Lab-on-a-Drone: Real-Time Electrochemical Sensing of Nitrate in Agricultural Watersheds

Abstract

Excess nitrate (NO₃^-) leaching from agricultural lands degrades water quality and contributes to algal blooms and hypoxic zones in surface waters, such as streams, lakes, and ocean bays. Monitoring of NO₃^- from agricultural watersheds is critical for optimizing fertilizer use and evaluating nutrient retention strategies. However, monitoring water quality in agricultural streams─particularly at subsurface tile drain outlets, which serve as major conduits for nitrate leaching from Midwestern U.S. farm fields─is often challenging due to rough terrain and shallow waterways. While unmanned aerial vehicles (UAVs) offer mobility to reach these remote sites, commercial systems typically only collect water samples for later testing, limiting real-time environmental assessments. This study presents a lab-on-a-drone platform capable of performing in-flight NO₃^- sensing within 7 min at each sampling site. The system integrates a custom peristaltic pumping mechanism, low-cost electrochemical NO₃^- sensors based on screen-printed carbon electrodes with ion-selective membranes, and a miniaturized open circuit potentiometric (OCP) device, all mounted on a DJI M350 UAV. The NO₃^- sensors demonstrated near-Nernstian sensitivity (-61.4 ± 0.92 mV dec^-1) and 94.5 ± 1.05% accuracy compared to a commercial benchtop potentiostat, with a detection limit of 40.3 μM (2.50 ppm). The UAV payload, operated via a Bluetooth-enabled ESP32 microcontroller, enables wireless control and data logging. Field tests showed successful NO₃^- quantification in hard-to-reach tile drainage outlets, with concentrations averaging 5.39 ± 0.62 ppm─values consistent with agricultural runoff in Iowa. This UAV-integrated platform improves sampling efficiency, eliminates lab testing, and offers a scalable, cost-effective sensing tool for real-time watershed monitoring.