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USGS2022

Advances in the study and understanding of groundwater discharge to surface water

Groundwater discharge is vitally important for maintaining or restoring valuable ecosystems in surface water and at the underlying groundwater-surface-water ecotone . Detecting and quantifying groundwater discharge is challenging because rates of flow can be very small and difficult to measure, exchange is commonly highly heterogeneous both in space and time, and surface-water hydrodynamics can influence the exchange and hinder measurements . Fortunately, a growing number of methods developed during the last several decades has led to advancements in our capabilities to identify and quantify groundwater discharge to surface water, including better use of seepage meters , application of tracers such as heat or isotopes , and improved groundwater-modeling capabilities . This progress has led to coalescence in characterizing the complex mix of hydrological, biological, and chemical processes that occur at the groundwater-surface water interface , along with relevant societal effects . Still, many uncertainties and assumptions show an incomplete knowledge of these processes, including the lack of studies in many regions of the world, insufficient sharing of practical methodologies between scientific disciplines , incomplete understanding of processes and parameters specific to the sediment-water interface , and challenges associated with measuring exchange at multiple scales of time and space.

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USGS2020

History and evolution of seepage meters for quantifying flow between groundwater and surface water: Part 1 – Freshwater settings

More than 75 years after its introduction, the seepage meter remains the only device for directly quantifying exchange across the sediment-water interface between groundwater and surface water. This device, first presented in the literature in the 1940s, has been in a state of near-constant improvement and design change, necessitating a review of the history and evolution of the device and a description of current best-measurement practices. Part 1 of this two-part review documents the evolution of seepage meters deployed in freshwater settings, including a listing of suggestions for best-measurement and deployment practices. Part 2 covers the same scope for seepage meters deployed in marine settings. Traditional seepage meters isolate a portion of the sediment bed; seepage commonly is determined by routing the volume of flow across that isolated interface to or from a submerged measurement bag over a known time interval. The time-integrated volume is then divided by the bed area covered by the meter to obtain a seepage flux expressed in distance per time. Both the instrument and the measurement are deceptively simple, leading some early users to question the viability of the measurement. Numerous sources of error have been identified and addressed over the decades, resulting in large improvements in measurement consistency and accuracy. Duration of each measurement depends on the seepage rate and can vary from minutes to days, leading to the erroneous and yet common assumption that seepage is relatively stable over time. Designs that replace the measurement bag with a flowmeter eliminate bag-related errors and provide much finer temporal resolution. Resulting data indicate seepage is highly variable in many settings and responds to numerous sub-daily processes, including evapotranspiration, rainfall, seiches and waves. Combining direct measurements from seepage meters with other measurements, such as vertical hydraulic gradients and vertical temperature profiles, provides far better understanding of the processes that control exchange between groundwater and surface water.

Earth-Science Reviews↗