Principal investigators: Amin Gorji1,4, Amy Kaleita2, and Nicola Bowler1,3,4 1Department 3Department
of Electrical and Computer Engineering; 2Department of Agricultural and Biosystems Engineering;
of Materials Science and Engineering; 4Center for Nondestructive Evaluation, Iowa State University, Ames, IA, USA
[email protected],
[email protected],
[email protected]
Towards Low-cost Sensors for Real-time Monitoring of Nitrate Concentration in Iowa Soil Water 1. Motivation
Fig. 3. News Headlines
The flow of nitrate ions into the Gulf of Mexico is a major environmental concern in the United States. Presence of excess of these ions causes explosive plant growth which uses up available oxygen in the water, leaving areas unable to support aquatic life. Furthermore, the excessive concentration of such ions in drinking water poses a threat to human health (in particular, it is a cause of “Blue Baby Syndrome”). The average annual export of nitrate in surface water in Iowa has been estimated to range from 204,000 to 222,000 Mg; one quarter of the nitrate that the Mississippi river delivers to the Gulf of Mexico! The efflux of ions from agricultural land in Iowa is a major contributor to this situation and has been a topic of recent controversy between the Des Moines Water Works and three Iowa counties, Fig. 1. Assessing the effectiveness of management strategies to control the ions’ efflux is hampered by the lack of affordable, effective, and in-field nitrate monitoring systems. Moreover, the ion concentration is tightly linked to the local hydrology and changes rapidly in time and space, so spot and send-to-lab analysis yields incomplete data. Thus the need for low-cost and real-time sensing becomes critical for complete monitoring.
2. Approach and Justification Dielectric spectroscopy at radio and microwave frequencies, and bulk electrical conductivity measurements, are useful for characterizing the physical and chemical properties of liquid media. The dielectric properties of a liquid are determined by its molecular structure which means that, by measuring the dielectric properties, we can infer other properties that influence the molecular structure, such as ion concentration. For ionic dilute solutions the relative permittivity (ԑT) can be modeled as:
RACCOON RIVER NITRATE LEVEL PEAKED SATURDAY (24 Nov. 2014)
WATER WORKS PLANS TO SUE THREE COUNTIES (6 Jan. 2015)
WILL DES MOINES WATER LAWSUIT CHANGE FARMING RULES? (19 Jan. 2015)
ISU News: Kaleita and Bowler research low-cost sensors to monitor nitrate concentration (2 Feb. 2015)
3. Preliminary Results The permittivity of nitrate, chloride, and sulphate solutions were measured. f = 200 MHz to 50 GHz; c = 10, 25, 50 (mg/L); T = 10, 15, 20, 25 ᵒC. ԑ’
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Fig. 4. Measured complex permittivity for various concentrations of nitrate, chloride and sulphate ions
s T j j 1 j 0 Debye Model
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Conductivity
Fig. 1. Raccoon River and counties involved in lawsuit ԑ’’
Fig. 5. Conductivity variation as a function of ionic concentration for three ion types
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Fig. 6. Measured complex relative permittivity (ԑT = ԑ’ - jԑ’’) and relaxation time (τ) variation as the temperature (T) changes
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4. Conclusion and Future Directions Na+
Fig. 2. Molecular structure of dilute ionic solution in water
Hydrogen-bond dipoles created between water molecules contribute to the polarization and relaxation properties. Free charges due to the presence of ions contribute to the conductivity.
To date, environmentally-relevant aqueous solutions of nitrate, chloride, and sulphate ions have been characterized in terms of their conductivity and Debye model dielectric properties. Ion-specific responses of conductivity and dielectric relaxation characteristics will be used to identify ions present in aqueous solution and to infer their concentration. Therefore the planned development of real-time in-field sensors will target these indicators, combining conductivity measurement with dielectric characterization at around 20 GHz. These sensors are practically intended to be low-cost and will be inserted into flowing tile-drained water for real-time monitoring.
This work is funded by USDA-National Institute of Food & Agriculture, Award Number 013025-00001
