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

Authors: Veith, Tameria - Tamie; GALL, HEATHER - Pennsylvania State University; Elkin, Kyle

Submitted to: Journal of Soil and Water Conservation Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/2/2019
Publication Date: 5/1/2020
Citation: Veith, T.L., Gall, H.E., Elkin, K.R. 2020. Characterizing transport of natural and anthropogenic constituents in a long-term agricultural watershed in the northeastern US. Journal of Soil and Water Conservation Society. 75(3):319-329. https://doi.org/10.2489/jswc.75.3.319.
DOI: https://doi.org/10.2489/jswc.75.3.319

Interpretive Summary: New equipment for collecting in-stream water samples have enabled scientists to more completely capture changes in water quality during both short-term and long-term stream flow changes. We collected and analyzed data using three different sampling strategies over four years of stream flow in a long-term USDA-ARS experimental watershed. Using a number of data analysis techniques that look at natural and anthropogenic drivers of contaminant transport, we found that agricultural management practices appear to have been effective in reducing legacy phosphorus values but have been less effective in reducing legacy nitrogen. These findings emphasize the importance of continued nutrient management of our agricultural fields to minimize increase of phosphorus in the soils and to prevent unnecessary additions of nitrogen into the surface and subsurface water.

Technical 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., sub-daily) 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 (12544 samples from 2015 to 2019) of water quality data for twelve constituents of interest were collected using three sampling strategies: (i) three times per week; (ii) high-temporal resolution flow-paced sampling to capture stormflow; and (iii) time-paced sampling with a time interval of 4 hours. Seasonal trends were investigated to understand concentration variability over time and concentration-discharge (C-Q) relationships were developed to determine whether the constituents exhibited accretion or dilution patterns or were independent of flow. Lorenz curves and Gini coefficients were employed to quantify the temporal inequality of the constituent loads discharged at the watershed outlet and understand the extent to which the transport behavior of geogenic constituents and those affected by anthropogenic activities differed. Overall, the results suggested that nearly all of the geogenic constituents, plus NO3-N and SO4-S, exhibited chemostatic dynamics with loads overwhelmingly that were controlled by flow variability, whereas Al, Fe, NO2-N, and PO4-P exhibited episodic transport dynamics largely controlled by source availability. Since the transport of NO3-N is similar to the transport of geogenic constituents, this suggests that decades of agricultural activities in the watershed have led to the emergence of legacy nitrogen sources while best management practices have likely prevented the emergence of phosphorus legacy sources.