Significance of isotopic and geochemical methods to determine the
evolution of inland brackish and bitter water: an example from the Zuli
river in the upper reaches of the Yellow River, China.
Abstract
With the increasing demand for water resources, the utilization of
marginal water resources of poor-quality has become a focus of
attention. The brackish water developed in the Loess Plateau is not only
salty but also famous for its “bitterness”. In the present work,
multi-isotope analysis (Sr, B) was combined with geochemical analysis to
gain insight into the hydrogeochemical evolution and formation
mechanisms of brackish water. These results demonstrate that groundwater
in the headwater is influenced by carbonate weathering. After the
confluence of several tributaries in the headwater, the total dissolved
solids (TDS) of water is significantly increased. The dissolution of
evaporates is shown to be the main source of salinity in brackish water,
which also greatly affects the Sr isotopic composition of water. This
includes the dissolution of Mg-rich minerals, which is the main cause of
the bitterness. Furthermore, the release of calcium from the dissolution
of gypsum may induce calcite precipitation and incongruent dissolution
of dolomite, which also contributes to the enrichment of magnesium. The
highly fractionated boron isotopic values observed in the upstream
groundwater were explained by boron interacting with clays, illustrating
the important role played by the cationic exchange reaction. The inflow
of brackish groundwater is the source of the observed quality of the
river water. River water with relatively enriched 11B contents reflects
the occurrence of evaporation along the flow path of the river. This
process further aggravates the salinization of river water, with water
quality evolving to saline conditions in the lower reach. When the river
reaches the valley plain, the 87Sr/86Sr ratios decreases significantly,
which is primarily related to erosion of the riverbanks during runoff.
These results indicate that water resource sustainability could be
enhanced by directing focus to mitigating salinization in the source
area of the catchment.