Spatial characteristics and controlling factors of chemical weathering of loess in the dry season in the middle Loess Plateau, China

Abstract

Hydrochemistry methods were used to decipher the weathering and geochemical processes controlling solute acquisition of river waters in the dry season in the middle Loess Plateau (MLP), one of the most severely eroded areas and turbid riverine systems in the world. River waters were neutral to slightly alkaline with pH varying from 7.6 to 9.6. The total dissolved solids decreased from northwest to southeast with a mean value of 804 mg/l, much higher than the global average and other large rivers in China. Ternary diagram showed that river waters were dominated by Na+, HCO3−, and Cl− with the main water‐type of HCO3−–Cl−–Na+. Saturation index values, Mg2+, Ca2+, and HCO3− analyses indicated the preferential Ca2+ removal by calcite precipitation. Gibbs plots and stoichiometry plots indicated that the dissolved solutes were mainly derived from rock weathering with minor anthropogenic and atmospheric inputs. Samples in the northwestern basin are also influenced by evaporation. A forward model of mass budget calculation showed that, owing to high soluble characteristics, evaporite dissolution was a major feature of river waters and contributed 41% to the total dissolved cations on average, while carbonate and silicate weathering contributed 28%,and 25% on average, respectively. Besides evaporite dissolution, cation exchange is also responsible for the high concentrations of Na+ in river water. Spatial variations showed that evaporite dissolution and silicate weathering were higher in the northern basin, whereas carbonate weathering was higher in the southern basin. Different from most rivers in the world, the physical erosion rates (varying from 117.7 to 4116.6 t/km2y) are much higher than the chemical weathering rates (varying from 3.54 to 6.76 t/km2y) in the MLP because of the loose structure of loess and poor vegetation in the basin. In the future, studies on comparison of water geochemistry in different seasons and on influence of different types of land use and soil salinization on water geochemistry, denudation rates, and water quality should be strengthened in the MLP. These results shed some lights on processes responsible for modern loess weathering and also indicate the importance of time‐series sampling strategy for river water chemistry. Copyright © 2016 John Wiley & Sons, Ltd.

Hydrochemical Characteristics, Controlling Factors, and Solute Sources of Streamflow and Groundwater in the Hei River Catchment, China

“…where ws is the water sample, atm is the atmospheric input, anth is the anthropogenic input, sil is silicate weathering, carb is carbonate weathering, and eva is evaporite dissolution. Detailed calculations for the forward modeling can be found in a previous study [4].…”

“…For example, our previous study showed that over 30% of streamflow or groundwater samples from the Wei River had water quality of Class V in the wet season [15]; the Fen River had even worse water quality [16]. The dissolved solutes in this river mostly come from rock weathering, and the effect of human activities is different than that in other rivers [4]. Specifically, the Wei and Fen rivers support large areas of farmlands, a large population, and an industrial base, while the Hei River is much smaller, with anthropogenic inputs mostly from agriculture and coal mining.…”

“…However, the majority of the data points in this study fell above the 1:1 line (Figure 6a), indicating the additional input of Na + from either sodium aluminosilicate (NaAlSi 3 O 8 ) weathering or anthropogenic inputs. The weathering of carbonate rocks or evaporites yields Ca 2+ , Mg 2+ , and SO 4 2− [4]. If Ca 2+ and Mg 2+ derive predominantly from carbonate weathering, the equivalence ratio of (Ca 2+ + Mg 2+ )/ HCO 3 − should be close to 1:1 [27].…”

“…Long-term focus on water problems has generally related to water quantity; however, since the 1990s, water pollution from human activities has increasingly threatened the majority of rivers [1,2]. Consequently, arid and semiarid regions are threatened by both limited water supply and water quality deterioration [3,4]. Water quality deterioration indirectly reduces the amount of available water, and thus exacerbates the water shortage crisis in arid regions [5,6].…”

“…As such, investigations of water quality are important, especially considering recent anthropogenic-induced environmental change. However, previous studies on water chemistry in the Loess Plateau have been predominantly based on single sampling or limited river cross section sampling [4,18]. Without sufficient data, it is difficult to characterize temporal and spatial changes in hydrochemical characteristics at the catchment scale.…”