Pore Water Residence Time Research Articles

Biogeochemical processes in the groundwater discharge zone of urban streams

The influence of biogeochemical processes on nitrogen and organic matter transformation and transport was investigated for two urban streams receiving groundwater discharge during the dry summer baseflow period. A multiple lines of evidence approach involving catchment-, and stream reach-scale investigations were undertaken to describe the factors that influence pore water biogeochemical processes. At the catchment-scale gaining stream reaches were identified from water table mapping and groundwater discharge estimated to be between 0.1 and 0.8 m3 m−2 d−1 from baseflow analysis. Sediment temperature profiles also suggested that the high groundwater discharge limited stream water infiltration into the sediments. At the stream reach-scale, dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations were higher in stream water than in groundwater. However, DOC and DON concentrations were greatest in sediment pore water. This suggests that biodegradation of sediment organic matter contributes dissolved organic matter (DOM) to the streams along with that delivered with groundwater flow. Pore water ammonium (NH4 +) was closely associated with areas of high pore water DOM concentrations and evidence of sulfate (SO4 2−) reduction (low concentration and SO4:Cl ratio). This indicates that anoxic DOM mineralization was occurring associated with SO4 2− reduction. However the distribution of anoxic mineralization was limited to the center of the streambed, and was not constrained by the distribution of sediment organic matter which was higher along the banks. Lower sediment temperatures measured along the banks compared to the center suggests, at least qualitatively, that groundwater discharge is higher along the banks. Based on this evidence anoxic mineralization is influenced by groundwater residence time, and is only measurable along the center of the stream where groundwater flux rates are lower. This study therefore shows that the distribution of biogeochemical processes in stream sediments, such as anoxic mineralization, is strongly influenced by both the biogeochemical conditions and pore water residence time.

Pore-water mobility: Distribution of δ37Cl, 36Cl/Cl, 129I/127I and dissolved 4He concentration in the core drilled in the Mobara gas field, Japan

We measured 36Cl/Cl and 129I/127I isotope ratios, dissolved 4He concentrations, and δ37Cl to estimate the residence time and flow characteristics of pore water in rocks of the Kazusa Group in the Mobara gas field, Japan. We deduced a residence time of 0.28–0.85Ma for the pore water, based on a secular equilibrium value of 7.05±1.58×10−15 for the 36Cl/Cl ratio in Pleistocene strata of the gas field. Furthermore, the concentration of 4He corrected by the solubility equilibrium model in the pore water indicated that residence times varied from 0.09 to 0.62Ma from the depth of 642 to the depth of 1742m in the core drilled in the gas field. The pore-water dating results have insignificant differences between the ages of the Kazusa Group formations, compared with the iodine ages from 129I/127I ratio. Variations of δ37Cl in the pore water suggested that mass transport in rocks of the Kazusa Group was dominantly controlled by diffusion.

Do Lab-Derived Distribution Coefficient Values of Pesticides Match Distribution Coefficient Values Determined from Column and Field-Scale Experiments? A Critical Analysis of Relevant Literature

In this study, we analyzed sorption parameters for pesticides that were derived from batch and column or batch and field experiments. The batch experiments analyzed in this study were run with the same pesticide and soil as in the column and field experiments. We analyzed the relationship between the pore water velocity of the column and field experiments, solute residence times, and sorption parameters, such as the organic carbon normalized distribution coefficient ( ) and the mass exchange coefficient in kinetic models, as well as the predictability of sorption parameters from basic soil properties. The batch/column analysis included 38 studies with a total of 139 observations. The batch/field analysis included five studies, resulting in a dataset of 24 observations. For the batch/column data, power law relationships between pore water velocity, residence time, and sorption constants were derived. The unexplained variability in these equations was reduced, taking into account the saturation status and the packing status (disturbed-undisturbed) of the soil sample. A new regression equation was derived that allows estimating the values derived from column experiments using organic matter and bulk density with an value of 0.56. Regression analysis of the batch/column data showed that the relationship between batch- and column-derived values depends on the saturation status and packing of the soil column. Analysis of the batch/field data showed that as the batch-derived value becomes larger, field-derived values tend to be lower than the corresponding batch-derived values, and vice versa. The present dataset also showed that the variability in the ratio of batch- to column-derived value increases with increasing pore water velocity, with a maximum value approaching 3.5.

