High-resolution monitoring of groundwater quality in unconsolidated aquifers using UV-Vis spectrometry

In many countries, groundwater quality is routinely monitored in yearly or quarterly intervals. Such frequencies, however, may not be sufficient for investigations into the temporal dynamics of water quality, due to seasonal variability or responses to extreme events. One approach to overcome this limitation is the use of in-situ sensors, such as UV-Vis spectrometers. With a focus on nitrate as an abundant groundwater contaminant, we explored the suitability of these sensors for high-resolution groundwater monitoring in shallow unconsolidated oxic aquifers. In-situ nitrate measurements in four selected wells compared well with values from lab analyses determined in pumped groundwater samples. Most importantly, our results revealed distinct seasonal patterns and, at two sites, pronounced short-term fluctuations in nitrate concentrations concomitant to patterns observed in precipitation, hydraulic head and groundwater temperature. Such dynamics in water quality, and thus the causal relationship with hydrological events and land use, will unfortunately be overlooked when following conventional sampling schemes with the risk of non-representative time series if routine sampling is timed unfavorably. Finally, we provide practical advice for deployment of the sensor.