Chemical Composition Of Pore Water Research Articles

Geochemical signature of aquitard pore water and its paleo-environment implications in Caofeidian Harbor, China

Oxygen-18 (18O), deuterium (D), and chemical patterns were determined in pore water samples extracted from 100-m depth in Caofeidian Harbor, China. The stable isotopic composition indicates that aquitard pore water was of meteoric origin and was basically not influenced by evaporation. Seawater diffusion was identified as the main influence on the isotope signature of pore water, according to the δ18O profile and positive correlation between δ18O and Cl–. The chemistry of aquitard pore water is characterized by high total dissolved solids (TDS) decreasing with depth, ranging from 26.89 to 7.26 g/L. The same trend was observed for Cl–, Na+, and Mg2+, as influenced by seawater. However, the typical ion ratios (i.e., Cl/Br, Sr/Ba) of pore water change significantly in different sedimentary facies along the study profile due to the long-term reaction between pore water and sediment, further supporting the finding that pore water was not replaced by modern water or seawater. Therefore, during long-term aquitard residence, the chemical composition of pore water would be influenced by other processes such as reduction of sulfates and cation exchange. The rare earth elements (REEs) of pore water are mainly affected by water–rock interaction. The shale-normalized REE fractionation patterns display enrichment of heavy REEs (HREE) relative to light REEs (LREE) and even greater fractionation degree in marine sediment pore waters (as larger La/YbNASC). The relative reduction environment in the aquitard and preferential mobilization of Eu2+ in the water–rock interaction lead to the development of positive Eu anomalies (1.13 < (Eu/Eu*)NASC < 1.98) and slightly positive Ce anomalies (–0.13 < (Ce/Ce*)NASC < 0.35).

A thermodynamic model for the prediction of pore water composition of clayey rock at 25 and 80 °C — Comparison with results from hydrothermal alteration experiments

This study proposes a thermodynamic model that is able to predict the pore water chemical composition of clayey rocks at temperatures up to 80°C. The model is based on equilibrium with a quartz/kaolinite/calcite/dolomite/Mg-chlorite mineralogical buffer and accounts for the sorption properties of both the montmorillonite and illite phases.Hydrothermal alteration experiments were also performed at 80°C on centimeter-sized samples from the Callovo-Oxfordian clay-rich formation (Paris Basin, France) to assess the validity of the proposed model at this temperature. The experiments were performed in both open and closed systems, and the initial CO2(g) partial pressure (pCO2(g)) used in each system was fixed to log pCO2(g)=−0.4. After 15months of alteration, the aqueous and gas phases were extracted at 80°C and analyzed. The evolution of the clay mineralogy was also characterized using X-ray diffraction profile modeling of experimental patterns and transmission electron microscopy (TEM).The composition of the Callovo-Oxfordian pore water predicted by our model at 25°C is in good agreement with that reported in the literature and measured in the geological formation. At 80°C, experiments in open systems were used to assess the kinetics of alteration process and the time needed to reach equilibrium for several dissolution–precipitation mineral reactions. For the closed system, it is shown that the proposed model well predicts the chemical composition of the fluid extracted at 80°C, considering the existing variability in the solubility constants for both Mg-chlorite and kaolinite minerals. Despite the need for additional alteration experiments to better constrain the temperature-dependence of the predicted pCO2(g) values, the proposed model provides reliable predictions of pore fluid composition at 80°C, a temperature expected in the framework of radioactive waste storage.