Groundwater residence time distributions in peatlands: Implications for peat decomposition and accumulation

Peat soils consist of poorly decomposed plant detritus, preserved by low decay rates, and deep peat deposits are globally significant stores in the carbon cycle. High water tables and low soil temperatures are commonly held to be the primary reasons for low peat decay rates. However, recent studies suggest a thermodynamic limit to peat decay, whereby the slow turnover of peat soil pore water may lead to high concentrations of phenols and dissolved inorganic carbon. In sufficient concentrations, these chemicals may slow or even halt microbial respiration, providing a negative feedback to peat decay. We document the analysis of a simple, one‐dimensional theoretical model of peatland pore water residence time distributions (RTDs). The model suggests that broader, thicker peatlands may be more resilient to rapid decay caused by climate change because of slow pore water turnover in deep layers. Even shallow peat deposits may also be resilient to rapid decay if rainfall rates are low. However, the model suggests that even thick peatlands may be vulnerable to rapid decay under prolonged high rainfall rates, which may act to flush pore water with fresh rainwater. We also used the model to illustrate a particular limitation of the diplotelmic (i.e., acrotelm and catotelm) model of peatland structure. Model peatlands of contrasting hydraulic structure exhibited identical water tables but contrasting RTDs. These scenarios would be treated identically by diplotelmic models, although the thermodynamic limit suggests contrasting decay regimes. We therefore conclude that the diplotelmic model be discarded in favor of model schemes that consider continuous variation in peat properties and processes.

A multiscale model for integrating hyporheic exchange from ripples to meanders

It is necessary to improve our understanding of the exchange of dissolved constituents between surface and subsurface waters in river systems in order to better evaluate the fate of water‐borne contaminants and nutrients and their effects on water quality and aquatic ecosystems. Here we present a model that can predict hyporheic exchange at the bed‐form‐to‐reach scale using readily measurable system characteristics. The objective of this effort was to compare subsurface flow induced at scales ranging from very small scale bed forms up to much larger planform geomorphic features such as meanders. In order to compare exchange consistently over this range of scales, we employed a spectral scaling approach as the basis for a generalized analysis of topography‐induced stream‐subsurface exchange. The spectral model involves a first‐order approximation for local flow‐boundary interactions but is fully three‐dimensional and includes the lateral hyporheic zone in addition to the flow directly beneath the streambed. The primary model input parameters are stream velocity and slope, sediment permeability and porosity, and detailed measurements of the stream channel topography. The primary outputs are the distribution of water flux across the stream channel boundary, the resulting pore water flow paths, and the subsurface residence time distribution. We tested the bed‐form‐exchange component of the model using a highly detailed two‐dimensional data set for exchange with ripples and dunes and then applied the model to a three‐dimensional meandering stream in a laboratory flume. Having spatially explicit information allowed us to evaluate the contributions of both gravitational and current‐driven hyporheic flow through various classes of stream channel features including ripples, dunes, bars, and meanders. The model simulations indicate that all scales of topography between ripples and meanders have a significant effect on pore water flow fields and residence time distributions. Furthermore, complex interactions across the spectrum of topographic features play an important role in controlling the net interfacial flux and spatial distribution of hyporheic exchange. For example, shallow exchange induced by current‐driven interactions with small bed forms dominates the interfacial flux, but local pore water flows are modified significantly by larger‐scale surface‐groundwater interactions. As a result, simplified representations of the stream topography do not adequately characterize patterns and rates of hyporheic exchange.

