Seepage Meters Research Articles

Further Development of a Specific Conductivity Approach to Measure Groundwater Discharge Area within Lakes

AbstractGroundwater exchanges with most lakes are rarely quantified because there are many technical challenges to quantification. We investigated a lakebed mapping approach to infer the relative areas of groundwater exchange in 12 prairie shallow lakes and five Laurentian mixed forest shallow lakes in Minnesota, USA in 2011. We used a relatively common approach (seepage meters) to provide baseline information on the magnitude and direction of flow at four locations in each lake. To expand from point measurements to the whole‐lake scale, we explored use of specific conductivity as a cheaper and more time efficient proxy for groundwater discharge to lakes. We validated the approach at near shore stations in each lake where seepage meter measurements and specific conductivity surveys overlapped. Specific conductivity surveys provided a similar assessment of groundwater discharge compared to seepage meters for 50% of the lake‐sampling period combinations. The lakebed mapping approach, when validated for a lake with a limited number of seepage meter (or alternative methods) measurements, offers the advantages of being more time and labor efficient over the use of a similar number of seepage meter monitoring locations; seepage meters (or piezometers, for example) are costlier in terms of equipment and labor, even for single‐lake studies. We show the combined approach could provide useful baselines for understanding and mapping groundwater exchange in shallow lakes.

As interações entre águas subterrâneas e superficiais em uma laguna costeira: Lagoa da Conceição, Florianópolis/SC, Brasil

As interações entre as águas subterrâneas e superficiais (Asub-Asup) ocorrem em todos os tipos de corpos hídricos, como rios, lagos, lagoas costeiras e oceanos. A importância deste processo é cada vez mais reconhecida e estudada, onde recentes investigações no sul do Brasil identificaram grande importância das descargas de águas subterrâneas (DAS) nos aportes de nutrientes e ciclos biogeoquímicos do oceano. A área de estudo configura-se em uma laguna costeira sufocada, localizada na Ilha de Santa Catarina. O método de identificação e quantificação deste trabalho foi o seepage meter, que consiste em medições diretas e pontuais dos fluxos na interface sedimento-água. A metodologia apresentou resultados satisfatórios, identificando padrões de interação Asub-Asup em cinco estações amostrais. Os resultados apresentaram taxas de DAS variando de -12,0 a 6,0 cm/d, levando à proposta de modelos conceituais dos regimes de interação Asub-Asup localmente, onde presume-se que estes sejam afetados diretamente pelo uso dos recursos hídricos e variações nos gradientes hidráulicos.

Linkages between hydrology and seasonal variations of nutrients and periphyton in a large oligotrophic subalpine lake

Periphyton is important to lake ecosystems, contributing to primary production, nutrient cycling, and benthic metabolism. Increases in periphyton growth in lakes can be indicative of changes in water quality, shifts in ecosystem structure, and increases in nutrient fluxes. In oligotrophic lakes, conservationists are interested in characterizing the influence of hydrological drivers on excessive periphyton growth along nearshore areas. We collected nutrient samples bi-weekly from groundwater and surface water during a 9-month monitoring period to evaluate the timing and availability of nutrients to eulittoral periphyton in Lake Tahoe. Groundwater discharge rates were measured synoptically using seepage meters and estimated indirectly using continuous head gradient measurements and aquifer properties estimated by slug tests. The discharge measurements made from the seepage meter measurements provide information about the spatial variability perpendicular from shore along and the change in groundwater discharge due to wave action. Algal biomass sampled from substrates and observed using underwater photographs were used to correlate seasonal growth and nutrient concentrations in groundwater and lake water. Results indicate that groundwater and nutrient discharge are temporally variable due to seasonal changes in recharge within the watershed, wave action, and lake stage. Groundwater discharge was enhanced by the seasonally-low lake stage and episodic recharge caused by precipitation falling as rain in the watershed. Increases in dissolved phosphorus and nitrate in the lake during winter are attributed to groundwater discharge and correlates to increases in algal biomass in the nearshore area. Results indicate that nutrient-rich groundwater discharge appears to stimulate seasonal periphyton blooms along the eulittoral zone of Lake Tahoe.

