Seepage Zones Research Articles

Electrical resistivity imaging and dye tracer test for the estimation of water leakage paths from reservoir of Akdeğirmen Dam in Afyonkarahisar, Turkey

The seepage occurrence from the reservoir on the right bank of the Akdegirmen Dam located in Afyonkarahisar province in the Turkey has been investigated. When the reservoir began to fill with water, a large amount of water seepage occurred at the dam. The seepage developed at the base of the spillway and the right downstream slope of the dam. The various attempts have been made to reduce the seepage using grouting. Although the additional grouting operations was reduced the seepage at the base of the spillway, there has not been a reduction in seepage at the water outlet location at the downstream slope. Electrical resistivity measurements along the eight lines with the dipole–dipole array and dye tracer tests were performed in order to identify the seepage locations. The interpretation of electrical resistivity data showed the distribution of strata and the seepage zone along the right downstream of the dam. Groundwater flow rates calculated from dye tests indicated that there is an excessive seepage south-eastwardly on the downstream slope of the dam. Integrated interpretation of resistivity data and dye tests indicate that the seepage paths are in the direction from NW to E and SE.

Emission of Methane and Heavier Alkanes From the La Brea Tar Pits Seepage Area, Los Angeles

AbstractNatural hydrocarbon (oil and gas) seeps are widespread in Los Angeles, California, due to gas migration, along faults, from numerous subsurface petroleum fields. These seeps may represent important natural contributors of methane (CH4) and heavier alkanes (C2‐C4) to the atmosphere, in addition to anthropogenic fossil fuel and biogenic sources. We measured the CH4 flux by closed‐chamber method from the La Brea Tar Pits park (0.1 km2), one of the largest seepage sites in Los Angeles. The gas seepage occurs throughout the park, not only from visible oil‐asphalt seeps but also diffusely from the soil, affecting grass physiology. About 500 kg CH4 d−1 is emitted from the park, especially along a belt of enhanced degassing that corresponds to the 6th Street Fault. Additional emissions are from bubble plumes in the lake within the park (order of 102–103 kg d−1) and at the intersection of Wilshire Boulevard and Curson Avenue (>130 kg d−1), along the same fault. The investigated area has the highest natural gas flux measured thus far for any onshore seepage zone in the USA. Gas migration, oil biodegradation, and secondary methanogenesis altered the molecular composition of the original gas accumulated in the Salt Lake Oil Field (>300 m deep), leading to high C1/C2+ and i‐butane/n‐butane ratios. These molecular alterations can be important tracers of natural seepage and should be considered in the atmospheric modeling of the relative contribution of fossil fuel (anthropogenic fugitive emission and natural geologic sources) versus biogenic sources of methane, on local and global scales.

Seafloor fluid seeps on Kimki Ridge, offshore southern California: Links to active strike-slip faulting

The Kimki Ridge fluid seeps are located in western Catalina Basin about 60km southwest of the southern California mainland and at a water depth of approximately 1100m. Multichannel seismic reflection profiles collected by the U.S. Geological Survey (USGS) in 2014 show acoustic transparency within the Kimki Ridge, suggesting the possibility of fluid seeps and possible sub-seafloor fluid pathways. Subsequent multibeam bathymetric and backscatter intensity data collected during a cooperative University of Washington/USGS cruise in early 2016 show subtle seafloor buildups with high acoustic backscatter (reflectivity) in three places along Kimki Ridge, supporting the existence of fluid seepage. A Remotely Operated Vehicle (ROV) dive, conducted as part of the Nautilus Exploration Program, took place in August 2016 to confirm the presence of these previously unknown seeps and document their characteristics as well as those of any associated biological communities. Two of the three seeps were explored by ROV, and showed abundant evidence of fluid seepage, including characteristic algal mats, chemosynthetic clams, and authigenic carbonate formation. The seeps are comprised of carbonate buildups 1–3m thick and 300–500m across. Within these areas, we interpret broad crater-like depressions 30–50m across and 1–2m deep to be individual seep vents. The seep areas appear to be broad zones of diffuse seepage that support chemosynthetic biologic communities; however, active venting was not observed. Geochemical analyses of rock samples collected from the seeps indicate microbially driven anaerobic oxidation of methane at or near the sediment water interface. Seismic-reflection profiles show chimney-like fluid pathways along the limbs and in the axis of the fold forming Kimki Ridge, and evidence of methane in shallow sediments can be traced into the adjacent Catalina Basin. A system of closely spaced faults located at the axis of the Kimki Ridge anticline may serve as pathways to allow fluid flow to the seafloor. Our data are consistent with other studies that suggest that transpression is an important component in the formation and localization of fluid seeps in a strike-slip setting, implying that seep formation may be a common occurrence at fault stepovers or transpressional bends in strike-slip systems.

