Seepage Rates Research Articles

Lisbon Field: Geology of microseepage

Certain satellite sensors can be used for mineral identification and thus detection of alteration indicative of hydrocarbon microseepage. However, care must be taken when placing these results into context because alteration also can be due to other factors. The authors review the rationale for multiscale and multispectral image selection as well as the benefit of iterative processing, to highlight alteration within a given formation. The authors determine using remote sensing results integrated with other geologic information to confirm that alteration exists, compare results with a conceptual microseepage model, reconcile alteration pattern and extent with the geologic setting and trap timing, and appraise the potential that other factors may have caused alteration. The authors used geologic information from the well-studied Lisbon Field including, but not limited to, geochemistry, petrography and diagenetic studies, and paleo-structure reconstruction. We evaluated whether the alteration model made sense by assessing the thermal history and estimating volumetrics and seepage rate based on the geologic setting and extent of alteration. The results confirm that the alteration at the Lisbon Field is compatible with hydrocarbon microseepage as its principal cause, however, it cannot rule out the possibility that some of it may be due to hydrothermal fluid migration.

Distribution, Magnitude, and Variability of Natural Oil Seeps in the Gulf of Mexico

The Gulf of Mexico is a hydrocarbon-rich region characterized by the presence of floating oil slicks from persistent natural hydrocarbon seeps, which are reliably captured by synthetic aperture radar (SAR) satellite imaging. Improving the state of knowledge of hydrocarbon seepage in the Gulf of Mexico improves the understanding and quantification of natural seepage rates in North America. We used data derived from SAR scenes collected over the Gulf of Mexico from 1978 to 2018 to locate oil slick origins (OSOs), cluster the OSOs into discrete seep zones, estimate the flux of individual seepage events, and calculate seep recurrence rates. In total, 1618 discrete seep zones were identified, primarily concentrated in the northern Gulf of Mexico within the Louann salt formation, with a secondary concentration in the Campeche region. The centerline method was used to estimate flux based on the drift length of the slick (centerline), the slick area, and average current and wind speeds. Flux estimates from the surface area of oil slicks varied geographically and temporally; on average, seep zones exhibited an 11% recurrence rate, suggesting possible intermittent discharge from natural seeps. The estimated average instantaneous flux for natural seeps is 9.8 mL s−1 (1.9 × 103 bbl yr−1), with an annual discharge of 1.73–6.69 × 105 bbl yr−1 (2.75–10.63 × 104 m3 yr−1) for the entire Gulf of Mexico. The temporal variability of average flux suggests a potential decrease following 1995; however, analysis of flux in four lease blocks indicates that flux has not changed substantially over time. It is unlikely that production activities in the Gulf of Mexico impact natural seepage on a human timescale. Of the 1618 identified seep zones, 1401 are located within U.S. waters, with 70 identified as having flux and recurrence rates significantly higher than the average. Seep zones exhibiting high recurrence rates are more likely to be associated with positive seismic anomalies. Many of the methods developed for this study can be applied to SAR-detected oil slicks in other marine settings to better assess the magnitude of global hydrocarbon seepage.

Influence of initial permeability on seepage behavior of the lubricant stored in hydrogel surface

This paper studied the unsteady seepage characteristics of lubricant stored in hydrogel under different initial permeability. Results show that the distribution of the permeability is determined by the deformation of hydrogel. The normal velocity and normal pressure gradient on the hydrogel surface have diffusion, fluctuation and stabilization processes. The normal pressure gradient is the internal cause that drives the change of the velocity. With the increase of the initial permeability, the seepage velocity increases significantly. Higher initial permeability leads to a higher exudation rate of hydrogel at the initial stage of loading, but the exudation is unsustainable. Considering the instantaneous seepage rate of the lubricant and the long-term lubrication, it is necessary to reasonably design the initial permeability.

