Non‐destructive monitoring of nitrate concentration in a laboratory flow experiment using time domain reflectometry (TDR)

Abstract

Nitrates, when found in excess in the groundwater, are a health hazard. Nitrates can easily be transported by groundwater movement and may eventually reach aquifers located far away from the contamination location. Recent research has focused on developing innovative techniques to remediate nitrate-contaminated soils. It is important to monitor the nitrate movement in the groundwater to predict the extent of nitrate contamination. Traditionally, nitrate movement is monitored by sampling soil pore water and analysing it in the laboratory. In this study a non-destructive method, using time domain reflectometry (TDR) to monitor nitrate movement in laboratory flow experiments, is presented. Three flow cells (replicates) packed with a silty loam soil were subjected to a constant hydraulic gradient inducing saturated water flow through the flow cells. A source of potassium nitrate solution, containing 500 ppm NO3-N, was connected at the inflow end, and the nitrate concentration change with time was monitored along the length of the flow cells by sampling the soil solution and analysing it in the laboratory for NO3-N concentration. At the time of sampling, TDR wave forms were recorded using the TDR mini-probes inserted at regular intervals along the length of the flow cells. The bulk soil electrical conductivity and the water content values extracted from the TDR wave forms were used to predict the nitrate concentrations at different locations. The nitrate concentration values predicted from the TDR-measured bulk electrical conductivity and water content data correlated well with the nitrate concentrations obtained by soil solution sampling method.

Decreasing Nitrate Leaching in Vegetable Crops with Better N Management

“…The use of TDR probes allows the estimation of available soil nitrogen and immobilised N in non-saline soils (De Neve and Hofmann 2001), and further calibration and development of specific TDR instruments could extend the use of this method, which has so far been limited to experimental work on study farms. The time and the frequency of a reflected TDR wave in the soil depends on its water content and on the quality and quantities of the ions in it, while mathematical models can be built to use the quality of the reflected wave to calculate the concentration of such ions (Krishnapillai andSri Ranjan 2009, Souza et al 2006). However, an extended comparison between chemical and EC based N measurements has proved how only the first one can give a constant reliable assessment of N soil (Baumgarten 2006), and factors such as soil texture, organic matter and stone content can influence the accuracy of results.…”

Response of Capacitance Probes to Soil Solution Nitrate Concentration

“…Calculated electrical conductivity of the pore water has been used to determine soil salinity (Bouksila et al, 2008;Zhang et al, 2004). Several studies have successfully demonstrated the potential to estimate nitrate (NO 3 ) concentration of soil solution from electrical conductivity measurement using time domain reflectometry (TDR) sensors (Krishnapillai and Ranjan, 2009;Payero et al, 2006;Nissen et al, 1998).…”

Estimating Soil Solution Nitrate Concentration from Dielectric Spectra Using Partial Least Squares Analysis

“…Dalton et al (1984) fi rst demonstrated the use of the TDR method for simultaneous measurement of soil moisture content and bulk electrical conductivity (EC). Later, the probes were adapted for continuous monitoring of NO 3 -N or other ionic solutes in soil and water in both laboratory and fi eld experiments (Nissen et al, 1998;Das et al, 1999;De Neve et al, 2003;Payero et al, 2006;Krishnapillai and Sri Ranjan, 2009). Th e operational principals and recent advancements in dielectric and EC measurements using TDR were reported by Robinson et al (2003).…”

“…Th e conductive behavior of soils has also been used by other methods, particularly the time domain refl ectometry (TDR) technique, to predict pore water NO 3 -N concentrations (Das et al, 1999;Krishnapillai and Sri Ranjan, 2009). Originally TDR probes were used to estimate the VWC of the soil by measuring the relative permittivity of the medium (Topp et al, 1980).…”