EXAMINING SAMPLING CHARACTERISTICS IN SURFACE WATER QUALITY DATA: BIAS DUE TO THE INFLUENCE OF BASEFLOW

The Mulberry River in north-central Arkansas was designated a Wild and Scenic River by Congress in 1992 and was noted specifically for outstanding recreational and scenic attributes. In 2008, a portion of the Upper Mulberry was listed as impaired on the 303 (d) list by the Arkansas Department of Environmental Quality (ADEQ) due to periodic low pH (acidic) conditions. Although continuous, long-term water quality records do not exist for the Upper Mulberry, ADEQ has routinely collected water quality samples at a station in the lower part of the watershed. Trends in pH at this lower station show an average linear decline of approximately 0.3 units per decade from 1990-2014. However, relying simply on long-term trends derived from random sampling over a wide variety of flow conditions in an area with significant differences in groundwater/surface water/rainfall pH introduces the possibility of a sampling bias in the analysis. As part of a larger study on identifying the cause(s) of acidity in the Mulberry River, this study used the local minimum method baseflow separation technique to determine if the observed downward pH trend is representative of actual conditions, or if an unintentional bias exists in the long-term record.