Faunal communities at sites of gas- and oil-bearing fluids in Lake Baikal

Macro- and meiofaunal communities were examined at four geomorphologically distinct sites with different gas- and oil-bearing fluid characteristics in the northern, central and southern basins of Lake Baikal. All sites had elevated concentrations of bicarbonate, nitrate, sulphate and chloride ions in pore fluids, with highest values at the Frolikha vent. Elevated levels of iron ions were found in pore waters of the St. Petersburg methane seep and the Gorevoy Utes oil seep. The chemical composition of pore waters at the Malenky mud volcano was similar to that reported in earlier work. Consistent with published data, the Frolikha vent (northern basin) and the St. Petersburg methane seep (central basin) were characterised by methane of mixed genesis (thermogenic + biogenic), whereas the methane source was mainly thermogenic at the Gorevoy Utes oil seep (central basin) and biogenic at the Malenky mud volcano (southern basin). In contrast to marine seep ecosystems, the macrofauna was dominated only by amphipods, giant planarians and oligochaetes, whereas bivalves were absent; the meiofauna was similar to its marine counterpart, being dominated by nematodes, cyclops, harpacticoids and ostracods. A statistically significant positive relationship was revealed between faunal abundance and the availability of bacterial mats on seep sediments. Moreover, ANOVA tests showed significant increases in both meiozoobenthic and macrozoobenthic densities at “hot spot” vent/seep sites relative to discharge-free reference sites. The isotopic composition of carbon and nitrogen at various trophic levels of these benthic vent/seep communities was found to differ markedly from that reported by earlier studies for the pelagic and other benthic food webs in Lake Baikal. As in marine seeps, the macrofauna had variable isotopic signatures. Light δ13C and δ15N values suggest the utilization of chemosynthetically fixed and/or methane-derived organic matter. By contrast, the heavy δ13C signatures of some mobile amphipods likely reflect consumption of photosynthetically derived carbon. These findings would at least partly explain why Lake Baikal is a notable outlier in global temperature–biodiversity patterns, exhibiting the highest biodiversity of any lake worldwide but at an extremely cold average temperature.

Early Diagenesis Records and Pore Water Composition of Methane-Seep Sediments from the Southeast Hainan Basin, South China Sea

Several authigenic minerals were identified by XRD and SEM analyses in shallow sediments from the Southeast Hainan Basin, on the northern slope of South China Sea. These minerals include miscellaneous carbonates, sulphates, and framboidal pyrite, and this mineral assemblage indicates the existence of gas hydrates and a methane seep. The assemblage and fabric features of the minerals are similar to those identified in cold-seep sediments, which are thought to be related to microorganisms fostered by dissolved methane. Chemical composition of pore water shows that the concentrations of SO42-, Ca2+, Mg2+, and Sr2+ decrease clearly, and the ratios of Mg2+ to Ca2+ and Sr2+ to Ca2+ increase sharply with depth. These geochemical properties are similar to those where gas hydrates occur in the world. All results seem to indicate clearly the presence of gas hydrates or deep water oil (gas) reservoirs underneath the seafloor.

Rare earth elements in cold seep carbonates from the Niger delta

The circulation of methane-rich fluids at cold seeps often leads to precipitation of authigenic carbonates mediated by microbial activity, which can be used as archives of fluid seepage on continental margins. In this study, we have investigated the potential of using rare earth element (REE) concentrations in authigenic carbonates for tracing fluid sources and providing information on associated biogeochemical processes at cold seeps. We have examined various chemical procedures for the analysis of REE in authigenic carbonates, and present a robust method using dilute acetic acid solution that allows the extraction of REE from carbonate phases with reduced contamination from Fe–Mn oxides and detrital minerals. Using this method, we analysed a series of carbonate samples collected at various deep-sea sites of active fluid seepage on the Niger Delta for their REE and trace element concentrations, δ 13C and, δ 18O values and 87Sr/ 86Sr isotope compositions. We show that the REE composition of cold seep carbonates is controlled primarily by that of the fluid from which they have precipitated. Our results also suggest that significant REE fractionation can occur during carbonate precipitation, which can be related to various degrees of REE complexation by organic versus carbonate ligands in response to changing pore water chemical composition for parameters such as carbonate alkalinity.

Plant community composition as a function of geochemistry and hydrology in three Appalachian wetlands

ABSTRACTAppalachian mountain wetlands are uncommon and diverse ecosystems; however, they are often susceptible to extensive alteration or destruction due to coal mining, highway construction, and quarrying. This study aimed to determine vegetation composition at three pristine wetlands and establish relationships with previously reported hydrologic, edaphic, and porewater characteristics to provide baseline data that could enhance wetland mitigation or restoration projects. Herbaceous vegetation was assessed by visually estimating percent cover for bryophyte and vascular species and by determining stem density for vascular taxa using 1 m2 quadrats located along transects. Multiple response permutation procedures (MRPPs) based upon importance values confirmed that species composition differed significantly (P &lt; 0·001) among the sites. Nonmetric multi‐dimensional scaling (NMDS) indicated that soil moisture conditions during the fall, soil chemistry, and porewater chemical composition influenced plant community composition. The first wetland (Martins Fork) was primarily dominated by Osmunda cinnamomea, Osmunda regalis, and Sphagnum palustre in response to drier soil during the fall season, higher soil pH, and weakly minerotrophic soil and porewater. The second wetland (Kentenia) was overwhelmingly dominated by S. palustre due to lower soil pH and persistently high water levels. Vegetation at the third wetland (Four Level) was distinguished by strongly dominant Scirpus polyphyllus and Glyceria striata and responded to higher soil and porewater Ca, Mg, and P concentrations as well as a light intensity gradient. The diverse vegetation and physico‐chemical characteristics indicate that these sites, although small in size, support regional biodiversity and are potential reference wetlands. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Rice Field Geochemistry and Hydrology: An Explanation for Why Groundwater Irrigated Fields in Bangladesh are Net Sinks of Arsenic from Groundwater