Stochastic evaluation of subsurface contaminant discharges under physical, chemical, and biological heterogeneities

A finite element 2D Monte Carlo approach is used to evaluate the sensitivity of groundwater contaminant discharges to a Damkohler number ω and spatial variability in aquifer hydraulic conductivity, initial microbial biomass concentrations, and electron acceptor/donor concentrations. Bioattenuation is most sensitive to spatial variations in incipient biomass and critical electron donors/acceptors for ω ≥ 1 (i.e., when pore-water residence times are high compared to the time needed for microbial growth or contaminant attenuation). Under these conditions, critical reaction processes can become substrate-limited at multiple locations throughout the aquifer; which in turn increases expected contaminant discharges and their uncertainties at monitored transects. For ω ≤ 0.2, contaminant discharge is not sensitive to incipient biomass variations. Physical heterogeneities expedite plume arrival and delay departure at transects and in turn attenuate peak discharges but do not affect cumulative contaminant discharges. Physical heterogeneities do, however, induce transect mass discharge variances that are bimodal functions of time; the first peak beings consistently higher. A simple stream tube model is invoked to explain the occurrence of peaks in contaminant discharge variance.

Biological and physical effects of non-native earthworms on nitrogen cycling in riparian soils

Global nitrogen cycling is being altered by anthropogenic disturbances including invasion by non-native species. European and Asian earthworms have invaded northern temperate forests in North America with dramatic consequences for litter thickness, forest floor plant diversity, and soil nitrogen cycling. Invasive earthworms present at the boundary of terrestrial and aquatic ecosystems (i.e., riparian zones) may alter the flux of nitrogen into adjacent aquatic ecosystems. We examined how nitrogen cycling in riparian soil responds to amendments of invasive earthworms or artificial earthworm burrows. In earthworm-free riparian plots (0.25 m 2), we established treatments of invasive earthworms (60 g fresh mass·m −2), artificial burrows (120 m −2), or control plots and sampled the plots after 30 days. Before and after treatment application we measured major soil characteristics (water-filled pore space, organic matter, and pH), nitrogen pools (exchangeable NH 4 + and NO 3 −), and nitrogen transformation rates (net N-mineralization, net nitrification, and denitrification). Exchangeable NH 4 + and NO 3 − changed through time but did not differ among treatments. Net N-mineralization and net nitrification rates did not change through time and were similar across all treatments. However, denitrification rates in plots with added earthworms were 4 times greater than rates in control and burrow-only plots, which represents a large rapid increase in gaseous nitrogen flux out of these riparian soils. For all response variables, artificial burrows responded similarly to control plots, suggesting that earthworm biological activity (i.e., feeding, excretion, and mucus production) rather than physical effects (i.e., burrowing and soil aeration) drove the changes in nitrogen cycling. Examination of soil nitrogen pool and flux measurements suggest that this increase in denitrification was coupled with NH 4 + consumption by nitrifying bacteria, but future studies are needed to confirm this hypothesis. We conclude that the activity of invasive earthworms in riparian zones can increase the flux of N out of riparian zones, but the hydrologic context of the riparian zone (e.g., pore-water residence time) ultimately controls whether denitrification or nitrate leaching is the dominant flux of N.

