A Wet/Wet Differential Pressure Sensor for Measuring Vertical Hydraulic Gradient

Abstract

Vertical hydraulic gradient is commonly measured in rivers, lakes, and streams for studies of groundwater-surface water interaction. While a number of methods with subtle differences have been applied, these methods can generally be separated into two categories; measuring surface water elevation and pressure in the subsurface separately or making direct measurements of the head difference with a manometer. Making separate head measurements allows for the use of electronic pressure sensors, providing large datasets that are particularly useful when the vertical hydraulic gradient fluctuates over time. On the other hand, using a manometer-based method provides an easier and more rapid measurement with a simpler computation to calculate the vertical hydraulic gradient. In this study, we evaluated a wet/wet differential pressure sensor for use in measuring vertical hydraulic gradient. This approach combines the advantage of high-temporal frequency measurements obtained with instrumented piezometers with the simplicity and reduced potential for human-induced error obtained with a manometer board method. Our results showed that the wet/wet differential pressure sensor provided results comparable to more traditional methods, making it an acceptable method for future use.

Conducting Slug Tests in Mini‐Piezometers

“…The mini‐piezometer is a useful tool for site characterization where a high‐spatial density of data is required (Fritz and Arntzen ; Mendoza et al ). For hydraulic measurements, mini‐piezometers are also typically used to measure vertical hydraulic gradient (Barbaro and Neupane ; Fritz and Mackley ; Rautio and Korkka‐Niemi ), and can be paired with temperature monitoring and modeling (Conant ; Lautz et al ). While the vertical gradient is a useful indicator of the direction of water flux, without information on hydraulic conductivity, the actual water flux cannot be calculated.…”

A Wet/Wet Differential Pressure Sensor for Measuring Vertical Hydraulic Gradient

“…Fritz and Mackley (2010) present a wet/wet differential pressure sensor for measuring vertical hydraulic gradients (VHGs) for studies of groundwater‐surface water (GW‐SW) interactions including a detailed comparison of their method with a “traditional” dual piezometer set‐up (whereby one piezometer is screened within the riverbed and one within the river itself). They conclude that “There did not appear to be any fundamental difference in the methods; if the instruments could measure the same point in space and time, the results would likely have been nearly identical.” However, an aspect of the wet/wet design that is not discussed by Fritz and Mackley (2010) and which may undermine this statement is dynamic head variations in the river.…”

“…Fritz and Mackley (2010) present a wet/wet differential pressure sensor for measuring vertical hydraulic gradients (VHGs) for studies of groundwater‐surface water (GW‐SW) interactions including a detailed comparison of their method with a “traditional” dual piezometer set‐up (whereby one piezometer is screened within the riverbed and one within the river itself). They conclude that “There did not appear to be any fundamental difference in the methods; if the instruments could measure the same point in space and time, the results would likely have been nearly identical.” However, an aspect of the wet/wet design that is not discussed by Fritz and Mackley (2010) and which may undermine this statement is dynamic head variations in the river. Following the Bernoulli equation, total streambed pressure expressed as hydraulic head h b [L] can be defined as where z is the elevation head [L] (elevation of point b), h p the static pressure head [L], and the dynamic pressure head, where v is the mean flow velocity [LT −1 ], g the gravitational acceleration [LT −2 ] and C [–] a perturbation coefficient representing transformations of momentum to potential energy due to turbulence, form drag or channel contraction/expansion (Tonina and Buffington 2009).…”

“…The traditional stilling well and, depending on the exact design, the piezometer screened within the river as used by Fritz and Mackley (2010) will damp out many of the variations in the dynamic head and give a measure of the static pressure head. However, the wet/wet system will be more subject to changes in the dynamic head, which will vary depending on the design and positioning of the pressure port opening in the river, and the degree to which the monitoring device itself interferes with the local flow conditions on the riverbed.…”

“…These changes may be of the order of several millimeters in magnitude depending on the flow conditions and geometry (e.g., Elliott & Brooks 1997; Vollmer et al 2002) and vary over seconds. Thus, using instantaneous measurements taken at a lower frequency (e.g., such as those at 5‐min intervals presented in Fritz and Mackley 2010 and Greswell et al 2009) may introduce an additional source of error in calculations of mean VHG driving GW‐SW interactions. The head gradients are typically large for the case described by Fritz and Mackley (2010) and the impact of small‐scale pressure head changes is likely to be unimportant; it seems likely that spatial and temporal dynamic pressure variations have contributed to the scatter seen in Figure 3.…”