Submarine groundwater discharge (SGD) links terrestrial and marine systems, but has often been overlooked in coastal nutrient budgets because it is difficult to quantify In this Review, we examine SGD nutrient fluxes in over 200 locations globally, explain their impact on biogeochemistry and discuss broader management implications SGD nutrient fluxes exceed river inputs in ~60% of study sites, with median total SGD fluxes of 60 mmol m−2 per day for dissolved inorganic nitrogen, 01 mmol m−2 per day for dissolved inorganic phosphorus and 65 mmol m−2 per day for dissolved silicate SGD nitrogen input (mostly in the form of ammonium and dissolved organic nitrogen) often mitigates nitrogen limitation in coastal waters, since SGD tends to have high nitrogen concentrations relative to phosphorus (76% of studies showed N:P values above the Redfield ratio) It is notable that most investigations do not distinguish saline and fresh SGD, although they have different properties Saline SGD is a ubiquitous, diffuse pathway releasing mostly recycled nutrients to global coastal waters, whereas fresh SGD is occasionally a local, point source of new nutrients SGD-derived nutrient fluxes must be considered in water quality management plans, as these inputs can promote eutrophication if not properly managed Submarine groundwater discharge transports nutrients from terrestrial to marine systems, but is often ignored in coastal biogeochemistry In this Review, the fluxes, impacts and management implications of this discharge are examined and compared with riverine fluxes globally

Submarine groundwater discharge impacts on coastal nutrient biogeochemistry

Lacustrine groundwater discharge (LGD) transports nutrients from a catchment to a lake, which may fuel eutrophication, one of the major threats to our fresh waters. Unfortunately, LGD has often been disregarded in lake nutrient studies. Most measurement techniques are based on separate determinations of volume and nutrient concentration of LGD: Loads are calculated by multiplying seepage volumes by concentrations of exfiltrating water. Typically low phosphorus (P) concentrations of pristine groundwater often are increased due to anthropogenic sources such as fertilizer, manure or sewage. Mineralization of naturally present organic matter might also increase groundwater P. Reducing redox conditions favour P transport through the aquifer to the reactive aquifer‐lake interface. In some cases, large decreases of P concentrations may occur at the interface, for example, due to increased oxygen availability, while in other cases, there is nearly no decrease in P. The high reactivity of the interface complicates quantification of groundwater‐borne P loads to the lake, making difficult clear differentiation of internal and external P loads to surface water. Anthropogenic sources of nitrogen (N) in groundwater are similar to those of phosphate. However, the environmental fate of N differs fundamentally from P because N occurs in several different redox states, each with different mobility. While nitrate behaves essentially conservatively in most oxic aquifers, ammonium's mobility is similar to that of phosphate. Nitrate may be transformed to gaseous N2 in reducing conditions and permanently removed from the system. Biogeochemical turnover of N is common at the reactive aquifer‐lake interface. Nutrient loads from LGD were compiled from the literature. Groundwater‐borne P loads vary from 0.74 to 2900 mg PO4‐P m−2 year−1; for N, these loads vary from 0.001 to 640 g m−2 year−1. Even small amounts of seepage can carry large nutrient loads due to often high nutrient concentrations in groundwater. Large spatial heterogeneity, uncertain areal extent of the interface and difficult accessibility make every determination of LGD a challenge. However, determinations of LGD are essential to effective lake management. Copyright © 2014 John Wiley & Sons, Ltd.

Groundwater - the disregarded component in lake water and nutrient budgets. Part 2: effects of groundwater on nutrients: EFFECT OF GROUNDWATER ON LAKE NUTRIENT BUDGETS

/pdf/radium-isotopes-as-submarine-groundwater-discharge-sgd-1qwv85znj4.pdf

Radium isotopes as submarine groundwater discharge (SGD) tracers: Review and recommendations

Submarine groundwater discharge as a source of nutrients and trace metals in a Mediterranean bay (Palma Beach, Balearic Islands)

Submarine groundwater discharge (SGD) is a rarely recognized pathway for nutrients and other solutes from land to sea. The sensitive coastal ecosystems around tropical islands could be particularly affected by nutrient discharge associated with SGD in relation to surficial nutrient transport by rivers, but have received comparatively little attention to date. This study reviews the findings of local assessments of submarine groundwater discharge from tropical islands. In addition, the ratio of coast length and land area of individual land bodies is suggested as an appropriate first-order estimate of the relevance of SGD versus river discharge, demonstrating the potential relative importance of SGD from tropical islands over rivers. The review highlights the need for targeted research of submarine groundwater discharge from tropical islands and highlights its relevance for biogeochemical fluxes in these geographic settings.

Submarine groundwater discharge from tropical islands: a review

Short and long-lived radium isotopes (223Ra, 224Ra, 226Ra, 228Ra) were used to quantify submarine groundwater discharge (SGD) and its associated input of inorganic nitrogen (NO3−), phosphorus (PO43−) and silica (SiO44−) into the karstic Alcalfar Cove, a coastal region of Minorca Island (Western Mediterranean Sea). Cove water, seawater and groundwater (wells and karstic springs) samples were collected in May 2005 and February 2006 for radium isotopes and in November 2007 for dissolved inorganic nutrients. Salinity profiles in cove waters suggested that SGD is derived from shallow brackish springs that formed a buoyant surface fresh layer of only 0.3 m depth. A binary mixing model that considers the distribution of radium activities was used to determine the cove water composition. Results showed that cove waters contained 20% brackish groundwater; of which 6% was recirculated seawater and 14% corresponded to freshwater discharge. Using a radium-derived residence time of 2.4 days, a total SGD flux of 150,000 m3 year−1 was calculated, consisting of 45,000 m3 year−1 recirculated seawater and 105,000 m3 year−1 fresh groundwater. Fresh SGD fluxes of NO3−, SiO44− and PO43− were estimated to be on the order of 18,000, 1,140 and 4 μmol m−2 day−1, respectively, and presumably sustain the high phytoplankton biomass observed in the cove during summer. The total amount of NO3− and SiO44− supplied by SGD was higher than the measured inventories in the cove, while the reverse was true for PO43−. These discrepancies are likely due to non-conservative biogeochemical processes that occur within the subterranean estuary and Alcalfar Cove waters.

An assessment of karstic submarine groundwater and associated nutrient discharge to a Mediterranean coastal area (Balearic Islands, Spain) using radium isotopes.

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An assessment of karstic submarine groundwater and associated nutrient discharge to a Mediterranean coastal area (Balearic Islands, Spain) using radium isotopes.