Measurements Of Submarine Groundwater Discharge Research Articles

Intercomparison of marine tracer and catchment-based submarine groundwater discharge estimates for a karst aquifer

Submarine groundwater discharge (SGD) is a major source of solutes to the ocean. Consequently, the accuracy of estimates of the magnitude and seasonal variability of SGD is critical for the management of coastal marine ecosystems. However, very few intercomparison studies between different conceptual approaches have been made to assess the reliability and consistency of SGD flow estimates while simultaneously gauging the applicability of underlying models and budget assumptions in real-world scenarios. To date, intercomparison efforts have focused on homogeneous aquifers, and less so on highly heterogeneous aquifers such as karst systems. Here we compare the results of tracer-based (salt mass balance, tidal prism and radon mass balances) and catchment-based methods (water balance, semi-distributed karst network model) to assess SGD at a data-rich coastal location. All approaches tested here reveal a positive relationship between SGD fluxes and groundwater level measured close to the preferential groundwater flow pathways in the phreatic karst aquifer (e.g. water-saturated conduits or fracture network). We use this finding to derive site-specific relationships between groundwater level and SGD flow rates. We derive these relationships from least square fitting of Darcian and Non-Darcian models and regression analysis. The relationships validate the findings derived independently from hydraulic/hydrogeological modelling of the complex karst networks. By combining various estimates made with distinct assumptions, this method produces robust estimates of the SGD seasonal variability, essential to develop optimal management strategies of SGD resources in coastal karstic regions. Where direct measurements of SGD are not possible, monitoring of coastal groundwater level close to preferential phreatic groundwater pathways can be thus used as a useful indicator to compare past groundwater flow measurements and derive site-specific relationships, particularly in settings where large flow rate changes are present such as karst aquifers.

The impact of future hydrology stresses and climate change on submarine groundwater discharge in arid regions: A case study of the Nile Delta aquifer, Egypt

Study regionClimate change is expected to severely impact Egypt's Nile Delta Aquifer (NDA). Despite its large freshwater reservoir, estimated at 400 Billion Cubic metres (BCM), climate-change-induced drivers (drought and sea-level rise) coupled with increasing groundwater over-abstraction will cause a gradual reduction of the available freshwater volume. This study used the numerical model SEAWAT to study the impact of the main hydrogeological and anthropogenic factors on the response of the submarine groundwater discharge (SGD) (i.e., the net fresh seaward groundwater flux) and seawater intrusion (SWI) in the NDA. Study focusFive scenarios were examined, including (i) a probable Sea-level rise (SLR), (ii) expected reduction in Nile hydrograph and its branches, (iii) freshwater overpumping, (iv) the combination of reduction in Nile hydrograph and overpumping, and (v) the combination of these scenarios in the years 2030, 2050 and 2070. New hydrological insights for the regionThe results show that the increasing saltwater head due to SLR coupled with a reduction in Nile flow and overpumping ultimately results in the landward shifting of the saltwater within the aquifer. In addition, the resulting salinity increase in the aquifer caused a significant increase in the deterioration of a large quantity of freshwater volume with a subsequent reduction of the SGD. Also, the salt mass variation (SMV) in scenario 5 increased to 7.09%, 10.69%, and 12.99%, while the groundwater discharges variation (SGDV) to the sea declined by 21.90%, 42.38%, and 61.95% in the years 2030, 2050 and 2070, respectively. Moreover, the coastal aquifers required the management of the SGD to keep the balance between the freshwater and saltwater interface. This study is useful for the future planning and water resources management in coastal regions for integrated management of SGD, SWI, and aquifer freshwater storage. Also, the applications of smart measurements of SGD and groundwater salinity are required for coastal aquifers management.