Irrigation of rice fields in Bangladesh with arsenic-contaminated groundwater transfers tens of cubic kilometers of water and thousands of tons of arsenic from aquifers to rice fields each year. Here we combine observations of infiltration patterns with measurements of porewater chemical composition from our field site in Munshiganj Bangladesh to characterize the mobility of arsenic in soils beneath rice fields. We find that very little arsenic delivered by irrigation returns to the aquifer, and that recharging water mobilizes little, if any, arsenic from rice field subsoils. Arsenic from irrigation water is deposited on surface soils and sequestered along flow paths that pass through bunds, the raised soil boundaries around fields. Additionally, timing of flow into bunds limits the transport of biologically available organic carbon from rice fields into the subsurface where it could stimulate reduction processes that mobilize arsenic from soils and sediments. Together, these results explain why groundwater irrigated rice fields act as net sinks of arsenic from groundwater.

Impact of hydrodynamics (ex- and infiltration) on the microbially controlled phosphorus mobility in running water sediments of a cultivated northeast German wetland

To gain insight into the impact of hydrodynamics (ex- and infiltration) on microbial activities, the microbial community structure, and the dynamics of phosphate and other chemical parameters in running water sediments of two transects in a German wetland have been studied. Flow directions and Darcy flow rates at canal bottom were determined using temperature profiles in the groundwater zone near and beneath the canal. Besides the chemical composition of pore water and sediment, a suite of enzyme activities and phosphorus fractions have been analyzed in the sediment. Hydrological analysis proved that at one transect infiltrating and stagnant conditions alter frequently, whereas at the other transect exfiltration dominates. We found significant differences between the transects in microbial activities and organic and iron-bound phosphorus as well as significant correlations between Darcy flux, microbial activities and phosphorus fractions. We conclude that not only the direction and rates of the flow, but also the quantity of changes between ex- and infiltration have an impact on microbial activity, and thus on phosphorus dynamics in the sediment of wetlands.

Geochemical and microbiological characteristics of sediments near the Malenky mud volcano (Lake Baikal, Russia), with evidence of Archaea intermediate between the marine anaerobic methanotrophs ANME-2 and ANME-3

Detailed lithological, biogeochemical and molecular biological analyses of core sediments collected in 2002–2006 from the vicinity of the Malenky mud volcano, Lake Baikal, reveal considerable spatial variations in pore water chemical composition, with total concentrations of dissolved salts varying from 0.1 to 1.8‰. Values of methane δ13С in the sediments suggest a biogenic origin (δ13Сmin. −61.3‰, δ13Сmax. −72.9‰). Rates of sulphate reduction varied from 0.001 to 0.7 nmol cm−3 day−1, of autotrophic methanogenesis from 0.01 to 2.98 nmol CH4 cm−3 day−1, and of anaerobic oxidation of methane from 0 to 12.3 nmol cm−3 day−1. These results indicate that methanogenic processes dominate in gas hydrate-bearing sediments of Lake Baikal. Based on clone libraries of 16S rRNA genes amplified with Bacteria- and Archaea-specific primers, investigation of microbial diversity in gas hydrate-bearing sediments revealed bacterial 16S rRNA clones classified as Deltaproteobacteria, Gammaproteobacteria, Chloroflexi and OP11. Archaeal clone sequences are related to the Crenarchaeota and Euryarchaeota. Baikal sequences of Archaea form a distinct cluster occupying an intermediate position between the marine groups ANME-2 and ANME-3 of anaerobic methanotrophs.

