The role of coastal aquifers in the composition of major elements in the ocean

Submarine groundwater discharge (SGD) is important to coastal aquifers' biogeochemistry and ecology. Most SGD is comprised of circulating seawater in the coastal aquifer. The circulating seawater is driven by several mechanisms with different spatial and temporal scales, from short-term/small-scale circulation driven by tides and waves through seasonal exchange driven by the sea- or groundwater-level changes and up to long-term/large-scale circulation driven by density differences. Although short-term circulation has been shown to affect groundwater chemistry and potentially modify the composition of seawater for some elements, long-term processes have the potential to affect elements (e.g., Na, Ca, K, Mg, Sr) that are controlled by long-term geochemical processes. These are not affected by the short-term/small-scale processes, thus allowing differentiation and quantifying the long-term density-driven circulation only. Being able to differentiate the different circulating seawater components is a critical step toward quantifying major elements fluxes from the coast into the ocean.

Our study presents a new compilation of worldwide coastal aquifers' data, which allows for determining the major elements' end-member composition of coastal aquifer groundwaters (see figure). Based on these compositions and their uncertainties, we could quantify the SGD flux of the long-term component due to density-driven circulation. Based on the Ca²⁺, K⁺, Sr²⁺, and ⁸⁷Sr/⁸⁶Sr ocean budgets, the calculated long-term SGD flux is 1117±487 km³/y (see figure). Although this flux is small compared to the global SGD water fluxes, it yields elemental fluxes of 8.0±3.5 Tmol Ca, -1.9±0.98 Tmol K, and 0.19±0.036 Tmol Sr per year, which are on the same order of magnitude as the fluxes through rivers.

End-member enrichment (+) and depletion (-)
Ca²⁺ - 13±8 mM
K⁺ - -1.48±1.11 mM
Sr²⁺ - 0.169±0.081 mM
⁸⁷Sr/⁸⁶Sr - 0.7089±0.00001

 

Figure 1: A global compilation of major elements enrichment and depletion in coastal aquifers’ groundwaters. The enrichment/depletion is calculated as the addition/deficit of a particular element compared to its expected concentration due to conservative mixing only.

Figure 2: Monte-Carlo simulation results of the long-term circulation (LTC) flux based on the different element and isotope budgets (up) and the combined distribution (bottom).

Major results:

Tmol/y	LTC	Rivers
Ca2+	8.0±3.5	13.2±1.3
K+	-1.9±0.98	1.9±0.4
Sr2+	0.19±0.036	0.033±0.006