Relations of dissolved-oxygen variability, selected field constituents, and metabolism estimates to land use and nutrients in high-gradient Boston Mountain streams, Arkansas

Continuous monitoring data can be extremely useful for assessing water-quality conditions particularly for variables, such as dissolved oxygen, that exhibit dynamic diel swings. As a means of evaluating stream dissolved oxygen criteria used by the Arkansas Department of Environmental Quality (ADEQ), we compared continuous dissolved oxygen (DO) data collected at five small- to moderate-sized (watersheds 10-100 mi²), high-gradient streams in the Boston Mountains distributed across a land-use and nutrient condition gradient. The sampled streams exhibit a general pattern established for other aquatic systems (e.g., larger streams, low-gradient streams, and lakes) where increasing land-use intensity results in increased nutrient concentrations, stream eutrophication, and increased DO variability. DO concentrations were < 6 mg/L for fewer than 4% of measurements at the two sites identified "a priori" as least disturbed by nutrient and land-use indices, while concentrations at the three sites identified as moderately and most disturbed were < 6 mg/L for 20 to 33% of measurements. These findings demonstrate that the current criterion (10% of the DO measurements are < 6 mg/L) employed by ADEQ was effective at identifying various degrees of DO impairment in Boston Mountain streams. Our analysis also demonstrated that continuous pH and specific conductance data and estimates of stream metabolism were helpful for attributing DO variability to anthropogenic or natural origins. Considerations that were useful for examining these relationships and evaluating ADEQ's DO criterion should be applicable to DO studies in other locations where stream and geologic characteristics are similar to those of the Boston Mountains.