Fresh and Recirculated Submarine Groundwater Discharge Evaluated by Geochemical Tracers and a Seepage Meter at Two Sites in the Seto Inland Sea, Japan

Submarine groundwater discharge (SGD) consists of fresh submarine groundwater discharge (FSGD) and recirculated submarine groundwater discharge (RSGD). In this study, we conducted simultaneous 25-hour time-series measurements of short-lived 222Rn and 224Ra activities at two sites with differing SGD rates in the central Seto Inland Sea of Japan to evaluate SGD rates and their constituents. At both sites, we also quantified the total SGD, FSGD, and RSGD using a seepage meter to verify the water fluxes estimated with 222Rn and 224Ra. SGD rates estimated using 222Rn and 224Ra at the site with significant SGD approximated the total SGD and RSGD measured by the seepage meter. However, SGD rates derived using 222Rn at the site with minor SGD were overestimated, since 222Rn activity at the nearshore mooring site was lower than that in the offshore area. These results suggest that the coupling of short-lived 222Rn and 224Ra is a powerful tool for quantification of FSGD and RSGD, although it is important to confirm that tracer activities in coastal areas are higher than those in offshore.

A conceptual model for groundwater discharge to a coastal brackish lagoon based on seepage measurements (Ringkøbing Fjord, Denmark)

AbstractGroundwater discharge to a brackish lagoon (Ringkøbing Fjord) was quantified with seepage meters along four transects perpendicular to the shore during four seasons in 1 year. The objectives were to develop a conceptual model of the spatiotemporal variability of seepage and how landscape characteristics and saltwater intrusion affect seepage fluxes. The electrical conductivity of the groundwater was measured in vertical sediment profiles up to 3 m below the lagoon bed to assess the effects of the freshwater–saltwater interface location on flux distribution. The main differences compared with existing theoretical models are a lower discharge near the shore line (0–5 m) and two discharge peaks more offshore (15–20 and >25 m), which allowed the development of a conceptual model that is different from the classical concepts introduced for marine or lake environments and which can be representative of other similar areas. These differences are explained by the vegetation and organic material deposition in the near‐shore environment reducing discharge, the presence of the saline wedge leading to upward flow, and the increase of recirculation of brackish water due to wave pumping in areas offshore most exposed to the wind. The seasonal variability in discharge is smaller than the spatial variability between and within transects along the shoreline. Based on the conceptual model, fresh water discharge in a 20‐m‐wide fringe was calculated to be between 66 and 388 L·day−1·m−1 shore line, corresponding to a difference of a factor of six due to the variations in coast morphology and local characteristics of the area. The seasonal changes were responsible for changes of only 8% to 75% depending on the location.

시피지미터를 이용한 지하수-지표수 상호유동량 측정: 전주시 덕진연못 사례연구

지하수와 지표수는 상호 유동적인 관계로 특히 갈수기에는 기저유출의 형태로 지하수가 지표수에 미치는 영향이 커진다. 하지만 지금까지 국내에서 지하수와 지표수의 상호유동량에 대한 정량적인 연구가 많이 부족하였다. 본 연구에서는 지하수-지표수의 상호유동량을 측정하기 위하여 시피지미터를 자체 제작하였고, 전주시 덕진연못을 대상으로 하여 2017년 3월부터 8월까지 지하수 유출입량을 측정하였다. 측정은 대부분의 기간 동안 주 1~2회였으나, 7월 3일부터 10일까지의 장마기간에는 1~2일 간격으로 이루어졌다. 측정된 지하수 유출입량을 덕진연못의 수위 및 측정기간의 평균 강수량과 비교하였으며, 결과는 지하수의 유출입량과 덕진연못의 수위는 상관관계가 있으며, 또한 측정기간 동안의 평균 강수량과도 상관관계가 있음을 보여주었다. 덕진연못 전체 면적에 대한 총 지하수 상호유동량은 -0.869∼+1.043 L/s로 계산되었다. 본 연구에서의 시피지미터를 이용한 지하수 유출입량 측정은 지표수에 기여하는 기저유출량 결정의 중요한 수단이 되며, 따라서 지하수와 지표수의 상호관계 이해에 기여할 수 있다.

