5.1.2 Formation of brackish groundwater in the upstream
The upstream groundwater shows an obvious salinization than groundwater
in headwater. The TDS in upstream groundwater varies from 1177 to 8216
mg/l (Table 1), which is classified as brackish water (Freeze & Cherry,
1979). According to the previous work, the upstream groundwater is
recharged by groundwater in headwater and precipitation. And the tritium
data shows a rapid circulation of groundwater (less than 40 years) in
the upstream area (Liu et al., 2019). Groundwater still evolves to
brackish water from headwater to the upstream area under such a rapid
replenishing, which is rare in most similar inland areas around the
world.
The δ11B values of upstream groundwater are
significantly lower than those of seawater (Barth, 1993), which
indicates the geochemical composition in upstream groundwater is of
non-marine origins (Fig. 7). The variation range of
δ11B value in these water load samples (3.39‰-5.42‰;
NO.61-63) and soil samples (acid-extracted) (0.6‰-7.42‰; NO.58-60) are
in the range of δ11B value for the non-marine
evaporates (Vengosh et al., 1994) (Table 1) (Fig. 7). The boron isotopic
characteristic indicates that the dissolution of non-marine evaporates
is the possible source of solutes in brackish groundwater.
In the quaternary loess layers of the study area, there are reserves a
large amount of soluble evaporates minerals (Tsunekawa et al., 2014;
Xiao et al., 2016), such as halite, gypsum, etc. The increase of
strontium concentration and87Sr/86Sr ratio observed in upstream
groundwater relative to the source groundwater (Fig. 5a) implies that
there exists a strontium input with higher87Sr/86Sr ratio during the evolution
of groundwater. However, there is no correlation between Sr/Cl ratio and
Sr isotopic composition in upstream groundwater (Fig. 5b), as would be
suggested weathering of carbonates rocks is no longer the major
geochemical process in water. Strontium and calcium belong to one family
and strontium usually reserve in Ca-containing minerals (Yokoo, Nakanob,
Nishikawac, & Quan, 2004). Strontium shows a significant correlation
with calcium (R2 = 0.9037; Fig. 5c) in upstream
groundwater, which means strontium origin from the dissolution of
Ca-containing mineral in the evolution of groundwater. The87Sr/86Sr ratios of upstream
groundwater are close to those reported for aquifers where evaporates
dissolution occurs (0.711; Palmer & Edmond, 1992). Therefore, the
dissolution of gypsum mineral is the main source of strontium, which
also is an important source of solutes in upstream water.
When there is a dissolution of non-marine evaporates mineral, the Cl/ Br
ratio will significantly increase with the increase of chloride
concentration in water (Brenot et al., 2015). The Cl/Br vs. Cl diagram
displays a well-defined relationship between these two conservative
indicators in the upstream groundwater and Cl/Br ratio of upstream
groundwater shows obvious high relative to that of source groundwater
(Fig. 4a), indicating progressive reaction derived from the dissolution
of non-marine evaporates with high Cl/Br ratio during the evolution of
groundwater. With the increase of Cl/Br ratio, the Na/Cl ratio of
groundwater samples gradually approaches 1 (Fig. 4b), which clearly
argues for the dissolution of halite mineral as an important source of
salinity in groundwater.
Nevertheless, the
δ11B values of upstream groundwater samples
(11.48‰-22.58‰; NO.5-7,11,16-17) are significantly higher than those of
water affecting by non-marine evaporates dissolution (-32‰-8‰) (Vengosh
et al., 1994) (Fig. 7), meaning that the boron isotopic signature is
altered by additional B inputs from other sources beyond the dissolution
of non-marine evaporates. Indeed, the loess layer is rich in clay
minerals which have a high cation exchange capacity (Cartwright, Weaver,
& Petrides, 2007; Tsunekawa et al., 2014), such as kaolinite and
illite. This characteristic usually promotes cationic exchange reaction
to be active in groundwater (Ghassemi et al, 1995). The preferential10B adsorption occurs on exchange phases (clay
minerals), while the 11B will remain in the liquid
phase (groundwater), which will lead to the relative enrichment of11B in water (Palmer et al., 1987). Since chloride
behaves as a conservative element, the low B/Cl ratio usually reflects a
loss of boron related to the preferential adsorption of10B in cation exchange (Cary et al., 2015). The B/Cl
ratio gradually decreased with the increase of TDS in upstream
groundwater also reflect the existence of the cation exchange reaction
(Fig. 6b).
As suggested above, the dissolution of non-marine evaporates is the main
source of solutes in the upstream groundwater. These soluble minerals
can dissolute into water rapidly during the flow path of groundwater and
lead to the salinization of water. In addition, the cation exchange
reaction occurs in the upstream groundwater significantly modify the
boron isotopic signature of water, which also influence the geochemical
composition of water.