Linking Hydrobiogeochemical Processes and Management Techniques to Close Nutrient Loops in an Arid River

Abstract

In this study, we explored opportunities to optimize food-energy-water (FEW) resources by closing nutrient loops in aridland rivers. We evaluated source and sink behavior of nitrogen as nitrate (NO 3-N) in three connected channels associated with an irrigation network, i.e., man-made delivery and drain canals, and the main stem of the Rio Grande river near Albuquerque, New Mexico, USA. All three channels are located downstream of a large wastewater treatment plant that is the main contributor of nutrients to this reach of the Rio Grande. We used a mass balance approach paired with stable isotope analysis to link sources and processing of NO 3-N with reaction pathways within the channels over time (a year) and through space (along ∼14-53 km reaches). Results indicated that the growing season was an important period of net sink behavior for the delivery channel and the Rio Grande, but the drain channel was a year-round net source. Stable isotope analyses of 15 N and 18 O found a distinct nitrate signature in the drain associated with biological processing, as well as sites along the Rio Grande impacted by agricultural outflow, but no equivalent signature was present in the delivery channel. Based on our findings, we provide recommendations to help close nutrient loops in our study system and in analogous aridland irrigation networks by (1) minimizing loss during the transfer of nutrients from wastewater facilities to agricultural areas, and (2) minimizing enrichment to downstream aquatic ecosystems by sequestering nutrients that would otherwise escape the nutrient loop.

Inputs from wastewater treatment plant effluent influence the temporal variability of nutrient uptake in an intermittent stream

“…Therefore, a better understanding of the temporal variability in nutrient uptake capacity of the receiving streams, and especially of the biogeochemical processes prevailing during low ow conditions, is important for improving the management of urban streams impacted by point-sources. In this context, our study conveys with the perspective of current studies suggesting management strategies for WWTP-receiving streams (i.e., Bicknell et al 2020). In particular, our study suggests that it is critical to design WWTP operation procedures taking into account both the dilution and the bioreactive capacity of receiving streams for an integrated management of water resources and their quality in urban landscapes, especially under water scarcity conditions.…”

“…The contribution of the WWTP uent to the ow of the receiving stream was highly variable (from < 1 to 100%) depending on the period of the year. This pattern is typically observed in many other intermittent streams across arid and semiarid regions (Arnon et al 2015;Martí et al 2010;Bicknell et al 2020), because their hydrologic regime is characterized by extreme events (i.e., oods and droughts). This nding explains the strong temporal shifts in the impact that WWTP e uent inputs have on the receiving streams of these regions as shown in our study.…”

“…Nevertheless, the extent to which hydrology will interact with chemistry to control temporal variation of nutrient uptake in receiving stream is not fully understood yet. In urban landscapes with water scarcity, such as the Mediterranean region, the temporal patterns of hydrology and dilution capacity in receiving streams as well as the biogeochemical derived effects could be magni ed because the dilution capacity of the receiving stream can largely vary on an annual basis from 0% to > 95% (Martí et al 2010;Bicknell et al 2020).…”

Wastewater treatment plant effluent inputs influence the temporal variability of nutrient uptake in an intermittent stream

“…The impact of the WWTP effluent in the study stream was dramatic for both physical and chemical variables, especially during summer and autumn when the dilution factor was well below the 40% threshold. WWTP effluent contribution to the receiving stream highly varied (from 0 to 100%) during the year, which it is common for intermittent streams across arid and semiarid regions (Arnon et al 2015;Martí et al 2010;Bicknell et al 2020), because extreme events (i.e., floods and droughts) characterize the hydrologic regime. During high flow conditions (i.e., winter and spring), landscape features and upstream conditions strongly influence stream physical and chemical characteristics, whereas these characteristics become increasingly determined by WWTP effluent inputs as the dilution capacity of the receiving stream decreases due to upstream flow reduction (i.e., summer) (Keller et al 2014).…”

“…However, understanding variation in in-stream nutrient processing over time in rivers that receive WWTP effluent inputs is essential for management of freshwater resources in urban landscapes. This is especially true for landscapes with water scarcity, such as the Mediterranean region, where the dilution capacity of receiving streams can vary from 0% to > 95% (Martí et al 2010;Bicknell et al 2020), encompassing large seasonal changes in both water mean residence time and stream nutrient concentrations downstream of WWTP effluent inputs. In this context, we aimed to assess the temporal variability and controlling factors of nutrient uptake in a Mediterranean stream with an intermittent hydrologic regime receiving WWTP effluent inputs.…”