Estimating groundwater discharge to a lowland alluvial stream using methods at point-, reach-, and catchment-scale

The groundwater contribution to streamflow along a lowland alluvial stream in Denmark was estimated using a variety of methods and at different spatial scales. At the point-scale (less than a few metres), groundwater discharge to the stream was measured using seepage meters. At the reach-scale (0.5–1.5 km), groundwater discharge to the stream was estimated using differential streamflow gauging. And at the catchment-scale (>1.5 km), groundwater discharge to the stream was estimated using hydrograph separation. The estimates of groundwater discharge obtained using point-scale measurements range from 12 to 41 cm/d which is lower than fluxes estimated at the reach-scale which range from 18 to 333 cm/d. This discrepancy was attributed to the partitioning of groundwater discharge into streambed seepage and bank seepage. On the other hand, the groundwater discharge estimates obtained using hydrograph separation were generally the highest and ranged from 194 to 289 cm/d. For this study, this discrepancy from the reach-scale estimates was attributed to the assumption that baseflow obtained using hydrograph separation represents groundwater discharge to the stream when part of the baseflow actually comes from artificial drainage systems. Anyhow, seepage meter measurements, differential streamflow gauging, and hydrograph separation showed similar trends. The increase in streamflow is mainly due to either groundwater discharge through the streambed and the banks in the lower part, or tile drainage discharge in the upper part of the catchment. Furthermore, estimation of uncertainty for the various groundwater discharge estimates showed that some of the flux estimates were insignificant compared with their propagated uncertainties. To this end, a novel method was developed to estimate the uncertainty of groundwater discharge estimates obtained by hydrograph separation. Overall, this paper shows that more than one method should be used to obtain a reliable estimate of groundwater discharge to a lowland alluvial stream from other discharge contributions such as tile drainage.

Current and historical rates of input of mercury to the Penobscot River, Maine, from a chlor-alkali plant

Mercury inputs by surface and ground water sources to Penobscot River from a defunct Hg-cell chlor-alkali plant were measured in 2009–10 and estimated for the entire period of operation of this facility. Over the measured interval (422 days) approximately 2.3 kg (5.4 g day−1) of mercury was discharged to the Penobscot River by the two surface streams that drain the site, with most of the combined loading (1.8 kg Hg, 78%) associated with a single storm with rainfall in excess of 100 mm. Groundwater seepage rates from the site, as estimated from both a radon tracer and seepage meter methods were in the range of 3 to 4 cm day−1 and, when combined with a best estimate of the area of groundwater discharge (11,000 m2) and average seepage/porewater mercury concentration (242 ng L−1, UCL95), yielded a loading of 0.11 g day−1 for site groundwater. None of the municipal or other industrial point sources of mercury to the river between Veazie and Bucksport, Maine exceeded 1 g day−1 individually, nor was the aggregate loading of all such sources >3 g day−1 (based on State of Maine data). Mercury loadings for the three largest tributaries downstream of Veazie Dam were estimated to contribute 4.2, 3.7 and 2.5 g day−1, respectively, to the Penobscot River. Based on sampling (total Hg ~ 2 to 4 ng L−1) and historical mean discharge data (340–460 m3 s−1), the Penobscot River upstream of the plant site contributes as much as 160 g day−1 to the downstream reach depending on river discharge. Estimates of historical (1967–2012) mercury loading using both generic emission factors and measured releases ranged from 2.6 to 27 MT while the mass of mercury found in downstream sediments amounted to 9 MT.

Evaluating long-term patterns of decreasing groundwater discharge through a lake-bottom permeable reactive barrier

Identifying and quantifying groundwater exchange is critical when considering contaminant fate and transport at the groundwater/surface-water interface. In this paper, areally distributed temperature and point seepage measurements are used to efficiently assess spatial and temporal groundwater discharge patterns through a glacial-kettle lakebed area containing a zero-valent iron permeable reactive barrier (PRB). Concern was that the PRB was becoming less permeable with time owing to biogeochemical processes within the PRB. Patterns of groundwater discharge over an 8-year period were examined using fiber-optic distributed temperature sensing (FO-DTS) and snapshot-in-time point measurements of temperature. The resulting thermal maps show complex and uneven distributions of temperatures across the lakebed and highlight zones of rapid seepage near the shoreline and along the outer boundaries of the PRB. Repeated thermal mapping indicates an increase in lakebed temperatures over time at periods of similar stage and surface-water temperature. Flux rates in six seepage meters permanently installed on the lakebed in the PRB area decreased on average by 0.021 md−1 (or about 4.5 percent) annually between 2004 and 2015. Modeling of diurnal temperature signals from shallow vertical profiles yielded mean flux values ranging from 0.39 to 1.15 md−1, with stronger fluxes generally related to colder lakebed temperatures. The combination of an increase in lakebed temperatures, declines in direct seepage, and observations of increased cementation of the lakebed surface provide in situ evidence that the permeability of the PRB is declining. The presence of temporally persistent rapid seepage zones is also discussed.

