Factors influencing streambed hydraulic conductivity and their implications on stream–aquifer interaction: a conceptual review

Abstract

The estimation and modeling of streambed hydraulic conductivity (K) is an emerging interest due to its connection to water quality, aquatic habitat, and groundwater recharge. Existing research has found ways to sample and measure K at specific sites and with laboratory tests. The challenge undertaken was to review progress, relevance, complexity in understanding and modeling via statistical and geostatistical approaches, literature gaps, and suggestions toward future needs. This article provides an overview of factors and processes influencing streambed hydraulic conductivity (K) and its role in the stream–aquifer interaction. During our synthesis, we discuss the influence of geological, hydrological, biological, and anthropogenic factors that lead to variability of streambed substrates. Literature examples document findings to specific sites that help to portray the role of streambed K and other interrelated factors in the modeling of hyporheic and groundwater flow systems. However, studies utilizing an integrated, comprehensive database are limited, restricting the ability of broader application and understanding. Examples of in situ and laboratory methods of estimating hydraulic conductivity suggest challenges in acquiring representative samples and comparing results, considering the anisotropy and heterogeneity of fluvial bed materials and geohydrological conditions. Arriving at realistic statistical and spatial inference based on field and lab data collected is challenging, considering the possible sediment sources, processes, and complexity. Recognizing that the K for a given particle size group includes several to many orders of magnitude, modeling of streambed K and groundwater interaction remain conceptual and experimental. Advanced geostatistical techniques offer a wide range of univariate or multi-variate interpolation procedures such as kriging and variogram analysis that can be applied to these complex systems. Research available from various studies has been instrumental in developing sampling options, recognizing the significance of fluvial dynamics, the potential for filtration, transfer, and storage of high-quality groundwater, and importance to aquatic habitat and refuge during extreme conditions. Efforts in the characterization of natural and anthropogenic conditions, substrate materials, sediment loading, colmation, and other details highlight the great complexity and perhaps need for a database to compile relevant data. The effects on streambed hydraulic conductivity due to anthropogenic disturbances (in-stream gravel mining, contaminant release, benthic activity, etc.) are the areas that still need focus. An interdisciplinary (hydro-geo-biological) approach may be necessary to characterize the magnitude and variability of streambed K and fluxes at local, regional scales.

Appendix

Definitions

Base flow: That part of the stream discharge that is not attributable to direct runoff from precipitation or melting snow; it is usually sustained by groundwater discharge (Isensee et al. 1989).

Colmation: It refers to the retention processes that can lead to the clogging of the top layer of channel sediments and decolmation refers to the resuspension of deposited fine particles (Brunke 1999).

Hydraulic conductivity: The volume of water at the existing kinematic viscosity that will move in a unit time under unit hydraulic gradient through a unit area measured at right angles to the direction of flow (in field aquifer tests) (ASTM D653-14 2014). The rate of discharge of water under laminar flow conditions through a unit cross-sectional area of a porous medium under a unit hydraulic gradient and standard temperature conditions [20 °C] (in laboratory testing).

Hydraulic gradient: The change in static head per unit distance in a given direction.

Hydrograph: A graph relating stage, flow, velocity, or other characteristics of water with respect to time.

Hydrologic unit: In hydrogeology, geologic strata can be distinguished on the basis of capacity to yield and transmit fluids. Aquifers and confining units are types of hydrologic units. Boundaries of a hydrologic unit may not necessarily correspond either laterally or vertically to lithostratigraphic formations.

Infiltration: The downward entry of water into the soil or rock.

Permeability: A measure of the relative ease with which a porous medium can transmit a fluid under a potential gradient and is a property of the medium alone.

Piping: The progressive removal of soil particles from a mass by percolating water, leading to the development of channels.

Porosity: The ratio, usually expressed as a percentage, of the total volume of voids of a given porous medium to the total volume of the porous medium.

Riverbank filtration (RBF): It is a low-cost water treatment/pre-treatment technology that is used in many countries around the world for water supply (Ray et al. 2008).

Radionuclide: A radioisotope (an unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation).

Seepage: The infiltration or percolation of water through rock or soil to or from the surface. The term seepage is usually restricted to the very slow movement of groundwater. Deep seepage is the water lost to groundwater (or more appropriately drainage below the depth of interest). The drainage of soil water downward by gravity below the maximum effective depth of the root zone toward storage in subsurface strata.