Fresh Submarine Groundwater Research Articles

Submarine fresh groundwater discharge into Laizhou Bay comparable to the Yellow River flux.

Near- and off-shore fresh groundwater resources become increasingly important with the social and economic development in coastal areas. Although large scale (hundreds of km) submarine groundwater discharge (SGD) to the ocean has been shown to be of the same magnitude order as river discharge, submarine fresh groundwater discharge (SFGD) with magnitude comparable to large river discharge is never reported. Here, we proposed a method coupling mass-balance models of water, salt and radium isotopes based on field data of 223Ra, 226Ra and salinity to estimate the SFGD, SGD. By applying the method in Laizhou Bay (a water area of ~6000 km2), we showed that the SFGD and SGD are 0.57 ~ 0.88 times and 7.35 ~ 8.57 times the annual Yellow River flux in August 2012, respectively. The estimate of SFGD ranges from 4.12 × 107 m3/d to 6.36 × 107 m3/d, while SGD ranges from 5.32 × 108 m3/d to 6.20 × 108 m3/d. The proportion of the Yellow River input into Laizhou Bay was less than 14% of the total in August 2012. Our method can be used to estimate SFGD in various coastal waters.

Determination of the distance of the fresh water–salt water interface extending into coastal confined aquifers

In a coastal zone an understanding of the distance of the fresh water–salt water interface and its extension inland is important for prevention of sea water intrusion. In this article estimating methods are described for calculating the distance of a fresh water–salt water interface in a coastal confined aquifer based on the submarine fresh groundwater discharge. This groundwater discharge is controlled not only by the aquifer properties and hydraulic head difference, but also by the position of the fresh water–salt water interface in the coastal zone. A homogeneous and isotropic coastal confined aquifer is considered and fresh groundwater flow in the confined aquifer is thought to be at a steady state. Two observation wells at different distances in a profile perpendicular to the coastline are required in calculation of the distance of the interface toe in the coastal zone. Four coastal confined aquifers with horizontal and sloping confining beds and with varying thickness are also considered. Reasonable results are obtained when examples are used to illustrate the application of the methods. The methods require hydraulic head data at the two wells and thickness of the confined aquifers, but the hydraulic conductivity and fresh groundwater flow rate of the confined aquifers are not needed.

Temporal changes and impacts of submarine fresh groundwater discharge to the coastal environment: A decadal case study in Toyama Bay, Japan

Recent dramatic changes of freshwater systems in high latitudes will allow Submarine Fresh Groundwater Discharge (SFGD) to play a more important role in the coastal environment; especially in that SFGD will directly effect heat flux. Toyama Bay, along the central western Japan coast, is a suitable and representative case study area to estimate SFGD flux using hydrographic properties since multiple high‐flow rate SFGD sites exist there. Salinities averaged over the water column (depth range 10–100 m) measured during research cruises in June 2003 and May 2005 show lower levels in the eastern than the western area in this bay. Together with monthly hydrographic properties over a 10 year period (1987–1998), the low‐salinity water mass in the eastern area exists consistently but distinctly and varies systematically, as does nutrient flux, affected by SFGD more than riverine input. SFGD fluxes in June 2003 (1 × 108 m3 month−1) and May 2005 (<1 × 108 m3 month−1) were estimated using a box model, which is divided into a shallow box (0–40 m) and a deeper box (40–100 m). The monthly flux ratio between the SFGD and the river inputs is 13% in June, comparable to higher values reported in other global studies. Our results demonstrate that the box model analysis, based on hydrographic observations in coastal areas, is an efficient approach that can be used to estimate SFGD fluxes between the land and ocean.

Understanding the hydrogeology of the Venice Lagoon subsurface with airborne electromagnetics

The occurrence of alternating dry/wet conditions in transitional environments, such as wetlands, deltas, and lagoons, usually challenges the use of traditional direct and geophysical surveys for comprehensive hydrogeologic investigations. Moreover, significant mixing between continental fresh groundwater and marine salty surface waters generally takes place in these flat coastal areas. Airborne electromagnetics (AEM) is a promising tool in this respect, as it provides, in a fast and cost effective manner, large-scale distribution of bulk electrical conductivities that can be used profitably to develop hydrogeologic models. The results of a SkyTEM AEM survey in the Venice Lagoon, Italy, show the capability of this technique to significantly improve the knowledge of the hydrogeologic setting of the lagoon and nearby coastland subsurface, irrespective of the different features characterizing the area. The environment consists of salt marshes, mud flats, shallows, tidal channels, islands, together with reclaimed farmlands crossed by natural watercourses and drainage channel networks. In particular, the AEM shows (i) the presence of fresh water (with resistivity larger than 20 Ω m) underneath the central part of the lagoon at depths from 10 to 25 m below m.s.l., (ii) the interface between different relevant stratigraphic units, e.g., the clayey layer bounding the Holocene–Pleistocene sedimentation, and (iii) the occurrence of areas with possible submarine fresh groundwater discharge. Moreover, the source and inland extent of the saltwater contamination in the shallow coastal aquifers along the southern margin of the lagoon are clearly revealed. AEM data were complemented with very high resolution seismic (VHRS) acquisitions. The integrated analysis of the two data sets allows distinguishing between lithostratigraphic heterogeneity and variability of the subsurface fluids.