Impacts of the Scale of Representation of Heterogeneity on Simulated Salinity and Saltwater Circulation in Coastal Aquifers

AbstractNumerical models of variable‐density groundwater flow and salt transport are a primary tool for predicting salinity distributions in coastal aquifers and estimating submarine groundwater discharge (SGD). Models are particularly useful to estimate the saline component of SGD, which can occur far offshore and is difficult to measure directly. Depending on the system and application, the level of geologic detail represented can range from homogeneous or layered to fully heterogeneous hydraulic conductivity fields. These features strongly affect model results, limiting understanding of subsurface salinity distributions and associated density‐driven saltwater circulation along coasts worldwide. In this study, the impact of the scale of representation of heterogeneity on salinity distributions and SGD was investigated using numerical simulations. Upscaling hydraulic conductivity can significantly modify salinity distributions and flow paths, resulting in unpredictable variations in simulated SGD, though the values for homogeneous fields with equivalent hydraulic conductivity show consistent trends. Simulated density distributions control both the rate and direction of subsurface saltwater circulation. The length of the mixing zone perimeter, a measure of salinity distribution complexity, is shown to correlate with both the rate of subsurface saltwater circulation and the amount of groundwater circulating in the reverse direction from homogeneous cases. Overall, the results demonstrate a strong dependence of salinity distributions and saltwater circulation on the scale and distribution of geologic heterogeneity represented in numerical models. This suggests that numerical models with simplified geologic structure may substantially underestimate saltwater circulation, and attempts to calibrate them using salinity distributions or SGD measurements may be problematic.

Small Unmanned Aircraft (sUAS)-Deployed Thermal Infrared (TIR) Imaging for Environmental Surveys with Implications in Submarine Groundwater Discharge (SGD): Methods, Challenges, and Novel Opportunities

Submarine Groundwater Discharge (SGD) represents a significant mode of chemical transport to water bodies, making it an important flux to understand. Small Unmanned Aircraft Systems-deployed thermal infrared sensors (sUAS-TIR) provide a financially and logistically inexpensive means of identifying SGD source zones and quantifying SGD thermal infrared (TIR) plume areas over regional scales at high spatial resolutions. sUAS-TIR additionally offers the unique capability of high temporal resolution measurements of SGD. As a developing science application, the use of sUAS-TIR to image SGD requires substantial background knowledge. We present a proposed methodological construct for implementing a sUAS-TIR program for SGD-TIR data gathering, with applications extending to other research fields that can benefit from airborne TIR. Several studies have used airborne TIR in combination with empirical SGD flux measurements to quantify SGD, reporting a consistently strong regression between SGD flux and SGD TIR plume area. We additionally discuss novel research opportunities for sUAS-TIR technologies, as applied to SGD flux. The combination of high spatial and temporal resolution capabilities, at relatively low costs, make sUAS-TIR a promising new technology to overcome the scaling challenges presented by empirical studies and modeling of SGD fluxes, and advance our understanding of the controls on SGD fluxes.

Microbial Source Tracking of Fecal Indicating Bacteria in Coral Reef Waters, Recreational Waters, and Groundwater of Saipan by Real-Time Quantitative PCR.

The Commonwealth of the Northern Mariana Islands (CNMI) recently identified the need to improve its capacity for detecting and tracking land-based sources of pollution (LBSP) in coastal waters, particularly microbial contaminants like fecal indicator bacteria (FIB). Reported here is a baseline study of a suite of host-specific FIB microbial source tracking (MST) markers in the coastal shoreline and reef waters around the island of Saipan. Three sampling campaigns were conducted in September 2017, March 2018, and August 2018. Samples were collected from the nearshore surface waters of Saipan, the reef waters of Saipan Lagoon, and groundwater from beaches along the Saipan Lagoon shoreline. Measurements of submarine groundwater discharge (SGD) into nearshore waters and isotopic source tracking of nitrogen inputs were conducted concurrently with MST. Environmental DNA was extracted from the samples and analyzed by quantitative polymerase chain reaction (qPCR) for MST gene markers of fecal Bacteroidales specifically associated with humans, dogs, cows, and pigs, and for an MST gene marker of Catellicoccus associated with seabirds. MST assessments were combined with local knowledge, assessments of sanitary infrastructure, and routine watershed surveys. This study identified hotspots of human FIB along the western Saipan Lagoon shoreline in both surface waters and groundwater, plus another hotspot of human FIB at a popular tourist bathing area known as the Grotto. FIB hotspots on the Lagoon shoreline coincided with areas of high SGD and nitrogen isotopic data indicating sewage-derived N inputs. It appears that faulty sanitary infrastructure may be contributing to inputs to Saipan Lagoon, while bather shedding is likely a primary input for the Grotto area. Moderate levels of dog fecal contamination were common and widespread across the island. High levels of seabird fecal contamination were more random, both spatially and temporally, and mostly concentrated along the less developed northeast region of Saipan. No significant levels of cow or pig fecal marker were detected in coastal water samples. This study provides demonstration and establishment of analytical capacity to resource management in CNMI for MST technology to aid in trouble-shooting water quality issues involving land-based sources of microbial contaminants to CNMI coastal waters.

