Modeling time series radon inventory and constraints on the submarine groundwater discharge mass balance of a well-mixed, highly dynamic estuary

Date

2022-05

Authors

Wolfe, William

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Abstract

Submarine groundwater discharge (SGD) is defined as any and all types of water flow from the seabed to the coastal ocean, which includes both terrestrial groundwater inputs and seawater recirculated through marine sediments. Although often diffuse and over-shadowed by surface-water inputs, SGD represents a significant source of freshwater and solutes to many sensitive marine environments. The naturally occurring radioisotope 222Rn (i.e., radon, t1/2 = 3.82 days) is an effective tracer of SGD due to its non-reactive nature and dominantly terrestrial source. However, poor constraints on radon variability in shallow-turbulent estuaries introduce large uncertainties to SGD rate estimation. This study collected high-resolution measurements of radon (30-minute, n=10,660) at a stationary platform near Ward Island, Texas in the western Gulf of Mexico over a 24-month period from 2019-2021. Continuous, publicly available hydroclimatic parameters (wind speed/direction, groundwater/tide levels, air/water temperature, creek discharge and precipitation, n= 35,088) were coupled with radon observations through high-powered statistical analyses. Multi-linear regression (MLR), generalized additive modeling (GAM), cross-correlation functions (i.e., lag times) and Random Forest modeling quantified atmospheric and hydrologic controls on radon tracer variability that would otherwise be obscured over shorter study periods. Lag times and wind speed-direction (i.e., wind vector) were incorporated into a novel mass balance for SGD that more accurately accounts for atmospheric evasion and mixing losses than previously published methods. The excellent performance of the Random Forest model (R2 = 0.93) allowed for gap-filling of radon throughout the study period, providing uninterrupted SGD estimates across multi-seasonal conditions. Results of this study reveal SGD in this region to be more of a perennial process (i.e., less variable in magnitude across wet-dry seasons) than previous studies have reported using shorter sampling campaigns. Additionally, correlations between groundwater levels, precipitation, and SGD estimates provide insight into the origin of groundwater (shallow or deeper aquifers) supplying SGD to regional estuaries.

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Keywords

groundwater, mass balance, radon, random forest, SGD

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