Linking Denitrification and Infiltration Rates during Managed Groundwater Recharge

We quantify relations between rates of in situ denitrification and saturated infiltration through shallow, sandy soils during managed groundwater recharge. We used thermal methods to determine time series of point-specific flow rates, and chemical and isotopic methods to assess denitrification progress. Zero order denitrification rates between 3 and 300 μmol L–1 d–1 were measured during infiltration. Denitrification was not detected at times and locations where the infiltration rate exceeded a threshold of 0.7 ± 0.2 m d–1. Pore water profiles of oxygen and nitrate concentration indicated a deepening of the redoxocline at high flow rates, which reduced the thickness of the zone favorable for denitrification. Denitrification rates were positively correlated with infiltration rates below the infiltration threshold, suggesting that for a given set of sediment characteristics, there is an optimal infiltration rate for achieving maximum nitrate load reduction and improvements to water supply during managed groundwater recharge. The extent to which results from this study may be extended to other managed and natural hydrologic settings remains to be determined, but the approach taken in this study should be broadly applicable, and provides a quantitative link between shallow hydrologic and biogeochemical processes

Historical and Operational Monitoring of Surface Sediments in the Lower Mekong Basin Using Landsat and Google Earth Engine Cloud Computing

Reservoir construction and land use change are altering sediment transport within river systems at a global scale. Changes in sediment transport can impact river morphology, aquatic ecosystems, and ultimately the growth and retreat of delta environments. The Lower Mekong Basin is crucial to five neighboring countries for transportation, energy production, sustainable water supply, and food production. In response, countries have coordinated to develop programs for regional scale water quality monitoring that including surface sediment concentrations (SSSC); however, these programs are based on a limited number of point measurements and due to resource limitations, cannot provide comprehensive insights into sediment transport across all strategic locations within the Lower Mekong Basin. To augment in situ SSSC data from the current monitoring program, we developed an empirical model to estimate SSSC across the Lower Mekong Basin from Landsat observations. Model validation revealed that remotely sensed SSSC estimates captured the spatial and temporal dynamics in a range of aquatic environments (main stem of Mekong river, tributary systems, Mekong Floodplain, and reservoirs) while, on average, slightly underestimating SSSC by about 2 mg·L−1 across all settings. The operational SSSC model was developed and implemented using Google Earth Engine and Google App Engine was used to host an online application that allows users, without any knowledge of remote sensing, to access SSSC data across the region. Expanded access to SSSC data should be particularly helpful for resource managers and other stakeholders seeking to understand the dynamics between surface sediment concentrations and land use conversions, water policy, and energy production in a globally strategic region