Submarine groundwater discharge into typical tropical lagoons:Acase study in easternHainanIsland,China

Abstract

Assessing submarine groundwater discharge (SGD) into lagoons and bays can be helpful to understand biogeochemical processes, especially nutrient dynamics. In the present paper, radium (Ra) isotopes were used to quantify SGD in two typical tropical lagoons (Laoye Lagoon (LY Lagoon) and Xiaohai Lagoon (XH Lagoon)) of eastern Hainan Island, China. The Ra mass balance model provided evidence that SGD plays an important role in the hydrology of the LY Lagoon and the XH Lagoon, delivering average SGD fluxes of 1.7 × 106 (94 L m−2 d−1) and 1.8 × 106 (41 L m−2 d−1) m3 d−1, respectively. Tidal pumping was one of the important driving forces for SGD fluxes in the LY and the XH Lagoons. Tidal‐driven SGD into the tidal channels of both lagoons can account for approximately 10% of the total SGD flux into the lagoons. In addition, the dissolved inorganic nutrient budgets were reassessed in the LY Lagoon and the XH Lagoon, which showed that SGD was the major source of nutrients entering the LY Lagoon and that the LY Lagoon behaved as a source for dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) and as a sink for dissolved silicate (DSi). Nutrient loads in the XH Lagoon were mainly derived from riverine inputs and SGD, and the XH Lagoon behaved as a source for DIP, but a sink for DIN and DSi.

<sup>226</sup>Ra and <sup>228</sup>Ra in the stratified estuary of the Krka River (Adriatic Sea, Croatia): implications for submarine groundwater discharge and its derived nutrients

“…Maximum and minimum salinities were found at high and low tide, respectively, and were in a range from 10.7 to 14.0 ( Figure 6 and Table 1 Wang et al, 2016). During the lower salinity period, the 226 Ra and 228 Ra activities were higher although some deviation due to hysteresis effect was observed.…”

“…Coastal Zone (Gordon et al, 1996), which has been widely used to evaluate the relative importance of external nutrient inputs versus the physical transports and internal biogeochemical processes within a body of water (e.g., Liu et al, 2009Liu et al, , 2011Wang et al, 2016). Except from river and SGD inputs, atmospheric deposition and wastewater were the other two sources for nutrient input to the KRE surface layer.…”

Net Submarine Groundwater‐Derived Dissolved Inorganic Nutrients and Carbon Input to the Oligotrophic Stratified Karstic Estuary of the Krka River (Adriatic Sea, Croatia)

“…During the 24‐hr time series observation, we did not obtain significant correlations between Ra activities and water depth variation (Figures b and c). This could be explained, at least in part, by the much smaller tidal changes in the KRE than in other regions around the world, where Ra activities and water depth variation showed a significant correlation (Garcia‐Orellana et al, ; Sadat‐Noori et al, ; Wang & Du, ). During the high tide, the stronger intrusion of the open seawater reduced the influence from submarine springs with high Ra activity.…”

“…From these calculations, we can conclude that tidal‐driven SGD accounts for approximately 49–61% of the total SGD flux, suggesting the importance of tidal pumping in driving SGD flux albeit the tidal range is very small in the KRE surface layer. It should be noted that majority of the SGD flux estimated in a tidal cycle could represent recirculated groundwater, which was derived from tidal and waves pumping (Burnett et al, ; Ji et al, ; Moore, ; Wang & Du, ), and to some extent affect terrestrial hydraulic gradients that drive fresh groundwater. Therefore, we considered the result of estimated SGD flux from time series observation as the recirculated groundwater discharge, which could represent a conservative flux of fresh groundwater discharge of (5.0–8.3) × 10 5 m 3 /day with an average of 6.6 × 10 5 m 3 /day.…”

“…In the KRE, we also conducted a continuous 24‐hr time series observation over one complete tidal cycle by sampling the surface water of 0–0.5 m at the time series (TS) station every 3 hr (Figure ). This method has been widely used to evaluate the SGD (Garcia‐Orellana et al, ; Peterson et al, ; Wang & Du, ). Groundwater samples were collected in springs and wells along the KRE.…”