Erodibility improvement and scour mitigation of beach sand by enzymatic induced carbonate precipitation

The main objective of this study was to evaluate the use of enzymatic induced carbonate precipitation (EICP) treatment for erodibility improvement, scour mitigation, and water infiltration capacity of beach sand at one, two, and three cycles of treatment. EICP is a biochemical process that produces calcium carbonate precipitation within a soil matrix to improve the geotechnical properties of the soil. Direct shear test (DST) was used to examine the shear behavior of bio-cemented beach sand after one and two cycle treatment, the results of which showed that the cohesion was increased substantially, especially after the second cycle; conversely, the friction angle was decreased. The mineralogy of treated soil was also assessed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The mini-jet test has been used to adequately measure the soil erosion characteristics of the biotreated sand (1–3 treatment cycles) in the laboratory, the results of which indicated a significant improvement compared to the control specimen after the third cycle of EICP treatment, with reduced erosion rate and scour depth in the order of approximately 88-fold and 23-fold respectively. After three treatment cycles, the seepage rate was decreased to 71 times lower than the untreated specimens. Strong positive correlations were found between CaCO3, seepage rate, erosion rate, and soil cohesion. The results of this study have confirmed that there is strong evidence for improving erodibility behavior and shear strength of beach sand via EICP treatment. The results of this study would be beneficial to engineers in better understanding bio-cementation techniques and their implementations.

A case study in controlled source electromagnetism: Near seabed hydrocarbon seep systems of Coal Oil Point, California, USA

Marine hydrocarbon seeps are found on all continental margins and release significant amounts of greenhouse gases and hydrocarbons into the atmosphere and hydrosphere. Most methods of studying seeps rely on seafloor or near-seafloor observations, but seepage rates and seep locations can be variable, leading to uncertainty. We exploit the fact that hydrocarbons are electrically resistive compared with surrounding sediments and use marine electromagnetic methods to study the deeper sources and accumulation sites, which are controlled by local geology and should be more stable than the seabed expressions. Our surface towed marine controlled source electromagnetic system used a horizontal electric dipole transmitter and floating electric dipole receivers spaced 100–400 m from the transmitter, collecting frequency domain amplitude and phase data at ∼2 Hz and harmonics. The survey targeted known and inferred hydrocarbon seeps within the Coal Oil Point seep field offshore Santa Barbara, California, USA. Two dimensional inversions of the data indicate that the method is sensitive to the shallowly buried (<400 m) structure of the marine hydrocarbon seeps and is an efficient and effective tool in their identification and characterization. The results show spatial variability of seafloor hydrocarbons along the Coal Oil Point seep field and indicate at least two previously unidentified intermediate depth accumulation sites. The depth and lateral extent of these hydrocarbon accumulation sites may improve seep emission models for the Coal Oil Point seep field.

Introducing a novel concept of wick drainage in masonry structures

A new line of research is presented in this study where wick drainage technique was implemented in masonry walls. Three different types of wicks, two lignocellulosic natural materials, namely, sisal and jute, and one synthetic, polypropylene were installed on the external side of the walls with the aim of reducing dampness on their interior structure. Two masonry walls with cement and lime mortars were used, and a moisture-blocking material was applied to their surfaces. The physical properties of both walls were thoroughly investigated in terms of moisture content, seepage/retention of water and draining efficiency in the presence of wicks and results were compared with control. Results indicated the ability of wicks to reduce the water content inside the walls with higher efficiency for sisal, followed by jute and polypropylene, respectively. Sisal wicks increased the seepage rate of water to the exterior surface of masonry by 5%, followed by jute and polypropylene wicks with 4% and 3%, respectively. Moreover, sisal wicks remarkably drained over 70% of the supplied water after 14 days of testing. On the other hand, 58%, 53% and 10% of the delivered water was drained by jute wicks, polypropylene wicks and control, respectively. This could be explained by the chemical constituents and morphology of lignocellulosic materials, which are prone to absorb more water.

