Seasonal and Spatial Variation in the Location and Reactivity of a Nitrate‐Contaminated Groundwater Discharge Zone in a Lakebed

Abstract

Groundwater discharge delivering anthropogenic N from surrounding watersheds can impact lake nutrient budgets. However, upgradient groundwater processes and changing dynamics in N biogeochemistry at the groundwater‐lake interface are complex. In this study, seasonal water‐level variations in a groundwater flow‐through lake altered discharge patterns of a wastewater‐derived groundwater contaminant plume, thereby affecting biogeochemical processes controlling N transport. Pore water collected 15 cm under the lakebed along transects perpendicular to shore varied from oxic to anoxic with increasing nitrate concentrations (10–75 μM) and corresponding gradients in nitrite and nitrous oxide. Pore water depth profiles of nitrate concentrations and stable isotopic compositions largely reflected upgradient groundwater N sources and N cycle processes, with minor additional nitrate reduction in the near‐surface lakebed sediments. Potential denitrification rates determined in laboratory microcosms were 10–100 times higher in near‐surface sediments (0–5 cm) than in deeper sediments (5–30 cm) and were correlated with sediment carbon content and abundance of denitrification genes (nirS, nosZI, and nosZII). Potential anammox‐driven N2 production was detectable in deeper anoxic sediments. Injection of bromide and nitrite in the lake sediments showed that the highest net nitrite consumption rates were within the top 10 cm. However, short transit times owing to rapid upward pore water velocities (4–5 cm hr−1) limited removal of the contaminant nitrate transiting through the sediments. Results demonstrate that local hydrologic and biogeochemical processes at the point of discharge affect the distribution and discharge rate of N through lakebed sediments, but processes in the upgradient groundwater can be more important for affecting N speciation and concentration.

Surface-water/groundwater boundaries affect seasonal PFAS concentrations and PFAA precursor transformations

“…These results are consistent with previously reported groundwater discharge patterns. 34,35,[49][50][51] In Ashumet Pond, mean PFAS concentrations were highest for peruorohexane sulfonate (PFHxS) (74 AE 6.5 ng L À1 ), peruorooctane sulfonate (PFOS) (50 AE 4.1 ng L À1 ), and peruorooctanoate (PFOA) (29 AE 2.8 ng L À1 ). No signicant temporal differences in concentrations of the sum of 23 targeted PFAS (S 23 PFAS) were observed between September and November 2017 (Mann-Whitney U test, p > 0.05), despite the collapse of the thermocline between the two sampling dates (Fig.…”

Evidence for interannual persistence of infectious influenza A viruses in Alaska wetlands

“…Standard water chemistry measures (pH, temperature, specific conductance, dissolved oxygen, and turbidity) were assessed in situ using a multiparameter water quality sonde (Yellow Springs Instruments; Yellow Springs, OH, USA) for each of our four sites. Approximately 4 l of water was also collected from each field site prior to the experiment (timepointzero or T0 henceforth; 5–7 September 2019) and sent to the lab to further characterize chemical concentrations of surface waters (inorganic anions, metals and trace elements, total dissolved nitrogen, dissolved organic carbon, and alkalinity; Table S1 ) using standard techniques ( Aiken, 1992 ; Barringer and Johnsson, 1996 ; Brinton et al, 1996 ; Garside, 1982 ; Repert et al, 2014 ; Smith et al, 2006 ; Smith et al, 2019 ; U.S. Environmental Protection Agency, 1993 ; U.S. Environmental Protection Agency, 1994 ). Quality assurance and quality control included the collection and analysis of three field blanks.…”

Tracing Bank Storage and Hyporheic Exchange Dynamics Using 222Rn: Virtual and Field Tests and Comparison With Other Tracers

“…It is noteworthy that Rn can be used as a tracer to quantify residence time (Hoehn & Von Gunten, 1989; Pittroff et al., 2017), hyporheic exchange flux (Cook et al., 2006), and exchange zone size (Gilfedder et al., 2019). While Rn application is limited to timescales of a few days (∼10–15 days maximum), most river corridor geochemical reactions of primary interest occur within this timeframe (Harvey et al., 2013; Sgouridis et al., 2011; Smith et al., 2019). With the improvement of measurement technology (Durejka et al., 2019; Gilfedder et al., 2013), the analysis of Rn is becoming more convenient.…”