New perspectives on radium behavior within a subterranean estuary

Abstract

Abstract:Over the past decade, radium isotopes have been frequently applied as tracers of submarine groundwater discharge (SGD). The unique radium signature of SGD is acquired within the subterranean estuary, a mixing zone between fresh groundwater and seawater in coastal aquifers, yet little is known about what controls Ra cycling in this system. The focus of this study was to examine controls on sediment and groundwater radium activities within permeable aquifer sands (Waquoit Bay, MA, USA) through a combination of field and laboratory studies. In the field, a series of sediment cores and corresponding groundwater profiles were collected for analysis of the four radium isotopes, as well as dissolved and sediment associated manganese, iron, and barium. We found that in addition to greater desorption at increasing salinity, radium was also closely tied to manganese and iron redox cycling within these sediments. A series of laboratory adsorption/desorption experiments helped elucidate the importance of 1) contact time between sediment and water, 2) salinity of water in contact with sediment, 3) redox conditions of water in contact with sediment, and 4) the chemical characteristics of sediment on radium adsorption/desorption. We found that these reactions are rapid (on the order of hours), desorption increases with increasing salinity and decreasing pH, and the presence of Fe and Mn (hydr)oxides on the sediment inhibit the release of radium.These sediments have a large capacity to sorb radium from fresh water. Combined with these experimental results, we present evidence from time series groundwater sampling 2 that within this subterranean estuary there are cyclic periods of Ra accumulation and release controlled by changing salinity and redox conditions.

Geochemical and physical sources of radon variation in a subterranean estuary — Implications for groundwater radon activities in submarine groundwater discharge studies

“…2C). Gonneea et al (2007) found that in the STZ Ra is sorbed onto Fe and Mn (hydr)oxides precipitating at the interface between the buffered saline water (high pH, low pε and high dissolved Mn and Fe concentrations) and fresh shallow groundwater (relatively low pH and high pε). Although Ra has higher affinity for Mn (hydr)oxides (Moore and Reid, 1973), Fe (hydr)oxides occur in one order of magnitude higher concentrations and may have just as much influence on the aquifer Ra distribution.…”

“…In the absence of Fe and Mn (hydr)oxides in sediments, Ra can quickly desorb as the salinity of the pore water increases (Li et al, 1976;Elsinger and Moore, 1980). When radium is bound to Fe and Mn (hydr)oxides, the potential release under reducing conditions is much greater than desorption driven only by increases in ionic strength (Gonneea et al, 2007).…”

“…The dissolved manganese pool is associated with the high salinity region of the STE, which also contains elevated dissolved Ra (Gonneea et al, 2007 andSholkovitz, 2006). Because we sampled at fixed horizons, this pool of water was captured in our time series observations only in the summer months, when the freshwater lens receded and the STZ moved to shallower depths (<4 m), as indicated by the decrease of groundwater level in the monitoring well (Fig.…”

“…The samples were analyzed on a Finnigan Element high resolution ICP-MS at Woods Hole Oceanographic Institution (Gonneea et al, 2007).…”

Using radium isotopes to characterize water ages and coastal mixing rates: A sensitivity analysis

“…The uncertainty associated with the groundwater end-member AR may also be reduced by collecting multiple (n 3) samples of the groundwater end-member and calculating the AR as the slope of the linear regression between short-and long-lived Ra isotope activities (Garcia-Solsona et al 2010b). However, we note that in some situations Ra isotope activities in groundwater are highly variable on a small spatial scale (e.g., Swearman et al 2006;Gonneea et al 2008) due to the multiple processes that affect groundwater chemistry in the subterranean estuary, making it difficult or impossible to identify which groundwater should be considered the discharging end-member or to define what the characteristics of this end-member are.…”