Pore Water Cl Research Articles

Chloride retention in forest soil by microbial uptake and by natural chlorination of organic matter

Inorganic chlorine (i.e. chloride; Cl in) is generally considered inert in soil and is often used as a tracer of soil and ground water movements. However, recent studies indicate that substantial retention or release of Cl in can occur in soil, but the rates and processes responsible under different environmental conditions are largely unknown. We performed 36Cl tracer experiments which indicated that short-term microbial uptake and release of Cl in, in combination with more long-term natural formation of chlorinated organic matter (Cl org), caused Cl in imbalances in coniferous forest soil. Extensive microbial uptake and release of Cl in occurred over short time scales, and were probably associated with changes in environmental conditions. Up to 24% of the initially available Cl in within pore water was retained by microbial uptake within a week in our experiments, but most of this Cl in was released to the pore water again within a month, probably associated with decreasing microbial populations. The natural formation of Cl org resulted in a net immobilization of 4% of the initial pore water Cl in over four months. If this rate is representative for the area where soil was collected, Cl org formation would correspond to a conversion of 25% of the yearly wet deposition of Cl in. The study illustrates the potential of two Cl in retaining processes in addition to those previously addressed elsewhere (e.g. uptake of chloride by vegetation). Hence, several processes operating at different time scales and with different regulation mechanisms can cause Cl in imbalances in soil. Altogether, the results of the present study (1) provide evidence that Cl in cannot be assumed to be inert in soil, (2) show that microbial exchange can regulate pore water Cl in concentrations and (3) confirm the controversial idea of substantial natural chlorination of soil organic matter.

Composition of pore water in lake sediments, research site “B”, Osage County, Oklahoma: Implications for lake water quality and benthic organisms

Shallow ground water at US Geological Survey research site B in northeastern Oklahoma is contaminated with NaCl-rich brine from past and present oil production operations. Contaminated ground water provides a potential source of salts, metals, and hydrocarbons to sediment and water of adjacent Skiatook Lake. A former brine storage pit 10 m in diameter that is now submerged just offshore from site B provides an additional source of contamination. Cores of the upper 16–40 cm of lake sediment were taken at the submerged brine pit, near an offshore saline seep, and at a location containing relatively uncontaminated lake sediment. Pore waters from each 2-cm interval were separated by centrifugation and analyzed for dissolved anions, cations, and trace elements. High concentrations of dissolved Cl − in pore waters (200–5000 mg/L) provide the most direct evidence of contamination, and contrast sharply with an average value of only about 37 mg/L in Skiatook Lake. Chloride/Br − mass ratios of 220–240 in contaminated pore waters are comparable to values in contaminated well waters collected onshore. Dissolved concentrations of Se, Pb, Cu and Ni in Cl −-rich pore waters exceed current US Environmental Protection Agency criteria for probable toxicity to aquatic life. At the submerged brine storage pit, the increase of Cl − concentration with depth is consistent with diffusion-dominant transport from deeper contaminated sediments. Near the offshore saline seep, pore water Cl − concentrations are consistently high and vary irregularly with depth, indicating probable Cl − transport by layer-directed advective flow. Estimated annual contributions of Cl − to the lake from the brine storage pit (∼20 kg) and the offshore seep (∼9 kg) can be applied to any number of similar sources. Generous estimates of the number of such sources at site B indicate minimal impact on water quality in the local inlet of Skiatook Lake. Similar methodologies can be applied at other sites of NaCl contamination surrounding Skiatook Lake and elsewhere.

Magnitudes of submarine groundwater discharge from marine and terrestrial sources: Indian River Lagoon, Florida

Magnitudes of terrestrial (fresh) and marine (saline) sources of submarine groundwater discharge (SGD) are estimated for a transect across Indian River Lagoon, Florida. Two independent techniques (seepage meters and pore water Cl− concentrations) show terrestrial SGD decreases linearly to around 22 m offshore, and these techniques, together with a model based on the width of the outflow face, indicate a cumulative discharge of between 0.02 and 0.9 m3/d per meter of shoreline. Seepage meters and models of the deficiencies in 222Rn activity in shallow sediments indicate marine SGD discharges of roughly 117 m3/d per meter of shoreline across the entire 1800‐m‐wide transect. Two surface streams nearest the transect have an average discharge of about 28 m3/d per meter of shoreline. Marine SGD is thus 4 times greater then surface water discharge and more than 2 orders of magnitude greater than terrestrial SGD. The magnitude of the terrestrial SGD is limited by the amount of regional precipitation, evaporation, recharge, and groundwater usage, while marine SGD is limited only by processes circulating marine water into and out of the sediments. The large magnitude of marine SGD means that it could be important for estuarine cycling of reactive components such as nutrients and metals with only slight modification from estuarine water compositions. The small magnitude of terrestrial SGD means that large differences from estuarine water composition would be required to affect chemical cycling.

