Fresh Submarine Groundwater Discharge Research Articles

Assessing the role of submarine groundwater discharge as a source of Sr to the Mediterranean Sea

Submarine groundwater discharge (SGD) has been identified as an important source of Sr to the ocean and the SGD-driven Sr flux to the global ocean has been recently re-evaluated (Beck et al., 2013). However, the uncertainty of this value is still high because of the uncertainties related to the determination of SGD flow rates and the paucity of 87Sr/86Sr data in SGD end-members. As carbonates have high Sr concentrations and are subjected to intense heightened weathering, they might significantly influence the SGD input of Sr to the ocean. Here we present data on Sr concentrations and 87Sr/86Sr ratios in three carbonate dominated sites of the western area of the Mediterranean Sea, a semi-enclosed basin characterized by abundant coastal carbonates. The 87Sr/86Sr ratios in groundwater were lower compared to modern seawater (∼0.70916), as expected for areas dominated by carbonate lithologies. Concentrations of Sr and 87Sr/86Sr ratios in groundwater showed conservative mixing in the studied subterranean estuaries. By using SGD flow rates reported in the literature for the study areas, a flow-weighted fresh SGD end-member characterized by a Sr concentration of 27–30μM and a 87Sr/86Sr ratio of 0.707834–0.708020 was calculated for the eastern coast of the Iberian Peninsula.Integrating these Sr data with literature data (i.e. values of Sr concentration and 87Sr/86Sr ratio from other lithologies as well as SGD flow rates), we also calculated the fresh SGD-driven Sr flux to the entire Mediterranean Sea, obtaining a value of (0.34–0.83)·109moly−1, with a 87Sr/86Sr of 0.7081–0.7086. Thus, for the entire Mediterranean basin, SGD is globally a source of Sr less radiogenic compared to seawater. The SGD Sr flux to the Mediterranean Sea represents 5–6% of the SGD Sr flux to the global ocean and the Mediterranean SGD end-member has higher Sr concentration (5–12μM) than the global SGD end-member (2.9μM). This confirms the significant role of carbonate lithologies on SGD-driven Sr fluxes to seawater.The fresh SGD-driven Sr flux to the Mediterranean Sea is about 20–50% of the riverine Sr input and significantly higher than the input through atmospheric dust deposition. Therefore SGD should be considered as an important continental source of Sr to the basin.

Estimation of the inland extending length of the freshwater–saltwater interface in coastal unconfined aquifers

ABSTRACTThe inland extending length of the freshwater–saltwater interface toe is useful in studies of seawater intrusion in coastal areas. The submarine fresh groundwater discharge in coastal zones is affected not only by hydraulic conductivity and hydraulic gradient of the aquifer, but also by the position of the interface. Two observation wells at different distances from the coast are required to calculate the fresh groundwater flow rate in coastal unconfined aquifers. By considering that the submarine groundwater discharge is equal to the groundwater flow rate, the length of the interface toe extending inland can be estimated when the groundwater flow is at a steady-flow state. Aquifers with horizontal and sloping confined beds and without/with unique surface vertical infiltration are considered. Examples used to illustrate the application of these methods indicate that the inland extending lengths of the interface toe in aquifers with vertical surface infiltration are much shorter than those in aquifers without vertical surface infiltration, and the length of the interface in aquifers with a horizontal confining lower bed are smaller than those in aquifers with a confining lower bed sloping towards the sea. The extent of the interface on the northwestern coast near the city of Beihai in southern Guangxi, China, on 18 January 2013 was estimated as 471–478 m.Editor M.C. Acreman Associate editor not assigned

Transport and transformations of groundwater-borne carbon discharging through a sandy beach to a coastal ocean

