Characterizing transport of natural and anthropogenic constituents in a long-term agricultural watershed in the northeastern United States

Abstract

As recent technologies enable water samples to be collected at increasingly shorter time intervals, water quality data can more fully capture the range of conditions a stream experiences over time. Various metrics can be employed with the large, high-temporal resolution (i.e., subdaily) data sets to gain insights into the hydroclimatic and biogeochemical processes affecting chemical fate and transport. These insights can be helpful in understanding the extent to which anthropogenic activities have impacted the natural response of some constituents, such as nutrients and salts, in managed landscapes. Here, nearly four years (12,544 samples from 2015 to 2019) of water quality data for 12 constituents of interest were collected using three sampling strategies: (1) low-frequency sampling three times per week, (2) high-temporal resolution flow-paced sampling to capture stormflow, and (3) high-frequency time-paced sampling with a time interval of four hours. Seasonal trends were investigated to understand concentration variability over time, and concentration-discharge (C-Q) relationships were developed to categorize the transport dynamics of each constituent. Lorenz curves and Gini coefficients were employed to quantify the temporal inequality of the constituent loads discharged at the watershed outlet and understand the extents to which the transport behaviors of geogenic constituents and those affected by anthropogenic activities differed. Overall, the results suggested that nearly all of the geogenic constituents, plus nitrate-nitrogen (NO3-N) and sulfate (SO4-S), exhibited chemostatic dynamics with loads overwhelmingly controlled by flow variability, whereas aluminum (Al), iron (Fe), nitrite-N (NO2-N), and orthophosphate (PO4-P) exhibited episodic transport dynamics that were likely controlled by limited source availability. Since the transport of NO3-N was found to be similar to the transport of common geogenic constituents for the region, this suggests that decades of agricultural activities in the watershed have led to the emergence of legacy N sources, while the episodic dynamics observed for PO4-P suggest that best management practices appear to have prevented the emergence of P legacy sources.