Relating estuarine geology to groundwater discharge at an oyster reef in Copano Bay, TX

Coastal bays with fresh submarine groundwater discharge (SGD) have been found to provide a favorable environment to shellfish (e.g., oysters). Significant variability in the spatial distribution and magnitude of SGD may occur because of subsurface heterogeneity. We investigated the relationship between stratigraphic controls (i.e. paleovalley systems) and the spatial distribution of SGD in relation to oyster reefs. In Copano Bay, South Texas, major oyster reefs have been shown to be oriented perpendicular to shore, along interfluves, and near paleovalley margins. A novel combination approach of radioactive geochemical tracers, stable isotopes, geophysical and petrophysical techniques were used to identify estuarine depositional environments for individual hydrogeologic characterization and evaluation of SGD distribution with implications to ecological features such as oyster reefs. A 1-km2 area within a paleovalley system, in Copano Bay, yielded discernible differences in sediment type, apparent resistivity, and SGD rates among four different depositional environments: paleovalley, paleovalley margin, oyster reef and interfluve. Electrical resistivity tomography revealed a direct connection between the oyster reef and the underlying, more permeable interfluve, and the presence of a fresher SGD component in the subsurface. Measurements of SGD from stationary and mobile continuous radon-222 surveys across each depositional environment showed that, by and large, areas in proximity to reefs have larger SGD rates (62.6 ± 11.3 cm/d in January and 28.7 ± 10.8 cm/d in July) when compared to paleovalleys (31.9 ± 8.9 cm/d in January and 19.5 ± 7.2 cm/d in July). The role of depositional environments as hydrologic controls on SGD to oyster reefs revealed in this study is further confirmed by significant differences in stable and radioactive isotopes observed over small spatial scales offshore when compared to the entire estuary. Although constant mixing and radon-222 degassing of bay waters are likely altering the relationships between the different depositional environments and magnitude of SGD, this study provides an improved understanding of possible pathways of freshwater and solute inputs to oyster reefs for more informed decisions on reef restoration efforts.

An alternative approach to conventional seepage meters: Buoy‐type seepage meter

AbstractWe propose a new type of seepage meter to allow sequential measurements of submarine groundwater discharge (SGD) rate and SGD‐driven chemical flux at a point on seawater and groundwater interface. The system works on the basis of Δh, the difference in head between groundwater and seawater at groundwater–seawater interface, and uses a vertical cylinder fixed to a buoy to derive groundwater outflow corresponding to Δh from the seepage meter chamber to the vertical cylinder. Laboratory tank tests showed that volumetric flow rate (Q) from the buoy‐type seepage meter (BSM) compared very well with conventional seepage meter (CSM). The linear relationship between Q measurements with the two methods is also statistically significant (R2 = 0.976). Two sample t‐test reveals that there is not a significant difference between means of Q measurements with the two methods. R2 values of linear regressions between volumetric flow rate (Q) and Δh are 0.993 with the BSM, and 0.972 with the CSM. Field tests with the BSM at Bangdu Bay of Jeju Island, Korea also show linear relationship (R2 = 0.86) between SGD flux and Δh, and SGD flux and SGD‐derived chemical fluxes are inversely related to tides. Both results of laboratory and field tests support that the proposed seepage meter can be an alternative to the conventional seepage meters for measuring SGD flux, and that it can be used to monitor temporal variations of SGD flux and SGD‐driven chemical fluxes simultaneously.