Submarine groundwater discharge (SGD) is now recognized as an important coastal process affecting local and regional sources of solutes to coastal oceans. The objective of this study was to compare SGD estimates through Martinique Beach (Îles-de-la-Madeleine, QC, Canada) using different sampling techniques in order to quantify SGD fluxes of dissolved organic and inorganic carbon (DOC, DIC) exported to a coastal embayment. These fluxes were estimated using hydrogeological and geochemical methods, including direct measurement via seepage meters and hydrogeological estimation using Darcy’s law. Darcy estimates led to a fresh SGD flow ranging from 0.020 m3/s at the shoreline to 0.030 m3/s at the beach face. Direct measurements by seepage meters revealed higher SGD flows, with a mean value of 0.090 m3/s. These SGD flows were mainly composed of recirculating seawater flushing by the falling tide. The unconfined aquifer of Martinique Beach is a biogeochemically reactive zone, where DOC and DIC are nonconservative. The groundwater residence time (~32 days), low oxygen conditions (~20%) and high DOC concentrations (~ 2 mmol/L) represent a suitable environment for biogeochemical reactions to occur, and subsequently alter groundwater-borne carbon concentrations. We calculated a fresh-groundwater carbon load of 147 kg/d of DIC and 27 kg/d of DOC at the beach face. Even though recirculating seawater dominated the volume of total SGD, fresh SGD was an important carbon pathway, accounting for 12 and 20% of total DIC and DOC, respectively. This site-specific study is the first attempt to estimate volumetric and chemical groundwater fluxes to a coastal Canadian ocean, and demonstrates the strong interaction between fresh groundwater and coastal systems.

Submarine groundwater discharge as a source of dissolved nutrients to an arid coastal embayment (La Paz, Mexico)

Submarine groundwater discharge (SGD) was investigated in the southeastern portion of La Paz Bay (Baja California, Mexico) using radon (222Rn) as a natural tracer. In the absence of permanent surface flows in this arid region, we hypothesize SGD is a major regional source of dissolved nutrients. Four spatial surveys showed higher radon and nutrient (NH4, NO3, NO2, PO4 and SiO4) values in the winter than summer lagging rainfall by 3–4 months. The surveys revealed two sites (Balandra and Merito) with a stronger radon signal. Intensive time series (12–24 h) measurements at those sites were used to estimate SGD fluxes using a radon mass balance approach. In Balandra, SGD was estimated to be 0.18 m3 m−2 day−1 and significant correlations between radon and nutrients (NO3 and SiO4) were observed. In Merito, SGD rates were estimated to be 0.10 m3 m−2 day−1 and no correlations between nutrients and 222Rn were observed. The difference between the two sites was interpreted to be related to different components dominating SGD (i.e., fresh SGD in Balandra and saline SGD in Merito). The estimated SGD-derived nutrients fluxes were 2–52, 0.04–0.94, 7–164 mmol m2 day−1 for dissolved inorganic nitrogen, phosphate, and silicate, respectively. These fluxes could explain between 5 and 20 % of the regional marine primary productivity values.

양식장 용수 추가 확보를 위한 수압파쇄 적용성 평가

Feasibility tests for the hydraulic fracturing were conducted in order to secure additional saline groundwater for irrigating to the land-based aquaculture farm. Two boreholes were placed to the aquaculture farm A and B, respectively. A hydraulic fracturing using single packer was applied to major fracture zones within two boreholes. To identify effects of hydraulic fracturing on securing additional saline groundwater, some selective methods including well logging methods, pumping tests, and groundwater quality analysis were commonly applied to the boreholes before and after the hydraulic fracturing. Enlarging/creating fracture zones, increasing water contents in bedrock near boreholes, and increasing transmissivity were observed after the hydraulic fracturing. Even though the hydraulic fracturing could be an alternative to secure additional saline groundwater to the land-based aquaculture farm, salinity of the groundwater did not meet optimal thresholds for each fingerling in two farms: Fresh submarine groundwater discharge flowed the more into borehole of the farm A that resulted in decreasing a salinity value. Increased saline groundwater quantity in the borehole of the farm B rarely affect to the salinity. Although salinity problem of groundwater limited its direct use for the farms, the mixing with seawater could be effectively used for the fingerlings during the early stage. A horizontal radial collector well placed in the alluvial layer could be an alternative for the farms as well.

