Evolution Mechanisms And Patterns Of Stream-groundwater Systems To Disconnection And Quantitively Identifying Streambed Permeability Transience

Seepage loss from streams is the main source of groundwater recharges in arid and semi-arid regions.A comprehensive and thorough understanding of the law of water seepage and groundwater recharge is the basis for the rational evaluation,sustainable use,management and protection of regional water resources.Previous studies mainly focused on the saturated flow between stream and groundwater.In recent years,with the emergence of a large number of water collection projects near a river,it is gradually recognized that a large water table decline can cause the disconnection between the river and groundwater,where the flow between them changes from saturated to unsaturated.To accurately quantify water resources,many studies have been carried out on the phenomenon of the disconnection between a stream and groundwater.However,the mechanisms and patterns by which the stream-groundwater systems change from connection to disconnection have not been fully understood at present.Based on numerical simulations and analytical methods whose results are compared with laboratory and field experiments data reported by previous studies,the present study examines the mechanisms and patterns to be a disconnected system under the conditions of static streambed permeability and bioclogging induced dynamic streambed permeability;In addition,to apply the patterns under the conditions of static and dynamic streambed permeability,the present study develops a new floodwave response method to quantitively identify streambed hydraulic conductivity transience.The key findings summarized from the present study include:?1?An inverted water table?IWT?can develop under a stream in response to the desaturation between the stream and underlying aquifer as the system changes from a connected to disconnected status.Previous studies have suggested that the IWT can only occur at the bottom of a low permeability streambed?The streambed in this study is simply defined as the seepage layer separating the stream water body from the aquifer?in which only the vertical flow between the stream and groundwater during disconnection was assumed.The present study revealed that the lateral flow induced by capillarity or heterogeneity also plays an essential role on interactions between streams and aquifers.Three pathways were identified for the transition from connection to disconnection in homogenous systems;notably,the lowest point of an IWT can develop not only at the bottom of the streambed but also within the streambed or the aquifer in response to the initial desaturation at,above,or below the interface between the streambed and aquifer?IBSA?,respectively.In wide streams,the lowest point of an IWT only occurs at the bottom of the streambed;In narrow streams the lowest point can also develop in a thick streambed under a shallow stream.In narrow streams,the lowest point also forms in the aquifer if the streambed is thin but relatively permeable and the stream is deep.Furthermore,in heterogeneous streambed systems,two or three pathways can simultaneously occur and various parts of the IWT occur at distinct positions relative to the IBSA.This challenges the commonly held assumption that streambed under a disconnected stream were always fully saturated,and limits the methods which introduce a negative or atmospheric pressure value at the IBSA to calculate seepage rate or assess stream-aquifer connectivity.?2?In most previous studies,static streambeds were assumed to examine the evolution of disconnection,or despite dynamic streambeds being considered,the feedbacks between nutrients transport and microbial growth were ignored.The present study shows that due to the interactions between nutrients in stream water from the effluent of sewage treatment plants and microbial growth,streambed permeability can intensively change over time and biomass growth and streambed permeability evolution are self-limiting.Thus,a new clogging layer can naturally develop due to these feedbacks and does not require prior clogging.With water table decline,the development of the new clogging layer promotes the occurrence of disconnection,i.e.,the flow regime between a stream and groundwater changes from saturated to unsaturated flow.These results illustrate that as bioclogging is a dominant process,previous static disconnection conditions cannot be used as criteria to predict whether disconnection can occur in a stream-aquifer system.Previous studies have suggested that the proliferation of microorganisms would inevitably lead to seepage rate keep decreasing.But the present study shows that after reaching the minimum,infiltration increases again with decline of the water table until achieving a constant at the moment of disconnection.?3?Because of neglecting the streambed storage effect,the previous method through a successive inversion of a flood wave response model is only valid to identify the transience in the hydraulic properties of a thin?less than 1 m?streambed.Significant under-estimations?1-2 orders of magnitude?can be seen.It is particularly true for a stream-confined aquifer system if the streambed is composed of fine deposits?e.g.,silt,clay?with a thicker thickness greater than 1 m corresponding to low permeability but high storativity,and the aquifer is characterized by relatively low permeability(e.g.,roughly 0.1-10 m d^-1)and relatively low storativity(roughly 1E-5 m^-1,e.g.,dense fine sands).The new method in which streambed storage effect is incorporated,can provide more reliable estimations of time-dependent streambed permeability for a stream-confined aquifer system,and it has been successfully applied to estimate the time-dependent streambed permeability of a reach of the Arkansas River,Kansas,USA.