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Abstract:The implementation and development of managed aquifer recharge (MAR) have been limited by the clogging attributed to physical, chemical, and biological reactions. In application field of MAR, physical clogging is usually the dominant type. Although numerous studies on the physical clogging mechanism during MAR are available, studies on the more detailed suspended clogging types and its prediction methods still remain few. In this study, a series of column experiments were inducted to show the process of suspended solids clogging process. The suspended solids clogging was divided into three types of surface clogging, inner clogging and mixed clogging based on the different clogging characteristics. Surface clogging indicates that the suspended solids are intercepted by the medium surface when suspended solids grain diameter is larger than pore diameter of infiltration medium. Inner clogging indicates that the suspended solids particles could transport through the infiltration medium. Mixed clogging refers to the comprehensive performance of surface clogging and inner clogging. Each suspended solids clogging type has the different clogging position, different changing laws of hydraulic conductivity and different deposition profile of suspended solids. Based on the experiment data, the ratio of effective medium pore diameter (D p ) and median grain size of suspended solids (d 50 ) was proposed as the judgment index for suspended solids clogging types.

Fencing enclosures play an important role in improving ecological quality. There is a direct impact of implementing fencing enclosures on the change in soil quality. The soil quality index was used to examine the effects of fencing enclosures for different years (7 and 11 years) on soil quality in Biru County of Qinghai–Tibet Plateau, China. The fencing enclosure significantly increased soil water content, non-capillary porosity, soil organic matter, total nitrogen, total phosphorus, and alkali-hydrolyzable nitrogen, and significantly decreased the soil bulk density. The soil quality gradually improved as the fencing enclosure time length increased, probably due to the increase of vegetation coverage and biomass under the fencing enclosure. The minimum data set was composed of soil organic matter, capillary porosity, total potassium, and non-capillary porosity. The minimum data set was significantly correlated with the total data set and could replace the total data set for soil quality evaluation in the fencing enclosure project area. In summary, our study reflects that fencing enclosures significantly improve soil quality, and the implementation of the fencing enclosure project will effectively curb land degradation in Biru County of the Qinghai–Tibet Plateau, China.

A hydrochemical assessment of the local harvested water and groundwater based on field data, lab experiments, and modeling was carried out for a groundwater artificial recharge (GAR) proposal in the Pinggu Basin aquifer. Harvested water chemically regulated by recharge water standard was chosen as an available recharge source in the basin. The sediments at the potential recharge site were characterized to determine the impact of GAR on subsurface hydrochemistry. To model the dynamic hydrochemical changes during GAR, transport process coupled with the geochemical equilibrium was resolved using PHREEQC and a reverse computing process. These were to simulate chemical reactions, soil soluble species dispersion, mineral dissolution and precipitation, and cation exchange in the recharge vadose zone in the Pinggu Basin. The model was validated by a column leaching experiment and applied for field conditions. The prediction results showed NO 3 À , SO 4 2À , Cl À , and total dissolved solid had peak breakthroughs during the first period of recharge indicating a "washout" phenomenon and then settled down, whilst changes in HCO 3 À , Ca 2þ , Mg 2þ , and pH resulted from carbonate buffering and cation exchange. In the long run, recharge by the harvested water was unlikely to have a negative impact on groundwater quality. Rather caution should be taken for potential chemical clogging induced by dolomite precipitation for a sustainable GAR management.