Degradation kinetics of ptaquiloside in soil and soil solution

Ptaquiloside (PTA) is a carcinogenic norsesquiterpene glycoside produced in bracken (Pteridium aquilinum (L.) Kuhn), a widespread, aggressive weed. Transfer of PTA to soil and soil solution eventually may contaminate groundwater and surface water. Degradation rates of PTA were quantified in soil and soil solutions in sandy and clayey soils subjected to high natural PTA loads from bracken stands. Degradation kinetics in moist soil could be fitted with the sum of a fast and a slow first-order reaction; the fast reaction contributed 20 to 50% of the total degradation of PTA. The fast reaction was similar in all horizons, with the rate constant k(1F) ranging between 0.23 and 1.5/h. The slow degradation, with the rate constant k(1S) ranging between 0.00067 and 0.029/ h, was more than twice as fast in topsoils compared to subsoils, which is attributable to higher microbial activity in topsoils. Experiments with sterile controls confirmed that nonmicrobial degradation processes constituted more than 90% of the fast degradation and 50% of the slow degradation. The lower nonmicrobial degradation rate observed in the clayey compared with the sandy soil is attributed to a stabilizing effect of PTA by clay silicates. Ptaquiloside appeared to be stable in all soil solutions, in which no degradation was observed within a period of 28 d, in strong contrast to previous studies of hydrolysis rates in artificial aqueous electrolytes. The present study predicts that the risk of PTA leaching is controlled mainly by the residence time of pore water in soil, soil microbial activity, and content of organic matter and clay silicates.

Long-term efficiency and kinetic evaluation of ZVI barriers during clean-up of copper containing solutions

Sixteen continuous column experiments were carried out under dynamic flow conditions in order to study the efficiency of zero-valent iron (ZVI) permeable reactive barriers (PRBs) to remove copper from solutions. The effect of various operational parameters, such as pH of the feed solution (2.5 and 4.5), initial copper concentration (5 and 50 mg/L), pore water velocity (30.48 and 152.40 cm/day) and residence time on iron corrosion and degree of copper removal was evaluated. Breakthrough curves showed that higher initial copper concentration and pore water velocity accelerated iron corrosion and slowed down the rate of copper removal due to the formation of mineral precipitates. The maximum removal rates recorded for initial copper concentrations of 5 and 50 mg/L were 2.68 and 13.33 mg/g of ZVI/sand mixture, after 440 and 227 pore volumes, respectively. The reduction of copper follows a pseudo-first-order kinetics while the rate constants ( k obs) decrease over time from 0.60 to 0.10 h −1, indicating thus progressive passivation of the ZVI/sand reactive bed. Mineralogical studies of the exhausted reactive mixture, using XRD and SEM/EDX, confirmed the deposition of copper on the ZVI surface and the formation of iron (oxy)hydroxides.

Surficial and deep pore water circulation governs spatial and temporal scales of nutrient recycling in intertidal sand flat sediment

This study addresses organic matter decomposition in permeable sediment of a sloping intertidal sand flat (German Wadden Sea) affected by current-induced pore water exchange and pore fluid drainage. Seasonal and spatial scales of aerobic and anaerobic mineralization were investigated at 2 sites, one near the water line and one on the upper flat. Hydrodynamic forcing during inundation caused deeper oxygen penetration through flushing of the uppermost sediment layer. This flushing resulted in higher areal oxygen consumption rates and lower depth integrated sulfate reduction rates in the submerged flat compared to the rates measured during exposure. Mineralization rates in the top 15 cm of the sediment were similar between both study sites and ranged from 38 (winter) to 280 mmol C m–2 d–1 (summer), with sulfate reduction contributing 3 to 25% to total mineralization, depending on the season. At the upper flat, these seasonal differences were reflected in the pore water concentrations of nutrients, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC). Near the low water line, however, pore water nutrient and DIC concentrations were independent of the season and up to 15 times higher compared to the values recorded in the upper flat. The differences in concentrations of metabolic products between the 2 sites resulted from a low tide drainage extending deep below the uppermost flushed layer and causing seepage of pore water near the low water line. Mineralization and nutrient release in these permeable intertidal sediments is affected by 2 circulation processes that work on distinctly different temporal and spatial scales: (1) rapid ‘skin circulation’ through the uppermost sediment layer during inundation that is characterized by short flow paths, low pore water residence time and immediate feedback to the ecosystem, and (2) slow ‘body circulation’ through deeper sediment layers during low tide that is characterized by long flow paths and pore water residence times, and acts as a buffered nutrient source to the ecosystem.

