Martha Conklin, Roger Bales & Josh Clemons

Department of Hydrology & Water Resources, University of Arizona

To effectively assess water quality changes over time, the frequency, accuracy and precision of the water quality measurements must be known. To evaluate the GLOBE data, we analyzed data from six measurements: temperature, pH, conductivity, alkalinity, dissolved oxygen and transparency. The recommended frequency for GLOBE water quality sampling is weekly basis throughout the entire year. However, some schools do measurements monthly. When an important event occurs (e.g. snowmelt or flood) more frequent sampling may be needed to capture the changes during the event and understand how the water body responds to the event. We will present suggestions for sampling frequency in order to capture information about a specific event. We are also evaluating the accuracy and precision of GLOBE data relative to that specified in the protocols. For example, according to its National Field Manual for the Collection of Water-Quality Data, the U.S. Geological Survey requires reproducibility of acid neutralizng capacity (comparable to GLOBE alkalinity) measurements on replicate samples of + 5%, or, if “large amounts of particulate matter” are present in the sample, + 10%. The maximum acceptable difference for alkalinity measurements between GLOBE student groups testing the same water sample is 4 mg/L as CaCO₃ when using one kit or one drop when using a second kit. The one-drop difference represents either 6.8 or 17 mg/L, depending on the range selected. If we examine the required accuracy at 60 mg/L as CaCO₃, the mean alkalinity for all GLOBE sites reporting, these translate to 7-11% (low range for second kit). For a concentration of 130 mg/L as CaCO₃, which is in the 75^th percentile for all GLOBE sites reporting, these translate to 3-13 % (high range for second kit). We will then discuss the impact of these accuracy and precision levels on use of data from GLOBE schools.