Groundwater Science Could Use a New Term: Transportivity

IF 2 4区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
Groundwater Pub Date : 2024-02-07 DOI:10.1111/gwat.13385
Ward Sanford
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However, two similar parameters, porosity and saturated thickness, although important for travel time calculations, have always been considered separately—never together as a single term. This editorial suggests that because the two frequently need to be considered together, a new term would be useful for this product. The term “transportivity” is suggested.</p><p>In reservoir theory, the age, or mean residence time, of discharging water at steady state is equal to the reservoir's volume divided by its volumetric discharge (or inflow) rate. This can be best envisioned in groundwater by imagining a closed-basin watershed with steady state recharge across the basin and base-flow discharge at its outlet. The volume of water in this case is computed by multiplying the saturated thickness by the porosity and the area of the watershed. Given that this system often has a well-defined area, it is often useful to divide the volume by the area and consider the mean residence time, or age, as the porosity times thickness divided by recharge. This is the most fundamental appearance of the combination of thickness times porosity—in the mean age of base flow discharge. This relation is often inverted to estimate recharge when age tracers are measured in shallow wells. In this case, the thickness is the depth to the well screen, or the distance between the water table and the well screen, depending upon the tracer. 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One can imagine this in a consolidated, fractured rock setting that is overlain by a weathered regolith. In such a watershed the variable porosity with depth is quite uncertain, and the thickness that contributes to discharge is not well defined. If one considers the travel time distribution at the outlet in this watershed, the mean age would be indicative of the total transportivity, and the median age indicate an effective transportivity. In addition, tracers that indicate age have nonlinear input signals, so their measurement in the stream's baseflow might be indicative of an apparent transportivity.</p><p>Darcy's Law further illustrates the analogy between transmissivity and transportivity. In Darcian flow the transmission of water horizontally through two different aquifer systems is directly proportional to their transmissivities, given the same hydraulic gradients. 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引用次数: 0

Abstract

The travel time for a parcel of groundwater from the water table to a well or stream is an important quantity for groundwater characterization. This is especially true if we want to understand and predict the movement of contaminants from sources at the land surface (e.g., fertilizer or road salt) through shallow aquifers. The migration and travel time of contaminant solutes depend on both the hydraulic and transport properties of the subsurface. Aquifer hydraulic conductivity, thickness, recharge rate, and porosity all influence the seepage velocities through the shallow subsurface, and thus the travel rates and times. In aquifer hydraulics, the transmissivity (hydraulic conductivity times saturated thickness) has long been recognized as an important parameter of the flow system. However, two similar parameters, porosity and saturated thickness, although important for travel time calculations, have always been considered separately—never together as a single term. This editorial suggests that because the two frequently need to be considered together, a new term would be useful for this product. The term “transportivity” is suggested.

In reservoir theory, the age, or mean residence time, of discharging water at steady state is equal to the reservoir's volume divided by its volumetric discharge (or inflow) rate. This can be best envisioned in groundwater by imagining a closed-basin watershed with steady state recharge across the basin and base-flow discharge at its outlet. The volume of water in this case is computed by multiplying the saturated thickness by the porosity and the area of the watershed. Given that this system often has a well-defined area, it is often useful to divide the volume by the area and consider the mean residence time, or age, as the porosity times thickness divided by recharge. This is the most fundamental appearance of the combination of thickness times porosity—in the mean age of base flow discharge. This relation is often inverted to estimate recharge when age tracers are measured in shallow wells. In this case, the thickness is the depth to the well screen, or the distance between the water table and the well screen, depending upon the tracer. The product of saturated thickness and porosity has the units of length, representing an apparent depth of water through which the solute passed.

Although we need not have a term for every combination of parameters, it is useful to do so when (1) we need a shorthand for frequent reference when that combination is an important control, and (2) the two conceptually distinct parameters are often difficult or impossible to measure separately in the field. It is for these reasons we have the term transmissivity in hydrogeology. Regarding reason (2), at many locations there is substantial vertical variation in the hydraulic conductivity and the thickness of the flow system is not well defined. Pump tests therefore measure the composite response (the effective transmissivity) of a vertical section. Porosity can also vary vertically within an aquifer and confound our ability to treat it separately at the field scale. One can imagine this in a consolidated, fractured rock setting that is overlain by a weathered regolith. In such a watershed the variable porosity with depth is quite uncertain, and the thickness that contributes to discharge is not well defined. If one considers the travel time distribution at the outlet in this watershed, the mean age would be indicative of the total transportivity, and the median age indicate an effective transportivity. In addition, tracers that indicate age have nonlinear input signals, so their measurement in the stream's baseflow might be indicative of an apparent transportivity.