A34 草津白根山火口湖「涸釜」の底質間隙水の化学組成(火山の物質科学(3),日本火山学会2008年秋季大会)

A34 草津白根山火口湖「涸釜」の底質間隙水の化学組成(火山の物質科学(3),日本火山学会2008年秋季大会)

Prediction of convective transport within unsaturated concrete utilizing pore size distribution data

The durability of concrete structures is often compromised by physical and chemical interaction with the external environment that leads to ongoing maintenance and, in the worst cases, can lead to reduced structural integrity and consequent asset replacement. Concrete is a porous material and most field-exposed concrete is partially saturated with water. Where the concrete is unsaturated and there is no external water pressure acting on a concrete surface, the primary mechanisms of transport into concrete are convective-diffusion ingress (i.e. uptake of water and water-borne agents due to capillary attraction). This paper assesses capillarity and outlines a predictive model of the uptake of water by concrete based on analysis of the pore size distribution. It is acknowledged that concrete has a multitude of internal pores with a broad range of lengths and cross-sectional shapes, surface roughness, tortuosities, random meeting and divergence with adjacent pores, microcracks and fractures, and variable pore-water chemical composition, however the prediction model shows reasonable agreement with water sorptivity test data.

Mechanism for calcite dissolution and its contribution to development of reservoir porosity and permeability in the Kela 2 gas field, Tarim Basin, China

This study is undertaken to understand how calcite precipitation and dissolution contributes to depth-related changes in porosity and permeability of gas-bearing sandstone reservoirs in the Kela 2 gas field of the Tarim Basin, Northwestern China. Sandstone samples and pore water samples are collected from well KL201 in the Tarim Basin. Vertical profiles of porosity, permeability, pore water chemistry, and the relative volume abundance of calcite/dolomite are constructed from 3600 to 4000 m below the ground surface within major oil and gas reservoir rocks. Porosity and permeability values are inversely correlated with the calcite abundance, indicating that calcite dissolution and precipitation may be controlling porosity and permeability of the reservoir rocks. Pore water chemistry exhibits a systematic variation from the Na2SO4 type at the shallow depth (3600–3630 m), to the NaHCO3 type at the intermediate depth (3630–3695 m), and to the CaCl2 type at the greater depth (3728–3938 m). The geochemical factors that control the calcite solubility include pH, temperature, pressure, Ca2+ concentration, the total inorganic carbon concentration (ΣCO2), and the type of pore water. Thermodynamic phase equilibrium and mass conservation laws are applied to calculate the calcite saturation state as a function of a few key parameters. The model calculation illustrates that the calcite solubility is strongly dependent on the chemical composition of pore water, mainly the concentration difference between the total dissolved inorganic carbon and dissolved calcium concentration (i.e., [ΣCO2]−[Ca2+]). In the Na2SO4 water at the shallow depth, this index is close to 0, pore water is near the calcite solubility. Calcite does not dissolve or precipitate in significant quantities. In the NaHCO3 water at the intermediate depth, this index is greater than 0, and pore water is supersaturated with respect to calcite. Massive calcite precipitation was observed at this depth interval and this intensive cementation is responsible for decreased porosity and permeability. In the CaCl2 water at the greater depth, pore water is under-saturated with respect to calcite, resulting in dissolution of calcite cements, as consistent with microscopic dissolution features of the samples from this depth interval. Calcite dissolution results in formation of high secondary porosity and permeability, and is responsible for the superior quality of the reservoir rocks at this depth interval. These results illustrate the importance of pore water chemistry in controlling carbonate precipitation/dissolution, which in turn controls porosity and permeability of oil and gas reservoir rocks in major sedimentary basins.

Inverse hydrochemical models of aqueous extracts tests

Aqueous extract test is a laboratory technique commonly used to measure the amount of soluble salts of a soil sample after adding a known mass of distilled water. Measured aqueous extract data have to be re-interpreted in order to infer porewater chemical composition of the sample because porewater chemistry changes significantly due to dilution and chemical reactions which take place during extraction. Here we present an inverse hydrochemical model to estimate porewater chemical composition from measured water content, aqueous extract, and mineralogical data. The model accounts for acid–base, redox, aqueous complexation, mineral dissolution/precipitation, gas dissolution/ex-solution, cation exchange and surface complexation reactions, all of which are assumed to take place at local equilibrium. It has been solved with INVERSE-CORE 2D and tested with bentonite samples taken from FEBEX (Full-scale Engineered Barrier EXperiment) in situ test. The inverse model reproduces most of the measured aqueous data except bicarbonate and provides an effective, flexible and comprehensive method to estimate porewater chemical composition of clays. Main uncertainties are related to kinetic calcite dissolution and variations in CO 2(g) pressure.