Hydrocarbon seeps in petroliferous basins in China: A first inventory

Natural hydrocarbon seepage is a widespread phenomenon in sedimentary basins, with important implications in petroleum exploration and emission of greenhouse gases to the atmosphere. China has vast petroleum (oil and gas) bearing sedimentary basins, but hydrocarbon seepage has rarely been the object of systematic studies and measurements. Based on the available Chinese literature, we report a first inventory of 932 hydrocarbon seeps or seepage zones (710 onshore seeps and 222 offshore seeps), including 81 mud volcanoes, 449 oil seeps, 215 gas seeps, and 187 solid seeps (bitumen outcrops). The seeps are located within the main 20 Mesozoic-Cenozoic petroliferous sedimentary basins, especially along the marginal, regional and local faults. The type of manifestations (oil, gas or mud volcano) reflects the type and maturity of the subsurface petroleum system and the sedimentary conditions of the basin. Oil seeps are particularly abundant in the Junggar Basin. Gas seeps mostly developed in the Lunpola Basin, in smaller basins of the eastern Guizhou and Yunnan provinces, onshore Taiwan and in the offshore Yinggehai Basin. Mud volcanoes developed in basins (Junggar, Qaidam, Qiangtang, onshore and offshore Taiwan) that experienced rapid sedimentation, which induced gravitative instability of shales and diapirism. In comparison to available global onshore seep data-bases, China results to be the country with the highest number of seeps in the world. The massive gas seepage in China could represent a considerable natural source of methane to the atmosphere, and a key process that may drive future hydrocarbon exploration.

Tracing dam seepage using nuclear logging and tracer techniques – a case study

Excessive and uncontrolled seepage through dams is a cause of concern towards dam safety. Dam instrumentation and its monitoring provide early warning of alarming conditions of excessive seepage. Hence, assessment of seepage is imperative to delineate path and source of seepage so that remedial measures can be adapted to reduce the seepage. This paper deals with the application of borehole nuclear logging and tracer techniques employed for tracing the suspected seepage zones and path through the body of dam. The potential of these methods in locating susceptible zones and tracing seepage path is highlighted with the case study of Indirasagar Project, Madhya Pradesh, India.

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.

Seepage Zone Identification at Sutami Dam by Means of Geoelectrical Resistivity Data

A study with entitled Seepage Zone Identification At Sutami Dam By Means Of Geoelectrical Resistivity Data has been conducted. The aim of this study is to get the distribution of seepage zones in the Sutami dam body. Data acquisition has been done by using Oyo-EL McOhm Model 2119C equipment on the tracks along the side of the dam body, both downstream (exhaust) side for first and upstream (reservoir) side for second with pseudodepth section dipole-dipole configuration. Each track has a length of 400m and 380m respectively. The distance between the point electrode consists of 1 track with space (a) is 10m (n=8) and 3 tracks with a space (a) is 20m (n=8). Based on the results of data processing obtained are low resistivity values (ρ ≤ 9.57 Ωm), the moderate resistivity value (9.57 Ωm < ρ ≤ 320 Ωm), and high resistivity (ρ > 320 Ωm). The distribution of fracture zones associated with seepage zones for Sutami dam distributed over the surface to depth of 8-18 m, both upstream (reservoir) or downstream (exhaust) indicated by resistivity values 0.922 Ωm to 9.57 Ωm. The results of the 3D data processing indicates that the fracture zone is also distributed between the upstream (reservoir) and downstream (exhaust) of the Sutami dam body.