A systematic approach for estimating water losses in irrigation canals

In order to determine water losses in irrigation canals, a systematic approach was developed, consisting of two main components: a seepage simulation model and a hydraulic simulation model. The SEEP/W module of the Geo-Studio software was used to simulate the seepage rate, and the Hydrologic Engineering Center-River Analysis System (HEC-RAS) hydrodynamic model was used for hydraulic simulation. Different operation scenarios were designed to investigate all possible situations in daily operation of water distribution and delivery systems. The seepage simulation results show that the seepage losses were higher at the bottom and corners of the canal, because the hydraulic gradient was affected by the hydraulic load. The hydraulic simulation results show that due to physical and management infrastructure (using non-automated and operator-based regulation structures), operational losses accounted for a significant volume of losses compared to seepage losses. In most operation scenarios, the maximum seepage loss was 10%, and the remaining 90% was related to operational losses. It is concluded that any factor (decrease or increase of inflow to the canal) that causes an increase or decrease of operational losses is ultimately a determining factor in reducing or increasing total losses. Therefore, management approaches should be adopted to improve performance of the system and reduce losses, especially operational losses, by improving the operation methods of water level regulation and off-take structures.

Monthly Variation in Flux of Inorganic Nutrients From Submarine Groundwater Discharge in a Volcanic Island: Significant Nitrogen Contamination in Groundwater

To determine nutrient fluxes derived from submarine groundwater discharge (SGD), we conducted monthly hydrological surveys on the coast of Jeju, a volcanic island located in the southern sea of Korea. The concentrations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicon (DSi) were significantly correlated with salinity, indicating that fresh SGD (FSGD) is a major nutrient source in Jeju Island where no other coastal freshwater origins exist. Based on a DSi-mass balance model, seepage rate of FSGD was found to depend on 5-day precipitation before sampling campaigns, which immediately permeatedviaporous aquifers. Thus, the FSGD-driven nutrient fluxes were generally higher in rainy season (July–August) and September 2019 when typhoons occurred. However, high DIN and DIP fluxes were found during spring (March–May), even at low seepage rate, perhaps by a fertilizer input from agriculture activity. This study highlights that large variation of the SGD-driven nutrient fluxes was caused by environmental and anthropogenic factors and emphasizes on the importance of long-term investigation.

Study on instability discriminating condition and permeability variation of triangle articulated structure in highly inclined goaf

The huge, inclined goaf formed by the mining of steeply inclined and extra thick coal seams has the characteristics of occupying a large space and being subject to complex breaking behavior of its overlying rock structure, especially the rock block structure characteristics of the interface area between the goaf and the lower working face which is difficult to be characterized, leading to a hidden danger when mining the lower working face. In response to the aforementioned problems, the following conclusions are drawing as follows: (1) with the aid of the contradiction method, the critical condition governing instability of the triangular articulated structure was obtained to explain the correctness of the assumption of the triangular hinged structure; (2) three-point truss structure was composed of key layer blocks; (3) in the stabilization stage of the triangular hinged structure, the fracture network area was extracted, and the centerline intensive equivalent method of a single fracture segment was adopted to directly measure the equivalent length. Meanwhile, the value of equivalent average width with the equivalent length of a single crack was calculated by integral operation and equivalent figure calculation; (4) the mathematical expressions of permeability which was positively exponential function related to the ratio of equivalent average width to equivalent length were derived. Furthermore, the vectorized solution of seepage rate was carried out respectively for the goaf area and the extracted network area by using COMSOL software. The distribution of the strong seepage area of CBM and the ratio of equivalent average width to equivalent length is found to be consistent to a large extent, which provides theoretical support for subsequent CBM management.

