Determining groundwater Ra end‐member values for the estimation of the magnitude of submarine groundwater discharge using Ra isotope tracers

Abstract

Radium isotopes (228Ra and 226Ra) are excellent tracers of submarine groundwater discharge (SGD). To estimate SGD magnitudes, information on the end‐member values of Ra concentrations in groundwater is critical; however, the distribution characteristics of Ra in coastal aquifers are poorly understood. In this study, we show that Ra concentrations in coastal groundwater are primarily dependent on salinity based on the data (n > 500) obtained from global coastal aquifers, although previous end‐member calculations averaged all Ra concentrations without considering salinity. If we assume that SGD is composed mainly of seawater infiltrating the aquifer, previous estimates of SGD for the Atlantic Ocean and the global ocean were overestimated twofold to threefold. This may be similar for other applications using different Ra isotopes. Our study highlights that the end‐members of Ra isotopes in groundwater should be carefully considered when estimating SGD using Ra isotope mass balances in the ocean.

Evaluation of Water Residence Time, Submarine Groundwater Discharge, and Maximum New Production Supported by Groundwater Borne Nutrients in a Coastal Upwelling Shelf System

“…Kwon et al () deployed a 228 Ra inverse model which led to a global SGD estimate of (12 ± 3) × 10 13 m 3 yr −1 . The global SGD estimate was further refined to be (4.4 ± 1.2) × 10 13 m 3 yr −1 by considering geographic and salinity effects on radium activities in groundwater end‐member (Cho & Kim, ). A modified 228 Ra inverse model has been recently developed, which leads to an update global SGD estimate of 1.3 – 14.7 × 10 13 m 3 yr −1 (Le Gland et al, ).…”

Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers: Controls on submarine groundwater discharge and chemical inputs to the ocean

“…Similarly, reviews that provide a synthesis of the growing number of SGD field studies are available Kwon et al, 2014;Cho and Kim, 2016]. Saltwater intrusion in coastal aquifers, a topic inextricably linked to nearshore groundwater dynamics and SGD, was extensively studied over the last 50 y with a comprehensive review recently provided by .…”

Ra Tracer‐Based Study of Submarine Groundwater Discharge and Associated Nutrient Fluxes into the Bohai Sea, China: A Highly Human‐Affected Marginal Sea

“…Usually, groundwater around the coast is recognized as the primary source of radium to coastal water. Cho and Kim () suggest that the geographical skewness of data distributions does not significantly affect the end‐member values in the groundwater. Therefore, in this study, we use the average values of 226 Ra and 228 Ra as the initial Ra input to the Bohai Sea, as 94 ± 43 and 565 ± 128 dpm 100 L −1 for 226 Ra and 228 Ra, respectively.…”

“…The reason for this trend may be increasing pollution from human activity, with some pollutants possibly penetrating into the groundwater. In addition, SGD is essentially ubiquitous in coastal areas; SFGD in particular usually occurs in the shallow water up to 1,000 m offshore (Cho & Kim, ). In the Bohai Sea, red tide is a major environmental problem, with frequent outbreaks over the last decade, especially along the coasts (State Oceanic Administration of China, SOA, http://www.coi.gov.cn/gongbao/huanjing/).…”