Using Automated Seepage Meters to Quantify the Spatial Variability and Net Flux of Groundwater to a Stream

Abstract

We utilized 251 measurements from a recently developed automated seepage meter (ASM) in streambeds in the Nebraska Sand Hills, USA to investigate the small‐scale spatial variability of groundwater seepage flux (q) and the ability of the ASM to estimate mean q at larger scales. Small‐scale spatial variability of q was analyzed in five dense arrays, each covering an area of 13.5–28.0 m2 (169 total point measurements). Streambed vertical hydraulic conductivity (K) was also measured. Results provided: (a) high‐resolution contour plots of q and K, (b) anisotropic semi‐variograms demonstrating greater correlation scales of q and K along the stream length than across the stream width, and (c) the number of rows of points (perpendicular to streamflow) needed to represent the groundwater flux of areas up to 28.0 m2. The findings suggest that representative streambed measurements are best conducted perpendicular to streamflow to accommodate larger seepage flux heterogeneity in this direction and minimize sampling redundancy. To investigate the ASM's ability to produce accurate mean q at larger scales, seepage meters were deployed in four stream reaches (170–890 m), arranged in three to six transects (three to eight points each) per reach across the channel. In each reach, the mean seepage flux from ASMs was compared to the seepage flux from bromide tracer dilution. Agreement between the two methods indicates the viability of a modest number of seepage meter measurements to determine the overall groundwater flux to the stream and can guide sampling for solutes and environmental tracers.

Spatial Variation in Transit Time Distributions of Groundwater Discharge to a Stream Overlying the Northern High Plains Aquifer, Nebraska, USA

“…where Q RBS and Q LBS were measured by channeling the bank seepage into a bottle of known volume and using a stopwatch to measure the rate of bottle fill and were 11.67 and 0.42 m 3 d 1 , respectively. Q Total was estimated from dilution tracer sampling as described in Humphrey et al (2022) and was 277.1 m 3 d 1 . Thus, F RBS and F LBS were 4.2% and 0.2%, respectively, and could be included with their corresponding apparent ages into the TTD as described in Section 3.3.…”

“…The deployment of the ASM and the groundwater flux results are presented in detail in Humphrey et al. (2022).…”

“…The term “study stream” refers to all three stream/river segments collectively. Groundwater flux along the study stream was generally positive (upward into the channel) and increased in magnitude from upstream to downstream sampling areas (Figure 3) (Humphrey et al., 2022).…”

“…Although the exchange of water between rivers and aquifers can be bi-directional, we believe that at the depth of our sampling (30-50 cm) hyporheic flow is minimal as evidenced by (a) a lack of Br in streambed samples after three days of steady-state Br injection into the stream, (b) only upward flow measured in 157/169 seepage measurements (Humphrey et al, 2022), and (c) the retention of gas tracers (e.g., tritiogenic 3 He) in streambed samples.…”

Theory of an Automatic Seepage Meter and Ramifications for Applications

“…These studies focused on hardware, simplified data interpretation for the steady-state stream stage using a basic model, ad hoc analyses of accuracy, and examples of possible ASM responses to time-variable stream stage. Numerous measurements using this instrument were made by Humphrey et al (2022).…”