Paleosols as deep regolith: implications for ground-water recharge across a loessial climosequence

Thick loess deposits consisting of paleosol sequences comprise the deep regolith of the Palouse region in eastern Washington and northern Idaho. Ground water is the principal water supply for the Palouse Basin, yet recharge mechanisms and rates through this regolith are poorly understood. Forecasting the sustainability of the water supply has been hampered because models that predict ground-water recharge are complicated by deep regolith that has complex stratigraphy and, in some instances, multiple buried hydraulically restrictive horizons. To assess water movement, pore-water Cl − and δ 18O distributions were measured to 6-m depths in three catchments representing a climosequence across the Basin, and interpreted using catchment and basin-wide stratigraphic relationships. Vertical facies between dense and soft horizons coincide with variations in [Cl −]. In the eastern Basin, pore-water [Cl −] ranged from 15 to 150 mg/L and increased with depth. In addition, shifts in isotopic signature of oxygen in pore water coincide with Cl −-rich horizons, and suggest long pore-water residence times. In contrast, [Cl −] in the central and western Basin ranges from 1 to 15 mg/L and changes little with depth. Tracer profiles illustrate three major hydrostratigraphic units in the Basin: (1) uplands with relatively homogeneous regolith that has short pore-water residence times; (2) uplands with heterogeneous regolith that has long pore-water residence times; and (3) valleys with heterogeneous regolith that display dynamic hydraulic processes. Regional relationships between deep regolith and surface soils were established in order to use the SSURGO database to estimate the spatial extent of each hydrostratigraphic unit. The degree of paleosol development mimics that of contemporary sola across the climosequence. Results indicate that recharge is less than 3 mm year −1 in 33% of the study area where precipitation is greatest and regolith is heterogeneous. Recharge increases to 10 mm year −1 in 37% of the study area where regolith is homogeneous. Valley positions having active hydraulic regimes constitute 10% of the Basin.

Implications of halide leaching on 36Cl studies at Yucca Mountain, Nevada

Chlorine 36 was generated from nuclear tests in the 1950s and 1960s and has been used to identify fast flow paths at Yucca Mountain, the proposed repository for high‐level nuclear waste [Fabryka‐Martin et al., 1997, 1998]. Bomb pulse 36Cl, brought into the subsurface by infiltrating rainwater, presumably resides along fracture surfaces because of the extremely low rock matrix permeability. However, leaching a rock sample to extract this salt inevitably extracts pore water chloride (Cl) and rock matrix chloride, thereby making it difficult to obtain reproducible measurements or detect the specific bomb pulse signatures. Complexities introduced by these sources of older chloride include dilution of bomb pulse 36Cl/Cl ratios for samples from strata with a high Cl concentration, variations in measured ratios as a function of leaching time, rock chip size, and the differing effects of active leaching from those of passive leaching. This work provides both a conceptual model and a mathematical solution for the leaching processes and examines the role of sample leaching in the 36Cl studies of Yucca Mountain rocks. An analytical solution is derived for the diffusion of Cl and 36Cl in composite media (rock matrix and water) to accommodate variable diffusivity. This solution is subsequently used to develop a leaching model that takes into account bomb pulse signal, matrix pore water, and relatively hard to leach components (isolated fluid inclusion and mineral boundary salts). The model is then applied to samples from stratigraphic units at Yucca Mountain to obtain leachate concentrations from different setup methods (protocols), including duration, chip size, and gravitational settling of the water‐rock mixture. The model results show that the probability of detecting a 36Cl/Cl bomb pulse signal is severely diminished under longer leaching times and smaller rock fragment sizes and that leaching times of 1 to 10 hours are most likely to be successful in detecting a bomb pulse signal. Bomb pulse 36Cl/Cl ratios are more likely to be observed if pore water Cl concentrations were initially low prior to the introduction of bomb pulse carrying water.

