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

Journal Article · Mon Nov 14 00:00:00 EST 2016 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2016.11.009· OSTI ID:1416299

Johannesson, Karen H. ^[1]; Palmore, C. Dianne ^[1]; Fackrell, Joseph ^[2]; Prouty, Nancy G. ^[3]; Swarzenski, Peter W. ^[4]; Chevis, Darren A. ^[1]; Telfeyan, Katherine ^[5]; White, Christopher D. ^[1]; Burdige, David J. ^[6]

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1. Tulane Univ., New Orleans, LA (United States). Dept. of Earth and Environmental Sciences
2. Univ. of Hawaii, Honolulu, HI (United States). Dept. of Geology and Geophysics
3. U.S. Geological Survey, Santa Cruz, CA (United States). Pacific Coastal and Marine Science Center
4. U.S. Geological Survey, Santa Cruz, CA (United States). Pacific Coastal and Marine Science Center; International Atmoic Energy Agency, Vienna (Austria)
5. Tulane Univ., New Orleans, LA (United States). Dept. of Earth and Environmental Sciences; Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
6. Old Dominion Univ., Norfolk, VA (United States). Dept. of Ocean, Earth and Atmospheric Sciences

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. Previously we saw 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 conducted 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 t-test, 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 light REE (LREE) > middle REE (MREE) > heavy REE (HREE). Release of REEs in the mixing experiments is driven by decreases in the free metal ion activity in solution and the concomitant increase in the amount of each REE that occurs in solution as dicarbonato complexes [i.e., Ln(CO₃)₂^-] as pH increases across the salinity gradient. Input-normalized REE patterns of Kona Coast groundwater and coastal seawater are nearly identical and relatively flat compared to North Pacific seawater, indicating that SGD is the chief source of these trace elements to the ocean along the Kona Coast. Additionally, REE concentrations of the coastal seawater are between 10 and 50 times higher than previously reported open-ocean seawater values from the North Pacific, further demonstrating the importance of SGD fluxes of REEs to these coastal waters. Altogether, these observations indicate that large-scale removal of REEs, which characterizes the behavior of REEs in the low salinity reaches of many surface estuaries, is not a feature of the subterranean estuary along the Kona Coast. A large positive gadolinium (Gd) anomaly characterizes groundwater from the vicinity of the WWTF. The positive Gd anomaly can be traced to the coastal ocean, providing further evidence of the impact of SGD on the coastal waters. Estimates of the SGD fluxes of the REEs to the coastal ocean along the Kona Coast (i.e., 1.3–2.6 mmol Nd day^-1) are similar to recent estimates of SGD fluxes of REEs along Florida’s east coast and to Rhode Island Sound, all of which points to the importance of SGD as significant flux of REEs to the coastal ocean.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: National Science Foundation (NSF)
• Grant/Contract Number: AC52-06NA25396
• OSTI ID: 1416299
• Report Number(s): LA-UR-17-27500; TRN: US1800907
• Journal Information: Geochimica et Cosmochimica Acta, Vol. 198, Issue C; ISSN 0016-7037
• Publisher: The Geochemical Society; The Meteoritical Society
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
rare earth elements
submarine groundwater discharge
coastal ocean
subterranean estuary
hawaii