Research on Influencing Factors of Effective Gas Extraction Radius in Coal Mine Based on Multiple Linear Regression

In order to analyze the influence degree of different factors on gas extraction radius, on the basis of coal seam gas flow theory and elastic mechanics, considering the influence of coal seepage rate change on gas flow, the COMSOL Multiphysics numerical simulation software is secondarily developed by using the stress-seepage coupling mathematical model of coal and rock containing gas. Taking coal seam No.2 of Juji Coal Mine as an example, the effective extraction radius under different extraction negative pressure, extraction borehole diameter, extraction time, and initial permeability of coal seam are simulated, respectively. Then, the orthogonal test method is used to design different calculation schemes, and the effective extraction radius is used as the judgment index. The simulation software is used to calculate the effective extraction radius corresponding to different influencing factors. SPSS statistical software is used to analyze the parameters. The multivariate linear regression equation of the influencing factors of the effective extraction radius is obtained, and the multivariate linear regression model is established. The results show that the negative pressure of extraction has little effect on the effective extraction radius; the effective extraction radius gradually expands with the increase of extraction time and extraction aperture, but it has certain timeliness. The initial permeability of the coal seam has a great influence on the effective extraction radius. The influence degree of different influencing factors from large to small is the initial permeability of coal seam, extraction time, borehole diameter, and extraction negative pressure. Therefore, the key to improving the effectiveness of gas extraction is to increase the initial permeability of the coal seam before preextraction, ensure sufficient extraction time, appropriately increase the borehole diameter, and improve the negative pressure of extraction. Finally, the gas flow rate is used for a field test, and the measured results are basically consistent with the simulation results, which proves the correctness of the mathematical model of gas-containing coal and the feasibility of using numerical simulation methods to study the effective extraction radius of coal seams.

Effects of microporous layer on electrolyte flooding in gas diffusion electrodes and selectivity of CO2 electrolysis to CO

Understanding the relationship between gas diffusion electrode (GDE) structures and the performance of electrochemical CO2 reduction reaction (CO2RR) is crucial to developing industrial-scale technologies to convert CO2 to valuable products. We studied how the microporous layer (MPL) on GDE's coated with silver nanoparticle catalysts affects the electrochemical CO2 conversion to CO in a flow cell electrolyser. We demonstrate a convenient method to measure the rate of catholyte seepage through a GDE during CO2RR experiments and used this method to show how the MPL thickness affects flooding of the GDE. We found the GDE with the thickest MPL (39BB) had the best selectivity for CO and stability at current densities above 100 mA cm−2 as the thick MPL minimized flooding. However, at low current densities the 39BB electrode achieved a lower CO selectivity than the GDE with thinner MPL. These results suggest opportunities to improve CO2 electrolyser performances at high current by optimisation of the MPL structure and wettability.

Restoration of annular zonal isolation using localized casing expansion (LCE) technology: Treatment of near-horizontal test sections containing a free-water channel

Fluid migration behind casings is a well integrity problem that can result in sustained casing pressure, undetected leaks to the environment, and potentially very difficult remediation attempts. Traditional methods for treating annular fluid migration are perf-and-squeeze cementing or section milling followed by recementing. The main disadvantage of the former is the limited penetrability of cement slurry into narrow cracks and microannuli, while for the latter it is the requirement of a full rig to perform the operation. Casing expansion is a recent alternative remediation technology that involves imposing a permanent radial expansion of the casing and external cement layer, with the purpose of mechanically closing pathways for migrating fluids. A necessary requirement for the treatment to be effective is that the annular cement is confined between casings or between casing and competent rock formations, such that the cement can sustain significant mechanical strains without failure. Recent laboratory experiments and field trials have shown casing expansion to be a promising alternative to traditional treatment methods. We build on these insights and perform controlled treatment experiments involving a 7-in Local Expander tool and full-scale cemented annulus test assemblies, which contained much larger-scale defects than previously tested. Prior to treatment, the test assemblies had continuous migration channels on the top side of the cement, leading to high effective permeabilities ranging from approximately 80 to hundreds of darcy. The origin of the defects was likely separation of free water from the cement slurry, which is considered a relevant failure mode for primary cementing of inclined wellbores. We study the effect of imposing single and multiple local dents on the annular seepage rates, and probe for potential alteration of the cement and casing properties using Vickers hardness testing. We find that casing expansion can be highly effective in treating relatively major defects in cemented annuli, even when the defect is several millimeters wide and located adjacent to the outer casing. The effectiveness of the treatment is found to be linked to the degree of casing expansion and the initial condition of the annular cement. The deformed cement shows a tendency toward increased Vickers hardness, which is likely linked to densification of the cement microstructure. Vickers hardness testing of casing steel indicated no significant changes in hardness at the dent locations compared to average hardness values away from these points.