Chloride Distributions as Indicators of Vadose Zone Stratigraphy in Palouse Loess Deposits

Chloride is often used as a conservative tracer to estimate groundwater recharge rates in arid and semiarid regions. Relationships between Cl− depth profiles and vadose zone stratigraphy have revealed new information on the behavior of this dissolved constituent in pore waters of heterogeneous materials. We measured pore-water Cl− in loess deposits of the eastern Palouse region in northern Idaho, where multiple sequences of buried soils extend to ∼20 m depth. Three cores were collected to bedrock at summit, side slope, and valley positions. Pore-water Cl− distribution, clay content, soil strength, and secondary Mnd/Fed ratios were measured to identify relationships between natural tracer migration and vadose zone stratigraphy. Characterization of deep strata revealed complex sequences of extremely dense paleosol fragipans interstratified with less dense leached horizons. Abrupt changes in Cl− concentration reflect boundaries between these stratigraphic units that display contrasting physical and morphological properties. Results illustrate that loess stratigraphy influences vadose zone water movement in the Palouse. In addition, Cl− depth profiles can be used as indicators of deep stratigraphy across various landscape positions.

Mineralization of Organic Matter in Intertidal Sediments of a Tropical Semi-enclosed Delta

Rates and pathways of organic matter decomposition in sediments were examined during winter and summer in two mangrove forests and two accreting mud flats in Hinchinbrook Channel, north-eastern Australia. Rates of O2consumption (range: 2·8–61·0mmolm−2day−1) and CO2release (range: 1·8–21·9mmolm−2day−1) were faster in winter than in summer. Low respiratory quotients (CO2/O2range: 0·24–1·08) and a comparison of other metabolic pathways with total carbon oxidation (Tcox) in summer suggests that most O2was used in oxidizing reduced solutes. Rates of total carbon oxidation were greater in the mangrove sediments than in the mud flats. In winter, sulphate reduction in the mangroves (range: 4·9–8·3mmolSm−2day−1) accounted for a much greater proportion (45–78%) of Tcoxthan in the mud flats where sulphate reduction (range: 0·8–1·0mmolSm−2day−1) was a minor (12–14% of Tcox) metabolic pathway. In summer, sulphate reduction accounted for a greater (62–>100%) proportion of Tcoxin both mangrove (range: 7·9–10·1mmolSm−2day−1) and mud flat (range: 0·8–2·3mmolSm−2day−1) sediments. Rates of denitrification in summer were rapid but highly variable at all four sites (range: 2·9–6·9mmolN2m−2day−1). Rates of Fe and Mn reduction were slow at all four sites (range: 0·0–0·27mmolCm−2day−1) suggesting that metal reduction was a minor decomposition pathway. No methane was detected in the porewater or released from sediments at any of the sites. There was no net microalgal primary production at any of the sites. Mineralization efficiency of organic carbon was low (9–14%) at the most sheltered mud flat, but efficiencies were greater (44–60%) and nearly equivalent at less sheltered mud flat and mangrove habitats, respectively. Imbalances of mineralized carbon between the sum of the various pathways and Tcox, particularly at the mangrove sites, suggest unaccounted losses of dissolved carbon. Large seasonal changes in porewater Cl−concentrations and lack of solute concentration differences with overlying tidal water, imply dilution and tidal advection and/or drainage of interstitial water by intense summer rains.

Thermodynamic and pore water halogen constraints on gas hydrate distribution at ODP Site 997 (Blake Ridge)

Marine sediment sequences with CH 4 hydrate are characterized by an atypical depth profile in dissolved Cl − squeezed from pore space: a shallow subsurface Cl − maximum overlies a lengthy and pronounced Cl − minimum. This pore water Cl − profile represents a combination of multiple processes including glacial–interglacial variations in ocean salinity, advection and diffusion of ions that are excluded during gas hydrate formation at depth, and release of fresh water from dissociation of hydrate during core recovery. In situ quantities of gas hydrate can be determined from a measured pore water Cl − profile provided the in situ pore water signature prior to core recovery can be separated. Ocean Drilling Program (ODP) Site 997 was drilled into a large CH 4 hydrate reservoir on the Blake Ridge in the western Atlantic Ocean. Previously we have constructed a high-resolution pore water Cl − profile at this location; here we present a `coupled chloride-hydrate' numerical model to explain basic trends in the Cl − profile and to isolate in situ Cl − concentrations. The model is based on thermodynamic and ecological considerations, and uses established equations for describing chemical behavior in marine sediment–pore water systems. The model incorporates four key concepts: (1) most gas hydrate is formed immediately below the SO 4 2− reduction zone; (2) fluid, dissolved ions and gas advect upward through the sediment column; (3) CH 4 hydrate dissociates at the base of hydrate stability conditions; and (4) seawater salinity fluctuates during glacial–interglacial cycles of the late Pliocene and Quaternary. Rates of upward advection in the model are sufficient to account for measured Br − and I − concentrations as well as CH 4 oxidation at the base of the SO 4 2− reduction zone. In situ pore water Cl − inferred from the model is similar to that determined by limited direct sampling; in situ CH 4 hydrate amounts inferred from the model (an average of about 4% of porosity) are broadly consistent with those determined by direct gas sampling and indirect geophysical techniques. The model also predicts production of substantial quantities of free CH 4 gas bubbles (>2.5% of porosity) at a depth immediately below the lowest accumulation of CH 4 hydrate in the sediment column. Our explanation for the pore water Cl − profile at Site 997 is important because it provides a theoretical mechanism for understanding the distribution of interstitial water Cl −, gas hydrate, and free gas in a marine sediment column.