Groundwater–seawater interactions off the coast of southern Taiwan: Evidence from environmental isotopes

We have conducted detailed hydrochemical surveys off the Pingtung Plain coast, SW Taiwan, using environmental isotope compositions (δ D, δ 18O, δ 13C DIC, tritium, and Δ 14C) as the main tracers to investigate groundwater–seawater interactions in the region. The δ 18O values of seawater in this area show significant variability with heavy isotope enrichment relatively to groundwater. The δ D and δ 18O data also confirmed that submarine groundwater discharge (SGD) originated from meteoric water and were mixed with seawater. Due to an overdraft of groundwater, seawater has intruded inwardly through the offshore outcrops for both the unconfined and confined aquifers at a depth of 300 m. However, δ 18O-depleted layer is found along deep parts of Kaoping Canyon. This result provides evidence for SGD in this region. This phenomenon is also reflected in depleted δ 13C DIC and lower Δ 14C values in deep canyon water than in adjacent seawater, and indicating that the discharge zone extends over a considerable distance to the offshore area (∼25 km). Based on these three parameters (δ 18O, δ 13C DIC and Δ 14C), we used two end-member mixing model to estimate that fresh submarine groundwater constitute. The submarine fresh groundwater contributed about 3%, and the estimated submarine fresh groundwater discharge was calculated as 4.5 × 10 9 m 3 per year. This result is comparable with groundwater annual loss in the Pingtung Plain. As documented by the isotopic composition of the collected samples, detailed results show that SGD in the Pingtung coastal zone raises a potential environmental concern and provides implication for the management of groundwater resource in the region.

Deep submarine groundwater discharge indicated by tracers of oxygen, strontium isotopes and barium content in the Pingtung coastal zone, southern Taiwan

We have conducted detailed hydrographic surveys off the Pingtung Plain coast, SW Taiwan, using environmental isotope and trace element compositions (δ 18O, Δ 87Sr, and Ba) as the main tracers to investigate submarine groundwater discharge (SGD) in the region. The δ 18O values of seawater in this area show significant variability with heavy isotope enrichment relatively to groundwater, and two δ 18O-depleted layers are found at depths of 400-700 m and 1200 m along deep parts of Kaoping Canyon. Assuming two end-member mixing model, we estimate that fresh submarine groundwater may constitute a few percent of the total SGD in the canyon. This is reflected in lower salinity, higher Δ 87Sr values and Ba concentrations in deep canyon water than in adjacent seawater. Pore water samples collected from the canyon floor have light δ 18O values and low chloride concentrations, consistent with the overlying δ 18O-depleted water. Elsewhere, seabed pore waters have normal seawater compositions. The results of this study indicate that the deep SGD in the Pingtung Plain coastal zone is mainly fed by recirculating seawater with some admixture of fresh groundwater in canyon area (down to ~ 1200 m). Areas with notable SGD are located a long distance from the shoreline (~ 25 km), and the ‘deep’ SGD may be widespread off southern Taiwan. This discharge must be considered as a significant source of trace elements and other chemical constituents to the coastal ocean.

Ancient Greek Hydromyths About the Submarine Transport of Terrestrial Fresh Water Through Seabeds Offshore of Karstic Regions

This study examined the relationship between ancient Greek texts and the physical possibility of focused, distal flow of terrestrial fresh water through the seabed, particularly offshore of karstic coasts. The four ancient texts which were analyzed describe powerful discharges from submarine springs in the eastern Black Sea; the local transport of groundwater through the bed of Turkey’s Bay of Miletus; alleged subterranean–submarine connections between coastal western Turkey and the Greek northeast Peloponnese; and alleged connections between the coastal western Peloponnese and southeastern coastal Sicily. The plausibility or implausibility of these legends was assessed in the context of modern reports indicating that seabed pathways can transport continental fresh water up to 60 km offshore. Other reports identify fresh water in the seabed as far as 160 km offshore, presumably due to marine-induced forces. These documented cases validated ancient claims of nearshore groundwater transport and legitimized transoceanic claims as mythologized extrapolations of local karstic hydrogeology. As submarine fresh groundwater becomes increasingly important in understanding material transport and in identifying potentially exploitable coastal water supplies, ancient stories from past civilizations may give clues to offshore sites meriting further exploration.

Submarine groundwater discharge from the Yellow River Delta to the Bohai Sea, China

Spatial and temporal variations of submarine groundwater discharge (SGD) have been evaluated by automated seepage meters from the Yellow River delta to 7 km offshore in the Bohai Sea, China. We identified three zones from the coast to offshore based on different relationships between tidal and SGD changes. Our results indicate that the point of maximum SGD shifted 2 km offshore from September 2004 to September 2006. This spatial change is thought to be caused by sediment deposition near the coast. Integrating submarine fresh groundwater discharge (SFGD) along the coastline of the Yellow River delta using measured values of SFGD per unit length of shoreline shows that discharge of fresh groundwater along the entire delta would be equivalent to 4.5 to 7.0 % of the river discharge.

