Rare earth element behavior during groundwater–seawater mixing along the Kona Coast of Hawaii

Abstract

Groundwater and seawater samples were collected from nearshore wells and offshore along the Kona Coast of the Big Island of Hawaii to investigate rare earth element (REE) behavior in local subterranean estuaries. Previous investigations showed that submarine groundwater discharge (SGD) is the predominant flux of terrestrial waters to the coastal ocean along the arid Kona Coast of Hawaii. Groundwater and seawater samples were filtered through 0.45 μm and 0.02 μm pore-size filters to evaluate the importance of colloidal and soluble (i.e., truly dissolved ionic species and/or low molecular weight [LMW] colloids) fractions of the REEs in the local subterranean estuaries. Mixing experiments using groundwater collected immediately down gradient from a wastewater treatment facility (WWTF) proximal to the Kaloko-Hanokohau National Historic Park, and more "pristine" groundwater from a well constructed in a lava tube at Kiholo Bay, were mixed with local seawater to study the effect of solution composition (i.e., pH, salinity) on the concentrations and fractionation behavior of the REEs as groundwater mixes with seawater in Kona Coast subterranean estuaries. The mixed waters were also filtered through 0.45 or 0.02 μm filters to ascertain the behavior of colloidal and soluble fractions of the REEs across the salinity gradient in each mixing experiment. Concentrations of the REEs were statistically identical (two-tailed Student ttest, 95% confidence) between the sequentially filtered sample aliquots, indicating that the REEs occur as dissolved ionic species and/or LMW colloids in Kona Coast groundwaters. The mixing experiments revealed that the REEs are released to solution from suspended particles or colloids when Kona Coast groundwater waters mix with local seawater. The order of release that accompanies increasing pH and salinity follows

Gadolinium as a new emerging contaminant of aquatic environments

“…In addition, a direct influence of Gd ant detected in surface water (contaminated with wastewater) on the amount of Gd ant in groundwater was observed. The positive Gd anomaly in groundwater of urban areas indicates their mixing with wastewater (e.g., discharging points from a WWTP facility, leaking waste pipes) or recycled water by bank filtration (Knappe et al 2005;Rabiet et al 2009;Birka et al 2016;Br€ unjes et al 2016;Johannesson et al 2017). Therefore, Gd anomaly phenomena can be used by hydrologists and geologists as tracers for anthropogenic impacts on aquatic ecosystems (M€ oller et al 2000;Lawrence and Bariel 2010;Telgmann et al 2013;Tepe et al 2014).…”

The Impact of Benthic Processes on Rare Earth Element and Neodymium Isotope Distributions in the Oceans

“…Budgetary estimates from diffuse benthic REE fluxes, such as SGD or from sedimentary pore water Greaves et al, 1999;Haley and Klinkhammer, 2003;Tachikawa et al, 2003;Johannesson and Burdige, 2007;Schacht et al, 2010;Johannesson et al, 2011Johannesson et al, , 2017Abbott et al, 2015a;Fröllje et al, 2016), suggest that they may dominate the flux of REEs to the oceans. Importantly, such diffuse, or at least hard to quantify, sources are consistent with deep-water isopycnal mixing of ε Nd signals off margins that cannot be explained through surface water sources (Grasse et al, 2012;Grenier et al, 2013).…”

Gadolinium as an Emerging Microcontaminant in Water Resources: Threats and Opportunities

“…However, no anomalous Gd concentrations in some urban rivers, such as the Chao Phraya that runs through a densely populated area in Thailand [14], implies that Gd anth inputs are mainly restricted to regions with a highly developed healthcare system and mass application of MRI tests. Positive Gd anomalies of variable size have been observed in the influent and effluent of WWTPs [13,[15][16][17], rivers [2,7,[9][10][11][12]18,19], seawaters [1,12,15,20], groundwaters [21], and tap waters [7,10] due to anthropogenic Gd input.…”