Seawater recirculation drives groundwater nutrient loading from a developed estuary shoreline with on-site wastewater treatment systems: Mobile Bay, USA

Previous studies have highlighted the influence of submarine groundwater discharge (SGD) on the environmental condition of coastal waters worldwide. Here, we focus on the quantity and quality of SGD from a developed estuarine shoreline in Mobile Bay to investigate the effects of residential development, specifically on-site wastewater treatment, on coastal water quality. A variety of techniques including a hydrologic balance, Darcy calculations, seepage meter observations, and coastal water radon-222 surveys were employed to quantify fresh and total components of SGD under varying hydrometeorological scenarios. Samples of groundwater from shallow wells, SGD from seepage meters, and surface water were analyzed for dissolved nutrients throughout the year to determine nutrient sources and loadings to the Bay. Comparisons of SGD yielded by Darcy’s calculations with seepage meter and radon-222 observations revealed that fresh, meteoric groundwater contributes only a minor fraction (~ 7%) of total SGD, thereby suggesting that seawater recirculation is the dominant mechanism driving SGD. Total dissolved nitrogen loading averaged 1960 mmol d−1 per m shoreline for the duration of the study; however, total dissolved nitrogen loading supported by the fresh, meteoric SGD component was only 11.2 mmol d−1 per m shoreline (less than 1% of the total loading), despite the presence of on-site wastewater treatment systems. These results demonstrate the importance of SGD as a source of nutrients to Mobile Bay and suggest that nutrient loading from regeneration and remineralization processes in the subterranean estuaries can exceed that of groundwater nutrient transport from developed watersheds.

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.

Tracing submarine groundwater discharge flux in Tolo Harbor, Hong Kong (China)

Submarine groundwater discharge (SGD) is an important pathway for groundwater and associated chemicals to discharge to the sea. Groundwater levels monitored along a transect perpendicular to the shoreline are used to calculate SGD flux from the nearshore aquifer to Tolo Harbor, Hong Kong (China). The calculated SGD flux—recharge/discharge measured with Darcy’s Law methods—agrees well with estimates based on geo-tracer techniques and seepage meter in Tolo Harbor during previous studies. The estimated freshwater SGD is 1.69–2.0 m2/d at the study site and 0.3 ± 0.04 cm/d for the whole of Tolo Harbor, which is comparable to the river discharge (0.25 ± 0.07 cm/d) and precipitation (0.45 ± 0.15 cm/d). The tide-driven SGD in the intertidal zone is 13.98–17.59 m2/d at the study site and 2.42 ± 0.56 cm/d for the whole of Tolo Harbor. The SGD occurring in the subtidal zone and the bottom of Tolo Harbor is 3.12 ± 4.63 cm/d. Fresh SGD accounts for ~5% of the total SGD, while the rest (~95%) is contributed by saline SGD driven by various forces. About 96% of the tide-driven SGD in the intertidal zone occurs in the ebbing tide period because the head difference between the groundwater level and sea level is great during this period. Tide-driven SGD in the spring tide is ~1.2 times that during neap tide. The tidal fluctuation amplitude and tide-driven SGD in the intertidal zone are positively correlated to each other; thus, a spring neap variation of the tide-driven SGD is observed.

Measurement of submarine groundwater discharge using diverse methods in Coleroon Estuary, Tamil Nadu, India

Submarine groundwater discharge (SGD) is described as submarine inflow of fresh and brackish groundwater from land into the sea. The release of sewages from point and non-point source pollutants from industries, agricultural and domestic activities gets discharged through groundwater to ocean creating natural disparity like decreasing flora fauna and phytoplankton blooms. Hence, to quantify fluxes of SGD in coastal regions is important. Quantification of SGD was attempted in Coleroon estuary, India, using three dissimilar methods like water budget, Darcy law and manual seepage meter. Three seepage meters were installed at two prominent litho units (alluvium and fluvio marine) at a distance of (0–14.7 km) away from Bay of Bengal. The water budget and Darcy law-quantified submarine seepage at a rate of 6.9 × 106 and 3.2 × 103 to 308.3 × 103 m3 year−1, respectively, and the seepage meter quantified seepage rate of 0.7024 m h−1 at an average. Larger seepage variations were isolated from three different techniques and the seepage rates were found to be influenced by hydrogeological characteristics of the litho units and distance from the coast.

