NOM degradation during river infiltration: Effects of the climate variables temperature and discharge

Abstract

Most peri-alpine shallow aquifers fed by rivers are oxic and the drinking water derived by riverbank filtration is generally of excellent quality. However, observations during past heat waves suggest that water quality may be affected by climate change due to effects on redox processes such as aerobic respiration, denitrification, reductive dissolution of manganese(III/IV)- and iron(III)(hydr)oxides that occur during river infiltration. To assess the dependence of these redox processes on the climate-related variables temperature and discharge, we performed periodic and targeted (summer and winter) field sampling campaigns at the Thur River, Switzerland, and laboratory column experiments simulating the field conditions. Typical summer and winter field conditions could be successfully simulated by the column experiments. Dissolved organic matter (DOM) was found not to be a major electron donor for aerobic respiration in summer and the DOM consumption did not reveal a significant correlation with temperature and discharge. It is hypothesized that under summer conditions, organic matter associated with the aquifer material (particulate organic matter, POM) is responsible for most of the consumption of dissolved oxygen (DO), which was the most important electron acceptor in both the field and the column system. For typical summer conditions at temperatures >20 °C, complete depletion of DO was observed in the column system and in a piezometer located only a few metres from the river. Both in the field system and the column experiments, nitrate acted as a redox buffer preventing the release of manganese(II) and iron(II). For periodic field observations over five years, DO consumption showed a pronounced temperature dependence (correlation coefficient r = 0.74) and therefore a seasonal pattern, which seemed to be mostly explained by the temperature dependence of the calculated POM consumption (r = 0.7). The river discharge was found to be highly and positively correlated with DO consumption (r = 0.85), suggesting an enhanced POM input during flood events. This high correlation could only be observed for the low-temperature range (T < 15 °C). For temperatures >15 °C, DO consumption was already high (almost complete) and the impact of discharge could not be resolved. Based on our results, we estimate the risk for similar river-infiltration systems to release manganese(II) and iron(II) to be low during future average summer conditions. However, long-lasting heat waves might lead to a consumption of the nitrate buffer, inducing a mobilization of manganese and iron.

Modeling the Impact of Stream Discharge Events on Riparian Solute Dynamics

“…These variations in stream stage induce the well-studied bank storage effect where water is temporarily stored in the riparian zone during high stream stage and subsequently released back to the stream when stream stage recedes to pre-event conditions (Squillace et al 1993;Chen and Chen 2003;McCallum et al 2010;Doble et al 2012;Grabs et al 2012;McCallum and Shanafield 2016). Along with the infiltration of river water into the riparian zone, river water constituents are transported into the riparian aquifer (Boutt and Fleming 2009;Sawyer et al 2014), where they potentially undergo transformations (Gu et al 2012;Diem et al 2013). For instance, riparian zones are known to be capable of removing elevated nutrient concentrations, like nitrogen species (Hill 1996).…”

Competing controls on groundwater oxygen concentrations revealed in multidecadal time series from riverbank filtration sites

“…We regard the following explanation of the observed long‐term trends and abrupt increases in the groundwater DO time series as the most likely. During periods in which river‐water infiltration is not affected substantially by alterations in the hydraulic regime (such as high pumping rates, extremes in river discharge, or changes in the state of the riverbed), increasing groundwater temperatures result in decreasing groundwater DO concentrations because of both a decrease in the physical solubility of oxygen, and an increase in microbial activity in the hyporheic zone and in the groundwater [ Chapelle , ; Greig et al ., ; Sprenger et al ., ; Diem et al ., ]. This may be compounded by a simultaneous decrease in DO concentration in the losing river.…”

Morphological, hydrological, biogeochemical and ecological changes and challenges in river restoration – the Thur River case study

“…Based on the travel-time estimates from EC fluctuations in the river and in observation wells as described above, Diem et al (2013c) were able to investigate the effects of temperature and discharge on degradation of natural organic matter during river infiltration. They developed a new modeling approach that allows efficiently estimating dissolved oxygen (DO) concentrations in groundwater from measured DO concentrations in the river under various temperature and discharge conditions (Diem et al, 2013a).…”