Xing Liang

Changes of vertical groundwater recharge with increase in thickness of vadose zone simulated by one-dimensional variably saturated flow model

The thickness of vadose zone plays a critical role in vertical groundwater recharge. The decline of water table since the past decades due to long-term groundwater over-exploitation has resulted in deep vadose zone in North China Plain. One-dimensional variably saturated flow models were established by Hydrus-1D software and simulations were run under steady and continuous declining water table respectively to estimate the impact of increase in thickness of vadose zone on recharge process, quantity and recharge time. Luancheng area was selected to estimate recharge quantity considering steady and continuous declining water table. The simulation results show that the increase in thickness of vadose zone delays recharge process to water table. The recharge quantity decreases first and then remains stable with the decline of water table. Under the condition of declining water table, the evaluation of recharge by the flux at water table overestimates the recharge quantity. The average annual recharge rate of Luancheng area is 134 mm/a.

Geochemistry of Clayey Aquitard Pore Water as Archive of Paleo-Environment, Western Bohai Bay

The record of paleo-environment in clayey aquitard pore water is much more effective relative to aquifer groundwater owing to the low permeability of clayey aquitard. Oxygen-18 (18O), deuterium (D), and chemical patterns were determined in pore water samples extracted from two 500 m depth boreholes, G1 and G2, in western Bohai Bay, China. Shallow pore water samples (depth<102 m) are saline water, with the TDS (total dissolved solids) of 3.69–30.75 g/L, and deeper ones (depth=102–500 m) are fresh water, with the TDS<1 g/L. Content of major ions (i.e., Cl−, Na+, K+, Mg2+, SO42−, Ca2+) is high in marine sediment pore water samples and gradually decrease towards to terrestrial sediment pore water, together with the Cl/Br and Sr/Ba ratios changing significantly in different sedimentary facies along the study profile, indicating that pore water may be paleo-sedimentary water and not replaced by modern water. δ18O profile and positive correlation between δ18O and Cl− of shallow saline pore water indicated diffusion as the main transport mechanism, and distinguished four transgressive layers since Late Quaternary (i.e., Holocene marine unit, two Late Pleistocene marine units and Middle Pleistocene marine unit), further supporting the finding that pore water retained the feature of paleo-sedimentary water. Climate was identified as the main influence on the isotopic signature of aquitard pore water and four climate periods were determined by δ18O profile.

Using EARTH model to estimate groundwater recharge at five representative zones in the Hebei Plain, China

Accurate estimation of groundwater recharge is essential for efficient and sustainable groundwater management in many semi-arid regions. In this paper, a lumped parameter model (EARTH) was established to simulate the recharge rate and recharge process in typical areas by the observation datum of weather, soil water and groundwater synthetically, and the spatial and temporal variation law of groundwater recharge in the Hebei Plain was revealed. The mean annual recharge rates at LQ, LC, HS, DZ and CZ representative zones are 220.1, 196.7, 34.1, 141.0 and 188.0 mm/a and the recharge coefficients are 26.5%, 22.3%, 7.2%, 20.4%, and 22.0%, respectively. Recharge rate and recharge coefficient are gradually reduced from piedmont plain to coastal plain. Groundwater recharge appears as only yearly waves, with higher frequency components of the input series filtered by the deep complicated unsaturated zone (such as LC). While at other zones, groundwater recharge series strongly dependent on the daily rainfall and irrigation because of the shallow water table or coarse lithology.

Analytical Study of Unsteady Nested Groundwater Flow Systems

Two analytical solutions using segregation variable method to calculate the hydraulic head under steady and unsteady flow conditions based on Toth’s classical model were developed. The impacts of anisotropy ratio, hydraulic conductivity (K), and specific yield () on the flow patterns were analyzed. It was found that the area of the equal velocity region increases and the penetrating depth of the flow system decreases at steady state with anisotropy ratio increases, which is defined as . In addition, stagnant zones can be found in the flow field where the streamlines have opposite directions. These stagnant zones move toward the surface as the horizontal hydraulic conductivity increases. The results of the study on transient flow indicate that a relative increase in hydraulic conductivity produces a positive impact on hydraulic head and a relative enhancement in specific yield produces a negative effect on hydraulic head at early times.

Origin and Evolution of Aquitard Porewater in the Western Coastal Plain of Bohai Bay, China

High-salinity paleowater from low-permeability aquitards in coastal areas can be a major threat to groundwater resources; however, such water has rarely been studied. The chemical and isotopic compositions of porewater extracted from a 200-m-thick Quaternary sedimentary sequence in the western coastal plain of Bohai Bay, China, were analyzed to investigate the salinity origin and chemical evolution of porewater in aquitards. Porewater samples derived at depths shallower than 32 m are characterized by Cl-Na type saline water (total dissolved solids [TDS], 10.9-84.3 g/L), whereas those at depths greater than 32 m comprise Cl·SO4 -Na type brackish water (TDS, 2.2-6.3 g/L). Saline porewater is interpreted as evaporated seawater prior to halite saturation, as evidenced by Cl-Br relationships. Although substantial dilution of saline porewater with meteoric water is supported by a wider Cl- range and δ2 H-δ18 O covariance, the original marine waters were not completely flushed out. The deeper brackish porewater is determined to be a mixture of fresher porewater and brine groundwater and had a component of old brine of less than 10%, as indicated by a mixing model defined using δ2 H and Cl- tracers. Porewater δ2 H-δ18 O relationships and negative deuterium excess ranging from -25.9‰ to -2.9‰ indicate the existence of an arid climate since Late Pleistocene in Tianjin Plain. The aquitard porewaters were chemically modified through water-rock interactions due to the long residence time.