## 2 Materials and Methods  
### Hydrogeological Background of the Case Study Area  
Information is reported in several references [19; 20; 21; 22]. The key point for the present study is that loess is generally considered a water-sensitive geologic material [22; 23; 24] whose bulk loess matrix has the potential to release fine loess particles when it is exposed to and in contact with water [16; 25]. Therefore, loess is used as an ideal medium for the present study.  
Loess is widely distributed in Guanzhong Plain, with sediments 1-70 m thick [26; 27], and groundwater is widely distributed there. Its shallow groundwater level depth has swiftly declined over the last few decades [28; 29], and this decline is more conducive to the infiltration of surface water and agricultural irrigation water, which might increase the risk of degrading the groundwater quality [28]. Chromium, arsenic, ammonium, nitrate and even micro-pollutants including organochlorine pesticides and aniline are present in the groundwater [30; 31]. These pollutants can be adsorbed on fine loess particles, implying that their transport in the groundwater system is likely to be realized in the form of particle adsorption [16; 17]. Loess has the potential to release endogenous fine loess particles (EFLPs) as responses to hydraulic and hydrochemical disturbances of the groundwater, adsorbing pollutants and then co-transporting them in groundwater systems. However, these releases occur at the beginning of these disturbances and continue [16; 17]. Therefore, it is essential that these particles be dispersed and kept in dispersion along the groundwater flow. Many studies have explored their transport characteristics and influencing factors [16; 17], most of which focused on deposition and transport in porous media for engineered nanoparticles [19; 32]; a few studies have been conducted on EFLPs, of which there are few on EFLPs in groundwater systems [33]. Thus, the knowledge of the generation, aggregation and sedimentation (AS) behaviors and suspension of EFLPs and their responses to hydrochemical conditions in groundwater systems is limited.  
The groundwater over Guanzhong Plain is mainly recharged by rainfall, river leakage and irrigation infiltration [7]. Generally, its hydraulic gradient is drawn from about 12 % in the western plain to 0.9 % in the eastern plain [34]. This means that the groundwater velocity is low and water flows slowly and can thus be considered as one-dimensional. It would have been difficult and time-consuming to change the fine loess particles along the groundwater flow path if a horizontal simulation had been employed in this study. So, one-dimensional vertical column tests were used in the present study.  
For the studied groundwater, numerous studies have shown that although the groundwater hydro-chemical characteristics are complex, fluoride, chloride and sulfate are the main anions, that originate from hydrogeological processes; nitrate is also a conventional anion, which has been reported as a pollutant and is considered a result of human activities, including agricultural activity [28; 35]. The Na+ ion is a major cation and its concentration is high enough to be listed among the types of groundwater hydrogeochemicals [18]. Given these, NaF, NaNO3 and Cu(NO3)2 were employed as typical representatives of conventional hydrochemical components; their concentrations and particle diameter sizes were selected as key parameters affecting the AS and suspension of EFLPs in groundwater in the present study.