Chloride Retention and Release in a Boreal Forest Soil: Effects of Soil Water Residence Time and Nitrogen and Chloride Loads

The common assumption that chloride (Cl-) is conservative in soils and can be used as a groundwater tracer is currently being questioned, and an increasing number of studies indicate that Cl- can be retained in soils. We performed lysimeter experiments with soil from a coniferous forest in southeast Sweden to determine whether pore water residence time and nitrogen and Cl- loads affected Cl- retention. Over the first 42 days there was a net retention of Cl- with retention rates averaging 3.1 mg CI- m(-2) d(-1) (68% of the added Cl- retained over 42 days). Thereafter, a net release of Cl- at similar rates was observed for the remaining experimental period (85 d). Longer soil water residence time and higher Cl- load gave higher initial retention and subsequent release rates than shorter residence time and lower Cl- load did. Nitrogen load did not affect Cl transformation rates. This study indicates that simultaneous retention and release of Cl- can occur in soils, and that rates may be considerable relative to the load. The retention of Cl- observed was probably due to chlorination of soil organic matter or ion exchange. The cause of the shift between net retention and net release is unclear, but we hypothesize that the presence of O2 or the presence of microbially available organic matter regulates Cl- retention and release rates.

Numerical modelling of groundwater vulnerability: the example Namibia

The UNSAT-H, HELP3 and MACRO4.3 computer codes, which have been developed for simulating the water balance of the unsaturated zone of soils and unconsolidated sediments, are also capable simulating water flow in low-porosity media such as fractured rock. The codes can be used to model the ability of rocks and overlying soils in the vadose zone to protect the groundwater in the uppermost aquifer. The net infiltration rates simulated by the different codes are compared against recharge determined by the chloride-balance method in Namibia. The dual-permeability code MACRO4.3 was found to produce more realistic estimates of net infiltration than the UNSAT-H and HELP3 codes, which are based on a single-permeability or effective-continuum method. The net infiltration rate together with the water storage in the unsaturated zone and the groundwater depth are used to calculate the residence time of pore water in the unsaturated zone. This parameter determines the intrinsic vulnerability of the aquifer.

Denitrification capacity in a subterranean estuary below a Rhode Island fringing salt marsh

Coastal waters are severely threatened by nitrogen (N) loading from direct groundwater discharge. The subterranean estuary, the mixing zone of fresh groundwater and sea water in a coastal aquifer, has a high potential to remove substantial N. A network of piezometers was used to characterize the denitrification capacity and groundwater flow paths in the subterranean estuary below a Rhode Island fringing salt marsh.15N-enriched nitrate was injected into the subterranean estuary (in situ push-pull method) to evaluate the denitrification capacity of the saturated zone at multiple depths (125–300 cm) below different zones (upland-marsh transition zone, high marsh, and low marsh). From the upland to low marsh, the water table became shallower, groundwater dissolved oxygen decreased, and groundwater pH, soil organic carbon, and total root biomass increased. As groundwater approached the high and low marsh, the hydraulic gradient increased and deep groundwater upwelled. In the warm season (groundwater temperature >12 °C), elevated groundwater denitrification capacity within each zone was observed. The warm season low marsh groundwater denitrification capacity was significantly higher than all other zones and depths. In the cool season (groundwater temperature <10.5 °C), elevated groundwater denitrification capacity was only found in the low marsh. Additions of dissolved organic carbon did not alter groundwater denitrification capacity suggesting that an alternative electron donor, possibly transported by tidal inundation from the root zone, may be limiting. Combining flow paths with denitrification capacity and saturated porewater residence time, we estimated that as much as 29–60 mg N could be removed from 11 of water flowing through the subterranean estuary below the low marsh, arguing for the significance of subterranean estuaries in annual watershed scale N budgets.