Effect of land use and groundwater flow path on submarine groundwater discharge nutrient flux

Study regionMaui, Hawaii, United States. Study focusWe investigated connections between land uses and submarine groundwater discharge (SGD) nutrient fluxes to coastal waters of Maui, Hawai'i. Nutrient contributions from agricultural lands, wastewater injection, and septic-cesspool systems were examined by combining a numerical groundwater model with δOH2O18, δNNO3−15, and δONO3−18 modeling to identify groundwater pathways, recharge elevations, and nitrate sources. Fresh and total SGD rates and nutrient fluxes were quantified using 222Rn mass balance modeling. New hydrological insights for the regionLow nitrate+nitrite (N+N) SGD fluxes (24mols/d) were measured where groundwater flowed beneath primarily undeveloped land on transit to the coast. By contrast, of all land use types, sugarcane and pineapple fields contributed the largest amount of N to coastal waters via SGD (3800mols/d). Despite their much smaller freshwater flux, these SGD sources provide substantially larger N fluxes than the State's largest rivers (avg. 700mols/d). Septic systems, cesspools, and near coast wastewater injection wells also contribute N+N to groundwater and coastal waters, although in much smaller quantities. This study demonstrates that numerical groundwater modeling combined with geochemical modeling can be used to determine sources and flux of nutrients in SGD and provides a unique, original, and practical framework for studying the effect of land use and its impact on nutrient delivery to coastal waters.

Identification and quantification of diffuse fresh submarine groundwater discharge via airborne thermal infrared remote sensing

Abstract Airborne thermal infrared (TIR) overflights were combined with shoreline radionuclide surveys to investigate submarine groundwater discharge (SGD) along the north shore of Long Island, NY between June 2013 and September 2014. Regression equations developed for three distinct geomorphological environments suggest a positive linear relationship between the rate of diffuse SGD and the spatial extent of the observed coastal TIR anomalies; such a relationship provides quantitative evidence of the ability to use TIR remote sensing as a tool to remotely identify and measure SGD. Landsat TIR scenes were unable to resolve any of the 18 TIR anomalies identified during the various airborne overflights. Two locations were studied in greater detail via 222 Rn time series and manual seepage meters in order to understand why specific shoreline segments did not exhibit a TIR anomaly. SGD at the first site, located within a large, diffuse TIR anomaly, was composed of a mixture of fresh groundwater and circulated seawater with elevated levels of nitrate. In contrast, SGD at the second site, where no coastal TIR anomaly was observed, was composed of circulated seawater with negligible nitrate. Despite the compositional differences in seepage, both sites were similar in discharge magnitude, with average time series 222 Rn derived SGD rates equal to 18 and 8 cm d − 1 for the TIR site and non-TIR site, respectively. Results suggest that TIR remote sensing has the ability to identify locations of a mixture between diffuse fresh and circulated seawater SGD. If TIR anomalies can be demonstrated to represent a mixture between fresh and circulated seawater SGD, then the cumulative area of the TIR anomalies may be used to estimate the fresh fraction of SGD relative to the cumulative area of the seepage face, and thus allows for improved SGD derived nutrient flux calculations on a regional scale.

Non-conservative behavior of fluorescent dissolved organic matter (FDOM) within a subterranean estuary

The role of submarine groundwater discharge (SGD) in releasing fluorescent dissolved organic matter (FDOM) to the coastal ocean and the possibility of using FDOM as a proxy for dissolved organic carbon (DOC) was investigated in a subterranean estuary in the northeastern Gulf of Mexico (Turkey Point, Florida). FDOM was continuously monitored for three weeks in shallow beach groundwater and in the adjacent coastal ocean. Radon (222Rn) was used as a natural groundwater tracer. FDOM and DOC correlated in groundwater and seawater samples, implying that FDOM may be a proxy of DOC in waters influenced by SGD. A mixing model using salinity as a seawater tracer revealed FDOM production in the high salinity region of the subterranean estuary. This production was probably a result of infiltration and transformation of labile marine organic matter in the beach sediments. The non-conservative FDOM behavior in this subterranean estuary differs from most surface estuaries where FDOM typically behaves conservatively. At the study site, fresh and saline SGD delivered about 1800mgd−1 of FDOM (quinine equivalents) to the coastal ocean per meter of shoreline. About 11% of this input was related to fresh SGD, while 89% were related to saline SGD resulting from FDOM production within the shallow aquifer. If these fluxes are representative of the Florida Gulf Coast, SGD-derived FDOM fluxes would be equivalent to at least 18% of the potential regional riverine FDOM inputs. To reduce uncertainties related to the scarcity of FDOM data, further investigations of river and groundwater FDOM inputs in Florida and elsewhere are necessary.