Evaluation of Water seepage Along Proposed Baghdad Metro Tunnel Across Tigris River

Water seepage can cause serious problems in geotechnical engineering especially for construction under the water level. Baghdad metro tunnel is one of the leading vital projects to solve the major problem of crowding roadways in a highly population increase city like Baghdad. In this study, the seepage rate that will flow toward different selected points along the tunnel section across Tigris River was calculated during the excavation process, with the consideration of three different water levels of River at maximum, moderate, and minimum water depths. A three-dimensional model of the study has been modeled using the finite element software (PLAXIS 3D V20). The water seepage was observed for six different locations on each route of the tunnel. The study showed that the change of water depth in the river has no significant effect on the seepage – time curve shape. However, increasing the water level in River from minimum to maximum leads to increase the seepage rate about 15%.

Riverbank Filtration – A Potential Water Source Exploitation for the Red River Delta Region

Riverbank filtration technology has been widely applied worldwide because of its high-capacitycollection and good water quality throughout natural purification processes. Infiltration water can beextracted from Holocene (qh) layer or the Pleistocene deep layer (qp), replenished with water from theriver through hydrogeological windows. Hydrodynamic and isotopic signatures were employed todetermine water seepage capacity. The results show that infiltrated water is found in the sand layers alongthe rivers. However, the seepage rate shows a heterogeneously spatial variation ranging from 30 m3/d inthe Dinh Dao river to 33,600 m3/d. Km along the shoreline in the Red River (RRD). Also, the exploitationcapacity of seepage water differs widely in order of large (&gt; 3,000 m3/d), medium (1,000-3,000 m3/d),small (500-1,000 m3/d), and very small capacity (200-500 m3/d). This study indicated that RRD couldapply riverbank filtration techniques to overcome freshwater scarcity in the delta due to increasing surfacepollution and discharge reduction.

Response of water fluxes and biomass production to climate change in permanent grassland soil ecosystems

Abstract. Effects of climate change on the ecosystem productivity and water fluxes have been studied in various types of experiments. However, it is still largely unknown whether and how the experimental approach itself affects the results of such studies. We employed two contrasting experimental approaches, using high-precision weighable monolithic lysimeters, over a period of 4 years to identify and compare the responses of water fluxes and aboveground biomass to climate change in permanent grassland. The first, manipulative, approach is based on controlled increases of atmospheric CO2 concentration and surface temperature. The second, observational, approach uses data from a space-for-time substitution along a gradient of climatic conditions. The Budyko framework was used to identify if the soil ecosystem is energy limited or water limited. Elevated temperature reduced the amount of non-rainfall water, particularly during the growing season in both approaches. In energy-limited grassland ecosystems, elevated temperature increased the actual evapotranspiration and decreased aboveground biomass. As a consequence, elevated temperature led to decreasing seepage rates in energy-limited systems. Under water-limited conditions in dry periods, elevated temperature aggravated water stress and, thus, resulted in reduced actual evapotranspiration. The already small seepage rates of the drier soils remained almost unaffected under these conditions compared to soils under wetter conditions. Elevated atmospheric CO2 reduced both actual evapotranspiration and aboveground biomass in the manipulative experiment and, therefore, led to a clear increase and change in seasonality of seepage. As expected, the aboveground biomass productivity and ecosystem efficiency indicators of the water-limited ecosystems were negatively correlated with an increase in aridity, while the trend was unclear for the energy-limited ecosystems. In both experimental approaches, the responses of soil water fluxes and biomass production mainly depend on the ecosystems' status with respect to energy or water limitation. To thoroughly understand the ecosystem response to climate change and be able to identify tipping points, experiments need to embrace sufficiently extreme boundary conditions and explore responses to individual and multiple drivers, such as temperature, CO2 concentration, and precipitation, including non-rainfall water. In this regard, manipulative and observational climate change experiments complement one another and, thus, should be combined in the investigation of climate change effects on grassland.