Advective flow through the upper continental shelf driven by storms, buoyancy, and submarine groundwater discharge

Data from monitoring wells on the Carolina, USA, continental shelf require a reevaluation of pore water exchange across the seafloor. Temperature, salinity, and 226Ra data from 3 offshore wells require rapid and frequent pore water exchange to depths of several meters through sands and other permeable sediments. The exchange is driven by tidal pumping, storms, buoyancy, and submarine groundwater discharge. Tidal pumping causes semidiurnal temperature cycles in one well that can only be explained by rapid fluid flow. The effect of large storms is evident in sudden temperature changes 2–4 m below the seabed in two wells. These changes require considerable pore water exchange. Sudden cooling of the ocean may cause pore waters to become buoyant with respect to the ocean and trigger pore water convection to several meters depth as observed in one well. The wells also display frequent large (factor of ten) changes in 226Ra activity. This requires a substantial input of 226Ra-enriched fluids to replace 226Ra that is lost during exchange events. These data support a fourth flow process, which is groundwater seeping from the underlying limestone. The pore spaces in the overlying sand provide a temporary reservoir for these fluids. We conclude that pore water residence times in 2–4 m thick sands on this continental margin are on the order of a few months or less.

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.

Zinc immobilization by zerovalent Fe: surface chemistry and mineralogy of reaction products

AbstractIn bioreactor systems for the treatment of metal-contaminated water, pretreatment with zerovalent Fe can be exploited for oxygen consumption and H2 production. In this study, a column experiment is used to investigate the changes in surface chemistry and solid phase products that result from the reaction of a Zn-sulphate-lactate solution with zerovalent Fe filings. The results of this study indicate that zerovalent Fe is very effective in immobilizing dissolved Zn with a porewater residence time of 1.3 –3.1 days. A combination of X-ray diffractometry, X-ray photoelectron spectroscopy, and mineral equilibria calculations indicates that Zn precipitates as Zn(OH)2 and zincite at pH 9 –10. At pH ≈6, Zn primarily adsorbs to abundant ferric oxyhydroxides, although incorporation in green rust is also considered. During the course of the experiment, the surface mineralogy changes from magnetitelepidocrocite- goethite to green rust-akaganéite-goethite. The results suggest that the zerovalent Fe surface becomes passivated by a surface film of ferric oxyhydroxides, green rust and organic material, so that the rate of electron transfer and proton consuming reactions (i.e. oxygen consumption, H2 generation) declines, resulting in a decrease in solution pH.

A Limestone Reactor for Fluoride Removal from Wastewaters

A two-column limestone reactor has been designed to reduce fluoride concentrations from wastewaters to below the maximum contaminant level (MCL limit) of 4 mg/L. The reactor functions by forcing calcite (CaCO3) to dissolve and fluorite (CaF2) to precipitate in the first column. The second column is not necessary to remove fluoride but is used to precipitate the calcite dissolved in the first column. This returns the treated water to its approximate initial composition. In laboratory experiments, the fluoride concentration of the effluent from feedwaters containing initial F amounts of up to 109 mg/L were brought below the MCL limit of 4 mg/L with a porewater residence time within the column of 2 h. Profile sampling shows that fluoride is reduced from 109 to 8 mg/L after only 35 min within the reactor. The major advantage of this potential technology over existing liming and ion exchange methods is that system monitoring is minimal, regular column regeneration is not required, and chemicals are not permanently added to the water. Because the CaCO3 dissolved in the first column is precipitated in the second, the reactor has potential application to reduce the concentrations from wastewaters of contaminants similar in charge and size to Ca2+ and CO32- through coprecipitation reactions. In a pilot experiment where fly ash leachate was spiked with mg/L levels of Cd, As, Cr, and Se and directed through the reactor, reductions in all elements except Cr occurred. Cd was the most notable. Influent concentrations from 2 to 30 mg/L were reduced to below detection (<0.01 mg/L).