Observations of presumable groundwater seepage occurrence in Puck Bay (the Baltic Sea)

Abstract We report the results of recent field measurements of seawater thermohaline structure and transparency carried out in early summer 2014 in Puck Bay. Near-bottom, less saline waters occurred almost in the entire study area. Their occurrence in the shallow part of Puck Bay was accompanied by an increase in the seawater transparency. The most likely explanation of these observations is a submarine fresh groundwater discharge.

Variations of Hydrodynamics and Submarine Groundwater Discharge in the Yellow River Estuary Under the Influence of the Water-Sediment Regulation Scheme

The “Water-Sediment Regulation Scheme” (WSRS) is critically important to the hydrologic evaluation of the Yellow River estuary since a huge pulse of water and sediment are delivered into the sea during a short period. We used the natural geochemical tracers radium (223Ra, 224Ra, 226Ra) and radon (222Rn) isotopes as well as other hydrological parameters to investigate the mixing variations and submarine groundwater discharge (SGD) in the Yellow River estuary under the influence of the 2013 WSRS. Dramatically elevated radium and radon isotopic activities were observed during this WSRS compared with activities measured during a non-WSRS period. Radium “water ages” indicated that the offshore transport rate nearly tripled when the river discharge increased from 400 to 3400 m3/s. We calculated the SGD flux in the Yellow River estuary based on a radium mass balance model as well as radium and radon time-series models. The SGD flux was estimated at 0.02~0.20 m/day during a non-WSRS period and 0.67~1.22 m/day during the 2013 WSRS period. The results also indicate that large river discharge tends to lead more intense SGD along the river channel direction with a large amount of fresh SGD.

Seasonal Changes in Submarine Groundwater Discharge and Associated Nutrient Transport into a Tideless Semi-enclosed Embayment (Obama Bay, Japan)

We carried out a seasonal study of fresh submarine groundwater discharge (SGD) and associated nutrient fluxes in a semi-enclosed bay along a tideless coastal zone using a 222Rn and salinity mass balance model for a whole bay scale. The resulting SGD rates showed large intra-annual variability from 0.05 × 106 to 0.77 × 106 m3 day−1, which were controlled by seasonal changes in the interaction of multiple driving forces, including water table height and seawater level. The highest SGD rate in early spring was induced by heavy snow and low sea level, whereas the seasonal increase in sea level gradually suppressed fresh SGD rates. In summer, an elevated water table may induce higher SGD rates (approximately 0.4 × 106 m3 day−1) regardless of high sea levels. The highest SGD fraction in total terrestrial freshwater fluxes also occurred in summer (>40 %), due to the decreasing rate of surface river discharge. The seasonally averaged SGD rate was 0.36 × 106 m3 day−1. This value was similar to the annual groundwater recharge rate (0.33 × 106 m3 day−1) estimated by the water balance method in the basin. Nutrient fluxes from SGD were approximately 42, 65, and 33 % of all terrestrial fluxes of dissolved inorganic nitrogen, phosphorous, and silicate, respectively. The average fraction of SGD in the water fluxes including terrestrial and oceanic water was low (0.3 %), but that of nutrient fluxes increased to 20–38 %. Higher nutrient concentrations in groundwater compensated for the lower volumetric flux of groundwater. Because primary production was mostly restricted by phosphorous throughout the year, phosphorous-enriched nutrient transport via SGD would play an important role in biological production.