Selenium and molybdenum enrichment in uranium roll-front deposits of Wyoming and Colorado, USA

Sandstone uranium (U) roll-front deposits of Wyoming and Colorado (USA) are important U resources, and may provide a terrestrial source for critical accessory elements, such as selenium (Se), molybdenum (Mo), and tellurium (Te). Due to their associated toxicity, MoSeTe occurrences in roll-fronts should also be carefully monitored during U leaching and ore processing. While elevated MoSe concentrations in roll-fronts are well established, very little is known about Te occurrence in such deposits. This study aims to establish MoSeTe concentrations in Wyoming and Colorado roll-fronts, and assess the significance of these deposits in an environmental and mineral exploration context.Sampled roll-front deposits, produced by oxidized groundwater transportation through a sandstone, show high MoSe content in specific redox zones, and low Te, relative to crustal means. High Se concentrations (up to 168ppm) are restricted to a narrow band of alteration at the redox front. High Mo content (up to 115ppm) is typically associated with the reduced mineralized nose and seepage zones of the roll-front, ahead of the U orebody. Elevated trace element concentrations are likely sourced from proximal granitic intrusions, tuffaceous deposits, and local pyritic mudstones. Elevated MoSe content in the sampled roll fronts may be regarded as a contaminant in U in-situ recovery and leaching processing, and may pose an environmental threat in groundwaters and soils, so extraction should be carefully monitored. The identification of peak concentrations of MoSe can also act as a pathfinder for the redox front of a roll-front, and help to isolate the U orebody, particularly in the absence of gamma signatures.

Tracer an effective tool in detecting canal seepage: case studies from Northern India

Canal irrigation is well developed in several provinces of India since independence. With the increase in demand and irregular rainfall, several areas are covered using canal network to suffice agricultural and drinking water needs. Besides its utility and functioning, several problems are associated with canal irrigation, such as seepage losses and water logging. Apart from geophysical and geological methods, tracer is a tool which can be used to study canal seepage. It facilitates in identifying maximum seepage zones and expected seepage path qualitatively and quantitatively without interrupting normal activity of the canal. In this paper, identification of canal seepage in detail with two case studies namely Indira Gandhi Main Canal (IGMC), Rajasthan, and canals in Kakrapar network, Gujarat, from Indian subcontinent is reported. Tracer studies revealed the vulnerable zone, i.e., fractures or cracks in the canal lining with excessive seepage in both the cases. Tracer technique is a definite, reliable and cost effective tool, which provides unique insight into canal seepage investigation.

Geological and geophysical investigation of water leakage from two micro-dam reservoirs: Implications for future site selection, northern Ethiopia

Water resources are essential to human development activities and to eradicate extreme poverty and hunger. Geological problems of two water harvesting Micro-Dam Reservoirs (MDRs) were evaluated from leakage perspectives in the northern part of Ethiopia, East Africa. Conventional geological mapping, discontinuity and weathering descriptions, test pits and geophysical methods were used to characterize the hydrogeological features of the MDRs. Vertical Electrical Sounding (VES) and Electrical Profiling (EP), were executed using Terrameter SAS (signal averaging system) 1000 manufactured by ABEM, Sweden, with Schlumberger and Wenner array configuration respectively. It was concluded that the foundations of both MDRs, except the right abutment for Adishuhu which is partly composed of dolerite, are pervious due to the presence of thin bedding planes, joints, weathered materials and fault. The presence of water in the downstream toe of the MDRs, at depressions, existing test pits and test pits excavated during the present study which lie within the seepage zone demarcated during surface geological mapping, correspond with the electrical resistivity study. The results of the electrical resistivity survey (EP and VES) were merged with the geological and structural mapping and the observation of seepage zones, for the delineation of weak zones responsible for leakage. Monitoring of the leakage (reservoir water and groundwater levels), both manually and using automatic divers, is recommended, along with monitoring of the stability of the embankments and the discharge or flow downstream of the MDRs.

