2015; Gamboa, Godfrey, Herrera,
Custodio, & Soler, 2019; Gil-Márquez, Barberá, Andreo, & Mudarra,
2017).The sustainable management of inland brackish water resources is a
global issue and requires a thorough assessment of the formation
mechanism of brackish water (Alcalá & Custodio, 2008; Cartwright, 2004;
Cary et al., 2015; Ghassemi, Jakeman, & Nix, 1995; Gil-Márquez et al.,
2017; Monjerezi, Vogt, Aagaard, Gebru, & Saka, 2011).
The Zuli River is a first-order tributary of the Yellow River, which is
located in the western Loess Plateau. Except in the headwater, the
groundwater and surface water in this catchment are both brackish, with
a characteristic of “bitterness”. Bitterness is an important factor
affecting the quality and usability of water resources, particularly in
inland areas (Gil-Márquez et al., 2017). However, few studies have been
conducted to investigate the formation mechanism of bitterness in water.
Our previous study showed that groundwater recharge in the Zuli River
catchment is produced by the infiltration of precipitation in the
headwaters (Liu, Tan, Shi, Xu, & I.Elenga, 2019). During this
recharging process, the upstream groundwater undergoes rapid
salinization. Several hypotheses have been advanced in previous studies
as to the source of salinity in brackish water.
Current knowledge typically
attributes the salinization of groundwater in most inland areas to the
mixing of old groundwater in deep strata (Herrera et al., 2018), which
usually means the salinity of groundwater originated from long periods
of water-rock interactions (Petrides, Cartwright, & R.Weaver, 2006;
Skrzypek, Dogramaci, & F.Grierson, 2013). However, our recent study
showed that even under the rapid renewal of groundwater, the fresh
groundwater from the headwaters still evolves into brackish water with
rapid salinization (Liu et al., 2019). Several hypotheses have been
advanced in previous studies as to the source of salinity in brackish
water. The large amounts of soluble salts in loess were identified as a
potential source of salinity (Luo et al., 2010). The contribution of
other hydrogeochemical processes still needs to be considered
comprehensively, especially the origin of the bitterness.
The shortage of freshwater resources in many areas has led to the use of
brackish water resources for agriculture in the Zuli River catchment.
The long-term utilization of this poor-quality water resources has
severely restricted local economic development. In recent years, the
government launched several agricultural water conservancy projects,
such as ”The Taohe River Diversion Project ” and ” The Yellow River
Diversion Project ” that divert freshwater flow into the Zuli River for
the dilution of saline river water, but they had minimal impact. How to
optimize water resources in a cost-effective and sustainable manner is a
critical question for this area.
The present contribution uses an integrated approach of geochemical and
isotopic analysis to identifying the origins of salinity and evaluating
salinization processes. The general water chemical methods are difficult
to discriminate the source of solutes in groundwater precisely. Research
increasingly suggests that a multi-isotope approach is necessary to
resolve the outstanding source of solutes in water (Cary et al., 2015;
Gamboa et al., 2019; Jørgensen et al., 2008). Such an approach has been
shown to be more effective in characterizing hydrogeochemical processes
(Bullen, Krabbenhoft, & Kendall, 1996; Cartwright, Weaver, Cendón, &
Swane, 2010; Cary et al., 2015; Harrington & Herczeg, 2003). The study
uses strontium isotopes and boron isotopes to investigate the possible
processes responsible for the formation of brackish water. As a
relatively stable element, strontium isotopes are minimally fractionated
in chemical and biochemical processes. Groundwater in equilibrium with
Sr-bearing minerals attains87Sr/86Sr values that reflect the
isotopic ratio of the minerals, leading to variation of the strontium
isotopic ratio in different settings (Harrington & Herczeg, 2003;
Monjerezi et al.,2011; Sahib et al., 2016). Therefore, the87Sr/86Sr ratio is classically used
to provide information on the water-rock interaction under different
geological contexts (Bullen et al., 1996; Brenot et al., 2015;
Cartwright, 2004; Cartwright et al., 2010; Monjerezi et al., 2011;
Palmer & Edmond, 1992; Pingitore & Eastman, 1986). On the contrary,
boron is a relatively reactive element and mainly enriched in various
types of rocks and water. Due to the isotopic distinctions, boron
isotopes are prone to fractionate in many geochemical processes because
of the larger relative difference in mass between its primary isotopes
(Palmer, Spivack, & Edmond, 1987; Vengosh, Heumann, Juraske, & Kasher,
1994). Boron isotopes have been gradually applied to distinguish the
original solute sources in groundwater, such as marine sources with
enriched δ11B values and continental sources with
significantly depleted δ11B values (Barth, 1998;
Palmer & Swihart, 1996; Vengosh, 2005). Therefore, boron isotope is
also an effective tracer for understanding the geochemical evolution
processes on a regional scale (Barth, 1993; Cary et al., 2015; Morell et
al., 2008; Palmer et al., 1987; Vengosh et al., 1994). Thus, the
combination of boron and strontium isotopes can be used more precisely
to understand the hydro-geochemical evolution processes in the
groundwater system on a regional scale. To our knowledge, this study is
the first to report on the strontium isotopes of brackish water as well
as the boron isotopes of local precipitation, river water, and
groundwater from the study area. It is anticipated that the results
obtained herein will fill in that gap in the isotope study of the Loess
Plateau.