Guidelines and standard procedures for high-frequency groundwater-quality monitoring stations—Design, operation, and record computation

Abstract

High-frequency water-quality monitoring stations measure and transmit data, often in near real-time, from a wide range of aquatic environments to assess the quality of the Nation’s water resources. Common instrumentation for high-frequency water-quality data collection uses a multi-parameter sonde, which typically has sensors that measure and record water temperature, specific conductance, pH, and dissolved oxygen. Nitrate, turbidity, and fluorescent dissolved organic matter can also be monitored at high frequency. High-frequency groundwater-quality monitoring stations provide high-resolution time-series data to improve understanding of the timing of water-quality changes in the subsurface, especially for aquifer systems with short groundwater-residence times. High-frequency time-series data are used to monitor surface-water to groundwater interaction, quantify contaminant transport rates, and study water-quality variability in relation to variability of precipitation and groundwater pumping rates. High-frequency monitoring for contaminants or their surrogates have the added benefit of providing an early warning to protect valuable or sensitive aquifer resources. High-frequency time-series data also reveal short-term trends in groundwater quality, which may not be identifiable from monthly or annual sampling programs which facilitate the interpretation of decadal conditions. Systematic application of water-quality sonde operational procedures and a standard record-computation process are part of the required quality assurance for producing and documenting complete and accurate high-frequency groundwater-quality monitoring records. To collect quality high-frequency groundwater times-series data, water-quality sondes and sensors require careful field operation, cleaning, and calibration, as well as specific procedures for data computation, evaluation, review, and publication of final records. This report provides guidelines for the use of water-quality sondes and sensors for high-frequency groundwater-quality monitoring and updates the guidance pertaining to standardized records computation procedures for a wide range of groundwater environments. This report builds on previous continuous surface-water-quality monitoring guidance documentation for water temperature, specific conductance, pH, dissolved oxygen, and nitrate. The specific groundwater-quality monitoring guidelines presented in this report address station selection, design, installation, and operations; sonde and sensor inspections and cleaning and calibration methods; troubleshooting procedures; data evaluations, data corrections, and record computations; and record review, approval, and auditing procedures for the groundwater environment.