Darcy's Law further illustrates the analogy between transmissivity and transportivity. In Darcian flow the transmission of water horizontally through two different aquifer systems is directly proportional to their transmissivities, given the same hydraulic gradients. In reservoir theory, the accumulation of age during transport in two different aquifer systems is proportional to their transportivities, given the same recharge rates. Both terms describe a fundamental property of the aquifer times its thickness. Given this analogy it seems the name “transportivity” is a logical choice for this combined term.

地下水科学需要一个新名词:运输。
地下水从地下水位到水井或溪流的流动时间是地下水特征描述的一个重要参数。如果我们想了解和预测污染物从地表源头(如肥料或路盐)通过浅含水层的移动情况,则更是如此。污染物溶质的迁移和传播时间取决于地下水的水力和传输特性。含水层的水力传导性、厚度、补给率和孔隙度都会影响通过浅层地下的渗流速度,从而影响迁移率和迁移时间。在含水层水力学中,渗透率(导水率乘以饱和厚度)一直被认为是水流系统的重要参数。然而,两个类似的参数,即孔隙度和饱和厚度,虽然对流动时间的计算很重要,但一直以来都是分开考虑的,从未作为一个单独的项一起考虑过。本社论认为,由于这两个参数经常需要放在一起考虑,因此应该为这一产品使用一个新的术语。在水库理论中,稳定状态下排水的年龄或平均停留时间等于水库容积除以其容积排水(或流入)率。在地下水中,这一点可以通过想象一个封闭流域来实现,该流域的补给处于稳定状态,出口处为基流排放。这种情况下的水量是由饱和厚度乘以孔隙度和流域面积计算得出的。由于该系统通常有一个明确的区域,因此将水量除以该区域,并将平均停留时间或水龄视为孔隙度乘以厚度除以补给量,通常是非常有用的。这是厚度乘以孔隙度的组合在基流排放平均年龄中最基本的体现。当在浅井中测量年龄示踪剂时,通常会反演这一关系来估算补给量。在这种情况下,根据示踪剂的不同,厚度是到井筛的深度,或地下水位与井筛之间的距离。饱和厚度与孔隙度的乘积以长度为单位,表示溶质通过的表观水深。虽然我们不需要为每种参数组合都设置一个术语,但在以下情况下这样做还是很有用的:(1)当参数组合是一个重要的控制因素时,我们需要一个速记符号以便经常参考;(2)这两个概念上不同的参数通常很难或不可能在现场分别测量。正是由于这些原因,我们在水文地质学中使用了透射率一词。关于原因 (2),在许多地方,水力传导性的垂直变化很大,流动系统的厚度也不十分明确。因此,泵测试测量的是垂直断面的综合响应(有效渗透率)。在含水层中,孔隙度也会在垂直方向上发生变化,使我们无法在实地范围内对其进行单独处理。我们可以想象一下在风化碎屑岩覆盖下的固结断裂岩石环境中的情况。在这样的流域中,随深度而变化的孔隙度是非常不确定的,而且造成排水的厚度也不是很明确。如果考虑到该流域出口处的流速分布,则平均流速可表示总流速,中位流速则表示有效流速。此外,指示龄期的示踪剂具有非线性输入信号,因此在溪流基流中测量到的龄期可能表示表观迁移率。在达西定律中,在相同的水力梯度条件下,水在两个不同含水层系统中的水平传输与它们的透射率成正比。在水库理论中,在补给率相同的情况下,两个不同含水层系统在输水过程中的年龄积累与它们的输移率成正比。这两个术语都描述了含水层乘以厚度的基本属性。有鉴于此,"迁移率 "这一名称似乎是这一组合术语的合理选择。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Groundwater
Groundwater 环境科学-地球科学综合
CiteScore
4.80
自引率
3.80%
发文量
0
审稿时长
12-24 weeks
期刊介绍: Ground Water is the leading international journal focused exclusively on ground water. Since 1963, Ground Water has published a dynamic mix of papers on topics related to ground water including ground water flow and well hydraulics, hydrogeochemistry and contaminant hydrogeology, application of geophysics, groundwater management and policy, and history of ground water hydrology. This is the journal you can count on to bring you the practical applications in ground water hydrology.
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