Assessing submarine groundwater discharge (SGD) and nitrate fluxes in highly heterogeneous coastal karst aquifers: Challenges and solutions

Groundwater discharge in coastal karst aquifers worldwide represents a substantial part of the water budget and is a main pathway for nutrient transport to the sea. Groundwater discharge to the sea manifests under different forms, making its assessment very challenging particularly in highly heterogeneous coastal systems karst systems. In this study, we present a methodology approach to identify and quantify four forms of groundwater discharge in a mixed lithology system in southern Spain (Maro-Cerro Gordo) that includes an ecologically protected coastal area comprised of karstic marble. We found that groundwater discharge to the sea occurs via: (1) groundwater-fed creeks, (2) coastal springs, (3) diffuse groundwater seepage through seabed sediments, and (4) submarine springs. We used a multi-method approach combining tracer techniques (salinity, 224Ra, and 222Rn) and direct measurements (seepage meters and flowmeters) to evaluate the discharge. Groundwater discharge via submarine springs was the most difficult to assess due to their depth (up to 15 m) and extensive development of the springs conduits. We determined that the total groundwater discharge over the 16 km of shoreline of the study area was at least 11 ± 3 × 103 m3 d−1 for the four types of discharge assessed. Groundwater-derived nitrate (NO3−) fluxes to coastal waters over ∼3 km (or 20%) in a highly populated and farmed section of Maro-Cerro Gordo was 641 ± 166 mol d−1, or ∼75% of the total NO3− loading in the study area. We demonstrate in this study that a multi-method approach must be applied to assess all forms of SGD and derived nutrient fluxes to the sea in highly heterogeneous karst aquifer systems.

How does the groundwater influence the water balance of a lowland lake? A field study from Lake Stechlin, north-eastern Germany

The subsurface catchment of Lake Stechlin is quite complex. Water level measurements in groundwater wells with different screen depths show that groundwater in the lake’s catchment may partly discharge to the lake and partly leak to deeper aquifers. Simple steady-state 2D vertical modelling of eight different scenarios (different leakage areas, different hydraulic conductivities) was conducted in order to explore the local and regional groundwater flow system around Lake Stechlin and the origin and possible magnitude of lacustrine groundwater discharge. A comparison of the different scenarios with in situ measurements of lacustrine groundwater discharge by temperature sticks and seepage meters revealed that only a part of the groundwater recharge in the catchment discharges to the lake while the rest leaks into deeper aquifers. This implies a differentiation of the subsurface catchment into a local and a regional flow system and impacts on travel times and chemical reaction time scales.

Validation of a new device to quantify groundwater-surface water exchange

Distributions of flow across the groundwater-surface water interface should be expected to be as complex as the geologic deposits associated with stream or lake beds and their underlying aquifers. In these environments, the conventional Darcy-based method of characterizing flow systems (near streams) has significant limitations, including reliance on parameters with high uncertainties (e.g., hydraulic conductivity), the common use of drilled wells in the case of streambank investigations, and potentially lengthy measurement times for aquifer characterization and water level measurements. Less logistically demanding tools for quantifying exchanges across streambeds have been developed and include drive-point mini-piezometers, seepage meters, and temperature profiling tools. This project adds to that toolbox by introducing the Streambed Point Velocity Probe (SBPVP), a reusable tool designed to quantify groundwater-surface water interactions (GWSWI) at the interface with high density sampling, which can effectively, rapidly, and accurately complement conventional methods. The SBPVP is a direct push device that measures in situ water velocities at the GWSWI with a small-scale tracer test on the probe surface. Tracer tests do not rely on hydraulic conductivity or gradient information, nor do they require long equilibration times. Laboratory testing indicated that the SBPVP has an average accuracy of ±3% and an average precision of ±2%. Preliminary field testing, conducted in the Grindsted Å in Jutland, Denmark, yielded promising agreement between groundwater fluxes determined by conventional methods and those estimated from the SBPVP tests executed at similar scales. These results suggest the SBPVP is a viable tool to quantify groundwater-surface water interactions in high definition in sandy streambeds.

