T193 Coastal Aquifers: Investigations of Submarine and Coastal Groundwater Discharge

Tuesday, 11 October 2022
1:30 PM - 5:30 PM
405

GEOCHEMICAL CHARACTERIZATION OF SURFACE AND GROUNDWATER IN A PROGLACIAL DELTA: IMPLICATIONS FOR COASTAL AND SUBMARINE GROUNDWATER DISCHARGE TO THE GULF OF ALASKA

Paper number: T193-10
Program: Technical Sessions
Time: 3:40 PM – 3:55 PM

Authors

Abstract

High latitude coastal catchments within the Gulf of Alaska region are undergoing rapid hydrologic modification driven by global climate change. These glacierized and non-glacierized watersheds deliver ecologically important organic and inorganic nutrients to the nearshore environment. A majority of the work describing the biogeochemical cycles of freshwater fluxes to the Gulf of Alaska focus on surface runoff. Recently, submarine groundwater discharge has been implicated as an important source of nutrient flux to the local intertidal ecosystems. We report here an investigation in the glaciofluvial delta of the Wosnesenski River that discharges to Kachemak Bay, situated in the northern Gulf of Alaska. We use geochemical data collected from the river and two shallow groundwater wells, with one well installed near the river bank approximately 3 km upstream from the outlet, and the second lower well 2.5 km down gradient within an abandoned portion of the delta. Specific conductance observed in wells increased with distance from the river. River water δ18O values averaged -14.5 ‰ while the upper well δ18O values averaged -14.2 ‰ and the lower well averaged -14.6 ‰ indicating a similar source for these waters. All waters were Ca-HCO3 type, with the lower well water having elevated Mg:Ca and Cl:HCO3 values. The sum of major cations and SiO2 (TDS) was such that the concentrations increased from the upper well to the lower, 38.72 mg/L to 66.87 mg/L respectively. River water had the lowest TDS, averaging 24.92 mg/L. These data reaffirm that coastal groundwaters are elevated in nutrients, illustrating the importance of incorporating groundwater geochemistry in studies of coastal catchments. The shallow groundwater within the delta eventually discharges to the ocean, delivering elevated concentrations of nutrients compared to the river water. Surface water, while extremely important, is only one piece of the nutrient flux to the intertidal. Studies aimed at investigating nutrient load and freshwater flux from the coast need to incorporate analysis that includes subsurface components in order to fully characterized the terrestrial to ocean nutrient connection. Geochemical characterization of coastal groundwater flux is virtually unexplored within the Gulf of Alaska and this work is a step towards filling the knowledge gap.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-380618

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