A GIS typology to locate sites of submarine groundwater discharge

Although many researchers agree on the importance of submarine groundwater discharge (SGD), it remains difficult to locate and quantify this process. A groundwater typology was developed based on local digital elevation models and compared to concurrent radon mapping indicative of SGD in the Niantic River, CT USA. Areas of high radon activity were located near areas of high flow accumulation lending evidence to the utility of this approach to locate SGD. The benefits of this approach are three-fold: fresh terrestrial SGD may be quickly located through widely-available digital elevation models at little or no cost to the investigator; fresh SGD may also be quantified through the GIS approach by multiplying pixelated flow accumulation with the expected annual recharge; and, as these data necessarily quantify only fresh SGD, a comparison of these data with SGD as calculated by Rn activity may allow for the separation of the fresh and circulated fractions of SGD. This exercise was completed for the Niantic River where SGD as calculated by the GIS model is 1.2 m3/s, SGD as calculated by Rn activity is 0.73–5.5 m3/s, and SGD as calculated via a theoretical approach is 1.8–4.3 m3/s. Therefore fresh, terrestrial SGD accounts for 22–100% of total SGD in the Niantic River.

Combining radon, short-lived radium isotopes and hydrodynamic modeling to assess submarine groundwater discharge from an anthropized semiarid watershed to a Mediterranean lagoon (Mar Menor, SE Spain)

Summary In highly anthropized watersheds, surface water tributaries may carry unexpected high quantities of radon and radium to coastal lagoons. Investigating submarine groundwater discharge (SGD) with radionuclide tracers is therefore a complex task. In order to quantify SGD and decipher the influence of the different water sources, we combined a radon ( 222 Rn) and short-lived radium ( 223 Ra, 224 Ra) survey with the hydrodynamic modeling of a lagoon. We applied it to the Mar Menor lagoon (SE Spain) where surface water tributaries and undocumented emissaries carry water from groundwater drainage and brines from groundwater desalinization. We identified the areas of influence of the plume of radionuclides from the river, located major areas of SGD and proposed a location for two submarine emissaries. Porewater, i.e. interstitial water from underlying sediments, was found to be the most representative SGD end member, compared to continental groundwater collected from piezometers. Mass balances in winter and summer seasons provided yearly SGD fluxes of water of 0.4–2.2 ⋅ 10 8 m 3 /y ( 222 Rn), 4.4–19.0 ⋅ 10 8 m 3 /y ( 224 Ra) and 1.3 ⋅ 10 8 m 3 /y ( 223 Ra, measured in winter only). Tidal pumping was identified as a main driver for recirculated saline groundwater, while fresh submarine groundwater discharge from the aquifer ranged between 2% and 23% of total SGD.

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.

Tide-induced salt-fingering flow during submarine groundwater discharge

This study investigated the stability of the upper saline plume (USP) within shallow tide-affected submarine groundwater discharge (SGD) zones. In contrast to earlier studies, numerical modeling revealed a number of realistic hydrological and hydrogeological conditions where the USP becomes unstable and salt-fingering flow occurs. These conditions were reasonably well identified in a stability diagram based on two dimensionless numbers that characterize the system. If fingering flow occurs, the SGD pattern is distinctly different from that of stable flow conditions: (i) freshwater discharge zones along the beach face are manifold and change their location with time, (ii) undulating freshwater/seawater interface that is extended along the groundwater flow path, and (iii) the total tide-averaged fresh SGD rate varies considerably in an irregular pattern. This has presumably important implications on reactive transport processes in the subterranean estuary, as well as on the interpretation of field data on water and solute fluxes during SGD.

Influence of porewater exchange on nutrient dynamics in two New Zealand estuarine intertidal flats