The influence of groundwater inputs and age on nutrient dynamics in a coral reef lagoon

Tritium dating of groundwater and a radon mass balance was used to assess the contribution of submarine groundwater discharge (SGD) to a nutrient budget in a tropical reef lagoon (Rarotonga, Cook Islands). Open ocean exchange accounted for the largest percentage of potential lagoon exports (nitrogen (N)=61%, phosphorous (P)=22%) with N export mostly in the form of dissolved organic nitrogen (DON) (93% of total N). SGD accounted for 29% and 11% of exported N and P respectively. Overall we could account for 92% of dissolved N but only 36% of dissolved P leaving the lagoon. However, if DON is assumed to be recalcitrant, SGD would be the major driver of lagoon N and would account for 81% of dissolved inorganic N (DIN) inputs and 47% of DIN exported form the lagoon. A small scale (~50m), high definition survey indicated areas of higher NH4+ towards the middle of the lagoon. This is indicative of SGD input away from the seepage face and may mean that SGD measurements made at the beach face underestimate the broader contribution of SGD to nutrient fluxes. Time series observations during a 100mm rain event indicated that these episodes can deliver high nutrient loads to the lagoon and may contribute to closing the phosphorous budget. Analysis of tritium concentrations in groundwater, surface waters and springs showed that old, deep groundwater (10–93years old) was the main source of SGD derived nutrients to the lagoon. This study demonstrates that there may be a long time lag between nutrient infiltration into aquifers and the discharge of nutrient loaded groundwater into the lagoon. As such, potential mitigation measures which decrease terrestrial nutrient loads may not result in decreased SGD nutrient fluxes for decades to come.

Isotopic, geophysical and biogeochemical investigation of submarine groundwater discharge: IAEA-UNESCO intercomparison exercise at Mauritius Island

Submarine groundwater discharge (SGD) into a shallow lagoon on the west coast of Mauritius Island (Flic-en-Flac) was investigated using radioactive ( 3H, 222Rn, 223Ra, 224Ra, 226Ra, 228Ra) and stable ( 2H, 18O) isotopes and nutrients. SGD intercomparison exercises were carried out to validate the various approaches used to measure SGD including radium and radon measurements, seepage rate measurements using manual and automated meters, sediment bulk conductivity and salinity surveys. SGD measurements using benthic chambers placed on the floor of the Flic-en-Flac Lagoon showed discharge rates up to 500 cm/day. Large variability in SGD was observed over distances of a few meters, which were attributed to different geomorphological features. Deployments of automated seepage meters captured the spatial and temporal variability of SGD with a mean seepage rate of 10 cm/day. The stable isotopic composition of submarine waters was characterized by significant variability and heavy isotope enrichment and was used to predict the contribution of fresh terrestrially derived groundwater to SGD (range from a few % to almost 100%). The integrated SGD flux, estimated from seepage meters placed parallel to the shoreline, was 35 m 3/m day, which was in reasonable agreement with results obtained from a hydrologic water balance calculation (26 m 3/m day). SGD calculated from the radon inventory method using in situ radon measurements were between 5 and 56 m 3/m per day. Low concentrations of radium isotopes observed in the lagoon water reflected the low abundance of U and Th in the basalt that makes up the island. High SGD rates contribute to high nutrients loading to the lagoon, potentially leading to eutrophication. Each of the applied methods yielded unique information about the character and magnitude of SGD. The results of the intercomparison studies have resulted a better understanding of groundwater–seawater interactions in coastal regions. Such information is an important pre-requisite for the protection and management of coastal freshwater resources.

Calibration and use of continuous heat-type automated seepage meters for submarine groundwater discharge measurements

Submarine groundwater discharge (SGD) assessments were conducted both in the laboratory and at a field site in the northeastern Gulf of Mexico, using a continuous heat-type automated seepage meter (seepmeter). The functioning of the seepmeter is based on measurements of a temperature gradient in the water between downstream and upstream positions in its flow pipe. The device has the potential of providing long-term, high-resolution measurements of SGD. Using a simple inexpensive laboratory set-up, we have shown that connecting an extension cable to the seepmeter has a negligible effect on its measuring capability. Similarly, the observed influence of very low temperature (≤3 °C) on seepmeter measurements can be accounted for by conducting calibrations at such temperatures prior to field deployments. Compared to manual volumetric measurements, calibration experiments showed that at higher water flow rates (>28 cm day −1 or cm 3 cm −2 day −1) an analog flowmeter overestimated flow rates by ≥7%. This was apparently due to flow resistance, turbulence and formation of air bubbles in the seepmeter water flow tubes. Salinity had no significant effect on the performance of the seepmeter. Calibration results from fresh water and sea water showed close agreement at a 95% confidence level significance between the data sets from the two media ( R 2 = 0.98). Comparatively, the seepmeter SGD measurements provided data that are comparable to manually-operated seepage meters, the radon geochemical tracer approach, and an electromagnetic (EM) seepage meter.

