Factors controlling phosphorus mobility in nearshore aquifers adjacent to large lakes.

Internal P stores in offshore lakebed sediments play an important role in lake nutrient dynamics. While P stores in nearshore aquifer sediments may also be important for nutrient dynamics, it is unclear whether P accumulates in these sediments, and if so, what factors control P accumulation and its potential later release from the sediments to nearshore waters. This knowledge gap was addressed by conducting field investigations at seven nearshore sites located along the shores of Lake Erie, Lake Huron and Lake Ontario, Canada, with more detailed dissolved and sediment phase characterization completed for two nearshore sites. PO4 concentrations were observed to be higher (>50 µg/L) in the more reducing nearshore aquifers compared to more oxidizing nearshore aquifers (<20 µg/L), despite similar total solid phase P concentrations at the sites. PO4 mobility in the nearshore aquifers was found to be closely linked to redox-driven Fe cycling. In the more reducing aquifers, dissolved PO4 was highest near the redox boundary present in the shallow sediments where oxic infiltrating surface water mixes with reducing groundwater. In the more oxidizing aquifers, solid phase characterization indicated that PO4 is sequestered to Fe oxide mineral phases throughout the nearshore aquifer which explains the low dissolved PO4. While pH was not found to be important for PO4 mobility at the study sites, batch laboratory experiments indicate that increased infiltration of more alkaline surface water into nearshore aquifers may promote PO4 release from the sediments. The study findings demonstrate that while internal P storage mechanisms in nearshore aquifer sediments may currently be limiting P loads to lakes, it is possible that P stores that build up over time may result in increased P loads to lakes in the future.

Occurrence of Arsenic in Nearshore Aquifers Adjacent to Large Inland Lakes.

Metal oxides that form near sediment-water interfaces in marine and riverine settings are known to act as a sediment trap for pollutants of environmental concern (e.g., arsenic and mercury). The occurrence of these pollutant traps near sediment-water interfaces in nearshore lake environments is unclear yet important to understand because they may accumulate pollutants that may be later released as environmental conditions change. This study evaluates the prevalence of pollutant sediment traps in nearshore aquifers adjacent to large lakes and the factors that affect the accumulation and release of pollutants, specifically arsenic. Field data from six sites along the Laurentian Great Lakes indicate widespread enrichment of arsenic in nearshore aquifers with arsenic sequestered to iron oxide phases. Arsenic enrichment at all sites (solid-phase arsenic >2 µg/g) suggests that this is a naturally occurring phenomenon. Arsenic was more mobile in reducing aquifers with elevated dissolved arsenic (up to 60 µg/L) observed, where reducing groundwater mixes with infiltrating oxic lake water. Dissolved arsenic was low (<3 µg/L) in all oxic nearshore aquifers studied despite high solid-phase arsenic concentrations. The findings have broad implications for understanding the widespread accumulation of reactive pollutants in nearshore aquifers and factors that affect their release to large lakes.

Spatiotemporal controls on septic system derived nutrients in a nearshore aquifer and their discharge to a large lake.

This study evaluates spatiotemporal variability in the behavior of septic system derived nutrients in a sandy nearshore aquifer and their discharge to a large lake. A groundwater nutrient-rich plume was monitored over a two-year period with the septic system origin of the plume confirmed using artificial sweeteners. High temporal variability in NO3-N attenuation in the nearshore aquifer prior to discharge to the lake (42-96%) reveals the complex behavior of NO3-N and potential importance of changing hydrological and geochemical conditions in controlling NO3-N discharge to the lake. While PO4-P was retarded in the nearshore aquifer, the PO4-P plume extended over 90 m downgradient of the septic system. It was estimated that the PO4-P plume may reach the lake within 10 years and represents a legacy issue whereby PO4-P loads to the lake may increase over time. To provide broader assessment of the contribution of septic systems to P and N loads to a large lake, a regional scale geospatial model was developed that considers the locations of individual septic systems along the Canadian Lake Erie shoreline. The estimated P and N loads indicate that septic systems along the shoreline are only a minor contributor to the annual P and N loads to Lake Erie. However, it is possible that nutrients from septic systems may contribute to localized algal blooms in shoreline areas with high septic system density. In addition, disproportionate P and N loads in discharging groundwater may change the N:P ratio in nearshore waters and promote growth of harmful cyanobacteria. The study provides new insights into factors controlling the function of the reaction zone near the groundwater-lake interface including its impact on groundwater-derived nutrient inputs to large lakes. Further, the study findings are needed to inform septic system and nutrient management programs aimed at reducing lake eutrophication.

Effect of Transient Wave Forcing on the Behavior of Arsenic in a Nearshore Aquifer.

Groundwater-coastal water interactions influence the fate of inorganic chemicals in nearshore aquifers and their flux to receiving coastal waters. This study evaluates the impact of variable wave conditions on the geochemical changes and distribution of mobile arsenic (As) in a nearshore aquifer. Field measurements in a sandy nearshore aquifer on Lake Erie, Canada, are presented with geochemical changes analyzed over a period of high waves. A numerical model of wave-induced groundwater flow dynamics, validated against field data, is used to provide insight into the physical flow processes underlying the observed geochemical changes. Rapid changes in dissolved As, Fe, Mn, and S demonstrate the importance of reactions as well as dynamic transport in controlling the behavior of reactive species, especially those that are redox sensitive. Field data suggest the presence of sediment traps, which under certain hydrological and geochemical conditions may result in a "hot moment" with episodic release of As. The study provides new insight into factors controlling the fate of reactive species in dynamic coastal environments as required to better predict chemical fluxes to coastal waters. Additionally, it highlights the need to pay particular attention to "hot moments" for reaction and transport caused by storms and waves.