We use concomitant radon (222Rn, a natural groundwater tracer) and nutrient time series observations upstream and downstream of two New Zealand estuarine intertidal flats to assess porewater exchange rates and/or submarine groundwater discharge (SGD) and their influence on surface water nutrient dynamics. A detailed radon mass balance model and an uncertainty analysis revealed porewater exchange rates of 27±7 and 14±6cmd−1 (or cm3cm−2d−1) in Waikareao and Te Puna, respectively. The upscaled porewater exchange rates were slightly higher than the creek input upstream of both intertidal flats. A water and salt balance compared to the radon balance indicated that about 16% (Waikareao) and 49% (Te Puna) of the total volume of porewater exchange consisted of fresh SGD and the remaining was related to seawater recirculation in intertidal sediments. Porewater exchange in Waikareao released about 1.9- and 1.6-fold more total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) than creek inputs at the upstream end of the intertidal flat even though observations followed a 140mm rain event. Dissolved organic nitrogen (DON) accounted for about 25% of TDN in shallow porewater. Nitrate dominated the nitrogen pool in Waikareao and ammonium was the main form of nitrogen in Te Puna porewaters. These dominant porewater N species were reflected in the surface waters that showed a relative enrichment of nitrate in Waikareao and ammonium in Te Puna as upstream waters travel to the downstream station and collect seeping porewater along the way. We suggest that porewater exchange may act as a buffered nutrient source to the estuary, continually releasing nutrients to surface waters and potentially sustaining primary production when other nutrient inputs cease. This study illustrates that combining bottom up (i.e., porewater exchange flux estimates) and top down (i.e., nutrient response and transformations in the water column) evidence may provide deeper insight when assessing the contribution of porewater to nutrient dynamics in estuaries.

Submarine groundwater discharge estimation in an urbanized embayment in Hong Kong via short-lived radium isotopes and its implication of nutrient loadings and primary production

224Ra and 223Ra are adopted as tracers to qualify submarine groundwater discharge (SGD) in Tolo Harbor, a highly urbanized embayment in Hong Kong. Based on the sampling data, a two-layered radium mass balance model is used to estimate lateral SGD and bottom SGD. Total SGD is estimated to be 1.2–3.0cmd−1, including lateral SGD of 5.7–7.9cmd−1 and bottom SGD of 0.3–2.0cmd−1. Fresh SGD is estimated to be (2.1–5.5)×105m3d−1. Nutrient fluxes (mold−1) from SGD are estimated to be (3–7.4)×104 (dissolved inorganic nitrogen), (2.4–6.2)×102 (dissolved inorganic phosphate) and (6.5–16)×104 (dissolved silicate). Primary productivity is estimated to be (1.5–15)×106gCd−1, 2–53% of which is supported by SGD-induced phosphate fluxes. The study indicates that SGD is a significant source of nutrients to coastal waters and may cause an obvious increase of primary production. These findings must be considered in future coastal ecological management.

Characterising the distribution of near-shore submarine groundwater discharge along a coastline using 222Rn and electrical conductivity

The spatial and temporal distribution of near-shore fresh submarine groundwater discharge (SGD) was characterised from the coastal aquifers of the Willunga Basin, South Australia, an extensive aquifer system that supports an important viticultural region. Measurements of electrical conductivity (EC) and 222Rn (radon) activity were collected at 19 sites along the coastline during the Southern Hemisphere spring (2011) and summer (2013). At each site, samples were collected from the surf zone as well asporewater from beach sediment in the intertidal zone. Surf-zone radon activity ranged from <5 to 70mBq L–1, and intertidal porewater radon ranged over two orders of magnitude (220–36 940 mBq L–1) along the Willunga Basin coastline during both surveys. Overall, surf-zone and porewater EC was lower in the spring 2011 survey than in the summer 2013 survey. Porewater EC was similar to that of coastal water at most sites along the coastline, except at three sites where porewater EC was found to be lower than coastal water during both surveys, and three sites where evaporated seawater was observed in the summer survey. Based on the patterns in radon and EC along the coastline, two sites of localised fresh SGD were identified, in addition to a groundwater spring that is known to discharge to the coast. The results indicate that near-shore fresh SGD occurs as localised seeps rather than diffuse seepage along the entire coastline. The apparent absence of groundwater discharge at most locations is also consistent with current evidence suggesting that extensive groundwater pumping within the basin has resulted in seawater intrusion across much of the coastline. These observations also suggest that previous studies are likely to have over-estimated SGD rates from the Willunga Basin because they assumed that SGD occurred along the entire coastline.

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 (&lt;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.