Effect of enhanced manganese oxidation in the hyporheic zone on basin‐scale geochemical mass balance

We determined the role of the hyporheic zone (the subsurface zone where stream water and shallow groundwater mix) in enhancing microbially mediated oxidation of dissolved manganese (to form manganese precipitates) in a drainage basin contaminated by copper mining. The fate of manganese is of overall importance to water quality in Pinal Creek Basin, Arizona, because manganese reactions affect the transport of trace metals. The basin‐scale role of the hyporheic zone is difficult to quantify because stream‐tracer studies do not always reliably characterize the cumulative effects of the hyporheic zone. This study determined cumulative effects of hyporheic reactions in Pinal Creek basin by characterizing manganese uptake at several spatial scales (stream‐reach scale, hyporheic‐flow‐path scale, and sediment‐grain scale). At the stream‐reach scale a one‐dimensional stream‐transport model (including storage zones to represent hyporheic flow paths) was used to determine a reach‐averaged time constant for manganese uptake in hyporheic zones, 1/λs, of 1.3 hours, which was somewhat faster but still similar to manganese uptake time constants that were measured directly in centimeter‐scale hyporheic flow paths (1/λh = 2.6 hours), and in laboratory batch experiments using streambed sediment (1/λ = 2.7 hours). The modeled depths of subsurface storage zones (ds = 4–17 cm) and modeled residence times of water in storage zones (ts = 3–12 min) were both consistent with direct measurements in hyporheic flow paths (dh = 0–15 cm, th = 1–25 min). There was also good agreement between reach‐scale modeling and direct measurements of the percentage removal of dissolved manganese in hyporheic flow paths (fs = 8.9%, andfh = 9.3%rpar;. Manganese uptake experiments in the laboratory using sediment from Pinal Creek demonstrated (through comparison of poisoned and unpoisoned treatments) that the manganese removal process was enhanced by microbially mediated oxidation. The cumulative effect of hyporheic exchange in Pinal Creek basin was to remove approximately 20% of the dissolved manganese flowing out of the drainage basin. Our results illustrate that the cumulative significance of reactive uptake in the hyporheic zone depends on the balance between chemical reaction rates, hyporheic porewater residence time, and turnover of streamflow through hyporheic flow paths. The similarity between the hyporheic reaction timescale (1/λs ≈ 1.3 hours), and the hyporheic porewater residence timescale (ts ≈ 8 min) ensured that there was adequate time for the reaction to progress. Furthermore, it was the similarity between the turnover length for stream water flow through hyporheic flow paths (Ls = stream velocity/storage‐zone exchange coefficient ≈ 1.3 km) and the length of Pinal Creek (L ≈ 7 km), which ensured that all stream water passed through hyporheic flow paths several times. As a means to generalize our findings to other sites where similar types of hydrologic and chemical information are available, we suggest a cumulative significance index for hyporheic reactions, Rs = λstsL/Ls (dimensionless); higher values indicate a greater potential for hyporheic reactions to influence geochemical mass balance. Our experience in Pinal Creek basin suggests that values of Rs &gt; 0.2 characterize systems where hyporheic reactions are likely to influence geochemical mass balance at the drainage‐basin scale.

Pore Water Velocity and Residence Time Effects on the Degradation of 2,4-D during Transport

Model predictions of fate and transport of organic solutes in soils and groundwater are sensitive to assumptions concerning rates of microbial degradation. We studied the independent effects of residence time (RT) and pore water velocity (ν) on the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) by employing a series of unsaturated soil column experiments (continuous pulse concentration = 1 mg L-1) with varying column lengths and ν. While 2,4-D degradation under batch conditions was best described by a logistic rate expression, analysis of the 2,4-D breakthrough curves (BTCs) showed that (i) observed 2,4-D degradation rates were consistent with a first-order kinetic model and (ii) a single set of independently determined rate parameters from batch experiments could not describe 2,4-D degradation for all transport conditions. Apparent first-order degradation rate constants obtained from column data were found to be independent of column RT, but increased with decreasing ν, especially at ν < 1 cm h-1. Variations in apparent degradation rate constants with changing ν may be due to effects of ν on microbial attachment and distribution, local opportunity times required for maximum 2,4-D degradation, or nutrient desorption rates from the soil solid phase. Results from this study emphasize the difficulty in obtaining accurate model predictions using a single set of degradation rate parameters for all transport conditions, and the need to develop a better understanding of coupled processes involving contaminant degradation and transport.

