Potential use of low-cost data loggers for groundwater temperature in a regulatory program for public water supply: impact of measurement frequency at a karst spring

A simple low-cost procedure for data collection and evaluation was developed to investigate the utility of continuous data loggers and the impact of measurement frequency on a water-quality monitoring method for use in a regulatory program for public water supply and groundwater under the direct influence of surface water (GWUDI). Data loggers were utilized to continuously measure groundwater temperature, descriptive statistics and the coefficient of variation to demonstrate rapid and significant variations. Temperature data were collected from a karst spring located in the Great Valley section of Pennsylvania and reported on daily and 15-min measurement frequencies. In response to five summer rain events, thermographs constructed with the 15-min measurements indicated rapid large-magnitude temperature variations. The average magnitude of change across the five events was 5.3 °C, but the shifts operated on quick timescales; thus, variations were not captured when data were reported on the daily frequency. This resulted in a distorted evaluation on how significant the groundwater temperature variations were, because results were dependent on measurement frequency. The average coefficient of variation percentage for the 15-min datasets was 12%, compared to 3% for the dataset of daily measurements. These findings could have important implications to a public water-supply regulatory program, because the regulatory definition for GWUDI includes those groundwater sources that display rapid and significant variations in water quality. Without high-frequency measurements, one alarming result could be that GWUDI sources, particularly those located in fast-acting hydrogeologic settings, are misevaluated when maximum or minimum water-quality variations are not included in the evaluation. Advancement in science and technology has resulted in data loggers for groundwater temperature that are economical, robust, and easy to install. Thus, their application in regulatory programs should no longer be underutilized, especially in karst spring settings where source construction deficiencies and focused recharge features can result in rain-induced rapid infiltration and water-quality variations on the minute-to-hour timescale.