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

Abstract

Continuous monitoring data can be extremely useful for assessing water quality conditions particularly for variables that exhibit dynamic diel swings such as dissolved oxygen. As a means of evaluating dissolved oxygen criteria used by the Arkansas Department of Environmental Quality (ADEQ) for assessing this stream class, we compared continuous dissolved oxygen (DO) data collected at five small- to moderate-sized (watersheds 10-100 mi 2 ), high-gradient streams in the Boston Mountains distributed across a land-use and nutrient condition gradient. The current DO criteria employed by ADEQ for Boston Mountains streams >10 mi2 consists of both an exceedance rate and a magnitude, in which, streams may be considered 'impaired' if greater than 10% of DO measurements during a period of record are < 6 mg/L. The 10% exceedance rate, however, is a commonly used “default” value that requires independent testing for different ecoregion stream classifications. Our findings for the five Boston Mountain streams fit a general pattern established for other aquatic systems (e.g. larger streams, low-gradient streams, and lakes) where increasing land-use intensity generally results in increased nutrient concentrations, which can lead to 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 10% exceedance rate currently 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 associating DO variability to anthropogenic or natural origins. Considerations that were useful for examining these relationships and evaluating ADEQ’s DO criteria should be applicable to DO studies in other locations where stream and geologic characteristics are like those of the Boston Mountains.