Local Sulphide Oxidation Drives Carbonate Dissolution in Modern Shallow Marine Carbonates: Evidence from Oxygen and Sulphur Isotope Composition of Pore Water Sulphate

We have investigated sulphur cycling in modem carbonate sediments from the Florida Platform, U.S.A. Relations among pore water chemistry (SO24 , ZCO2, Ca2+/C1 ) and oxygen and sulphur stable isotope composition of SO 2require a direct coupling between sulphur redox cycling and syndepositional carbonate dissolution. Importantly, despite rapid rates of sulphate reduction, organic-rich carbonate sediment pore waters typically maintain SO2-/C1 ratios within 5% of seawater values, suggesting local reoxidation of H2S. Shallow shelf carbonates comprise 93% of the entire Phanerozoic carbonate rock record and about 38% of the modem oceanic carbonate accumulation, so quantifying sulphur and carbon coupling during the early marine diagenesis is vital in deciphering biogeochemical cycling (e.g. Ronov, 1980; Milliman, 1993). In low-Fe carbonates, H2S-O2 reactions can lower pH values and pore water saturation states promoting carbonate dissolution (e.g. Boudreau, 1991). Given that platform carbonate dissolution fluxes are roughly half of gross biogenic carbonate production, sulphur cycling may greatly affect carbonate preservation and accumulation (Walter and Burton, 1990; Walter et al., 1993). Pore water sulphate ~180 and 834S values were markedly shifted from the established seawater values of +9.5%o (SMOW) and +21.0%o (CDT), respectively. Chemical evolution was least in environments where the sediment-pore water system was relatively 'open' to the overlying seawater. The greatest chemical evolution was observed in bioturbated sediments colonized by Thalassia grass. Here, there was a ~lSOso 4 shift of 5%0, var iab le ~348SO 4 values (+17.7 to +23.3%o), and exceptionally high Ca2+/C1 ratios. These shifts in pore water sulphate isotopic compositions exist despite the apparent lack of net sulphate reduction (Fig. 1). The natural system isotope trend is in marked contrast to the closed system isotope evolution observed in sediment incubation experiments (Fig. 1). Importantly, sulphide oxidation is required as an acid source because pore water Ca+2/C1 and ZCO2 values exceed those attributable to the utilization of dissolved oxygen and the small amount of net sulphate reduction. The observed degree of carbonate dissolution (up to 2.3 mM excess Ca 2+) and sulphate isotopic values are best explained by a redox cycle where bacterial sulphate reduction is followed by efficient H2S oxidation resulting in relatively invariant SO24-/C1 ratios. The 02 required to locally oxidize pore water H2S may come from oxygenated seawater advection or from marine grass rhizome systems which enhance oxygen supply rates. A mass balance model demonstrates that the observed 81SOso4 values are consistent with measured sulphate reduction rates and the short pore water residence times. Pore water geochemistry and sediment incubation experiments confirm that sulphur cycling has a profound effect on the early marine diagenesis of shallow platform carbonates. The important diagenetic processes are oxic respiration, bacterial sulphate reduction, sulphide oxidation, and carbonate dissolution. In most sediments, pore water SO2-/C1 ratios are near overlying ocean values, yet in many cases SO24 has been recycled from HzS. The regularity of pore water evolution and sediment environment becomes more apparent when the oxygen and sulphur isotope compositions of SO 2are examined. Diagenetic effects are most