Atmospheric deposition and mass balance of major and trace elements in two oceanic peat bog profiles, northern Scotland and the Shetland Islands

A mass balance was performed on the major (Si, Al, Fe, Na, K, Mg, Ca) and some trace elements (Rb, Sr, Cl, Br, S) in peat cores from two oceanic blanket bogs in northern Scotland: Loch Laxford (LL) is 15 km east of the open ocean in the extreme northwest of the Scottish mainland; Fleck's Loch (FL) is on the small (15 km 2) island of Foula, 50 km west of the ‘mainland’ of Shetland. The supply of these elements to the peats can be explained in terms of two main atmospheric sources: mineral aerosols from weathering of crustal rocks, and sea salt spray. The average Al/Sc and Fe/Sc ratios of the peats at both sites are comparable to the corresponding ratios in soil-derived aerosols suggesting that crustal inputs account for essentially all of the Al and Fe in the profiles. The Si/Sc ratios of the peats from LL again correspond with the value for soil-derived aerosols, but at FL there is an excess of Si in the uppermost part of the profile mainly because of quartz from sources immediately around the bog. In contrast to Al, Fe, Si, the concentrations of Na, Mg, Ca, and Sr in the peats exceed the corresponding ratios in soil-derived aerosols by 5 to 30 times, reflecting the importance of marine aerosols for these elements. The conservative behaviour of Cl − allows the concentration of this species in the pore waters to be used as a quantitative indicator of the relative importance of sea salt inputs to the two bogs. In contrast, the concentrations of other important pore water species (SO 4 2−, Br −, Na +, K +, Mg 2+, and Ca 2+) do not simply reflect their concentrations in rainwater because of biological transformations (S, Br, K), ion exchange (Na, K, Mg, Ca), and possibly mineral dissolution (Na, Mg, Ca). The average pore water Cl − concentrations at FL (97.0 ± 15.2 mg/1) show that marine inputs are 4 times greater than at LL (24.0 ± 2.6 mg/1). The concentrations of other major and trace elements in the rainwater supplied to the bogs were calculated using the known abundances of the elements in seawater, and normalizing to the pore water Cl − concentrations at the two sites. These estimated rainwater compositions were then combined with rainfall data to calculate the annual inputs to the bogs. The peat core inventory of major and trace elements (corrected for crustal contributions) was compared with these inputs to determine the percentage of each element retained in the peat profiles. These findings indicate that 91.5 to 99.9% of the major elements (Cl, Na, Mg, K, Ca) and trace elements (Br, Sr, Rb) supplied to the bogs by rainwater are not retained by the peat. Thus, none of these elements in the solid phase (peat) can be used as a quantitative measure of sea salt inputs to the bogs.

Geochemical and geothermal evidence for fluid migration in the Barbados Accretionary Prism (ODP leg 110)

Measured geochemical and geothermal effects of fluid migration in the northern Barbados accretionary prism indicate that: (1) fluid flows laterally along low‐angle active faults; (2) the entire length of the Leg 110 transect is a fluid discharge zone; (3) migration results in heating of sediments across the transect but most strongly at the deformation front; and (4) migration results in dilution of pore water‐Cl and 18O enrichment more in the arcward than the seaward side of the prism, suggesting that the Cl‐poor, 18O‐rich fluids that intrude the prism are derived from smectiteillite reactions occurring arcward. These conclusions have strengthened those derived from the initial appraisal of shipboard data, modified ideas about where fluids are heated most and where the geochemistry is strongly altered, and provided new ideas about the sources of fluids and their relative contributions.