Groundwater connections under a barrier beach: A case study in the Venice Lagoon

The flow of groundwater beneath barrier islands has been cited as a possible pathway for salt water and chemical exchange between a protected embayment and the open sea. Evidence is presented that identifies an exchange of groundwater through a highly permeable paleoinlet along the barrier beach of Cavallino, which separates the northern Venice Lagoon from the Adriatic Sea. We utilized both point measurements of submarine groundwater discharge (SGD) and a geophysical investigation of the subsurface resistivity to analyze the movement of saline groundwater. Discharge of groundwater and associated nutrients, was higher at the site of a former inlet than at a similar site along the barrier and modulated by the difference in tidal water level between the lagoon and Adriatic Sea. If the measured conditions are typical, storm surge barriers could potentially result in a saline groundwater flow of up to 1.5×10 6 m 3 d −1 into the lagoon.

Eddy correlation measurements of submarine groundwater discharge

This paper presents a new, non-invasive means of quantifying groundwater discharge into marine waters using an eddy correlation approach. The method takes advantage of the fact that, in virtually all aquatic environments, the dominant mode of vertical transport near the sediment–water interface is turbulent mixing. The technique thus relies on measuring simultaneously the fluctuating vertical velocity using an acoustic Doppler velocimeter and the fluctuating salinity and/or temperature using rapid-response conductivity and/or temperature sensors. The measurements are typically done at a height of 5–15 cm above the sediment surface, at a frequency of 16 to 64 Hz, and for a period of 15 to 60 min. If the groundwater salinity and/or temperature differ from that of the water column, the groundwater specific discharge (cm d − 1 ) can be quantified from either a heat or salt balance. Groundwater discharge was estimated with this new approach in Salt Pond, a small estuary on Cape Cod (MA, USA). Estimates agreed well with previous estimates of discharge measured using seepage meters and 222Rn as a tracer. The eddy correlation technique has several desirable characteristics: 1) discharge is quantified under in-situ hydrodynamic conditions; 2) salinity and temperature can serve as two semi-independent tracers of discharge; 3) discharge can be quantified at high temporal resolution, and 4) long-term records of discharge may be possible, due to the low power requirements of the instrumentation.

Direct measurements of submarine groundwater discharge (SGD) over a fractured rock aquifer in Flamengo Bay Brazil

Measurements of submarine groundwater discharge (SGD) along the South American coast and over fractured rock aquifers are rare. The rate and distribution of SGD was measured using three types of vented benthic chambers on the floor of Flamengo Bay located at the southeast coast of Brazil. Discharge rates were found up to almost 400 cm day −1, although typically less than 100 cm d −1. Large variations in SGD rates were seen over distances of a few meters which are attributed to the geomorphologic features of the fracture rock aquifer underlying a thin blanket of coastal sediments; clustering of fractures and the topography of the rock–sediment interface might be focusing or dispersing the discharge of groundwater. SGD was modulated by the tides with the highest values occurring at times of low tide, but the interaction was non-linear and, the correlation was weak at tidal ranges less than 1 m. The effect was masked by devices that integrated the SGD, but detected on continuously recording devices.

Submarine groundwater discharge measured by seepage meters in sicilian coastal waters

Seepage measurements of submarine groundwater discharge (SGD) were carried out in March 2002 in the Donnalucata boat basin in the southeastern Sicily. SGD rates measured by manual seepage meters ranged from 5.5 to 19.3 L/min/m (with an average of 12.1 L/min/m), which was 0.6 and 5.9 times the SGD determined in Florida and Perth, respectively. The spatial distribution of SGD was found to be highly variable along ∼300 m long coast area. Semi-diurnal SGD variations which anti-correlated with tide were found using newly developed automated seepage meters that can provide continuous SGD data with high resolution. These variations may be attributed to the tidal effect, when a greater hydraulic gradient, observed at low tide, moves groundwater from land to the ocean, and an opposite effect is observed at high tide.

