Groundwater Science Could Use a New Term: Transportivity

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.