Numerical Canal Seepage Loss Evaluation for Different Lining and Crack Techniques in Arid and Semi-Arid Regions: A Case Study of the River Nile, Egypt

Owing to the potential negative impacts of climatic changes and the grand Ethiopian renaissance dam, water scarcity has become an urgent issue. Therefore, the Egyptian Ministry of Water Resources and Irrigation has started a national project of the lining and rehabilitation of canals, to reduce seepage losses and for efficient water resource management. This study presents a new approach for assessing three different lining and crack techniques for the Ismailia canal, the largest end of the river Nile, Egypt. A 2-D steady state seep/w numerical model was developed for the Ismailia canal section, in the stretch at 28.00–49.00 km. The amount of seepage was significantly dependent on the hydraulic characteristics of the liner material. The extraction from aquifers via wells also had a considerable impact on the seepage rate from the unlined canals; however, a lesser effect was present in the case of lined canals. The concrete liner revealed the highest efficiency, followed by the geomembrane liner, and then the bentonite liner; with almost 99%, 96%, and 54%, respectively, without extraction, and decreasing by 4% for bentonite and geomembrane liners during extraction; however, the concrete lining efficiency did not change considerably. Nevertheless, the efficiency dramatically decreased to 25%, regardless of the lining technique, in the case of deterioration of the liner material. The double effect of both deterioration of the liner material and extraction from the aquifer showed a 16% efficiency, irrespective of the utilized lining technique.

Aquiclude Stability Evaluation and Significance Analysis of Influencing Factors of Close-Distance Coal Seams: A Case Study of the Yili No. 4 Coal Mine in Xinjiang, China

Aquiclude stability is vital for the realization of water-preserving coal mining. And its evaluation, influencing factors, and their significance analysis are quite topical for the ecosystem conservation. The purpose of this paper was to establish an evaluation index system of weakly cemented aquiclude stability. An evaluation index system was built based on three evaluation factors (subsidence, seepage, and deformation), three subfactors (subsidence gradient, seepage rate, and horizontal deformation), and four evaluation criteria (unstable, weakly stable, medium stable, and stable). The evaluation method was applied to evaluate the index for the case study of Yili No. 4 Coal Mine in Xinjiang, China. Based on the geological conditions of the close-distance coal seams in the mine under study, the main influencing factors and subordinate functions of evaluation index S t a were analyzed. The above three factors’ weights were assessed as 0.1095, 0.3090, and 0.5815, respectively, and the proposed evaluation method’s feasibility was verified by the water level variation in the observation hole. The range and variance analyses were performed to assess the significance of the mining heights of the upper and lower coal seams and the coal seam spacing. The results showed that the aquiclude stability negatively correlated with the mining heights and positively correlated with the coal seam spacing. The decreasing order of influence significance on the aquiclude stability was as follows: upper coal seam mining height, lower coal seam mining height, and coal seam spacing. Water protection mining was an effective measure to control the S t a , and the findings provided a reference value and academic significance for the ecosystem conservation.

Enhanced landfill's characterization by using an alternative analytical model for diffusion tests.

