## 1 Introduction  
With rapid population growth and economic development, the utilization and the exploitation of groundwater have sharply increased, causing continuous declines in groundwater levels, land subsidence, and seawater intrusion [1]. To alleviate the groundwater crisis, reclaimed water is widely used for artificial groundwater recharge (AGR) in many arid and semiarid regions [2, 3]. However, AGR implementation is often hampered by clogging that occurs because of physical, chemical, biological, and mechanical processes or their interactions [4, 5]. Clogging causes a sharp drop in the recharge rate, which in turn significantly reduces the working life of the recharge facility [6]. Physical clogging from suspended solids (SSs) is considered the most common form of clogging, accounting for 70.0% of all clogging cases in a study examining 40.0 recharged injection wells [7]. Likewise, a series of artificial recharge tests in Southern Australia found that SS-induced clogging accounted for over 92.0% of total clogging [8]. Thus, the foremost goal for preventing physical clogging is to limit SS concentrations in recharge water. Column experiments that injected SSs into porous media found that the clogging rate is further influenced by numerous factors, including temperature [1], flow velocity [9], ionic strength [10], porous media structure [11], particle concentration [12], roughness [13], and particle-size ratio [14]. Understanding the interactions between these factors will benefit the development of anticlogging strategies. In addition to SSs, turbidity is another frequently used indicator of physical clogging during AGR. Many countries require turbidity measurements in their statutory regulations because the variable is easily measured [15]. The thresholds for SSs and turbidity in recharge water vary across quality standards (Table S1) in different regions/countries, and the two indicators can be used individually or simultaneously. For example, turbidity limits vary from 5.00 to 12.0 NTU in Chinese standards, but they are lower than 0.100 NTU in US standards [15; 16]. Additionally, Chinese standards employ only turbidity, whereas foreign standards tend to jointly use SSs and turbidity [16]. Column experiments have also been useful in determining critical SS concentrations that cause clogging. For example, physical clogging was found to still occur when the influent SS concentration was only 3.00-4.00 mg/L\({}^{1}\)[8]. Another study suggested that SS concentrations should be less than 2.00 mg/L\({}^{1}\) to prevent the clogging of calcareous aquifers [17]. Evidence has also suggested that clogging could be completely avoided with a turbidity level lower than 3.00 NTU [18]. The differences in critical NTU values are due mainly to variations in the properties of particulate matter from different recharge-water sources. Previously, we conducted simulated AGR experiments using saturated coarse-sand columns [19]. The experiments fed chlorinated secondary effluent (SE) taken from a wastewater treatment plant (WWTP) through the columns. Suspended particles in chlorinated SEs were composed mainly of activated sludge flocs. Effluent turbidity (3.09 NTU) and SSs (17.5 mg/L) complied with requirements in the Water Quality Standard for Urban Sewage Recharge (GB/T19, 772-2005) and the Water Quality Standard for Sewage Recharge (SL368-2006), respectively. After continuous injection for 50.0 h, \(K/K_{0}\) decreased by 85.8%, indicating severe clogging in the porous medium. However, inorganic particles with the same size distribution and concentration (either SSs or turbidity) did not cause clogging, where \(K/K_{0}\) decreased by only 1.80% and 4.03%, respectively. We inferred that the water-quality standards for AGR are insufficient in China, and more research is needed to determine water turbidity thresholds for preventing SE-related clogging. Therefore, this study aimed to determine the turbidity threshold that avoids clogging during AGR.