Chemical composition of pore waters of bottom sediments in different Baikal basins

Results of study of pore waters of bottom sediments from different Baikal basins are presented. The most typical ion distribution patterns reflecting the Baikal sediment diagenesis are given. We have established that in areas with regular sedimentation, in the absence of faults and inflows, the sediment pore waters of three lake basins inherit the chemical composition of the Baikal water, which is stable in time and space. Changes in pore water composition mark general natural anomalies, such as the presence of active faults, tectonic movements, and inflows along permeable zones. In areas with the subsurface occurrence of gas hydrates, thorough long-term research has revealed an anomalous composition of pore waters. It has been established that the anomalies are caused by a discharge of deep-level mud-volcanic fluids. The ejected mud-volcanic waters differ from each other in mineralization, ion composition, and sources, which determines the difference in hydrate formation and the composition of gas hydrates.

Analysis of the porewater chemical composition of a Spanish compacted bentonite used in an engineered barrier

Compacted bentonites are being considered in many countries as a backfill material in high-level radioactive waste disposal concepts. A knowledge of the porewater chemistry in the clay barrier is essential since the porewater composition influences the release and transport of the radionuclides. However, quantification of the water chemistry in compacted bentonite under repository conditions is difficult. The methodology followed to obtain the porewater composition of the FEBEX bentonite is described in this paper. It is based on the characterisation of the solid phase, determination of the physico-chemical properties of the montmorillonite component and geochemical modelling. The FEBEX bentonite has a high cation exchange capacity (∼1 eq/kg), high surface area (∼725 m 2/g total surface area and 62 m 2/g external surface area) and accessory minerals such as carbonates, sulphates, pyrite, etc.; and organic matter. The chloride inventory in the FEBEX bentonite is ∼22 mmol/kg. The montmorillonite, together with the other mineral phases present, will determine the composition of the porewater. However, in order to calculate a unique aqueous chemistry, two further quantities are required, the chloride concentration and the pH. Water vapour adsoption/desorption isotherms, together with c-lattice spacing determinations, were used to identify the different states and location of water. Most of the water in the as received bentonite resides in the interlayer space. However, the measurements indicate that about 0.053 l/kg may be regarded as free water, implying a chloride concentration of 0.42 M. The pH of the system is fixed by equilibrium with the atmosphere ( P CO 2 =10 −3.5 bar) and saturation with the carbonate phases present. The porewater calculated to be in equilibrium with the as received FEBEX bentonite powder is a Na–Ca–Mg chloride type with a high ionic strength, 0.66 M, and a pH of ∼7.4. Likewise, in order to calculate the porewater composition of compacted re-saturated bentonite, the volume of free water is required. This value is taken to be the chloride accessible porosity obtained from Cl − through-diffusion tests (due to anion exclusion, Cl − anions can not move through the interlayer and overlapping double layer regions). The amount of free water in compacted bentonite determined in this manner was 0.03 l/kg at a dry density of ρ d=1650 kg/m 3. The corresponding chloride concentration is thus ∼0.73 M. Arguments are presented that the initial pH is fixed in the compacted material by the high buffering capacity afforded by the amphoteric edge sites, SOH sites, of the montmorillonite. The pH of the porewater depends directly on the speciation of these sites, i.e., the proportions of sites present as SOH, SOH 2 +, SO −, and this is fixed in the powdered source material through equilibration with air (compaction will not alter the state of the SOH sites). The porewater of compacted FEBEX bentonite at ρ d=1650 kg/m 3 was calculated to be a Na–Ca–Mg chloride type with a high ionic strength, 0.90 M, and a pH of ∼7.4.

