Study On Control Factors And Evolution Law Of Groundwater Flow Systems

The temporal and spatial changes of groundwater flow systems are complex, and investigating the control factors and evolution law has great significance for understanding the distribution, circulation and utilization of groundwater. Affected by geological environment and sea-level, the temporal and spatial evolution law of groundwater flow systems in Hebei Plain is extremely complicated since late Pleistocene. In this study, the control factors (the ratio of infiltration intensity to hydraulic conductivity; the ratio of length to depth of a basin; aquitard) and distribution characteristics of groundwater flow systems are analyzed using analytic solution and numerical simulation, and the evolution law of groundwater flow systems in Hebei Plain since late Pleistocene are investigated by numerical simulation combined with previous results.The results of analytical solution for steady flow with given head upper boundary show that when increasing anisotropy ratio, the groundwater penetration depth becomes shallower, and the stagnation zones near the reverse flow become closer to the surface. The results of numerical simulation with flux upper boundary indicate that when reducing infiltration intensity with the rest conditions remain the same, the groundwater velocity decreases, the residence time increases and the circulation and regeneration capacity decrease, accompanied with the transformation of groundwater flow from nested local-intermediate-regional to nested local-regional and regional flow system. The ratio of length to depth of a basin has a similar effect with the ratio of infiltration intensity to hydraulic conductivity. The large ratio of length to depth and aquitard (such as the Hebei plain) are not conducive to the development of the regional flow systems. The influence of various factors on the patterns of groundwater flow systems could be attributed to the changes of the driving force (the ratio of infiltration intensity to hydraulic conductivity) of the groundwater flow.The parameter sensitivity analyses for analytic solution of unsteady groundwater flow systems show that the increasing of hydraulic conductivity (specific yield) could lead to the positive (negative) changes of hydraulic head, and then it gradually approximates to the steady result with the given upper boundary. Periodic infiltration could make the flow systems lag and lack (compared with the corresponding steady patterns), which has a greater influence on the shallow local flow system than the deep regional flow system. In addition, the exploitation would cause the change of flow patterns.The temporal and spatial variations of groundwater flow systems in Hebei plain are influenced by both environment and artificial factors. The changes of paleoclimate and sea-level are the main control factors for the characteristics and evolution of groundwater flow system since late Pleistocene, and human activities have strongly affected the modern flow system. Based on the paleoenvironmental records of clay pore water and pollen of 130 m depth borehole in Hengshui City, and through the sea-level and coastline changes of east China reconstructed by a large number of documents, it can be considered that the groundwater flow systems of Hebei Plain has experienced four stages since the late Pleistocene.Combined with previous research results, the simulation of two-dimensional steady flow in divided periods show that the regional flow system penetrated through the bottom of the Quaternary during the low sea-level period due to the large topographic potential difference. At the rapid sea-level rising stage, the difference of topographic potential became weakened, thus the regional flow system became stagnation zone due to the low flow velocity; low plain area (such as Hengshui) became the concentrated discharge zone; shallow salt water was formed due to long-term evaporation, and the early intermediate flow system through the third aquifer developed. At the stage of high sea-level without strong influence of human activities, the surface river flooding eroded local topography, and the difference of regional topographic potential became more weakened; the penetration depth of intermediate groundwater flow systems decreased, and the local flow systems developed. The strong artificial exploitation produced new sources and sinks, which leaded to separation of flow systems. The late flow systems replaced the preliminary flow systems partly, and the evolution of the flow system in different periods is similar to strata stacking patterns of different depositional systems in stratum.