Investigation of submarine groundwater discharge to tidal rivers: Evidence for regional and local scale seepage

AbstractFluxes of submarine groundwater discharge (SGD) were investigated into two tidal rivers on the north and south shore of Long Island, NY, during July 2015. Ground‐based handheld thermal infrared (TIR) imagery, combined with direct push‐point piezometer sampling, documented spatially heterogeneous small‐scale intertidal seepage zones. Pore waters were relatively fresh and enriched in nitrogen (N) within these small‐scale seeps. Pore waters sampled just 20 cm away, outside the boundary of the ground‐based TIR‐located seepage zone, were more saline and lower in N. These ground‐based TIR‐identified seeps geochemically represented the terrestrial fresh groundwater endmember, whereas N in pore waters sampled outside of the TIR‐identified seeps was derived from the remineralization of organic matter introduced into the sediment by tidal seawater infiltration. A 222Rn (radon‐222) time‐series was used to quantify fresh SGD‐associated N fluxes using the N endmembers sampled from the ground‐based TIR pore water profiles. N fluxes were up‐scaled to groundwater seepage zones identified from high‐resolution airborne TIR imagery using the two‐dimensional size of the airborne TIR surface water anomalies, relative to the N flux from the time‐series sampling location. Results suggest that the N load from the north‐shore tidal river to Long Island Sound is underrepresented by at least 1.6–3.6%, whereas the N load from SGD to a south‐shore tidal river may be up to 9% higher than previous estimates. These results demonstrate the importance of SGD in supplying nutrients to the lower reaches of tidal rivers and suggest that N loads in other tidal river environments may be underestimated if SGD is not accounted for.

Estimation of pore space methane hydrate in marine sediments of Cascadia Margin

The first key controlling factor to estimate the methane yield is the percentage of organic carbon that can be utilized by methanogenic bacteria. Total Organic Carbon (TOC) analysis is one of the most common proxies which provide clues for methane rich seep zones. Bacteria and bacterial activity play a vital role in the degradation of organic matter. It has been revealed that the presence of sulphate reducing bacteria at bottom layers of the core, suggests the presence of methane from the subsurface layers where methanogenesis process takes place. The present study is resulted from Integrated Ocean Drilling Program (IODP), Hole 1325B, during the Pleistocene sequence of North-Eastern Pacific Ocean and further supporting results for entire 311 transect obtained from published records of Shipboard Scientific Party. The total carbon (TC), inorganic carbon (IC) and TOC content is examined individually for 357 samples from various depths, measures the rate of deposition of total organic carbon. The average values of TC, IC and TOC shows quite fluctuating trends in the study region with 0.67 wt%, 0.11 wt% and 0.56 wt% respectively. Many published results suggest that 0.5 wt% TOC is required for hydrate formation by determining the average TOC content at hydrate bearing sediments. The outcome of proposed study put forward that the TOC content is an important aspect for the formation of methane hydrate in marine sediment. The rate of methane generated, dissolved methane are also estimated in order to calculate the pore space methane hydrate in the study area. This accumulation can roughly explain the average amount of hydrates as 0.85% in the sediments at the study region and 0.88% particularly from Hole 1325B where the amount of methane generated at 146.48 mM.

Identifying geochemical hot moments and their controls on a contaminated river floodplain system using wavelet and entropy approaches

Geochemical hot moments are defined here as short periods of time that are associated with disproportionally high levels of concentrations (biogeochemically-driven or transport-related) relative to longer intervening time periods. We used entropy and wavelet techniques to identify temporal variability in geochemical constituents and their controls along three transects within a contaminated floodplain system near Rifle CO. Results indicated that transport-dominated hot moments drove overall geochemical processing in the contaminated groundwater and seep zones. These hot moments were associated with seasonal hydrologic variability (∼4 months) in the contaminated aquifer and with annual hydrologic cycle and residence times in the seep zone. Hot moments associated with a naturally reduced zone within the aquifer were found to be biogeochemically-driven, with a different dominant frequency (∼3 months) and no correlation to hydrologic or weather variations, in contrast to what is observed in other regions of the floodplain.

