Using Tidal Fluctuation‐Induced Dynamics of Radium Isotopes (²²⁴Ra, ²²³Ra, and ²²⁸Ra) to Trace the Hydrodynamics and Geochemical Reactions in a Coastal Groundwater Mixing Zone

Abstract

The reactive transport of radium isotopes (224Ra, 223Ra, and 228Ra) in coastal groundwater mixing zones (CGMZs) is sensitive to shifts of redox conditions and geochemical reactions induced by tidal fluctuation. This study presents a spatial distribution and temporal variation of radium isotopes in the CGMZ for the first time. Results show that the activity of radium isotopes in the upper saline plume (USP) is comparatively low due to a short residence time and mixing loss induced by the infiltration of low radium seawater whereas the activity of radium isotopes in the salt wedge (SW) is comparatively high due to a long residence time in the aquifer. The spatial distribution of radium isotopes is determined by the partitioning of radium isotopes, groundwater residence time, and relative ingrowth rates of radium isotopes. In addition, the variation of radium isotopes in the USP lags slightly (∼0 h) whereas the fluctuation of radium isotopes in the SW lags significantly (∼12 h) behind sea level oscillation. Tidal fluctuation affects the partitioning of radium isotopes through controlling seawater infiltration and subsequently influences the dynamics of radium isotopes in the USP. Concurrently, seawater infiltration significantly affects geochemical processes such as the production of nutrients and total alkalinity. Therefore, radium dynamics in the USP have implications for these geochemical processes. The variation of radium isotopes in the USP also has potential implications for transformation of trace metals such as iron and manganese because of the close affinity of radium isotopes to manganese and iron oxides.

Spatial Characteristics Reveal the Reactive Transport of Radium Isotopes (224Ra, 223Ra, and 228Ra) in an Intertidal Aquifer

“…The 224 Ra activity was higher than other locations in the transition zone that have higher salinity (e.g., L3-1 and L3-2) despite higher salinity typically causing greater 224 Ra desorption from the sediment surface coatings and thus high dissolved phase activities. This result is consistent with our previous study (Liu, Jiao, Liang, & Luo, 2018) and is likely because the more saline groundwater at the shallower locations (L3-1 and L3-2) has a shorter residence time and high mixing (dilution) with newly infiltrating seawater. In contrast, the deeper groundwater in the transition zone may be upwelling from the salt-wedge with a long residence time and negligible mixing loss.…”

“…These studies evaluating the long‐term variations in radium isotopes used only one observation piezometer and therefore did not explore variations in the distribution of radium isotopes across the intertidal aquifer although geochemical reactions are known to have clear spatial patterns linked to the groundwater flow patterns and origins of the water (Robinson et al, ; Santos et al, ). More recently, Liu, Jiao, Liang, and Luo () conducted high spatial resolution sampling across the transition zone of the intertidal aquifer in Tolo Harbor, Hong Kong, illustrating the dissimilar behavior of the radium isotopes in the different zones of the transition zone and identifying that radium isotopes are comparatively low in the upper saline plume compared to the salt‐wedge due to short groundwater residence times. The focus of Liu, Jiao, and Liang () was evaluating the short‐term variations in radium isotopes induced by semidiurnal tidal fluctuations and the implications of these variations on other chemical constituents.…”

Seasonality of Nutrient Flux and Biogeochemistry in an Intertidal Aquifer

“…It is essential to recognize the zonation of groundwater in the intertidal aquifer since there are different nutrient compositions and geochemical reactions in groundwater of different zones due to distinct nutrient sources and biogeochemical transformation pathways in these zones (Y. Liu, Jiao, Liang, & Luo, ). The groundwater in the intertidal aquifer is separated into four water zones, that is, the shallow fresh groundwater (shallow FGW) zone ( S < 1 psu, δ 18 O > −5.1‰), the deep fresh groundwater (deep FGW) zone ( S < 1 psu, δ 18 O < −5.1‰), the transition zone (1 psu < S < 25 psu), and the high‐salinity zone ( S > 25 psu) according to the stable isotopic (δ 2 H and δ 18 O) composition and salinity (Y. Liu, Jiao, Liang, & Luo, ).…”

Radium isotopes and radon-222 as tracers of sediment-water interaction in Arctic coastal and lacustrine environments

“…The ratios were 19 to 47 across the Beaufort shelf, with little variation beyond analytical uncertainties (Figure 12a). 224 Ra/ 223 Ra ARs were greater than those of Mediterranean beach sands and less than Tolo Harbor (Hong Kong) sediments measured using similar methods, and slightly enriched compared to average bulk continental crust (Liu et al, 2018;Tamborski et al, 2019).…”

“…Panel (a) shows surface available activity ratios of 224 Ra/ 223 Ra. In light blue are values fromTamborski et al (2019) andLiu et al (2018), and in gray is the average value for continental crust. Panels b-d are bulk sediment radium activities measured via gamma spectrometry.…”