WICKIE - an in-situ monitoring system to measure groundwater recharge flux and water quality

Groundwater contamination in intensively cultivated catchments frequently threatens drinking‑water supplies. However, continuous, low‑cost monitoring of recharge fluxes and the related water quality remains elusive. We present WICKIE (WICK‑sampler In‑situ Estimations), an on-line, passive system that quantifies percolating water directly in the field and allows to take water samples or in-situ water quality monitoring systems. The device combines 2 m‑long fiberglass wicks, fixed in a collector to operate at field‑capacity suction, with a 3D‑printed PETG tipping‑bucket that records each drainage event as an absolute volume. A stainless‑steel collector (1 m × 0.12 m) is inserted horizontally into a pre‑excavated pit wall, preserving the surrounding pore structure and providing an active sampling area of 900 cm².

In addition, we introduce an in-situ, low-cost optical sensor for real-time, in-situ monitoring of nitrate (NO₃^-) and dissolved organic carbon (DOC) concentrations in natural water. Utilizing absorbance and fluorescence at specific wavelengths with LEDs and photodiodes, this sensor system offers a practical alternative to expensive and complex laboratory or in situ spectrometer methods. Although the sensor's accuracy does not yet fully match that of much more expensive commercial sensors, it maintains strong predictive capabilities with comparative accuracies and correlations. Challenges such as the interference of DOC and turbidity with the nitrate absorbance signal, intense calibration procedures and site-specific variability remain, necessitating further refinement.

A half-year field trial at a cropping–grassland interface in southwestern Germany demonstrated that WICKIE effectively captures the temporal lag between precipitation and percolation, yielding high‑resolution flux data with low external power supply. WICKIE’s key advantages are reduced soil disturbance compared to lysimeter, enhanced lateral representativeness compared to alternative methods, direct volumetric flux measurement facilitating water balance calculations, inexpensive material cost, and modular adaptation to diverse soils and land uses. WICKIE facilitates precise groundwater recharge assessments and promotes sustainable agricultural and aquifer‑management through the provision of continuous, real‑time, cost-effective recharge and water quality data.