The use of electrical conductivity to develop temporally precise breakthrough curves in tracer injection experiments

Authors: SCOTT, ISIS S. P. C. - Purdue University; Huang, Chi Hua; BOWLING, LAURA - Purdue University

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/2020
Publication Date: 4/25/2020
Citation: Scott, I., Huang, C., Bowling, L.C. 2020. The use of electrical conductivity to develop temporally precise breakthrough curves in tracer injection experiments. Journal of Hydrology. 588. Article 124998. https://doi.org/10.1016/j.jhydrol.2020.124998.
DOI: https://doi.org/10.1016/j.jhydrol.2020.124998

Interpretive Summary: Tracer injection is a common technique used in studying chemical or pollutant transport in streams and channels. However, tracer studies require sample collection and analyses and the sampling interval is usually limited, which may not portrait the rapid changes in tracer concentration to fully understand the transport process. The electrical conductivity (EC) which reflects the amount of soluble compounds in water is a potential alternative because EC can be easily measured at a very low cost. Two injection studies were conducted in two separate flumes: an instantaneous injection of phosphorus (P) and bromide (Br) in a 20-m flume and a constant injection of P in a 10-m flume. Water samples were collected intensively at multiple locations and EC was automatically recorded continuously. We used different numbers of measured tracer data to derive solute concentration-EC equations and found good correlations. However, the EC measurement should not be used as a replacement for actual tracer sample collection. EC measurements allow for reducing the number of collected samples. This study also shows a potential complication in interpreting chemical transport processes from a tracer study when additional solute sources are present in the stream channel.

Technical Abstract: Tracer injection is a common technique used in studying chemical transport in streams and channels. One of its limitations is the requirement of frequent sampling for a representative tracer breakthrough data. Frequent collection and analysis of water samples are normally expensive and time-consuming. The measurement of electrical conductivity (EC) is a potential alternative to sample collection and analyses, due to its ease of data acquisition. But whether the EC value can characterize the injected solute concentration requires further evaluation. This article assesses the ability of EC to predict solute concentrations in tracer injections. Two injection studies were conducted in two separate flumes: an instantaneous injection of phosphorus (P) and bromide (Br) in a 20-m flume and a constant injection of P in a 10-m flume. Water samples were collected intensively at multiple locations and EC was automatically and continuously recorded. Using a linear model, a solute breakthrough was calculated based on recorded EC and compared to the measured solute breakthrough. The results showed that the EC-solute relationship can reduce the amount of water samples needed for an accurate solute breakthrough. A small sample of paired recorded EC and solute concentrations can be used to build a model for each individual tracer study. By collecting at least 9 water samples at specific times, i.e., at shoulder, peak and tail of a breakthrough curve, a simple regression model can efficiently predict P concentrations. The correlation is 80% or greater between predicted and observed concentrations, confirming that EC successfully traced P presence in water and can be used to facilitate tracer injection studies.