Fluorescent tracer tests for detection of dam leakages: The case of the Bumbuna dam - Sierra Leone

The first impounding of the Bumbuna dam was subject to unexpected high seepages discharge (up to 550l/s) from the foundation drains. Fluorescent tracer tests, commonly used for hydrogeological study of porous and karst aquifer, were performed to locate the seepage zones. Solutions of 3 different fluorescent tracers (Uranine, Sulforhodamine-B and Tinopal CBS-X) were injected in the reservoir at regularly spaced points down to a depth of 80m. Tracer concentration in the seepage water from the foundation drains was continuously monitored by high resolution fluorometers with detection limit as low as 0.02ppb.A total of 107 tests were performed in two separate campaigns conducted in January–February and October–December 2009. Concentration curves recorded from each test were analyzed to determine the time of first response to the injection (detection time) and the quantity of tracer returned from each test (restitution). Maps of minimum detection time and maximum tracer restitution were generated to identify the position of leakages.Seepage mitigation works were planned on the base of test results and completed in the dry season after the drawdown of the reservoir. Works consisted in the additional grouting along the grout curtain, sealing of fissures on the bituminous layer and installation of a PVC membrane on the dam upstream face.Inspection of the upstream dam face confirmed the presence of damages in the areas identified by tracer tests. Since completion of seepage mitigation works the dam operated satisfactorily with maximum seepage from the foundation of 70l/s.

Effect of gas content and actual stresses on coalbed permeability

The analytical review covers the key factors that influence permeability of a coalbed under mining-induced alteration of stress state. The authors put forward an analytical relation between coal permeability, stresses and adsorbed gas to define parameters of gas leakage zone parameters and laws of mass transfer. The analytically derived relation is compared with the known model of coal structure and with the experimental results. The proposed model of coalbed permeability allows mechanism of leakage and localization of seepage zones in coal provided that the model parameters are properly selected or found experimentally.

ERT-based Investigation of a Sinkhole in Greene County, Missouri

Investigating sinkhole morphology and formation mechanisms is key to understanding their long term impact and susceptibility to development, and aids in the design of effective mitigation measures. In this study, ERT (electrical resistivity tomography), MASW (multichannel analysis of surface waves) and borehole data were used to image the subsurface morphology of an active sinkhole in Greene County, Missouri. The study reveals that the sinkhole developed along a natural surface drainage pathway above a pervasively fractured limestone. The subsurface image of the sinkhole depicts a zone of near-vertical water seepage and soil piping. Based on the nature of the overburden material, and the morphology and current/past surface expression of the sinkhole, it is concluded that the sinkhole is predominantly a cover subsidence type of sinkhole. However, it is possible that minor cover collapse occurred locally and in an area slightly to the north of the current active sinkhole.

ERT-based Investigation of a Sinkhole in Greene County, Missouri

Investigating sinkhole morphology and formation mechanisms is key to understanding their long term impact and susceptibility to development, and aids in the design of effective mitigation measures. In this study, ERT (electrical resistivity tomography), MASW (multichannel analysis of surface waves) and borehole data were used to image the subsurface morphology of an active sinkhole in Greene County, Missouri. The study reveals that the sinkhole developed along a natural surface drainage pathway above a pervasively fractured limestone. The subsurface image of the sinkhole depicts a zone of near-vertical water seepage and soil piping. Based on the nature of the overburden material, and the morphology and current/past surface expression of the sinkhole, it is concluded that the sinkhole is predominantly a cover subsidence type of sinkhole. However, it is possible that minor cover collapse occurred locally and in an area slightly to the north of the current active sinkhole.