Submarine groundwater discharge: An important source of new inorganic nitrogen to coral reef ecosystems

Submarine groundwater discharge: An important source of new inorganic nitrogen to coral reef ecosystems

Submarine groundwater discharge into Venice Lagoon, Italy

Venice Lagoon, Italy, rests on a series of aquifers that are 1,000 m thick. Measurements of submarine groundwater discharge (SGD) were made in Venice Lagoon using benthic chambers vented to a plastic collection bag. Two hundred measurements taken in a pristine northern lagoon site (Isola la Cura) revealed flow rates as high as 200 cm d−1 with an average of 30 cm d−1. Over 100 measurements taken adjacent to a bulkhead shoreline in the Porto Marghera industrial zone (Fusina) showed flow rates as high as 30 cm d−1 and averaging 6 cm d−1. These flow rates, if representative of even a fraction of the lagoon floor, are easily able to account for the 15% deficit previously calculated between precipitation and runoff for the entire Venice Lagoon drainage basin. Land elevation surrounding the Venice Lagoon is < 10 m within 20 km of the shoreline and is unable to support any substantial onshore water table. Submarine groundwater discharge most likely represents upward artesian discharge from deeper partially confined aquifers. Over 60 samples were collected in total from both sites for nutrient analysis. Ammonium concentration was found to be 2–8 fold higher in the device water than in the lagoon water at the northern site depending on season, and 10–30 times higher at the industrial zone site. These numbers suggest that SGD may be the primary pathway for nutrients and perhaps other contaminants to enter Venice Lagoon.

A hydrogeologic model of submarine groundwater discharge: Florida intercomparison experiment

A hydrogeologic model of submarine groundwater discharge (SGD) to the near-shore environment at a site on the northeast Gulf of Mexico has been developed to provide a basis for comparison with measurements of SGD made using seepage meters, and with estimates derived from chemical tracers. The hydrogeologic model incorporates the seaward movement of fresh water and the recirculation of sea water at the fresh water–salt water interface. The hydrostratigraphy at the site includes the Surficial Aquifer, a thin confining unit known as the Intracoastal Formation, and the underlying Upper Floridan Aquifer. It is not possible to explain either the magnitude or spatial distribution of SGD recorded by the seepage meters, or the magnitude of SGD estimated using radium and radon tracers, if only steady state flow in the Surficial Aquifer is considered. Nor does it appear likely that the difference between the model-based prediction of SGD and the field-based estimates can be fully resolved by leakage across the Intracoastal Formation from a source in the Floridan Aquifer. These results suggest that processes driven by variations in fluid pressure in the marine water column, which occur on a variety of time scales, be examined to quantify their contribution to fluid circulation within and discharge from that segment of the Surficial Aquifer located beyond the low tide line.

Tidal effects on submarine groundwater discharge into the ocean

Semi‐monthly, diurnal and semi‐diurnal variations of Submarine Groundwater Discharge (SGD) were found in Osaka bay, Japan, by continuous measurements of SGD using newly developed automated seepage meters. SGD increased sharply from neap to spring tide with semi‐monthly period. SGD also changes with diurnal and semi‐diurnal periods due to changes in hydraulic gradient between the groundwater and seawater in the coastal zone. SGD variations with semi‐monthly period by neap‐spring tidal pumping is much larger than those with diurnal and semi‐diurnal variations by tidal height changes.

Offshore Passive Soil Vapor Survey at a Contaminated Coastal Site

A high-resolution passive soil vapor survey was recently conducted at a coastal military base in southern California where a chlorinated and aromatic hydrocarbon plume threatens San Diego Bay. An existing sampling grid was extended offshore the entire length of the contaminated site. PETREX sampling tubes were emplaced conventionally onshore and were emplaced offshore in sandy bay-bottom sediments by divers working in 3 to 17 m (10 to 55 ft) of water. Results of mass spectrometric analysis of passive vapor samples were consistent with submarine ground water discharge measurements made previously with seepage meters, yet also produced one unexpected result: evidence of in situ degradation. Tetrachloroethene (PCE), widespread onshore, was not detected offshore. Trichloroethene (TCE), the primary contaminant onshore, was detected in only three points offshore. Dichloroethene (DCE), present only at trace levels onshore, was the dominant constituent measured in the offshore plume. Results were confirmed by laboratory analyses of sediment samples. The study demonstrated the utility of extending the passive soil vapor technique offshore in up to 17 m (55 ft) of sea water in a heavily trafficked waterway with a strong tidal current.