Landfills have been extensively used as waste deposits in most of the big cities in the world. Therefore, considerably large urban areas have contamination threats. Engineering solutions applied to prevent or contain soil, and water contamination often involve the application of liners, which are low permeability barriers made of materials such as compacted clay and geomembranes. In many liner applications, once reduced rates of seepage are expected, diffusion has proved to be relevant, if not dominant, to the process of contaminant transport. Normally, diffusivity parameters can be assessed by a single-reservoir pure diffusion test, where a contaminant solution is placed above a saturated soil sample and the solution's concentration is monitored over time. Once the temporal variation of concentration is measured, the process of back-calculating diffusivity parameters is not standardized. In this paper, an analytical model of the diffusive transport of contaminants is revisited considering the initial and boundary conditions of the pure diffusion test. In this model, the contaminant solution reservoir is included in the analysis domain as an equivalent contaminated soil layer. The analytical solution relies on a series evaluation, which may be a drawback to everyday engineering situations. Therefore, we build a high-accuracy exponential approximation to the solution. Expedited evaluation procedures are proposed to provide reasonable estimates for the fitting parameters. Also, in order to illustrate the applicability of the new solution, test datasets of a soil around the Jockey Club Landfill (JCL) site, one of the major landfills in Latin America, have been modeled. We discuss possible issues of considering linear isotherms to model the sorption characteristics of soils, indicating that convex isotherms, if linearly modeled, may lead to overestimated values of the diffusivity parameter.

Visual Simulation of Soil-Structure Destruction with Seepage Flows

This paper introduces a method for simulating soil-structure coupling with water, which involves a series of visual effects, including wet granular materials, seepage flows, capillary action between grains, and dam breaking simulation. We develop a seepage flow based SPH-DEM framework to handle soil and water particles interactions through a momentum exchange term. In this framework, water is seen as a seepage flow through porous media by Darcy's law; the seepage rate and the soil permeability are manipulated according to drag coefficient and soil porosity. A water saturation-based capillary model is used to capture various soil behaviors such as sandy soil and clay soil. Furthermore, the capillary model can dynamically adjust liquid bridge forces induced by surface tension between soil particles. The adhesion model describes the attraction ability between soil surfaces and water particles to achieve various visual effects for soil and water. Lastly, this framework can capture the complicated dam-breaking scenarios caused by overtopping flow or internal seepage erosion that are challenging to simulate.

Submarine Groundwater Discharge From Non-Tidal Coastal Peatlands Along the Baltic Sea

Submarine groundwater discharge (SGD) is an important pathway for water and materials within the land-ocean transition zone that can impact coastal environments and marine life. Although research from sandy shorelines has rapidly advanced in recent years, there is very little understanding of coastal areas characterized by a low hydraulic conductivity, such as carbon-rich coastal peatlands. The objective of this study was to determine the magnitude and location of terrestrial SGD to be expected from a non-tidal low-lying coastal peatland located along the Baltic Sea and to understand the controlling factors using numerical modeling. We employed the HYDRUS-2D modeling package to simulate water movement under steady-state conditions in a transect that extends from the dune dike-separated rewetted fen to the shallow sea. Soil physical properties, hydraulic gradients, geological stratifications, and topography were varied to depict the range of properties encountered in coastal peatlands. Our results show that terrestrial SGD occurs at the study site at a flux of 0.080 m2 d−1, with seepage rates of 1.05 cm d−1 (upper discharge region) and 0.16 cm d−1 (lower discharge region above submerged peat layer). These calculated seepage rates compare to observations from other wetland environments and SGD sites in the Baltic Sea. The groundwater originates mainly from the dune dike—recharged by precipitation and infiltration from ponded peatland surface water—and to a lesser extent from the sand aquifer. The scenario simulations yielded a range of potential SGD fluxes of 0.008–0.293 m2 d−1. They revealed that the location of terrestrial SGD is determined by the barrier function of the peat layer extending under the sea. However, it has little impact on volume flux as most SGD occurs near the shoreline. Magnitude of SGD is mainly driven by hydraulic gradient and the hydraulic conductivity of peat and beach/dune sands. Anisotropy in the horizontal direction, aquifer and peat thickness, and peatland elevation have little impacts on SGD. We conclude that SGD is most probable from coastal peatlands with high water levels, large Ks and/or a dune dike or belt, which could be an essential source for carbon and other materials via the SGD pathway.