Phát hiện nước ngọt dưới đất ngoài biển bằng phương pháp ảnh điện (ĐSĐ 2D)

Detection of submarine fresh groundwater by electrical imaging method

Assessment of terrestrial factors controlling the submarine groundwater discharge in water shortage and highly deformed Island of Taiwan, Western Pacific Ocean

Submarine groundwater discharge (SGD) from seventeen sites was investigated on the coasts around Taiwan in order to verify its occurrences and to understand its characteristics. The fresh water fraction (FWF) in most pore water samples, except the Fangsan sampling site, range from 0 to 59% based on the salinity evidence. The relative low FWF evidence implies that the recirculated saline groundwater discharge (RSGD) is more important than the submarine fresh groundwater discharge (SFGD) in most sampling sites. Fangsan, which is located at the southwestern coast of Taiwan, has nearly 100% FWF in SGD. Evaluation of the relationship between the magnitude of SGD flux and geological type of the Taiwanese coast proves to be difficult because complex hydrological factors affect the SGD, rather than the coastal topography. According to the hydrological evidence, the modeled-SGD discharge is enhanced by high precipitation but is generally reduced by severe groundwater pumping. In addition, modeled-SGD has an inverse relationship with river base flow, indicating the river effect. The almost fresh SGD with flow rates ranged from 34 to 42 m/year at Fangsan deserves attention because a seven-year-long groundwater budget calculation implies that the aquifer could not supply so much fresh SGD over a large area. This suggests that a significant amount of fresh water SGD at Fangsan is derived from a point source via a fault passage, considering its geological background. Although the rudimentary salinity and stable isotope results indicate that RSGD plays an important role in SGD, the type of submarine spring discharge via fault zones may very well be the most prominent in highly deformed areas elsewhere in the world, too.

Temporal variability of water quality of submarine groundwater discharge in Ubatuba, Brazil

Measurements of electric resistivity of the seabed and of the electric conductivity of submarine groundwater discharge (SGD) collected in seepage meters have been made to evaluate temporal variability of the origin and quality of submarine groundwater discharge in Ubatuba, Brazil. A diurnal variation of SGD conductivity was found under the condition of semi-diurnal tidal changes over a period of 4 days. Both the ground resistivity and the conductivity of the discharged groundwater indicated that the freshwater–saltwater interface moved towards offshore at high tide, i.e. in the direction opposite to commonly made observations. SGD comprised a combination of Submarine Fresh Groundwater Discharge (SFGD) of terrestrial origin, and of Recirculated Saline Groundwater Discharge (RSGD) of marine origin. The maximum of the terrestrially derived fraction SFGD/SGD was found at a distance of 50 m offshore. A lower SFGD/SGD ratio was found closer to shore, where the highest SGD flux was measured. SGD conductivity and ground resistivity displayed a diurnal cyclicity at semi-diurnal tidal water level variations, indicating that tidal water level fluctuations may not be the primary driver of SGD flux, but preferential flow in volcanic hydrogeologic setting at Ubatuba.

Estimating submarine discharge of fresh groundwater from a volcanic island using a freshwater budget of the coastal water column

The magnitude of submarine fresh groundwater discharge (SFGD) was estimated using a freshwater budget of the coastal water, which was calculated from salinity anomalies off a volcanic island, Jeju, Korea. We obtained high‐resolution profiles of salinity from the entire coastal area of Jeju Island using accurate CTD sensors. Lower salinity anomalies relative to the pristine offshore seawaters were observed and assumed to be a result of SFGD on the basis of negligible surface runoffs from the island and the relatively high levels of 222Rn in the low‐salinity waters. The SFGD flux during the study period calculated from a freshwater budget of the water column was 2.5−3.8 × 106 m3 d−1, which is close to the long‐term average based on the inland water balance. Our study shows that SFGD can be accurately and easily measured using a freshwater budget of the coastal waters off a highly permeable coastal zone with little surface runoff.

Dynamics of submarine groundwater discharge and freshwater‐seawater interface

Relationships between submarine groundwater discharge (SGD) and the freshwater‐saltwater interface are evaluated by continuous measurements of SGD rates, conductivity and temperature of SGD, and resistivity measurements across the coastal aquifer. Our measurements show that the processes of SGD differ between the offshore and nearshore environments. SGD and submarine fresh groundwater discharge (SFGD) rates were largest just landward of the saltwater‐freshwater interface. SGD variations landward the saltwater‐freshwater interface had negative correlations with tidal variations, because of the connections of terrestrial groundwater in the land and the ocean. SGD in the nearshore can be explained mainly by connections of terrestrial groundwater, while offshore SGD rate is controlled mostly by oceanic process such as recirculated saline groundwater discharge.

沿岸海底から湧出する淡水性地下水の探査および陸域地下水との関連に関する検討

沿岸海底から湧出する淡水性地下水の探査および陸域地下水との関連に関する検討