Chemistry of oil-field brines in relation to diagenesis of reservoirs 1. Use of mineral stability fields to reconstruct in situ water composition. Example of the Mahakam basin

During mineral diagenesis, the chemical composition of pore water can be strongly influenced by mineral reactions, especially under geological conditions where water/rock ratios are low. To model and eventually to predict diagenesis, it is important to have an understanding of formation water composition. However, available water analyses in many basins are not complete enough for this purpose. In most cases, reliable values for pH, dissolved silica, and aluminium are lacking. Such data can be reconstructed using various relations developed herein. These data are constrained by petrographic observations. Because of their well preserved, or authigenic character, some alumino-silicates are considered as relatively stable and can be thought of as effectively saturated with respect to them. The saturation state of the other minerals of the matrix can be discussed according to these constraints. The sub-arkosic series of the Mahakam basin resevoirs (Mio-Pliocene) are used as a reference example to perfect a method capable of reconstructing consistent compositions of diagenetic waters, suitable for input into diagenetic modelling.

Near-Field Chemical Composition of Porewaters in a Near-Surface Low-Level Radioactive Waste Vault

ABSTRACTA long-term waste degradation experiment has been performed with actual low-level radioactive wastes (LLRW) at the Chalk River Laboratories (CRL), to support the licensing and modelling efforts for near-surface disposal. The wastes consist of paper, mop heads, paper towels, used clothing, etc. The wastes were compacted into bales and sealed into separate steel containers, which were connected to leachate collection systems for sampling. The leachates collected had a composition typical of landfill leachates. The major inorganic ions were Na, Ca, Cl, and Fe, and the ionic strength was ∼0.05 M. The relative distribution of inorganic ions in the leachates was remarkably similar between bales. Volatile fatty acids (VFA) were the major species of dissolved organic carbon (DOC, total DOC up to 7000 mg/L). A typical composition of leachates is proposed, which can be used in geochemical and source term modelling.

Marine sediments in a region of recent volcanic activity in the Bay of Plenty, New Zealand

Abstract The chemical composition of recent marine sediments and pore waters in areas of shallow underwater volcanic vents in the Bay of Plenty, New Zealand, was studied. Values of pH in the sediments in active sites were higher than in control sites, and concentrations of Fe, Cu, and Ni were elevated; however, the general chemical composition of the sediments and pore waters was rather similar to that of common shelf sediments. The chemical composition of pore waters agrees well with the usual diagenesis of these sediments; the increase in temperature might influence the rates of diagenetic processes. No signs of uniform seeping of volcanic waters through the sediments were detected. Hydrothermal waters and gases are preferentially transported through sulfur tubes. These tubes and sulfide concretions replacing magnetite were collected at and near the hydrothermal vents.

Relationships between authigenic mineral transformation and variation in vitrinite reflectance during diagenesis: an example from the Tertiary of northern Kyushu, Japan

AbstractThe Tertiary sequences in northern Kyushu, Japan, contain several types of zeolite and clay minerals together with carbonaceous materials. A correlation between authigenic mineral zones and vitrinite reflectance reveals the delay in mineral authigenesis relative to the increase in vitrinite reflectance compared with similar studies in other regions. This may be ascribed to causes such as high paleogeothermal gradient, and chemical composition of pore-waters and host sediment.

Low‐temperature hydrothermal alteration in near‐surface sediments, Salton Sea Geothermal Area

As part of the Lawrence Livermore/Sandia National laboratories shallow drilling (19 wells) into Salton Sea sediments, four 1‐m cores were recovered from 76 m subbottom depth. The cored wells are along a transect from the margin toward the interior of the Mullet Island thermal anomaly. The thermal gradients range from 0.191°C/m near the margin to 0.816°C/m near the interior of the thermal anomaly. Bottom‐hole temperature in a cored well close to the center of the thermal anomaly is 87°C. Pore water chemical composition and bulk and clay size sediment mineralogical and chemical compositions are influenced by hydrothermal reactions which intensify with increasing thermal gradient. X ray diffraction and petrographic microscopic analyses of bulk and clay size fractions of the sediments indicate that the relative proportions of smectite to kaolinite plus illite and of dolomite to calcite increase from the margin toward the interior of the thermal anomaly region. Clay mineral recrystallization and dolomitization are indicated by increased K/Al and decreased Ca/Mg molar ratios in the clay size sediment fraction. Calcium, silica, potassium, lithium, and strontium are released from sediments into pore fluids. These data suggest that even at temperatures less than 100°C, hydrothermal activity significantly alters the pore water chemistry and moderately alters the sediment mineralogical and chemical composition.