Seawater‐groundwater mixing in and fluxes from coastal sediment overlying discrete fresh seepage zones: A modeling study

AbstractSubmarine fresh groundwater discharging from discrete zones such as conduits overlain by thick sediment remains poorly documented and understood despite common anecdotal accounts of it. We analyzed this phenomenon by a suite of variable‐density, variably saturated groundwater flow simulations guided by field studies. The field observations included point flux and salinity measurements using seepage meters spread across the intertidal and the subtidal zones. The discretely measured seepage fluxes in the subtidal region were as high and the salinity was as fresh as those from the intertidal zone. Both fluxes and salinities varied with tides. Two‐dimensional simulations of the idealized beach‐ocean vertical section with discrete freshwater springs at the base showed the development of seawater recirculation cells along the edge of freshwater discharge plumes emanating from the outlets, causing mixing. Brackish water discharges at the sediment‐water interface above where the plume mixes with the recirculating seawater. Sensitivity analyses showed that stronger terrestrial fresh groundwater flow increases the freshwater plume size but does not affect the mixing zone width. Tidal oscillations and a dynamic fresh groundwater flow widened the mixing zones and increased pore water flux across the sediment‐water interface. Sediment thickness had a minor effect on the width of both the freshwater plume and its surrounding mixing zone. The subtidal freshwater plume and mixing zone likely represent a unique but potentially ubiquitous setting which hosts dramatic ecological and chemical gradients. Their local biogeochemical niche and coastal scale ecosystem impacts will need to be considered in investigations of coastlines.

Air Compressibility Effect on Bouwer and Rice Seepage Meter

Measuring a disconnected streambed seepage flux using a seepage meter can give important streambed information and help understanding groundwater-surface water interaction. In this study, we provide a correction for calculating the seepage flux rate with the consideration of air compressibility inside the manometer of the Bouwer and Rice seepage meter. We notice that the effect of air compressibility in the manometer is considerably larger when more air is included in the manometer. We find that the relative error from neglecting air compressibility can be constrained within 5% if the manometer of the Bouwer and Rice seepage meter is shorter than 0.8 m and the experiment is done in a suction mode in which air is pumped out from the manometer before the start of measurement. For manometers longer than 0.8 m, the relative error will be larger than 5%. It may be over 10% if the manometer height is longer than 1.5 m and the experiment is done in a no-suction mode, in which air is not pumped out from the manometer before the start of measurement.

An attempt to identify and estimate the subsurface groundwater discharge in the south east coast of India

Abstract An attempt has been made to study the subsurface groundwater discharge (SGD) in the coastal Cuddalore region of south east India. Measurement for Radon, water level, Electrical Conductivity (EC) and pH in surface water for a total of twenty hours by hourly interval has been attempted and further correlated with tidal values calculated by WX Tide 32 software. The SGD measurements were made by using a modified seepage meter. The study reveals a match with water level variation and tide with minor variation due to influx of surface water. Saline discharges, fresh groundwater discharges and surface water mixing processes were identified along the coast. Lower SGD (37.24–79.16 cm/day) was observed during fresh groundwater discharge.

High-resolution mapping and time-series measurements of 222Rn concentrations and biogeochemical properties related to submarine groundwater discharge along the coast of Obama Bay, a semi-enclosed sea in Japan

High-resolution mapping along the coast and time-series measurements of the radon-222 (222Rn) concentrations in the shallow zone in a semi-enclosed sea, Obama Bay, Japan, were undertaken in 2013. The temporal and spatial variations in the 222Rn concentrations were analyzed in parallel with meteorological conditions, physical–biogeochemical characteristics, and the submarine groundwater discharge (SGD) flux measured with a seepage meter. These data indicate that the groundwater influences the water properties of the bay and that the groundwater supply pathways are not limited to the local SGD. The concentrations of 222Rn flowing into the bay from rivers was known to be relatively high because groundwater seeps from the river bed. High-222Rn water was almost always present around the river mouth, and northward advection of the water affected the distribution of 222Rn concentrations in the bay. The southward wind suppressed the advection of the high-222Rn water and largely controlled the temporal variations in 222Rn concentrations at a station located on the north side of the river mouth, whereas the local SGD affected the short-term changes in the 222Rn concentrations. The concentrations of 222Rn and chlorophyll-a, an indicator of phytoplankton biomass, show a significant positive correlation in the surface layer along the coastline in seasons when the nutrient supply was the main factor limiting primary productivity.