Natural and unnatural oil slicks in the Gulf of Mexico

When wind speeds are 2–10 m s−1, reflective contrasts in the ocean surface make oil slicks visible to synthetic aperture radar (SAR) under all sky conditions. Neural network analysis of satellite SAR images quantified the magnitude and distribution of surface oil in the Gulf of Mexico from persistent, natural seeps and from the Deepwater Horizon (DWH) discharge. This analysis identified 914 natural oil seep zones across the entire Gulf of Mexico in pre‐2010 data. Their ∼0.1 µm slicks covered an aggregated average of 775 km2. Assuming an average volume of 77.5 m3 over an 8–24 h lifespan per oil slick, the floating oil indicates a surface flux of 2.5–9.4 × 104 m3 yr−1. Oil from natural slicks was regionally concentrated: 68%, 25%, 7%, and <1% of the total was observed in the NW, SW, NE, and SE Gulf, respectively. This reflects differences in basin history and hydrocarbon generation. SAR images from 2010 showed that the 87 day DWH discharge produced a surface‐oil footprint fundamentally different from background seepage, with an average ocean area of 11,200 km2 (SD 5028) and a volume of 22,600 m3 (SD 5411). Peak magnitudes of oil were detected during equivalent, ∼14 day intervals around 23 May and 18 June, when wind speeds remained <5 m s−1. Over this interval, aggregated volume of floating oil decreased by 21%; area covered increased by 49% (p < 0.1), potentially altering its ecological impact. The most likely causes were increased applications of dispersant and surface burning operations.

Seepage-zone recognition of river water based on Cl(-) spatial difference.

Based on Cl(-) monitoring data of river water and shallow groundwater samples on both sides of the Dashi River, we analyzed spatial distribution of Cl(-) content in surface water and groundwater, and evaluated the retardation strength, pCl(-), the average retardation strength, pA, and the average residual retardation strength, pAR, of surface-groundwater Cl(-) content. We introduce the difference, Q, of the average retardation strength and the average residual retardation strength, which can be used to identify the seepage zone and the transition seepage zone of the river, and to quantitatively express the seepage range. The results show that Cl(-) content in the river gradually increased in areas with industrial agglomeration, but decreased in areas with non-agglomeration, and gradually decreased along the river flow. If Q ≥ 0, then there is no seepage zone in the study area, but if Q < 0, then there is a seepage zone in the study area. The sampling point is in the seepage zone when pCl(-) ≤ pA, in the transition seepage zone when pA < pCl(-) ≤ pAR, and in the no-seepage zone when pCl(-) > pAR. The recognition results are more consistent with field investigation.

A comparison of thermal infrared to fiber-optic distributed temperature sensing for evaluation of groundwater discharge to surface water

Groundwater has a predictable thermal signature that can be used to locate discrete zones of discharge to surface water. As climate warms, surface water with strong groundwater influence will provide habitat stability and refuge for thermally stressed aquatic species, and is therefore critical to locate and protect. Alternatively, these discrete seepage locations may serve as potential point sources of contaminants from polluted aquifers. This study compares two increasingly common heat tracing methods to locate discrete groundwater discharge: direct-contact measurements made with fiber-optic distributed temperature sensing (FO-DTS) and remote sensing measurements collected with thermal infrared (TIR) cameras. FO-DTS is used to make high spatial resolution (typically m) thermal measurements through time within the water column using temperature-sensitive cables. The spatial–temporal data can be analyzed with statistical measures to reveal zones of groundwater influence, however, the personnel requirements, time to install, and time to georeference the cables can be burdensome, and the control units need constant calibration. In contrast, TIR data collection, either from handheld, airborne, or satellite platforms, can quickly capture point-in-time evaluations of groundwater seepage zones across large scales. However the remote nature of TIR measurements means they can be adversely influenced by a number of environmental and physical factors, and the measurements are limited to the surface “skin” temperature of water features. We present case studies from a range of lentic to lotic aquatic systems to identify capabilities and limitations of both technologies and highlight situations in which one or the other might be a better instrument choice for locating groundwater discharge. FO-DTS performs well in all systems across seasons, but data collection was limited spatially by practical considerations of cable installation. TIR is found to consistently locate groundwater seepage zones above and along the streambank, but submerged seepage zones are only well identified in shallow systems (e.g. <0.5 m depth) with moderate flow. Winter data collection, when groundwater is relatively warm and buoyant, increases the water surface expression of discharge zones in shallow systems.